Climate is a natural resource vital to our well-being, health and prosperity. The information gathered, managed and analysed under the coordination of WMO by the NMHSs, in collaboration with other regional and international organizations and programmes, helps decision-makers and users plan and adapt their activities and projects to expected conditions. In this way, decisions may be taken in planning which reduce risks and optimize socio-economic benefits.
Climate System
What is climate
At the simplest level the weather is what is happening to the atmosphere at any given time. Climate in a narrow sense is usually defined as the "average weather," or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time.
In a broader sense, climate is the status of the climate system which comprises the atmosphere, the hydrosphere, the cryosphere, the surface lithosphere and the biosphere. These elements determine the state and dynamics of the earths climate.
The atmosphere is the envelope of gas surrounding the earth. The hydrosphere is the part of the climate system containing liquid water at earths surface and underground (e.g oceans, rivers, lakes,). The cryosphere contains water in its frozen state (e.g. glaciers, snow, ice,). The surface lithosphere is the upper layer of solid Earth on land and oceans supporting volcanic activity which influence climate. The biosphere contains all living organisms and ecosystems over land and oceans.
Climate Classification
There are many ways of classifying the climate depending on the user and the level of understanding of the climate system at a given time. Most atlases have maps of temperature and precipitation around the world, and some contain maps of atmospheric pressure, prevailing winds, ocean currents and extent of sea ice throughout the year. Many countries have more detailed classifications for various reasons. For example, the average dates of the first and last killing frost are of value to farmers and growers, as is the average length of the frost-free growing season. In colder places, the number of days below freezing affects building design. The number of degree-days below or above some reference value (such as 18°C for heating and 22°C for cooling) provides a measure of the energy demand for heating, air conditioning and refrigeration in homes and offices.
The most often used classification scheme is that of Vladimir Köppen, first presented in the early 1900s, and revised frequently since representing five principal climate classes: tropical rain forest; hot desert flora; temperate deciduous forest; boreal forest and tundra;
tropical rain forest is a dense forest of trees containing other plants and animal species in regions of heavy year round rainfall in the tropics ( e.g amazon, Congo,) and the midlatitudes ( eg. Eastern Australia, florida, south Japan, );
hot desert flora are plants mainly composed of ground-hugging shrubs and short woody trees found in tropical arid lands;
Temperate deciduous forest are plants mostly found in temperate climate. The dominant species are broad-leaved deciduous trees;
Boreal forest or taiga are found in the northern hemisphere over areas at the interface between temperate and polar climates. The dominant plant species are coniferous trees.
Tundra is characterized by tree growth hindered low temperatures and short growing season. The vegetation contains dwarf shrubs, sedges and grasses.
All the lesser formations such as the bushlands of the maquis and the chaparral represented subdivisions of one of the main climatic types.
Perceptions of climate (you get used to the climate where you live)
Most people come to enjoy the climate where they live. However, they may have to face the challenge of climatic variability, and possibly more radical climate change. The argument that the developed world is becoming increasingly independent of climatic variability is not entirely true: despite a marked decline in mortality and social disruption resulting from climate extremes, the financial consequences of climate variability are increasing. One reason is that rising incomes are enabling people to buy properties in more vulnerable locations, such as close to the seashore, on the flood plains of rivers, or high in the mountains. The losses incurred as a result of extreme weather events in these parts of the world are rising steeply.
In many parts of the world, crowded cities with inadequate services are increasingly susceptible to weather disasters. In particular, building in flood-prone areas, particularly shantytowns without adequate early warning services and infrastructure for evacuation, increases vulnerability, especially to flash floods and mudslides such as those recently experienced in China, Madagascar, Mozambique and Venezuela.
Earth's Energy Budget
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All parameters of the earths climate (winds, rain, clouds, temperature, etc) are the result of energy transfer and transformations within the atmosphere, at the earths surface and in the oceans. Over time, the Earths climate remains largely stable because the energy received is equal to that lost. Earth is bathed in an average solar influx of 1370 watts per square meter (W/m2). As earth is spherical each square meter receives on average only about 342 W/m2.
The temperature of the earth results from a balance between energy coming into the earth from the sun (solar radiation) and the energy leaving the earth to the outer space. About half of the solar radiation striking the earth and its atmosphere is absorbed at the surface. The other half is absorbed by the atmosphere or reflected back into space by clouds, small particles in the atmosphere, snow,ice and deserts at the earth surface.
Part of the energy absorbed at the earth surface is radiated back (or re-emitted) to the atmosphere and space in the form of heat energy. The temperature we feel is a measure this heat energy. In the atmosphere, not all radiation emitted by the earth surface reaches the outer space. Part of it is reflected back to the earth surface by the atmosphere (greenhouse effect) leading to a global average temperature of about 14°C well above -19°C which would have been felt without this effect.
Given the spherical form of the earth and its position in the solar system, more solar energy is absorbed in the tropics creating differences in temperatures from the equator to the poles. Atmospheric and oceanic circulation contributes to reducing these differences by transporting heat from the tropics to the midlatitudes and the Polar Regions. These equators to pole exchanges are the main driving force of the climate system. Many changes (e.g. increase in the greenhouse effect) and feedbacks in the climate system modify the energy budget.
Circulation Patterns
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Atmospheric
In the tropical regions the planet is girdled by a belt of intense convective activity and rising air, known as the intertropical convergence zone (ITCZ). Here hot air rises, releasing its latent heat energy to the atmosphere. As this rising air cools, moisture condenses to form clouds and rainfall. Where the air rises buoyantly within short-lived convective clouds (called hot towers), rainfall can be very intense. When the rising air reaches an altitude of around 12 to 15 km and virtually all the moisture has been extracted it spreads out. Descending air on each side of the ITCZ creates zones of dry, hot air that maintain the deserts in the subtropical regions of the world. At the surface the Trade Winds flow back towards the ITCZ. First interpreted by George Hadley in 1735, this basic circulation pattern now bears his name.
Further poleward, the middle latitude depressions swirl endlessly around the globe, often steered by concentrated cores of strong westerly winds aloft known as jet streams. These rivers of air are usually found between altitudes of 9 and 12 km. Wind speeds are at a maximum during winter and often average near 180 km/h, although peak speeds can exceed twice this value. Jet streams can be very turbulent and hazardous for aircraft.
Ocean circulation
Ocean circulations transport roughly the same amount of energy towards the poles as does the atmosphere. The basic form in both hemispheres is a basin-wide vortex or ciculation known as a gyre, with wind-driven westward flow in low latitudes close to the equator and poleward-directed currents along the western margins. Beyond about 35°N and 35°S the major currents sweep eastward carrying warm water to higher latitudes. This pattern is seen most clearly in the North Atlantic and North Pacific in the form of the Gulf Stream/North Atlantic Current and the Kuroshio/North Pacific Current. To balance the poleward flow there are returning currents of cold water moving toward the equator on the eastern sides of the ocean basins. In the Southern Hemisphere, because of the virtual absence of land between 35°S and 60°S, the ocean gyres are linked with a strong circumpolar current around Antarctica. There are also regions of significant vertical motion associated with these global ocean circulations.
Hydrological cycle
The continual recycling of water between the oceans, land surface, underground aquifers, rivers and the atmosphere (the hydrological cycle) is an essential part of the climate system. Ice requires much energy to melt (latent heat of fusion) and water needs even more energy to evaporate (latent heat of vaporization), so the cycling of water through the atmosphere by evaporation and its subsequent precipitation is a significant mechanism through which energy is transported throughout the climate system.
Influences on the Earth's climate
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Atmosphere-ocean interactions
Covering some 71 per cent of the Earths surface, the oceans are a fundamental component of the climate system. Interactions between the rapidly mixing atmosphere and the slowly changing ocean basins are largely responsible for the climatic variations. The high heat capacity of the oceans damps the much higher temperature changes that would otherwise occur each day, each season and each year both in coastal areas and often farther inland. The oceans are the birthplace of all tropical cyclones and most mid-latitude storms. Half the heat transported to the poles is carried by ocean currents, which is why western Europe, for example, is such a hospitable place. The oceans are also the single most important sink for carbon dioxide produced by human action. Their carbon pool is 50 times larger than the atmospheres and, when equilibrium between these reservoirs prevails, the oceans can absorb up to 85 per cent of the additional atmospheric load. Present high emission rates, however, prevent this equilibrium and only about 30 per cent of anthropogenic emissions now seems to enter the oceans.
Land surface-atmosphere interactions
Within the atmospheric boundary layer (the first few tens of metres above the ground) there are many complex physical processes at work. Understanding these processes is an essential part of improving our knowledge of climate, developing better climate forecasting models, estimating the impact of human activities on climate, and understanding how a changing climate might affect us. On a hot day, it is cooler within a canopy of leafy trees than where the soil or grass is exposed to direct sunlight. In winter, ground frost develops first on exposed grass rather than under trees.
Until recently, the representation of the land surface in computer models of weather and climate was quite inadequate. Hoever, most coupled models now employ some representation of how vegetation controls evaporation and most can estimate river runoff for the ocean component of the model. Freshwater runoff and local rainfall affect the salinity distribution of the oceans and together are an important part of the development of the latest climate models.
The feedback process whereby climate-induced changes in vegetation affect the climate system, which further affects vegetation, potentially has large climatic implications. So far, however, it has proven difficult to incorporate this feedback process adequately in the coupled-model experiments used to estimate climate sensitivity. Also, the amount of carbon that is either extracted from the soil or stored in it by decaying vegetation is another source of considerable uncertainty. Snow, with its high reflectivity, is an important component of the land surface. Current climate models have some capability in simulating the seasonal cycle of snow extent but tend to underestimate interannual variability. These weaknesses limit confidence in the details of changes, particularly at middle and high latitudes, simulated by current climate models.
Volcanoes
Volcanoes can inject vast amounts of dust and, more significantly, sulphur dioxide into the upper atmosphere where aerosol particles remain suspended for up to several years and are spread round the entire globe forming a veil. The particles absorb sunlight and locally heat the stratosphere but at lower levels cause compensating cooling as less solar radiation reaches the Earths surface.
After large explosive tropical eruptions, the Southern Hemisphere shows a cooling (somewhat smaller than the Northern Hemisphere) in the three years following the eruptions, but the spatial patterns of the responses have been less well studied than in the Northern Hemisphere. The fact that climatically significant eruptions have, in recent centuries, occurred roughly every decade means that they are a significant factor in understanding climatic variability and climate change. Two recent eruptions, El Chicon (Mexico) in 1982, and Mount Pinatubo (Philippines) in 1991, provided the opportunity to make more detailed measurements. Mount Pinatubo appears to have injected the greatest amount of sulphur compounds into the stratosphere in the 20th century. This eruption also produced an extensive dust veil and generated significant cooling for several years. Somewhat surprisingly, however, a warming was observed over the continents of the Northern Hemisphere at higher latitudes in the first winter after the Mount Pinatubo eruption. Overall, the eruption of Mount Pinatubo caused quite a strong cooling of the global surface temperature (about 0.2°C) and in the troposphere (perhaps 0.4°C) from late 1991 to 1994.
The Sun
The output of the Sun varies on all timescales. The best-known variation is the regular fluctuation in the number of sunspots, which show up as small dark regions on the solar disk, and affect the energy output of the Sun. Other aspects of solar activity include changes in the solar magnetic field, which influence the number of cosmic rays entering the Earths atmosphere from deep space, and variations in the amount of ultraviolet radiation from the Sun that may lead to photochemical changes in the upper atmosphere. All these variations have the potential to induce fluctuations in the climate.
Does the Suns energy output vary enough to affect the climate Ground-based efforts during the first half of the century to show that there were appreciable changes in the output were plagued by problems in correcting for the effects of atmospheric absorption. It was only in 1980, with the launching of specialized satellite instruments, that it was possible to measure accurately the changes in energy radiated by the Sun. Observations now show a modulation of about 1.5 W/m2 in the solar output received by the Earth over the 11-year solar cycle. This is equivalent to about 0.1 per cent of the average incoming solar radiation (1370 W/m2). These changes cannot, however, be explained in terms of sunspots alone. Sunspots are areas of lower temperature and an increase in their number might be expected to coincide with reduced solar output. On the contrary, the energy output from the Sun peaks when the sunspot number is high.
It is now known that solar output is a balance between increases due to the development of bright areas, known as faculae, at times of high solar activity and the decrease resulting from increased sunspots. Overall the heating effect of the faculae outweighs the cooling effect of the sunspots. Estimates have also been made of the longer-term fluctuations in solar energy output over the past two or three centuries. The possibility that the Suns energy output may have varied more appreciably in the past could explain the marked parallel between these changes and estimates of the Earths surface temperature over much of the past four centuries.
Human influences
Land-use changes have led to changes in the amount of sunlight reflected from the ground (the surface albedo). The scale of these changes is estimated to be about one-fifth of the forcing on the global climate due to changes in emissions of greenhouse gases. About half of the land use changes are estimated to have occurred during the industrial era, much of it due to replacement of forests by agricultural cropping and grazing lands over Eurasia and North America. The largest effect of deforestation is estimated to be at high latitudes where the albedo of snow-covered land, previously forested, has increased. This is because snow on trees reflects only about half of the sunlight falling on it, whereas snow-covered open ground reflects about two-thirds.
Overall, the increased albedo over Eurasian and North American agricultural regions has had a cooling effect. Other significant changes in the land surface resulting from human activities include tropical deforestation which changes evapotranspiration rates, desertification which increases surface albedo, and the general effects of agriculture on soil moisture characteristics. All of these processes need to be included in climate models, but for climate change studies there are few reliable records of past changes in land use. One way to build up a better picture of the effects of past changes is to combine surface records of changing land use with satellite measurements of the properties of vegetation cover. Such analyses show that forest clearing for agriculture and irrigated farming in arid and semi-arid lands are two major sources of climatically important land cover changes. The two effects tend, however, to cancel out, because irrigated agriculture increases solar energy absorption and the amount of moisture evaporated into the atmosphere, whereas forest clearing decreases these two processes.
Human activity is also changing the composition of the atmosphere. The graph below shows the rising levels of carbon dioxide in the atmosphere caused by such factors as the increased levels of fossil fuel use.
Past Climates
Our knowledge of changes in the climate over the last few thousand years was transformed during the 20th century. A lack of appreciation that changing climate was a possibility can be attributed to, in part, an interpretation of classical literature, which appeared to describe the climate in similar terms to current experience. Studies of written works from Classical Greece through to the early years of the 20th century had concluded that there had been no change in the climate since the fifth century BC. These analyses were based on descriptions of the fertility of the country, the nature of streams and rivers, and the dates of sowing and harvests. This view changed gradually during subsequent decades, thanks principally to the painstaking work of several scholars who produced a variety of evidence to show that the climate had indeed changed on almost every timescale and in every part of the world.
The broad picture is that by around 6 000 years ago the post-glacial warming trend reached a peak. This was mainly a Northern Hemisphere summer phenomenon. On the basis of evidence of pollen from trees, the average summer temperature in middle latitudes of the Northern Hemisphere was 2°C to 3°C warmer than at present, largely because of the increased levels of summer sunshine that peaked, as a result of the Milankovitch effect, about 9 000 years ago.
Around 5 500 years ago, in the early stages of the development of the ancient civilizations of Egypt and the Middle East, the climate began to cool gradually and become drier. These changes were small compared with the sudden shifts at the end of, and during, the last Ice Age. Nevertheless, some of the changes were profound. The Sahara dried up. Desert formed where giraffes, elephants and antelope had roamed. More generally, there is evidence of a decline in rainfall in the Middle East and North Africa setting around 4 000 years ago. At the same time the poleward extent of the treeline across the Canadian Arctic and Siberia started to shift southward. This trend towards cooler, drier conditions continued until near the end of the first millennium AD but was punctuated by warmer periods. There is also considerable evidence of mountain glaciers around the world expanding around 2500 BC, but then receding to high elevations around 2000 BC. Further glacial expansions occurred around 1400 BC to 1200 BC, and between 200 BC and around AD 500800. In between these colder episodes the climate was warmer with glaciers receding to higher elevations around the world.
Although there is no sudden change in the amount of historical evidence on the climate at around AD 1000, much points to the fact that many conditions in northern Europe and around the North Atlantic became warmer during the 9th and 10th centuries. In part, this is inferred from the expansion of economic and agricultural activity throughout that region. Grain was grown farther north in Norway than is now possible. Similarly, crops were grown at levels in upland Britain that have proven uneconomic in recent centuries. The Norse colonization of Iceland in the 9th century and of Greenland at the end of the 10th century is also seen as evidence of a period of more benign climate in this region. There is little doubt the warmer conditions over much of northern Europe extended into the 11th and 12th centuries, but the geographical evidence is complicated.
The far past: palaeoclimatology
Much of what we know about climate and how it is varying is based on bitter experience. The patient collection of data, day in and day out, forms the basis for many insights on how the climate works as well as how it can change. When it comes to exploring our vulnerability to the varying climate, however, it is the extreme events that provide the most important messages. Extreme events will form a significant focus of the rest of this section and much of the next one, because they have a disproportionate impact on the lives we lead, and so represent a vital aspect of climate variability and change.
Historical documentary sources
For times before instrumental measurements of the climate were made, some of the gaps can be filled in with documentary records of events that relate to the weather. These include direct references to the weather, agricultural records, wine harvest dates and phenological records. The latter consist of compilations of annual records such as leaf opening, flowering, fruiting and leaf-fall together with climatic observations. Phenological calendars were used long ago in both China and the Roman Empire. A lot of work was done on this subject up to the mid-20th century, especially in Europe, but fell out of fashion. Recently there has been a revival of interest in the subject in both Europe and North America as part of efforts to identify the effects of global warming on flora and fauna.
Proxy data
Where we do not have instrumental observations or documentary records, much of our information about the climate has been obtained from what are known as proxy data. These are obtained by analysing a wide variety of materials whose properties are influenced by the surrounding climate. Tree rings, ice cores, ocean sediments, coral growth rings and pollens are the best known examples. Proxy data rarely ever provide a direct measure of a single meteorological parameter. For instance, the width of tree rings is a function of temperature and rainfall over the growing season, and also of ground water levels reflecting rainfall in earlier seasons. Only where the trees are growing near their climatic limit can most of the growth be attributed to a single parameter (e.g. summer temperature). For other records (e.g. analysis of the pollen content in lake sediments, or the creatures deposited in ocean sediments), drawing climatic conclusions depends on knowing the sensitivity of the plants or creatures to the climate and how their distribution might be a measure of the climate at the time.
Climate Variability and Extremes
Types of measures of variability
With adequate measurements of climatic conditions covering many years, it is possible to define what is considered normal and what is an extreme event for any part of the world. Data gathered over the 30-year period from 1961 to 1900 define the latest Normals used for climate reference. At any given time of the year, an extreme high temperature might be defined as one that occurs only once in every 30 years. A cold winter or hot summer can be specified in a similar way, or in terms of the number of days below or above defined exceptional values. This means that when there is a succession of extremes, or more extreme events over a period such as a season, it is possible to estimate whether these extremes are part of the normal expectation for the locality, or are so unlikely that they can only be explained in terms of some more radical shift in climate.
The basic properties of any data series, for example temperature, can be defined in terms of the mean over time and the amount of variance about the mean. Other meteorological variables exhibit more complicated statistical properties. For instance, rainfall is episodic. In many parts of the world, much of the annual rainfall falls in a short rainy season. In addition, most of that rain may be concentrated in a few heavy falls and small shifts in the large-scale weather patterns from year to year may significantly alter the amount and distribution of seasonal rainfall. More complex techniques usually are needed to interpret variations in rainfall.
Some facts on global climate extremes
Since extreme meteorological events may be good markers of climatic change or variability, it is important to keep good records of such extremes. A worldwide collection of such events has been assimilated by WMO in conjunction with the University of Arizona. In 2006, the WMO Commission for Climatology has developed of a world archive for verifying, certifying and storing world weather extremes. Existing record extremes are available to the general public on http://wmo.asu.edu. They cover temperature, pressure, rainfall, hail, aridity, wind, tornados and cyclones, and are displayed on maps of the world, the hemispheres and the continents.
From Climate extremes to disasters
On 10 February 1935 35 cm of snow fell on Laghouat on the edge of the Algerian Sahara. While this was certainly an extreme event, it was no disaster. Disasters occur frequently as a result of extreme climatic events, however, and also as a result of the accumulation of extreme events that constitute climatic variability or change. The word disaster is used to describe such events when they cause human sickness, death or migration on a large scale, or when they cause severe economic damage.
Although human misery cannot be adequately represented by statistics, it is helpful to have some measure of the global scale of the impact of weather-related disasters. Many data have been collected by the International Federation of the Red Cross and Red Crescent Societies (IFRC). The important features of these figures are:
droughts killed more people than all other disasters combined;
droughts and floods affected about an equal number of people, and far more than high winds (including hurricanes, cyclones, typhoons, storms and tornadoes);
floods were, however, by far the greatest cause of homelessness; and
for the limited time covered (1973-97), there were large variations in the numbers of people affected by different forms of disaster in successive five-year periods. This makes it difficult to draw any definite conclusions about trends, apart from noting that the number of people affected by floods appears to be rising.
Examples of Climate Events and extremes
When it comes to exploring our vulnerability to the varying climate, however, it is the extreme events that provide the most important messages. Extreme events have a disproportionate impact on human populations, and so represent a vital aspect of climate variability and change.
Tropical cyclones
Tropical cyclones are areas of very low atmospheric pressure over tropical and sub-tropical waters which build up into a huge, circulating mass of wind and thunderstorms up to hundreds of kilometres across. Surface winds can reach speeds of 200 km/h or more. On average 80 tropical cyclones form every year. They are called differently depending on where they are formed: typhoons in the western North Pacific and South China Sea; hurricanes in the Atlantic, Caribbean and Gulf of Mexico, and in the eastern North and central Pacific Ocean; and tropical cyclones in the Indian Ocean and South Pacific region.
Mid-latitude winter storms
Heavy rain and snow are dangerous for vulnerable communities. They can exacerbate rescue and rehabilitation activities after a major disaster, such as the earthquake in Pakistan in October 2005. They bring havoc to road and rail transportation, infrastructure and communication networks. An accumulation of snow can cause the roofs of buildings to collapse. Strong winds are a danger for aviation, sailors and fishermen, as well as for tall structures such as towers, masts and cranes. Blizzards are violent storms combining below-freezing temperatures with strong winds and blowing snow. They are a danger to people and livestock. They cause airports to close and bring havoc to roads and railways.
As an example, the East Asian winter is dominated by cold and relatively dry winds. The Siberian anticyclone is a persisting climatic feature that blows Arctic air over Siberia and northern China. Part of the flow sweeps out toward the North Pacific and part southward through China to the equatorial regions. The winter monsoon throughout this region is characterized by successive outbursts of cold air, called cold waves, that produce sharp falls in temperature of more than 10°C and are accompanied by snow and, in the south, rain. Snowstorms can be especially violent over northern China with sub-zero temperatures and gales lasting many days. The snowstorms can be damaging to communities and are particularly disruptive to transport, including coastal shipping. Periods of frost following the cold outbreak last several days and are a major hazard over the south of China as they have far-reaching effects on agriculture, especially on plants and crops. The frequency of cold waves varies greatly from year to year; as many as 10 per year or as few as one have been experienced. An active period is associated with higher pressure within the Siberian anticyclone and an intense low pressure system near the Aleutian Islands.
Droughts and duststorms
The primary cause of any drought is deficiency of rainfall. Drought is different from other hazards in that it develops slowly, sometimes over years, and its onset can be masked by a number of factors. Drought can be devastating: water supplies dry up, crops fail to grow, animals die and malnutrition and ill health become widespread.
Duststorms and sandstorms are ensembles of particles of dust or sand lifted to great heights by strong and turbulent wind. They occur mainly in parts of Africa, Australia, China and the USA. They threaten lives and health, especially of persons caught in the open and far from shelter. Transportation is particularly affected as visibility is reduced to only a few metres.
Floods
Floods can occur anywhere after heavy rains. All floodplains are vulnerable and heavy storms can cause flash flooding in any part of the world. Flash floods can also occur after a period of drought when heavy rain falls onto very dry, hard ground that the water cannot penetrate. Floods come in all sorts of forms, from small flash floods to sheets of water covering huge areas of land. They can be triggered by severe thunderstorms, tornadoes, tropical and extra-tropical cyclones (many of which can be exacerbated by the El Niño phenomenon), monsoons, ice jams or melting snow. In coastal areas, storm surges caused by tropical cyclones, tsunamis or rivers swollen by exceptionally high tides can cause flooding. Dikes can flood when the rivers feeding them carry large amounts of snowmelt. Dam breaks or sudden regulatory operations can also cause catastrophic flooding. Floods threaten human life and property worldwide.
Monsoons
The term monsoon, of Arabic origins, refers to a steady seasonal wind, and became widely associated with the Indian subcontinent and the onset of the main rainfall season. In fact, monsoon systems are a major feature of the general circulation of the atmosphere in subtropical latitudes of most regions of the world, including India, East Asia, Australia, and both North and South America ( map of the global monsoon system) .
Monsoon forecasts have improved since the early 1980s. This is the result of a growing understanding of the empirical relationships between indicators around the world and the subsequent monsoon. One reason for these advances has been the rising quality of data. Recent satellite observations have also revived interest in Himalayan snow cover as a predictor. They show that the relationship first identified by Blandford is a useful guide, but that the extent of the all-Eurasian winter snow cover was a better indicator, given the geographically uneven and variable nature of snow cover over the Himalayas, Tibet and Siberia.
Heatwaves amd cold waves/frost
Heat waves are most deadly in mid-latitude regions, where they concentrate extremes of temperature and humidity over a period of a few days in the warmer months. The oppressive air mass in an urban environment can result in many deaths, especially among the very young, the elderly and the infirm. In 2003, much of western Europe was affected by heat waves during the summer months. In France, Italy, The Netherlands, Portugal, Spain and the United Kingdom, they caused some 40 000 deaths. Extremely cold spells cause hypothermia and aggravate circulatory and respiratory diseases.
El Niño, La Niña and the El Niño Southern Oscillation (ENSO)
For centuries fishing people of the coastal communities of northern Peru and Ecuador have used the term El Niño to describe an annual warming of the offshore waters during December. El Niño is now used to describe extensive warming of the ocean surface across the eastern and central equatorial Pacific lasting three or more seasons. When this oceanic region switches to below normal temperatures, it is called La Niña. The Southern Oscillation which is a fluctuation of atmospheric pressure over the tropical indo-pacific region and El Niño are closely linked and are collectively known as the El Niño/Southern Oscillation phenomenon. ENSO swings between warm (El Niño) and cold (La Niña) conditions. WMO publishes an El Niño/La Niña update.
Modern thinking about ENSO rests on a hypothesis first put forward by Jacob Bjerknes in the mid-1960s. He noted that in normal conditions, the persistent tropical Trade Winds push the oceans surface water westward causing upwelling of cold subsurface water off the coast of Peru. During an El Niño event, the appearance of positive sea surface temperature anomalies over the eastern equatorial Pacific Ocean is accompanied by falls of atmospheric pressure and a reduction of the normal sea-level pressure gradient that drives the Trade Winds. The Trade Winds are weakened and the upwelling of cold water off the coast of Peru is reduced, thus reinforcing the initial positive temperature anomaly. The net effect of these interactions gives the appearance of large quantities of warm water slowly sloshing back and forth across the equatorial Pacific and a large east-west oscillation in the heat supply to the atmosphere from the Pacific Ocean. At the peak of an El Niño event, the tropical Pacific Ocean is warmer than normal and the global near-surface air temperature warms up as the ocean gives up heat to the atmosphere.
Madden-Julian Oscillations (MJO)
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In 1971-72, Roland Madden and Paul Julian reported that there was a roughly periodic nature to tropical convection. Over the Indian and Pacific Oceans, in particular, tropical convection was often active for about a week or so and then there was a relatively quiet period of around a month. The active regions of convection travel from west to east close to the equator with a period of roughly 30-60 days. Much of the rainfall in the tropics occurs during the active convection phases of this cycle, during many tropical cyclones are also formed. This cycle is now known as the Madden-Julian Oscillation (MJO) or Intra-seasonal Oscillation. The MJO is at its strongest from September to May. Moreover, if one of these bouts of activity hits the western Pacific just as an El Niño event is ready to hatch, it can stimulate its rapid development. The trigger factors for the MJO are not fully understood and they are largely chaotic like other aspects of atmospheric weather behaviour. However, once an active area is identified, satellite imagery can be used to predict the short-term future movement of the active regions. The MJO itself can be strong in some years and almost absent in others, thus making the forecasting of its behaviour that more difficult.
South Pacific Convergence Zone
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In the South West Pacific, there is an interesting region where the generally warm sea surface temperatures produce lower surface air pressure, and where converging, rising air produces cloud and rainfall. Its position and the associated rainfall have a major impact on the peoples of many Pacific Island countries in the region. Known as the South Pacific Convergence Zone, it runs diagonally southeast from the Solomon Islands to Samoa and beyond. While it usually shifts little during the year, its position is linked to ENSO variations. During El Niño events it is displaced east, and during La Niña events west, of its mean position. Over the longer term the zone has shown a striking eastward displacement since 1977, compared with the period 194876, again reflecting the more prevalent occurrences of El Niño events during the later period.
Intertropical Convergence Zone (ITCZ)
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In the tropical regions the planet is girdled by an extensive belt of intense convective activity and rising air, known as the intertropical convergence zone (ITCZ), that shifts with the seasonal latitude of maximum solar heating. During the northern summer the ITCZ develops into the more sustained expansion of the monsoon over much of southern Asia. Activity around the ITCZ can erupt into tropical cyclones. Similar processes operate in the Southern Hemisphere during the southern summer, but the definition of the ITCZ is less pronounced and there the number of tropical cyclones is only about half the northern figure.
In the tropics, particularly in the ITCZ, the air primarily rises buoyantly within short-lived convective clouds (called hot towers) and rainfall is often very intense. When the rising air reaches an altitude of 12 to 15 km and virtually all the moisture has been extracted it spreads out. Descending air on each side of the ITCZ creates zones of dry, hot air that maintain the deserts in the subtropical regions of the world. At the surface the Trade Winds flow back towards the ITCZ.
North Atlantic Oscillation (NAO)
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An important mode of variability in the extratropics of the Northern Hemisphere is the North Atlantic Oscillation (NAO). The NAO is a measure of the surface westerlies across the Atlantic. Positive values of the NAO index indicate stronger-than-average westerlies over the middle latitudes with low pressure anomalies in the Icelandic region and high pressure anomalies across the subtropical Atlantic. The positive phase is associated with cold winters over the northwest Atlantic and mild winters over Europe, as well as wet conditions from Iceland to Scandinavia and drier winter conditions over southern Europe. A negative index indicates weaker westerlies, a more meandering circulation pattern, often with blocking anticyclones occurring over Iceland or Scandinavia, and colder winters over northern Europe. Over the past two decades of the 20th century the pattern of wintertime atmospheric circulation variability over the North Atlantic apparently shifted in an unprecedented manner. A sharp change in the index began around 1980, since when the NAO has tended to remain in a highly positive phase. The recent upward trend in the NAO accounts for much of the regional surface warming over Europe and Asia, as well as the cooling over the northwest Atlantic. There are also clear indications that the thermohaline circulation of the North Atlantic Ocean varies significantly with the variations in the circulation patterns of the overlying atmosphere. These changes are linked not only to the NAO but also to wider circulation patterns involving both the North Pacific and ENSO, and including variations on decadal timescales.
Interdecadal Pacific Oscillation (IPO)
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The Interdecadal Pacific Oscillation (IPO) is a lengthy interdecadal fluctuation in atmospheric pressure. When the IPO is low, cooler than average sea surface temperatures occur over the central North Pacific, and vice versa. These changes extend over the entire Pacific Basin and exhibited three major phases during the 20th century. The IPO had positive phases (southeastern tropical Pacific warm) from 1922 to 1946 and 1978 to 1998, and a negative phase between 1947 and 1976. The two phases of the IPO appear to modulate year-to-year ENSO precipitation variability over Australia, and bi-decadal climate shifts in New Zealand. The positive phase enhances the prevailing west to southwest atmospheric circulation in the region, and the negative phase weakens this circulation.
Antarctic Oscillation (AAO)
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The Antarctic Oscillation (AAO) is an oscillation of atmospheric pressure in far southern latitudes. It is characterized by pressure anomalies of one sign centered in the Antarctic and anomalies of the opposite sign centered about 40-50°S. The AAO is also referred to as the Southern Annular Mode (SAM). There is a Northern Hemisphere equivalent called the Arctic Oscillation or Northern Annular Mode.
The fact that Antarctica appears to have cooled during the 1990s is claimed to relate to the fact that the AAO was largely in its positive phase during that time. Typically the Antarctic Oscillation alternates between phases about every month. But in the 1990s the positive phase occurred more often. Without the influence of the Antarctic Oscillation, it is likely the Antarctic would show the same kind of warming as the rest of the Southern Hemisphere. Before 1975, Antarctica appears to have warmed at about the same rate as the rest of the hemisphere, about 0.25 degree C per century. But since 1975, while the Antarctic showed overall cooling, the Southern Hemisphere has warmed at a rate of about 1.4 degrees per century. It has been claimed that ozone depletion in the Southern Hemisphere is keeping the Antarctic Oscillation in its positive phase for longer periods.
Indian Ocean Dipole (IOD)
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The Indian Ocean Dipole (IOD) is a coupled ocean-atmosphere phenomenon in the Indian Ocean. It is normally characterized by anomalous cooling of the sea surface in the south-eastern equatorial Indian Ocean and anomalous warming of the sea surface in the western equatorial Indian Ocean. Associated with these changes the normal convection situated over the eastern Indian Ocean warm pool shifts to the west and brings heavy rainfall over east Africa and severe droughts/forest fires over the Indonesian region.
Source: JAMSTEC
Climate System
What is climate
At the simplest level the weather is what is happening to the atmosphere at any given time. Climate in a narrow sense is usually defined as the "average weather," or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time.
In a broader sense, climate is the status of the climate system which comprises the atmosphere, the hydrosphere, the cryosphere, the surface lithosphere and the biosphere. These elements determine the state and dynamics of the earths climate.
The atmosphere is the envelope of gas surrounding the earth. The hydrosphere is the part of the climate system containing liquid water at earths surface and underground (e.g oceans, rivers, lakes,). The cryosphere contains water in its frozen state (e.g. glaciers, snow, ice,). The surface lithosphere is the upper layer of solid Earth on land and oceans supporting volcanic activity which influence climate. The biosphere contains all living organisms and ecosystems over land and oceans.
Climate Classification
There are many ways of classifying the climate depending on the user and the level of understanding of the climate system at a given time. Most atlases have maps of temperature and precipitation around the world, and some contain maps of atmospheric pressure, prevailing winds, ocean currents and extent of sea ice throughout the year. Many countries have more detailed classifications for various reasons. For example, the average dates of the first and last killing frost are of value to farmers and growers, as is the average length of the frost-free growing season. In colder places, the number of days below freezing affects building design. The number of degree-days below or above some reference value (such as 18°C for heating and 22°C for cooling) provides a measure of the energy demand for heating, air conditioning and refrigeration in homes and offices.
The most often used classification scheme is that of Vladimir Köppen, first presented in the early 1900s, and revised frequently since representing five principal climate classes: tropical rain forest; hot desert flora; temperate deciduous forest; boreal forest and tundra;
tropical rain forest is a dense forest of trees containing other plants and animal species in regions of heavy year round rainfall in the tropics ( e.g amazon, Congo,) and the midlatitudes ( eg. Eastern Australia, florida, south Japan, );
hot desert flora are plants mainly composed of ground-hugging shrubs and short woody trees found in tropical arid lands;
Temperate deciduous forest are plants mostly found in temperate climate. The dominant species are broad-leaved deciduous trees;
Boreal forest or taiga are found in the northern hemisphere over areas at the interface between temperate and polar climates. The dominant plant species are coniferous trees.
Tundra is characterized by tree growth hindered low temperatures and short growing season. The vegetation contains dwarf shrubs, sedges and grasses.
All the lesser formations such as the bushlands of the maquis and the chaparral represented subdivisions of one of the main climatic types.
Perceptions of climate (you get used to the climate where you live)
Most people come to enjoy the climate where they live. However, they may have to face the challenge of climatic variability, and possibly more radical climate change. The argument that the developed world is becoming increasingly independent of climatic variability is not entirely true: despite a marked decline in mortality and social disruption resulting from climate extremes, the financial consequences of climate variability are increasing. One reason is that rising incomes are enabling people to buy properties in more vulnerable locations, such as close to the seashore, on the flood plains of rivers, or high in the mountains. The losses incurred as a result of extreme weather events in these parts of the world are rising steeply.
In many parts of the world, crowded cities with inadequate services are increasingly susceptible to weather disasters. In particular, building in flood-prone areas, particularly shantytowns without adequate early warning services and infrastructure for evacuation, increases vulnerability, especially to flash floods and mudslides such as those recently experienced in China, Madagascar, Mozambique and Venezuela.
Earth's Energy Budget
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All parameters of the earths climate (winds, rain, clouds, temperature, etc) are the result of energy transfer and transformations within the atmosphere, at the earths surface and in the oceans. Over time, the Earths climate remains largely stable because the energy received is equal to that lost. Earth is bathed in an average solar influx of 1370 watts per square meter (W/m2). As earth is spherical each square meter receives on average only about 342 W/m2.
The temperature of the earth results from a balance between energy coming into the earth from the sun (solar radiation) and the energy leaving the earth to the outer space. About half of the solar radiation striking the earth and its atmosphere is absorbed at the surface. The other half is absorbed by the atmosphere or reflected back into space by clouds, small particles in the atmosphere, snow,ice and deserts at the earth surface.
Part of the energy absorbed at the earth surface is radiated back (or re-emitted) to the atmosphere and space in the form of heat energy. The temperature we feel is a measure this heat energy. In the atmosphere, not all radiation emitted by the earth surface reaches the outer space. Part of it is reflected back to the earth surface by the atmosphere (greenhouse effect) leading to a global average temperature of about 14°C well above -19°C which would have been felt without this effect.
Given the spherical form of the earth and its position in the solar system, more solar energy is absorbed in the tropics creating differences in temperatures from the equator to the poles. Atmospheric and oceanic circulation contributes to reducing these differences by transporting heat from the tropics to the midlatitudes and the Polar Regions. These equators to pole exchanges are the main driving force of the climate system. Many changes (e.g. increase in the greenhouse effect) and feedbacks in the climate system modify the energy budget.
Circulation Patterns
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Atmospheric
In the tropical regions the planet is girdled by a belt of intense convective activity and rising air, known as the intertropical convergence zone (ITCZ). Here hot air rises, releasing its latent heat energy to the atmosphere. As this rising air cools, moisture condenses to form clouds and rainfall. Where the air rises buoyantly within short-lived convective clouds (called hot towers), rainfall can be very intense. When the rising air reaches an altitude of around 12 to 15 km and virtually all the moisture has been extracted it spreads out. Descending air on each side of the ITCZ creates zones of dry, hot air that maintain the deserts in the subtropical regions of the world. At the surface the Trade Winds flow back towards the ITCZ. First interpreted by George Hadley in 1735, this basic circulation pattern now bears his name.
Further poleward, the middle latitude depressions swirl endlessly around the globe, often steered by concentrated cores of strong westerly winds aloft known as jet streams. These rivers of air are usually found between altitudes of 9 and 12 km. Wind speeds are at a maximum during winter and often average near 180 km/h, although peak speeds can exceed twice this value. Jet streams can be very turbulent and hazardous for aircraft.
Ocean circulation
Ocean circulations transport roughly the same amount of energy towards the poles as does the atmosphere. The basic form in both hemispheres is a basin-wide vortex or ciculation known as a gyre, with wind-driven westward flow in low latitudes close to the equator and poleward-directed currents along the western margins. Beyond about 35°N and 35°S the major currents sweep eastward carrying warm water to higher latitudes. This pattern is seen most clearly in the North Atlantic and North Pacific in the form of the Gulf Stream/North Atlantic Current and the Kuroshio/North Pacific Current. To balance the poleward flow there are returning currents of cold water moving toward the equator on the eastern sides of the ocean basins. In the Southern Hemisphere, because of the virtual absence of land between 35°S and 60°S, the ocean gyres are linked with a strong circumpolar current around Antarctica. There are also regions of significant vertical motion associated with these global ocean circulations.
Hydrological cycle
The continual recycling of water between the oceans, land surface, underground aquifers, rivers and the atmosphere (the hydrological cycle) is an essential part of the climate system. Ice requires much energy to melt (latent heat of fusion) and water needs even more energy to evaporate (latent heat of vaporization), so the cycling of water through the atmosphere by evaporation and its subsequent precipitation is a significant mechanism through which energy is transported throughout the climate system.
Influences on the Earth's climate
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Atmosphere-ocean interactions
Covering some 71 per cent of the Earths surface, the oceans are a fundamental component of the climate system. Interactions between the rapidly mixing atmosphere and the slowly changing ocean basins are largely responsible for the climatic variations. The high heat capacity of the oceans damps the much higher temperature changes that would otherwise occur each day, each season and each year both in coastal areas and often farther inland. The oceans are the birthplace of all tropical cyclones and most mid-latitude storms. Half the heat transported to the poles is carried by ocean currents, which is why western Europe, for example, is such a hospitable place. The oceans are also the single most important sink for carbon dioxide produced by human action. Their carbon pool is 50 times larger than the atmospheres and, when equilibrium between these reservoirs prevails, the oceans can absorb up to 85 per cent of the additional atmospheric load. Present high emission rates, however, prevent this equilibrium and only about 30 per cent of anthropogenic emissions now seems to enter the oceans.
Land surface-atmosphere interactions
Within the atmospheric boundary layer (the first few tens of metres above the ground) there are many complex physical processes at work. Understanding these processes is an essential part of improving our knowledge of climate, developing better climate forecasting models, estimating the impact of human activities on climate, and understanding how a changing climate might affect us. On a hot day, it is cooler within a canopy of leafy trees than where the soil or grass is exposed to direct sunlight. In winter, ground frost develops first on exposed grass rather than under trees.
Until recently, the representation of the land surface in computer models of weather and climate was quite inadequate. Hoever, most coupled models now employ some representation of how vegetation controls evaporation and most can estimate river runoff for the ocean component of the model. Freshwater runoff and local rainfall affect the salinity distribution of the oceans and together are an important part of the development of the latest climate models.
The feedback process whereby climate-induced changes in vegetation affect the climate system, which further affects vegetation, potentially has large climatic implications. So far, however, it has proven difficult to incorporate this feedback process adequately in the coupled-model experiments used to estimate climate sensitivity. Also, the amount of carbon that is either extracted from the soil or stored in it by decaying vegetation is another source of considerable uncertainty. Snow, with its high reflectivity, is an important component of the land surface. Current climate models have some capability in simulating the seasonal cycle of snow extent but tend to underestimate interannual variability. These weaknesses limit confidence in the details of changes, particularly at middle and high latitudes, simulated by current climate models.
Volcanoes
Volcanoes can inject vast amounts of dust and, more significantly, sulphur dioxide into the upper atmosphere where aerosol particles remain suspended for up to several years and are spread round the entire globe forming a veil. The particles absorb sunlight and locally heat the stratosphere but at lower levels cause compensating cooling as less solar radiation reaches the Earths surface.
After large explosive tropical eruptions, the Southern Hemisphere shows a cooling (somewhat smaller than the Northern Hemisphere) in the three years following the eruptions, but the spatial patterns of the responses have been less well studied than in the Northern Hemisphere. The fact that climatically significant eruptions have, in recent centuries, occurred roughly every decade means that they are a significant factor in understanding climatic variability and climate change. Two recent eruptions, El Chicon (Mexico) in 1982, and Mount Pinatubo (Philippines) in 1991, provided the opportunity to make more detailed measurements. Mount Pinatubo appears to have injected the greatest amount of sulphur compounds into the stratosphere in the 20th century. This eruption also produced an extensive dust veil and generated significant cooling for several years. Somewhat surprisingly, however, a warming was observed over the continents of the Northern Hemisphere at higher latitudes in the first winter after the Mount Pinatubo eruption. Overall, the eruption of Mount Pinatubo caused quite a strong cooling of the global surface temperature (about 0.2°C) and in the troposphere (perhaps 0.4°C) from late 1991 to 1994.
The Sun
The output of the Sun varies on all timescales. The best-known variation is the regular fluctuation in the number of sunspots, which show up as small dark regions on the solar disk, and affect the energy output of the Sun. Other aspects of solar activity include changes in the solar magnetic field, which influence the number of cosmic rays entering the Earths atmosphere from deep space, and variations in the amount of ultraviolet radiation from the Sun that may lead to photochemical changes in the upper atmosphere. All these variations have the potential to induce fluctuations in the climate.
Does the Suns energy output vary enough to affect the climate Ground-based efforts during the first half of the century to show that there were appreciable changes in the output were plagued by problems in correcting for the effects of atmospheric absorption. It was only in 1980, with the launching of specialized satellite instruments, that it was possible to measure accurately the changes in energy radiated by the Sun. Observations now show a modulation of about 1.5 W/m2 in the solar output received by the Earth over the 11-year solar cycle. This is equivalent to about 0.1 per cent of the average incoming solar radiation (1370 W/m2). These changes cannot, however, be explained in terms of sunspots alone. Sunspots are areas of lower temperature and an increase in their number might be expected to coincide with reduced solar output. On the contrary, the energy output from the Sun peaks when the sunspot number is high.
It is now known that solar output is a balance between increases due to the development of bright areas, known as faculae, at times of high solar activity and the decrease resulting from increased sunspots. Overall the heating effect of the faculae outweighs the cooling effect of the sunspots. Estimates have also been made of the longer-term fluctuations in solar energy output over the past two or three centuries. The possibility that the Suns energy output may have varied more appreciably in the past could explain the marked parallel between these changes and estimates of the Earths surface temperature over much of the past four centuries.
Human influences
Land-use changes have led to changes in the amount of sunlight reflected from the ground (the surface albedo). The scale of these changes is estimated to be about one-fifth of the forcing on the global climate due to changes in emissions of greenhouse gases. About half of the land use changes are estimated to have occurred during the industrial era, much of it due to replacement of forests by agricultural cropping and grazing lands over Eurasia and North America. The largest effect of deforestation is estimated to be at high latitudes where the albedo of snow-covered land, previously forested, has increased. This is because snow on trees reflects only about half of the sunlight falling on it, whereas snow-covered open ground reflects about two-thirds.
Overall, the increased albedo over Eurasian and North American agricultural regions has had a cooling effect. Other significant changes in the land surface resulting from human activities include tropical deforestation which changes evapotranspiration rates, desertification which increases surface albedo, and the general effects of agriculture on soil moisture characteristics. All of these processes need to be included in climate models, but for climate change studies there are few reliable records of past changes in land use. One way to build up a better picture of the effects of past changes is to combine surface records of changing land use with satellite measurements of the properties of vegetation cover. Such analyses show that forest clearing for agriculture and irrigated farming in arid and semi-arid lands are two major sources of climatically important land cover changes. The two effects tend, however, to cancel out, because irrigated agriculture increases solar energy absorption and the amount of moisture evaporated into the atmosphere, whereas forest clearing decreases these two processes.
Human activity is also changing the composition of the atmosphere. The graph below shows the rising levels of carbon dioxide in the atmosphere caused by such factors as the increased levels of fossil fuel use.
Past Climates
Our knowledge of changes in the climate over the last few thousand years was transformed during the 20th century. A lack of appreciation that changing climate was a possibility can be attributed to, in part, an interpretation of classical literature, which appeared to describe the climate in similar terms to current experience. Studies of written works from Classical Greece through to the early years of the 20th century had concluded that there had been no change in the climate since the fifth century BC. These analyses were based on descriptions of the fertility of the country, the nature of streams and rivers, and the dates of sowing and harvests. This view changed gradually during subsequent decades, thanks principally to the painstaking work of several scholars who produced a variety of evidence to show that the climate had indeed changed on almost every timescale and in every part of the world.
The broad picture is that by around 6 000 years ago the post-glacial warming trend reached a peak. This was mainly a Northern Hemisphere summer phenomenon. On the basis of evidence of pollen from trees, the average summer temperature in middle latitudes of the Northern Hemisphere was 2°C to 3°C warmer than at present, largely because of the increased levels of summer sunshine that peaked, as a result of the Milankovitch effect, about 9 000 years ago.
Around 5 500 years ago, in the early stages of the development of the ancient civilizations of Egypt and the Middle East, the climate began to cool gradually and become drier. These changes were small compared with the sudden shifts at the end of, and during, the last Ice Age. Nevertheless, some of the changes were profound. The Sahara dried up. Desert formed where giraffes, elephants and antelope had roamed. More generally, there is evidence of a decline in rainfall in the Middle East and North Africa setting around 4 000 years ago. At the same time the poleward extent of the treeline across the Canadian Arctic and Siberia started to shift southward. This trend towards cooler, drier conditions continued until near the end of the first millennium AD but was punctuated by warmer periods. There is also considerable evidence of mountain glaciers around the world expanding around 2500 BC, but then receding to high elevations around 2000 BC. Further glacial expansions occurred around 1400 BC to 1200 BC, and between 200 BC and around AD 500800. In between these colder episodes the climate was warmer with glaciers receding to higher elevations around the world.
Although there is no sudden change in the amount of historical evidence on the climate at around AD 1000, much points to the fact that many conditions in northern Europe and around the North Atlantic became warmer during the 9th and 10th centuries. In part, this is inferred from the expansion of economic and agricultural activity throughout that region. Grain was grown farther north in Norway than is now possible. Similarly, crops were grown at levels in upland Britain that have proven uneconomic in recent centuries. The Norse colonization of Iceland in the 9th century and of Greenland at the end of the 10th century is also seen as evidence of a period of more benign climate in this region. There is little doubt the warmer conditions over much of northern Europe extended into the 11th and 12th centuries, but the geographical evidence is complicated.
The far past: palaeoclimatology
Much of what we know about climate and how it is varying is based on bitter experience. The patient collection of data, day in and day out, forms the basis for many insights on how the climate works as well as how it can change. When it comes to exploring our vulnerability to the varying climate, however, it is the extreme events that provide the most important messages. Extreme events will form a significant focus of the rest of this section and much of the next one, because they have a disproportionate impact on the lives we lead, and so represent a vital aspect of climate variability and change.
Historical documentary sources
For times before instrumental measurements of the climate were made, some of the gaps can be filled in with documentary records of events that relate to the weather. These include direct references to the weather, agricultural records, wine harvest dates and phenological records. The latter consist of compilations of annual records such as leaf opening, flowering, fruiting and leaf-fall together with climatic observations. Phenological calendars were used long ago in both China and the Roman Empire. A lot of work was done on this subject up to the mid-20th century, especially in Europe, but fell out of fashion. Recently there has been a revival of interest in the subject in both Europe and North America as part of efforts to identify the effects of global warming on flora and fauna.
Proxy data
Where we do not have instrumental observations or documentary records, much of our information about the climate has been obtained from what are known as proxy data. These are obtained by analysing a wide variety of materials whose properties are influenced by the surrounding climate. Tree rings, ice cores, ocean sediments, coral growth rings and pollens are the best known examples. Proxy data rarely ever provide a direct measure of a single meteorological parameter. For instance, the width of tree rings is a function of temperature and rainfall over the growing season, and also of ground water levels reflecting rainfall in earlier seasons. Only where the trees are growing near their climatic limit can most of the growth be attributed to a single parameter (e.g. summer temperature). For other records (e.g. analysis of the pollen content in lake sediments, or the creatures deposited in ocean sediments), drawing climatic conclusions depends on knowing the sensitivity of the plants or creatures to the climate and how their distribution might be a measure of the climate at the time.
Climate Variability and Extremes
Types of measures of variability
With adequate measurements of climatic conditions covering many years, it is possible to define what is considered normal and what is an extreme event for any part of the world. Data gathered over the 30-year period from 1961 to 1900 define the latest Normals used for climate reference. At any given time of the year, an extreme high temperature might be defined as one that occurs only once in every 30 years. A cold winter or hot summer can be specified in a similar way, or in terms of the number of days below or above defined exceptional values. This means that when there is a succession of extremes, or more extreme events over a period such as a season, it is possible to estimate whether these extremes are part of the normal expectation for the locality, or are so unlikely that they can only be explained in terms of some more radical shift in climate.
The basic properties of any data series, for example temperature, can be defined in terms of the mean over time and the amount of variance about the mean. Other meteorological variables exhibit more complicated statistical properties. For instance, rainfall is episodic. In many parts of the world, much of the annual rainfall falls in a short rainy season. In addition, most of that rain may be concentrated in a few heavy falls and small shifts in the large-scale weather patterns from year to year may significantly alter the amount and distribution of seasonal rainfall. More complex techniques usually are needed to interpret variations in rainfall.
Some facts on global climate extremes
Since extreme meteorological events may be good markers of climatic change or variability, it is important to keep good records of such extremes. A worldwide collection of such events has been assimilated by WMO in conjunction with the University of Arizona. In 2006, the WMO Commission for Climatology has developed of a world archive for verifying, certifying and storing world weather extremes. Existing record extremes are available to the general public on http://wmo.asu.edu. They cover temperature, pressure, rainfall, hail, aridity, wind, tornados and cyclones, and are displayed on maps of the world, the hemispheres and the continents.
From Climate extremes to disasters
On 10 February 1935 35 cm of snow fell on Laghouat on the edge of the Algerian Sahara. While this was certainly an extreme event, it was no disaster. Disasters occur frequently as a result of extreme climatic events, however, and also as a result of the accumulation of extreme events that constitute climatic variability or change. The word disaster is used to describe such events when they cause human sickness, death or migration on a large scale, or when they cause severe economic damage.
Although human misery cannot be adequately represented by statistics, it is helpful to have some measure of the global scale of the impact of weather-related disasters. Many data have been collected by the International Federation of the Red Cross and Red Crescent Societies (IFRC). The important features of these figures are:
droughts killed more people than all other disasters combined;
droughts and floods affected about an equal number of people, and far more than high winds (including hurricanes, cyclones, typhoons, storms and tornadoes);
floods were, however, by far the greatest cause of homelessness; and
for the limited time covered (1973-97), there were large variations in the numbers of people affected by different forms of disaster in successive five-year periods. This makes it difficult to draw any definite conclusions about trends, apart from noting that the number of people affected by floods appears to be rising.
Examples of Climate Events and extremes
When it comes to exploring our vulnerability to the varying climate, however, it is the extreme events that provide the most important messages. Extreme events have a disproportionate impact on human populations, and so represent a vital aspect of climate variability and change.
Tropical cyclones
Tropical cyclones are areas of very low atmospheric pressure over tropical and sub-tropical waters which build up into a huge, circulating mass of wind and thunderstorms up to hundreds of kilometres across. Surface winds can reach speeds of 200 km/h or more. On average 80 tropical cyclones form every year. They are called differently depending on where they are formed: typhoons in the western North Pacific and South China Sea; hurricanes in the Atlantic, Caribbean and Gulf of Mexico, and in the eastern North and central Pacific Ocean; and tropical cyclones in the Indian Ocean and South Pacific region.
Mid-latitude winter storms
Heavy rain and snow are dangerous for vulnerable communities. They can exacerbate rescue and rehabilitation activities after a major disaster, such as the earthquake in Pakistan in October 2005. They bring havoc to road and rail transportation, infrastructure and communication networks. An accumulation of snow can cause the roofs of buildings to collapse. Strong winds are a danger for aviation, sailors and fishermen, as well as for tall structures such as towers, masts and cranes. Blizzards are violent storms combining below-freezing temperatures with strong winds and blowing snow. They are a danger to people and livestock. They cause airports to close and bring havoc to roads and railways.
As an example, the East Asian winter is dominated by cold and relatively dry winds. The Siberian anticyclone is a persisting climatic feature that blows Arctic air over Siberia and northern China. Part of the flow sweeps out toward the North Pacific and part southward through China to the equatorial regions. The winter monsoon throughout this region is characterized by successive outbursts of cold air, called cold waves, that produce sharp falls in temperature of more than 10°C and are accompanied by snow and, in the south, rain. Snowstorms can be especially violent over northern China with sub-zero temperatures and gales lasting many days. The snowstorms can be damaging to communities and are particularly disruptive to transport, including coastal shipping. Periods of frost following the cold outbreak last several days and are a major hazard over the south of China as they have far-reaching effects on agriculture, especially on plants and crops. The frequency of cold waves varies greatly from year to year; as many as 10 per year or as few as one have been experienced. An active period is associated with higher pressure within the Siberian anticyclone and an intense low pressure system near the Aleutian Islands.
Droughts and duststorms
The primary cause of any drought is deficiency of rainfall. Drought is different from other hazards in that it develops slowly, sometimes over years, and its onset can be masked by a number of factors. Drought can be devastating: water supplies dry up, crops fail to grow, animals die and malnutrition and ill health become widespread.
Duststorms and sandstorms are ensembles of particles of dust or sand lifted to great heights by strong and turbulent wind. They occur mainly in parts of Africa, Australia, China and the USA. They threaten lives and health, especially of persons caught in the open and far from shelter. Transportation is particularly affected as visibility is reduced to only a few metres.
Floods
Floods can occur anywhere after heavy rains. All floodplains are vulnerable and heavy storms can cause flash flooding in any part of the world. Flash floods can also occur after a period of drought when heavy rain falls onto very dry, hard ground that the water cannot penetrate. Floods come in all sorts of forms, from small flash floods to sheets of water covering huge areas of land. They can be triggered by severe thunderstorms, tornadoes, tropical and extra-tropical cyclones (many of which can be exacerbated by the El Niño phenomenon), monsoons, ice jams or melting snow. In coastal areas, storm surges caused by tropical cyclones, tsunamis or rivers swollen by exceptionally high tides can cause flooding. Dikes can flood when the rivers feeding them carry large amounts of snowmelt. Dam breaks or sudden regulatory operations can also cause catastrophic flooding. Floods threaten human life and property worldwide.
Monsoons
The term monsoon, of Arabic origins, refers to a steady seasonal wind, and became widely associated with the Indian subcontinent and the onset of the main rainfall season. In fact, monsoon systems are a major feature of the general circulation of the atmosphere in subtropical latitudes of most regions of the world, including India, East Asia, Australia, and both North and South America ( map of the global monsoon system) .
Monsoon forecasts have improved since the early 1980s. This is the result of a growing understanding of the empirical relationships between indicators around the world and the subsequent monsoon. One reason for these advances has been the rising quality of data. Recent satellite observations have also revived interest in Himalayan snow cover as a predictor. They show that the relationship first identified by Blandford is a useful guide, but that the extent of the all-Eurasian winter snow cover was a better indicator, given the geographically uneven and variable nature of snow cover over the Himalayas, Tibet and Siberia.
Heatwaves amd cold waves/frost
Heat waves are most deadly in mid-latitude regions, where they concentrate extremes of temperature and humidity over a period of a few days in the warmer months. The oppressive air mass in an urban environment can result in many deaths, especially among the very young, the elderly and the infirm. In 2003, much of western Europe was affected by heat waves during the summer months. In France, Italy, The Netherlands, Portugal, Spain and the United Kingdom, they caused some 40 000 deaths. Extremely cold spells cause hypothermia and aggravate circulatory and respiratory diseases.
El Niño, La Niña and the El Niño Southern Oscillation (ENSO)
For centuries fishing people of the coastal communities of northern Peru and Ecuador have used the term El Niño to describe an annual warming of the offshore waters during December. El Niño is now used to describe extensive warming of the ocean surface across the eastern and central equatorial Pacific lasting three or more seasons. When this oceanic region switches to below normal temperatures, it is called La Niña. The Southern Oscillation which is a fluctuation of atmospheric pressure over the tropical indo-pacific region and El Niño are closely linked and are collectively known as the El Niño/Southern Oscillation phenomenon. ENSO swings between warm (El Niño) and cold (La Niña) conditions. WMO publishes an El Niño/La Niña update.
Modern thinking about ENSO rests on a hypothesis first put forward by Jacob Bjerknes in the mid-1960s. He noted that in normal conditions, the persistent tropical Trade Winds push the oceans surface water westward causing upwelling of cold subsurface water off the coast of Peru. During an El Niño event, the appearance of positive sea surface temperature anomalies over the eastern equatorial Pacific Ocean is accompanied by falls of atmospheric pressure and a reduction of the normal sea-level pressure gradient that drives the Trade Winds. The Trade Winds are weakened and the upwelling of cold water off the coast of Peru is reduced, thus reinforcing the initial positive temperature anomaly. The net effect of these interactions gives the appearance of large quantities of warm water slowly sloshing back and forth across the equatorial Pacific and a large east-west oscillation in the heat supply to the atmosphere from the Pacific Ocean. At the peak of an El Niño event, the tropical Pacific Ocean is warmer than normal and the global near-surface air temperature warms up as the ocean gives up heat to the atmosphere.
Madden-Julian Oscillations (MJO)
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In 1971-72, Roland Madden and Paul Julian reported that there was a roughly periodic nature to tropical convection. Over the Indian and Pacific Oceans, in particular, tropical convection was often active for about a week or so and then there was a relatively quiet period of around a month. The active regions of convection travel from west to east close to the equator with a period of roughly 30-60 days. Much of the rainfall in the tropics occurs during the active convection phases of this cycle, during many tropical cyclones are also formed. This cycle is now known as the Madden-Julian Oscillation (MJO) or Intra-seasonal Oscillation. The MJO is at its strongest from September to May. Moreover, if one of these bouts of activity hits the western Pacific just as an El Niño event is ready to hatch, it can stimulate its rapid development. The trigger factors for the MJO are not fully understood and they are largely chaotic like other aspects of atmospheric weather behaviour. However, once an active area is identified, satellite imagery can be used to predict the short-term future movement of the active regions. The MJO itself can be strong in some years and almost absent in others, thus making the forecasting of its behaviour that more difficult.
South Pacific Convergence Zone
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In the South West Pacific, there is an interesting region where the generally warm sea surface temperatures produce lower surface air pressure, and where converging, rising air produces cloud and rainfall. Its position and the associated rainfall have a major impact on the peoples of many Pacific Island countries in the region. Known as the South Pacific Convergence Zone, it runs diagonally southeast from the Solomon Islands to Samoa and beyond. While it usually shifts little during the year, its position is linked to ENSO variations. During El Niño events it is displaced east, and during La Niña events west, of its mean position. Over the longer term the zone has shown a striking eastward displacement since 1977, compared with the period 194876, again reflecting the more prevalent occurrences of El Niño events during the later period.
Intertropical Convergence Zone (ITCZ)
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In the tropical regions the planet is girdled by an extensive belt of intense convective activity and rising air, known as the intertropical convergence zone (ITCZ), that shifts with the seasonal latitude of maximum solar heating. During the northern summer the ITCZ develops into the more sustained expansion of the monsoon over much of southern Asia. Activity around the ITCZ can erupt into tropical cyclones. Similar processes operate in the Southern Hemisphere during the southern summer, but the definition of the ITCZ is less pronounced and there the number of tropical cyclones is only about half the northern figure.
In the tropics, particularly in the ITCZ, the air primarily rises buoyantly within short-lived convective clouds (called hot towers) and rainfall is often very intense. When the rising air reaches an altitude of 12 to 15 km and virtually all the moisture has been extracted it spreads out. Descending air on each side of the ITCZ creates zones of dry, hot air that maintain the deserts in the subtropical regions of the world. At the surface the Trade Winds flow back towards the ITCZ.
North Atlantic Oscillation (NAO)
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An important mode of variability in the extratropics of the Northern Hemisphere is the North Atlantic Oscillation (NAO). The NAO is a measure of the surface westerlies across the Atlantic. Positive values of the NAO index indicate stronger-than-average westerlies over the middle latitudes with low pressure anomalies in the Icelandic region and high pressure anomalies across the subtropical Atlantic. The positive phase is associated with cold winters over the northwest Atlantic and mild winters over Europe, as well as wet conditions from Iceland to Scandinavia and drier winter conditions over southern Europe. A negative index indicates weaker westerlies, a more meandering circulation pattern, often with blocking anticyclones occurring over Iceland or Scandinavia, and colder winters over northern Europe. Over the past two decades of the 20th century the pattern of wintertime atmospheric circulation variability over the North Atlantic apparently shifted in an unprecedented manner. A sharp change in the index began around 1980, since when the NAO has tended to remain in a highly positive phase. The recent upward trend in the NAO accounts for much of the regional surface warming over Europe and Asia, as well as the cooling over the northwest Atlantic. There are also clear indications that the thermohaline circulation of the North Atlantic Ocean varies significantly with the variations in the circulation patterns of the overlying atmosphere. These changes are linked not only to the NAO but also to wider circulation patterns involving both the North Pacific and ENSO, and including variations on decadal timescales.
Interdecadal Pacific Oscillation (IPO)
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The Interdecadal Pacific Oscillation (IPO) is a lengthy interdecadal fluctuation in atmospheric pressure. When the IPO is low, cooler than average sea surface temperatures occur over the central North Pacific, and vice versa. These changes extend over the entire Pacific Basin and exhibited three major phases during the 20th century. The IPO had positive phases (southeastern tropical Pacific warm) from 1922 to 1946 and 1978 to 1998, and a negative phase between 1947 and 1976. The two phases of the IPO appear to modulate year-to-year ENSO precipitation variability over Australia, and bi-decadal climate shifts in New Zealand. The positive phase enhances the prevailing west to southwest atmospheric circulation in the region, and the negative phase weakens this circulation.
Antarctic Oscillation (AAO)
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The Antarctic Oscillation (AAO) is an oscillation of atmospheric pressure in far southern latitudes. It is characterized by pressure anomalies of one sign centered in the Antarctic and anomalies of the opposite sign centered about 40-50°S. The AAO is also referred to as the Southern Annular Mode (SAM). There is a Northern Hemisphere equivalent called the Arctic Oscillation or Northern Annular Mode.
The fact that Antarctica appears to have cooled during the 1990s is claimed to relate to the fact that the AAO was largely in its positive phase during that time. Typically the Antarctic Oscillation alternates between phases about every month. But in the 1990s the positive phase occurred more often. Without the influence of the Antarctic Oscillation, it is likely the Antarctic would show the same kind of warming as the rest of the Southern Hemisphere. Before 1975, Antarctica appears to have warmed at about the same rate as the rest of the hemisphere, about 0.25 degree C per century. But since 1975, while the Antarctic showed overall cooling, the Southern Hemisphere has warmed at a rate of about 1.4 degrees per century. It has been claimed that ozone depletion in the Southern Hemisphere is keeping the Antarctic Oscillation in its positive phase for longer periods.
Indian Ocean Dipole (IOD)
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The Indian Ocean Dipole (IOD) is a coupled ocean-atmosphere phenomenon in the Indian Ocean. It is normally characterized by anomalous cooling of the sea surface in the south-eastern equatorial Indian Ocean and anomalous warming of the sea surface in the western equatorial Indian Ocean. Associated with these changes the normal convection situated over the eastern Indian Ocean warm pool shifts to the west and brings heavy rainfall over east Africa and severe droughts/forest fires over the Indonesian region.
Source: JAMSTEC
