A fairly small increase in average global temperature can lead to changes in rainfall patterns, rises in sea levels, melting of the ice caps and widespread flooding. Evidence continues to accumulate that increases in atmospheric carbon dioxide and other "greenhouse gases" (carbon dioxide, chlorofluorocarbons, methane, nitrogen oxides, ozone, halocarbons, and others) are substantially raising the global temperature.
 A global surface warming of between 0.3 and 0.6°C (degrees Celsius) has taken place over the past century. Estimated global temperature for 1992 and 1993 remain about 0.13 and 0.19°C warmer than 1951-80 norms. 1997 was the warmest year on record. The warmest years of the century have all occurred since 1979.
The UN intergovernmental panel on climate change, underpinned by thousands of scientists worldwide, has concluded that there is a "discernible human influence" on the global climate. Corresponding to a doubling of pre-industrial levels of carbon dioxide (CO2) in the atmosphere by 2050, the IPPC has forecast that global average temperatures will rise by between 1.4 to 5.8 degrees centigrade by 2100. It is worth remembering that the last ice age, when much of the Northern Hemisphere was buried under an ice-pack thousands of feet thick, was triggered by a fall in temperature of only some five degrees centigrade.
The climate of the earth is primarily conditioned by solar radiation. The earth's atmosphere is almost transparent to the incoming heat radiation from the sun but strongly absorbs and reflects back infrared radiation from the Earth's surface. Global warming can occur when the fine balance between heat input and release is disrupted, for example through changes in the composition of the atmosphere which affect its heat transference, or the heat reflective properties of the earth (albedo), or release of extra heat from the earth's surface.
There have been many historic periods of global warming. For example, the warming referred to as the "latest Palaeocene thermal maximum" or LPTM, occurred over a 10,000 to 20,000-year interval and corresponds to the appearance of numerous mammals (including primates) and the extinction or temporary disappearance of many deep-sea species. There is strong evidence that this extraordinary warming event was linked to a massive release of methane which had been buried as a frozen solid (the form of methane clathrates) in the upper layers of the continental slope sediments. Its precipitous release within a gradual warming cycle added massive amounts of carbon to all reservoirs of the global carbon cycle and triggered exceptional changes in oxygen levels. Over several hundred thousand years, global carbon and oxygen cycles gradually returned to equilibrium conditions after the LPTM, although marine and terrestrial ecosystems were forever changed.
Compared with historic incidents of global warming, the current concern is prompted by the amount of energy released (particularly through the burning of fossil fuels) over large areas of industrially developed countries is now almost equal to the amount of solar energy that reaches these areas, causing an "industrial warming" of the surface of the planet. At the same time, many modern processes lead to the production and accumulation of certain gases in the atmosphere which allow incoming solar radiation to penetrate, but prevent heat from escaping as easily as in pre-industrial times. Thus they trap heat within the atmosphere in the same way as a greenhouse. The heat can therefore progressively build up. This can raise surface temperatures and considerably affect climatic patterns.
It is agreed that a number of influences, both natural and man-made, cause the planets temperature to vary. Natural causes include changes in solar radiation, and sulphate droplets called aerosols cast aloft by erupting volcanoes, which cool the atmosphere by reflecting sunlight. Human influences stem mostly from the emission of waste industrial gases like carbon dioxide, which trap heat in the atmosphere, and sulphate aerosols from industrial smokestacks. The combination of these processes causes complex and distinctive weather patterns. Analysis of these patterns led the IPPC in 1995 to abandon its previous view that global warming observed over the last 100 years might as easily be natural as human induced. The Hadley Center for Climate Prediction and Research, a British government organization, found that in the earlier part of the century the rise in temperature could be explained either by an increase in solar radiation or a combination of stronger solar radiation and heat trapping greenhouse gases emitted by industrial economies. For the period after the 1970's when about half of the centuries warming took place, the warming resulted from the greenhouse gases. Other scientists still feel that experts had insufficient knowledge of the magnitude of natural climatic variations, especially solar radiation, to gauge how large the human impact was by comparison. Research at the Massachusetts Institute of Technology suggests that although there was "accumulating evidence that humans were having an influence on the climate system" it would not be possible to discern its magnitude until the degree of natural climate variability could be pinned down better. The Universities of Massachusetts and Arizona tried to reconstruct the average annual surface temperatures of the northern hemisphere for the last 1000 years. Acknowledging that the margin of error was large enough to render earlier centuries data untrustworthy, concluded that the 20th century was the warmest century of the millennium, and the warmest years occurred in the 1990's particularly 1998 as the warmest year on record even when disregarding the effects of El Nino.
Global warming causes more evaporation from the tropical oceans, putting more water vapour into the atmosphere. Warmer air holds more water vapour; more water evaporates from the ocean, meaning more clouds, more rain, more snow. This leads to increased precipitation over cooler parts of the globe, including the polar regions where, in the short term at least, it is still cold enough for that precipitation to fall as snow. This increases the size of the polar ice sheets. The build up of carbon dioxide may also explain the increase in plankton blooms to an extent that two to three times more organic matter is being produced by photosynthesis than has been measured before.
With more water in the atmosphere, there is less in the soil. Those parts of the continent that are normally dry -- the eastern sides of mountains, the plains and deserts -- are even drier, as the higher average temperatures evaporate more of what rain does fall. Plants wilt and drought arrives faster than it would otherwise; and when the rain does come, it is often so intense that much of it runs off before it can soak into the soil. One consequence could be severe water shortages. According to the IPCC, the frequency of severe droughts that now occur only five percent of the time in the USA could rise to 50 percent by 2050.
What is clear is that fundamental shifts are under way in the operation of the planet. Changes in the highlands are most dramatic. The Lewis glacier on Mount Kenya has lost forty percent of its mass; in the Ruwenzori range all the glaciers are in massive retreat; Patagonia is similar. Plants distributions moving up the mountains. In the last decade it has been observed that more vegetation seems to be be growing in spring, stimulated by higher temperatures. This is matched by an increased release of carbon dioxide to the atmosphere in autumn, with the breakdown of plant materials. The earth appears to be "breathing deeper." The earth is breathing earlier, too. Spring is starting about a week earlier in the 1990s than it was in the 1970s. Migrating red-winged blackbirds arrive three weeks earlier in Michigan in 1997 than they did in 1960.
Climate change will continue so long as the greenhouse gases accumulate in the atmosphere. There is a delayed effect, and the consequences of past and present activities will be accumulating even if all greenhouse gas production were to cease today; it will take at least hundreds of years for excess carbon dioxide to be reabsorbed into the oceans or into other carbon storage. In 1985, the concentration of carbon dioxide in the atmosphere had increased by more that 25% since pre-industrial times. By 2050, if energy demand remains constant, it may increase by a further 40%, or by as much as 80% if energy consumption increases in line with population growth.
While considerable uncertainty exists concerning the rate and ultimate magnitude of the acceleration of the greenhouse effect and consequent temperature rise, estimates range from a global average 1°C rise over the next 50 years to a 4.5°C increase within the next 20 to 30 years. (A warming rate of 0.4°C per decade is believed faster than most planets are able to accommodate.) The most commonly used model forecasts a global temperature rise of 2°C by 2050 (warmer than previously recorded in the history or experienced over the last 5,000 years). It is also largely agreed that the warmer average temperatures predicted for the middle of the next century cannot be avoided.
Such high rates of change would be sufficiently disruptive that no country would likely benefit in toto from climate change. Marked regional variations in the amount of warming are expected. For example, at high latitudes the warming may be twice the global average. Also, the warming would be accompanied by changes in the amount and distribution of rainfall and in atmospheric and ocean circulation patterns. The natural variability of the atmosphere and climate will continue and be superimposed on the long-term trend forced by human activities.
A temperature rise of up to 5.8°C could melt the glaciers and the Greenland ice sheet. As the ocean warms, sea level will also rise due to thermal expansion. The IPPC predicts a sea level rise of 60 to 70 cm by 2100. This would inundate low-lying coastal lands and islands. Existing mangrove forests and coral reefs will be heavily affected and coastal water supplies would be damaged by increased salt water intrusion. Many densely populated deltas and adjacent agricultural lands would be threatened. The frequency of tropical cyclones may increase and storm tracks may change, exposing to devastating damage the less-prepared and more-populated inland areas and producing magnified impacts on coastal areas and islands by floods and storm surges. A warmer ocean would also have other potential impacts such as altering hurricane frequency, diminishing sea ice and affecting fish and ocean biota. The exact nature of these results cannot be predicted with any certainty, and will emerge slowly over several decades, but it is clear that living systems will need to adjust in a variety of ways. But in the next century, the most noticeable effect would be an increase in the frequency of extreme weather events, more and more severe floods, storms, and so on.
For many of the earth's plants and animals, a few degrees make the difference between survival and extinction, and changes of a fraction of a degree could have the long-term potential to change environmental fitness and ecological relationships. The range for non-tropical forests, for example, will shrink from 59 percent of global land area to about 47 percent. Boreal forests will virtually disappear as climatic boundaries shift towards the poles an estimated 35 kilometres every decade. By 2050, a Dutch study predicts, 24 percent of the world's parks and protected areas could see major vegetation changes because of warming. Already, alpine plants that thrive on cold weather are being found only at higher and higher elevations. The habitat range of two thirds of species of European butterflies has shifted northward by 30 to 240 kilometres. 20 species of European birds are now laying their eggs earlier in the spring. In the Antarctic, warming temperatures have affected the supply of tiny, shrimp-like krill, subsequently causing a 40 percent population drop among the Adelie penguins that feed on them.
Warming would alter current patterns of agriculture: the major wheat belts of North America and Europe may move onto the poorer northern soils, while the coffee-growing areas of Africa may disappear. There would be increased failure rates of some crops; others would mature more rapidly, but the implications for their nutritional value are as yet unknown. Algal growth in coastal areas is likely to increase. Existing erosion and desertification problems would be aggravated, and the incidence of drought could increase. Deforestation and desertification are reducing the biological storage of carbon dioxide, thereby contributing to a "vicious cycle" in the atmospheric increase of this greenhouse gas. Unsustainable agricultural practices are also contributing additional greenhouse gases, such as nitrous oxide and methane.
The rise in summertime temperatures will make certain areas more difficult to live in, although the changes in winter temperatures and snowfall patterns could make currently uninhabitable land more favoured, whilst threatening marginal alpine ecosystems and ski resorts. Heat-related mortality could increase in large cities, particularly those that come under the influence of hot humid air masses. The summer increase in mortality would not be offset by the effect of warmer winters because most winter deaths are due to infectious diseases. The spread of vector-borne disease may be greater. The is likely to be greater exposure to moulds, pollens and dust, which may caused greater morbidity in people with asthma and allergies, and increased risk of dehydration which can lead to the development of kidney stones.
In 1994, it was reported that trend studies in regions as diverse as North America, India, the Middle East and New Zealand have consistently showed strong but seasonally and geographically variable warming (with greatest warming generally in the spring season). While studies comparing these observed trends against a combination of climate forcings, including solar, volcanic, El NiÃ±o and greenhouse gases, still fail to prove conclusively that the trends are caused by the enhanced greenhouse effect, they do provide strong evidence that it is a significant factor.
Scientists at the US National Oceanic and Atmospheric Administration and the British Meteorological Office have produced evidence that 1997 was the warmest recorded year since record keeping began. Bolstered by exceptionally warm temperatures in December, the global average temperature for 1997 was at 62.45 degrees Fahrenheit (16.92 degrees centigrade) - three-quarters of a degree higher the "normal" average for the past 30 years, and 0.15 degrees warmer than the previous record high set in 1990.
While there are local differences, the average temperature in Europe has increased by 0.8 degrees centigrade since 1990. Greater precipitation has been documented in the northern half of Europe, with increases ranging from 10% to 50%. In an area stretching from the Mediterranean through central Europe into the European part of the Russian Federation and Ukraine, by contrast, precipitation has decreased by as much as 20%.
On 12 January 1999 NASA scientists announced that 1998 was the warmest recorded year in history. "There should no longer be an issue about whether global warming is occurring, but what is the rate of warming, what is its practical significance and what should be done about it," said Dr. James Hansen, a scientist with the NASA Goddard Institute for Space Studies. The NASA report concluded that the average worldwide temperature in 1998 was 58.496 degrees, topping the record of 58.154 degrees set in 1995. NASA researchers collecting data from thousands of meteorological stations around the world have concluded that although some natural phenomena may have contributed to the warming, the planet is exiting the range of normal climatic variability.
An interagency report, compiled by the US Environmental Protection Agency, notes that the warming of the US is expected to be greater in the 21st century than in the 20th, and will affect nearly every region of the country. Sea levels would rise quickly, threatening low-lying and storm-raked areas like Long Island and coastal wetlands. Storm surges will pose a greater threat to coastal communities. There will be more stifling heat waves and the disruption of snow-fed water supplies. In the mountains of New York and New England, winter would disappear and the forests that make the fall spectacular would wither. Some treasured ecosystems, such as the Rocky Mountain meadows and certain coral reefs and barrier islands, are likely to disappear entirely. Some areas are already experiencing a shorter duration of lake ice and there has been a northward shift in the distributions of some species of butterflies.
In Fairbanks Alaska the number of days 40-below in the late 1990s is half what it was in the 1950s. The permafrost is warming by an estimated two to four degrees centigrade. Sea ice is shrinking.EPn the Bering sea it has reduced by about 5% over the last 40 years.
The southern half of Greenland's ice sheet is melting according to scientists. Between 1993 and 1998 the ice cap on Greenland has shrunk substantially, by an average of two cubic miles of ice each year. Whilst acknowledging that five years is not enough to establish a trend, scientists are surprised by the magnitude of the shrinkage. Even a partial melting of the ice cap could have significant effects on the worlds climate and regulatory systems.
Almost two million homes in England and Wales are at risk from floods, and Britain will experience a 65% increase in river flooding if defences do not account for climate change.