1. An introduction to climate change
3. Greenhouse gases and aerosols
4. How will greenhouse gas levels change in the future?
5. How will the climate change?
6. Has climate change already begun?
7. The evidence from climate models
8. The evidence from past climates
9. Adapting to the impacts of climate change
10. Agriculture and food security
11. Sea levels, oceans, and coastal areas
12. Biological diversity and ecosystems
15. Infrastructure, industry, and human settlements
16. Climatic disasters and extreme events
17. The international response to climate change: a history
18. The Climate Change Convention
19. The Conference of the Parties (COP)
20. Sharing and reviewing critical information
22. How human actions produce greenhouse gases
23. The challenge for policymakers
24. Crafting policies that cost-effective
25. New energy technologies and policies
26. New transportation technologies and policies
27. New approaches to forestry and agriculture
28. Financing action under the Convention
29. Global cooperation on technology
30. Data on Greenhouse gas emissions and sources
Updated in July 1999. Sponsored by the UN Environment Programme, the UN Development Programme, the UN Department of Economic and Social Affairs, the UN Institute for Training and Research, the World Meteorological Organization, the World Health Organization, and the Climate Change Secretariat (UNFCCC). For additional copies, please contact UNEP's Information Unit for Conventions, International Environment House (Geneva), C.P. 356, 1219 Châtelaine, Switzerland; tel. (+41-22) 917-8244/8196/1234; fax (+41-22) 797 3464; e-mail iuc@unep.ch; web site http://www.unep.ch/iuc/. These information sheets were written by Roberto Acosta, Myles Allen, Anilla Cherian, Sarah Granich, Irving Mintzer, Alevino Suarez, and David von Hippel, and edited by Michael Williams. Each sheet was reviewed by at least two experts.
The most profound global threat facing humanity today is the prospect that our economic activities will result in global warming, with serious consequences for the earth’s entire ecosystem and for the way of life in rich and poor societies alike.
The expected consequences – rising sea levels, depleted agriculture, reduced water flows, increased health hazards, turbulent weather, social strains – suggest that both developed and developing countries have good reason to worry about climate change.
Many scientists suspect that recent temperature increases and changes in climate variability in various parts of the world may be the first signals of such global climate change.
The stakes are high. We cannot allow damage to the systems that support human life to become irreversible knowing that the cost of implementing future adaptive measures will be prohibitive.
In this regard, the Kyoto Protocol to the Climate Change Convention goes beyond mere calls for action. It relies on legally binding commitments to arrest and then reverse the upward surge in emissions that started in the industrialized countries 150 years ago.
Now it is up to policymakers everywhere to refine and launch the many “win-win” solutions available to them. Abandoning counterproductive incentives and subsidies, removing barriers to market efficiency, and promoting investments in energy efficiency can limit emissions while benefiting the national economy.
Economists have much to offer policymakers by analyzing win-win policies, market mechanisms, and other solutions. The Kyoto Protocol is the first instance where governments have agreed to use economic instruments to implement their commitments. Developing these instruments will give stake-holders more opportunities for achieving cost efficiencies.
One of the most important tasks facing policymakers will be to engage the energies of business, local government, and civil society. Industry leaders must be convinced to adjust their investment and marketing strategies and to develop more energy-efficient vehicles, consumer goods, and production processes. At the local government and community level, the Protocol should be seen as a harbinger of increased pressure to make urban transport systems, public buildings, and town planning more energy efficient and environmentally friendly.
Most importantly, individual households must contribute to emissions reduction through their power of consumer choice and their personal lifestyle decisions.
For its part, the United Nations Environment Programme is fully committed to strengthening its support to the Intergovernmental Panel on Climate Change and its contribution to Convention-related activities, including public information services. Only by working together in this way can the international community effectively address the global challenge of climate change.
Klaus Töpfer
Executive Director
United Nations Environment Programme (UNEP)
Human activities are releasing greenhouse gases into the atmosphere. Carbon dioxide is produced when fossil fuels are used to generate energy and when forests are cut down and burned. Methane and nitrous oxide are emitted from agricultural activities, changes in land use, and other sources. Artificial chemicals called halocarbons (CFCs, HFCs, PFCs) and other long-lived gases such as sulphur hexafluoride (SF6) are released by industrial processes. Ozone in the lower atmosphere is generated indirectly by automobile exhaust fumes.
Rising levels of greenhouse gases are expected to cause climate change. By absorbing infrared radiation, these gases control the flow of natural energy through the climate system. The climate must somehow adjust to the “thicker blanket” of greenhouse gases in order to maintain the balance between energy arriving from the sun and energy escaping back into space.
Climate models predict that the global temperature will rise by about 1-3.5·C by the year 2100.This projected change is larger than any climate change experienced over the last 10,000 years. It is based on current emissions trends and assumes that no efforts are made to limit greenhouse gas emissions. There are many uncertainties about the scale and impacts of climate change, particularly at the regional level. Because of the delaying effect of the oceans, surface temperatures do not respond immediately to greenhouse gas emissions, so climate change will continue for many decades after atmospheric concentrations have stabilized. Meanwhile, the balance of the evidence suggests that the climate may have already started responding to past emissions.
Climate change is likely to have a significant impact on the global environment. In general, the faster the climate changes, the greater will be the risk of damage. The mean sea level is expected to rise 15-95 cm by the year 2100, causing flooding of low-lying areas and other damage. Climatic zones (and thus ecosystems and agricultural zones) could shift towards the poles by 150-550 km in the mid-latitude regions. Forests, deserts, rangelands, and other unmanaged ecosystems would face new climatic stresses. As a result, many will decline or fragment, and individual species will become extinct.
Human society will face new risks and pressures.Food security is unlikely to be threatened at the global level, but some regions are likely to experience food shortages and hunger. Water resources will be affected as precipitation and evaporation patterns change around the world. Physical infrastructure will be damaged, particularly by sea-level rise and by extreme weather events. Economic activities, human settlements, and human health will experience many direct and indirect effects. The poor and disadvantaged are the most vulnerable to the negative consequences of climate change.
People and ecosystems will need to adapt to future climatic regimes. Past and current emissions have already committed the earth to some degree of climate change in the 21st century. Adapting to these effects will require a good understanding of socio-economic and natural systems, their sensitivity to climate change, and their inherent ability to adapt. Many strategies are available for adapting to the expected effects of climate change.
Stabilizing atmospheric concentrations of greenhouse gases will demand a major effort.Based on current trends, the total climatic impact of rising greenhouse gas levels will be equal to that caused by a doubling of pre-industrial CO2 concentrations by 2030, and a trebling or more by 2100. Freezing global CO2 emissions at their current levels would postpone CO2-doubling to 2100; emissions would eventually have to fall to about 30% of their current levels for concentrations to stabilize at doubled-CO2 levels sometime in the future. Given an expanding world economy and growing populations, this would require dramatic improvements in energy efficiency and fundamental changes in other economic sectors.
The international community is tackling this challenge through the Climate Change Convention. Adopted in 1992 and now boasting over 175 members, the Convention seeks to stabilize atmospheric concentrations of greenhouse gases at safe levels. It commits developed countries to take measures aimed at returning their emissions to 1990 levels by the year 2000. It further requires all countries to limit their emissions, gather relevant information, develop strategies for adapting to climate change, and cooperate on research and technology.
The 1997 Kyoto Protocol will require stronger action in the post-2000 period. The Parties to the Convention have agreed by consensus that developed countries will have a legally binding commitment to reduce their collective emissions of six greenhouse gases by at least 5% compared to 1990 levels by the period 2008-2012. The Protocol also establishes an emissions trading regime and a "clean development mechanism".
Many options for limiting emissions are available in the short- and medium-term.Policymakers can encourage energy efficiency and other climate-friendly trends in both the supply and consumption of energy. Key consumers of energy include industries, homes, offices, vehicles, and farms. Efficiency can be improved in large part by providing an appropriate economic and regulatory framework for consumers and investors. This framework should promote cost-effective actions, the best current and future technologies, and “no regrets” solutions that make economic and environmental sense irrespective of climate change. Taxes, regulatory standards, tradable emissions permits, information programmes, voluntary programmes, and the phase-out of counterproductive subsidies can all play a role. Changes in practices and lifestyles, from better urban transport planning to personal habits such as turning out the lights, are also important.
Energy efficiency gains of 10-30% above baseline trends can be realized over the next 20-30 years at no net cost. Some researchers believe that much greater gains are also feasible during this period and beyond. Improvements over the baseline can be achieved in all major economic sectors with current knowledge and with today’s best technologies. In the longer term, it will be possible to move close to a zero-emissions industrial economy – with the innumerable environmental and economic benefits that this implies.
Reducing uncertainties about climate change, its impacts, and the costs of various response options is vital. In the meantime, it will be necessary to balance concerns about risks and damages with concerns about economic development. The prudent response to climate change, therefore, is to adopt a portfolio of actions aimed at controlling emissions, adapting to impacts, and encouraging scientific, technological, and socio-economic research.
The earth's climate is driven by a continuous flow of energy from the sun. This energy arrives mainly in the form of visible light. About 30% is immediately scattered back into space, but most of the 70% that is absorbed passes down through the atmosphere to warm the earth's surface.
The earth must send this energy back out into space in the form of infrared radiation. Being much cooler than the sun, the earth does not emit energy as visible light. Instead, it emits infrared, or thermal radiation. This is the heat thrown off by an electric fire or grill before the bars begin to glow red.
"Greenhouse gases" in the atmosphere block infrared radiation from escaping directly from the surface to space. Infrared radiation cannot pass straight through the air like visible light. Instead, most departing energy is carried away from the surface by air currents and clouds, eventually escaping to space from altitudes above the thickest layers of the greenhouse gas blanket.
The main greenhouse gases are water vapour, carbon dioxide, ozone, methane, nitrous oxide, and the chlorofluorocarbons (CFCs).Apart from CFCs all of these gases occur naturally. Together, they make up less than 1% of the atmosphere. This is enough to produce a "natural greenhouse effect" that keeps the planet some 30oC warmer than it would otherwise be - essential for life as we know it.
Levels of all key greenhouse gases (with the possible exception of water vapour) are rising as a direct result of human activity.Emissions of carbon dioxide (mainly from burning coal, oil, and natural gas), methane and nitrous oxide (due to agriculture and changes in land use), ozone (generated by the fumes in automobile exhausts) and CFCs (manufactured by industry) are changing how the atmosphere absorbs energy. Water vapour levels may also be rising because of a "positive feedback". This is all happening at an unprecedented speed. The result is known as the "enhanced greenhouse effect".
The climate system must adjust to rising greenhouse gas levels to keep the global "energy budget" in balance. In the long term, the earth must get rid of energy at the same rate at which it receives energy from the sun. Since a thicker blanket of greenhouse gases helps to reduce energy loss to space, the climate must change somehow to restore the balance between incoming and outgoing energy.
This adjustment will include a "global warming" of the earth's surface and lower atmosphere. But this is only part of the story. Warming up is the simplest way for the climate to get rid of the extra energy. But even a small rise in temperature will be accompanied by many other changes: in cloud cover and wind patterns, for example. Some of these changes may act to enhance the warming (positive feedbacks), others to counteract it (negative feedbacks).
Meanwhile, industrially-generated "sulphate aerosols" may have a local cooling effect. Sulphur emissions from coal― and oil―fired power stations produce clouds of microscopic particles that reflect sunlight back out into space. This partly compensates for greenhouse warming. These sulphate aerosols, however, remain in the atmosphere for a relatively short time compared to the long-lived greenhouse gases. They also cause problems, such as acid rain. This means we should not rely on sulphate aerosols to keep the climate cool indefinitely.
Climate models predict that the global average temperature will rise by about 2oC (3.6oF) by the year 2100 if current emission trends continue. This projection uses 1990 as a baseline. It also takes into account climate feedbacks and the effects of sulphate aerosols as they are presently understood. Because there are still many uncertainties, current estimates of how much it will warm during the 21st century range from 1 to 3.5oC.
Past emissions have already committed us to some climate change. The climate does not respond immediately to emissions. It will therefore continue to change for many years even if greenhouse gas emissions are reduced and atmospheric levels stop rising. Some important impacts of climate change, such as a predicted rise in sea level, will take even longer to be fully realized.
There is evidence that climate change has already begun. The climate varies naturally, making it difficult to identify the effects of rising greenhouse gases. But the pattern of temperature trends over the past few decades does resemble the pattern of greenhouse warming predicted by models. These trends are unlikely to be due entirely to known sources of natural variability. While many uncertainties remain, scientists believe that "the balance of the evidence suggests a discernible human influence on global climate."
It is still too early to predict the size and timing of climate change in specific regions. Current climate models are only able to predict patterns of change for the continental scale. Predicting how climate change will affect the weather in a particular region is much more difficult. Thus the practical consequences of "global warming" for individual countries or regions remain very uncertain.
Greenhouse gases (GHGs) control energy flows in the atmosphere by absorbing infra―red radiation.These trace gases comprise less than 1% of the atmosphere. Their levels are determined by a balance between "sources" and "sinks". Sources are processes that generate greenhouse gases; sinks are processes that destroy or remove them. Humans affect greenhouse gas levels by introducing new sources or by interfering with natural sinks.
The largest contributor to the natural greenhouse effect is water vapour. Its presence in the atmosphere is not directly affected by human activity. Nevertheless, water vapour matters for climate change because of an important "positive feedback". Warmer air can hold more moisture, and models predict that a small global warming would lead to a rise in global water vapour levels, further adding to the enhanced greenhouse effect. On the other hand, it is possible that some regions may become drier. Because modelling climate processes involving clouds and rainfall is particularly difficult, the exact size of this crucial feedback remains unknown.
Carbon dioxide is currently responsible for over 60% of the "enhanced" greenhouse effect, which is responsible for climate change.This gas occurs naturally in the atmosphere, but burning coal, oil, and natural gas is releasing the carbon stored in these "fossil fuels" at an unprecedented rate. Likewise, deforestation releases carbon stored in trees. Current annual emissions amount to over 7 billion tonnes of carbon, or almost 1% of the total mass of carbon dioxide in the atmosphere.
Carbon dioxide produced by human activity enters the natural carbon cycle. Many billions of tonnes of carbon are exchanged naturally each year between the atmosphere, the oceans, and land vegetation. The exchanges in this massive and complex natural system are precisely balanced; carbon dioxide levels appear to have varied by less than 10% during the 10,000 years before industrialization. In the 200 years since 1800, however, levels have risen by almost 30%. Even with half of humanity's carbon dioxide emissions being absorbed by the oceans and land vegetation, atmospheric levels continue to rise by over 10% every 20 years.
A second important human influence on climate is aerosols. These clouds of microscopic particles arenot a greenhouse gas. In addition to various natural sources, they are produced from sulphur dioxide emitted mainly by power stations, and by the smoke from deforestation and the burning of crop wastes. Aerosols settle out of the air after only a few days, but they are emitted in such massive quantities that they have a substantial impact on climate.
Aerosols cool the climate locally by scattering sunlight back into space. Aerosol particles block sunlight directly and also provide "seeds" for clouds to form, and often these clouds also have a cooling effect. Over heavily industrialized regions, aerosol cooling may counteract nearly all of the warming effect of greenhouse gas increases to date.
Methane is a powerful greenhouse gas whose levels have already doubled. The main "new" sources of methane are agricultural, notably flooded rice paddies and expanding herds of cattle. Emissions from waste dumps and leaks from coal mining and natural gas production also contribute. The main sink for methane is chemical reactions in the atmosphere, which are very difficult to model and predict.
Methane from past emissions currently contributes 15―20% of the enhanced greenhouse effect. The rapid rise in methane started more recently than the rise in carbon dioxide, but methane's contribution has been catching up fast. However, methane has an effective atmospheric lifetime of only 12 years, whereas carbon dioxide survives much longer. This means that the relative importance of methane versus carbon dioxide emissions depends on the "time horizon". For example, methane emitted during the 1980s is expected to have about 80% of the impact of that decade's carbon dioxide emissions over the 20―year period 1990―2010, but only 30% over the 100―year period 1990―2090 (see figure).
Nitrous oxide, chlorofluorocarbons (CFCs), and ozone contribute the remaining 20% of the enhanced greenhouse effect. Nitrous oxide levels have risen by 15%, mainly due to more intensive agriculture. CFCs increased rapidly until the early 1990s, but levels of key CFCs have since stabilised due to tough emission controls introduced under the Montreal Protocol to protect the stratospheric ozone layer. Ozone is another naturally-occurring greenhouse gas whose levels are rising in some regions in the lower atmosphere due to air pollution, even as they decline in the stratosphere.
Humanity's greenhouse gas emissions have already disturbed the global energy budget by about 2.5 Watts per square metre. This equals about one percent of the net incoming solar energy that drives the climate system. One percent may not sound like much, but added up over the earth's entire surface, it amounts to the energy content of 1.8 million tonnes of oil every minute, or over 100 times the world's current rate of commercial energy consumption. Since greenhouse gases are only a by-product of energy consumption, it is ironic that the amount of energy humanity actually uses is tiny compared to the impact of greenhouse gases on natural energy flows in the climate system.
Future greenhouse gas emissions will depend on global population, economic, technological, and social trends.The link to population is clearest: the more people there are, the higher emissions are likely to be. The link to economic development is less clear. Rich countries generally emit more per person than do poor countries. However, countries of similar wealth can have very different emission rates depending on their geographical circumstances, their sources of energy, and the efficiency with which they use energy and other natural resources.
As a guide to policymakers, economists produce "scenarios" of future emissions. A scenario is not a prediction. Rather it is a way of investigating the implications of particular assumptions about future trends, including policies on greenhouse gases. Depending on the assumptions, a scenario can project growing, stable, or declining emissions.
Most scenarios suggest that future growth in emission rates will be dominated by what happens in developing countries. The bulk of emissions to date have come from industrialized countries. However, most future growth is likely to come from emerging economies where economic and population growth is fastest - and for which projections are most uncertain.
In a typical "non―intervention" scenario, carbon dioxide emissions rise from 7 billion tonnes of carbon per year in 1990 to 20 billion in 2100. "Non―intervention" means that no new policies are adopted to reduce emissions in response to the threat of climate change. It does not mean that nothing else changes: in this particular scenario (known as IS92a), world population doubles by 2100 while economic growth continues at 2―3% per year. (Remember that scenarios are based on assumptions, which may be quite wrong.)
This scenario leads to the equivalent of a doubling of pre―industrial CO2 concentrations by 2030, and a trebling by 2100.This includes the effects of other greenhouse gas emissions, translated into their carbon-dioxide equivalents. Even a doubling of pre-industrial carbon dioxide would take levels of long-lived greenhouse gases higher than they have been for several million years.
Different assumptions about sources and sinks give very different results. Future emissions are uncertain, and they have to be translated into future atmospheric concentrations using models of the carbon cycle and atmospheric chemistry. This introduces more uncertainty, since it is unclear how key sinks (processes that absorb or destroy greenhouse gases) will respond to a changing climate. Rising carbon dioxide levels, for example, cause plants to grow faster (the "CO2―fertilisation effect") and absorb more carbon dioxide through photosynthesis. CO2 fertilisation, together with forest re-growth in northern countries, may be absorbing up to 25% of the carbon dioxide currently produced by human activity. No-one knows how this sink will behave in the future: if more land is required for food production, the trend may reverse.
"Intervention" scenarios are designed to examine the impact of efforts to reduce greenhouse gas emissions. They depend not only on assumptions about population and economic growth, but also about how future societies will respond to the introduction of policies such as taxes on carbon-rich fossil fuels.
Existing international commitments could slightly reduce the rate of growth in emissions through the 21st century. Under the Climate Change Convention, developed countries are trying to return their greenhouse gas emissions to 1990 levels by the year 2000. If they were to succeed, the date of CO2 doubling would be postponed by less than five years. A goal of making more substantial reductions in atmospheric concentrations would clearly require all countries to make dramatically stronger cuts in their emissions.
Freezing global emissions at current levels would postpone CO2―doubling to 2100. While such a scenario is far beyond any proposals now being considered, it still would not be enough to prevent greenhouse gas concentrations from continuing to rise far beyond the year 2100. Stabilising carbon dioxide at double its pre-industrial concentration sometime in the 22nd century would require emissions to fall eventually to less than 30% of their current levels, despite growing populations and an expanding world economy.
Reducing uncertainties about climate change impacts and the costs of various response options is vital for policymakers. Stabilising or reducing emissions world-wide would have an impact on almost every human activity. To decide if it is worthwhile, we need to know how much it would cost, and how bad things will get if we let emissions grow. There are tough moral questions too: how much are we prepared to pay for the climate of the 22nd century, which only our children's children will see?
If nothing is done to reduce emissions, current climate models predict a global warming of about 2oC between 1990 and 2100.This projection takes into account the effects of aerosols and the delaying effect of the oceans. This oceanic inertia means that the earth's surface and lower atmosphere would continue to warm by a further 1-2oC even if greenhouse gas concentrations stopped rising in 2100.
The range of uncertainty in this projection is 1oC to 3.5oC. Even a 1oC rise would be larger than any century-time-scale trend for the past 10,000 years. Uncertainties about future emissions, climate feedbacks, and the size of the ocean delay all contribute to this uncertainty range.
The earth's average sea level is predicted to rise by about 50 cm by 2100. The uncertainty range is large - 15 to 95 cm - and changing ocean currents could cause local and regional sea levels to rise much more or much less than the global average. The main cause of this rise is the thermal expansion of the upper layers of the ocean as they warm, with some contribution from melting glaciers. Slightly faster melting of the Greenland and Antarctica ice sheets is likely to be balanced by increased snowfall in both regions. As the warming penetrates deeper into the oceans and ice continues to melt, sea level will continue rising well after surface temperatures have levelled off.
Regional and seasonal warming predictions are much more uncertain. Although most areas are expected to warm, some will warm much more than others. The largest warming is predicted for cold northern regions in winter. The reason is that snow and ice reflect sunlight, so less snow means more heat is absorbed from the sun, which enhances any warming: a strong positive feedback effect. By the year 2100, parts of northern Canada and Siberia are predicted to warm by up to 10oC in winter, but less than 2oC in summer.
Inland regions are projected to warm faster than oceans and coastal zones. The reason is simply the ocean delay, which prevents the sea surface from warming as fast as the land. The size of this delay depends on how deep any warming penetrates into the oceans. Over most of the oceans, the uppermost few hundred metres do not mix with the water beneath them. These upper layers will warm within just a few years, while the deep ocean stays cold. Water mixes down into the ocean depths in only a few very cold regions, such as the Atlantic south of Greenland and the Southern Ocean near Antarctica. In these regions, warming will be delayed because much more water needs to be warmed up to get the same temperature change at the surface.
Aerosols may counteract some of the effects of greenhouse warming in the vicinity of major industrialised regions. Clouds of superfine sulphate particles from burning coal and oil should counteract greenhouse warming over much of the Eastern USA, Eastern Europe, and parts of China. But since some action is likely to reduce sulphur emissions because of acid rain, the size of this effect is unpredictable.
Total precipitation is predicted to increase, but at the local level trends are much less certain.Wintertime precipitation in the far north is likely to rise, but what happens in mid-latitudes and in the tropics depends very much on the details of the particular climate model and the emissions scenario. Including the effects of aerosols, for example, significantly weakens the Asian summer monsoon in the two models which have so far run this experiment.
More rain and snow will mean wetter soil conditions in high-latitude winters, but higher temperatures may mean drier soils in summer. Local changes in soil moisture are clearly important for agriculture, but models still find it difficult to simulate them. Even the sign of the global change in summertime soil moisture - whether there will be an increase or a decrease - is uncertain.
The frequency and intensity of extreme weather events such as storms and hurricanes may change.However, models still cannot predict how. The models used to simulate climate change cannot themselves simulate these extreme weather events, so the evidence is indirect. There is some concern that patterns of extreme weather may change because the models predict changes in ocean surface temperatures and other factors that are known to affect storm and hurricane development. However, it will be many years before scientists can predict whether individual regions will become more or less stormy.
Rapid and unexpected climate transitions cannot be ruled out. The most dramatic such change, the collapse of the West Antarctic ice sheet, which would lead to a catastrophic rise in sea level, is now considered unlikely in the next 100 years. There is evidence that changes in ocean circulation which have a significant impact on regional climate (such as a weakening of the Gulf Stream that warms Europe) can take place in only a few decades, but it is unknown whether or not greenhouse warming could trigger any such change. External factors, such as a series of volcanic eruptions or a change in the power output of the sun, could also have a major impact, but the consensus is that climate change over the 21st century as a whole is likely to be dominated by the effects of greenhouse gas emissions.
The earth's climate is already adjusting to past greenhouse gas emissions. The climate system must adjust to changing greenhouse gas concentrations in order to keep the global energy budget balanced. This means that the climate is changing and will continue to change as long as greenhouse gas levels keep rising. But this is not very useful. The real question is how large the change is likely to be relative to the natural climate fluctuations that human societies and natural ecosystems have learned to adapt to.
Measurement records indicate a warming of 0.3o-0.6oC in global average temperature since 1860. This is in line with model projections of the size of warming to date, particularly when the cooling effect of sulphur emissions is included. But observations are sparse before 1900 and much of the warming occurred between 1910 and 1940, before the largest rise in greenhouse gases. There is clearly more going on than a simple, direct response to emissions. This is to be expected as the climate is a complicated and chaotic system.
Mean sea level has risen by 10 to 25 cm and mountain glaciers have retreated. As the upper layers of the oceans warm, water expands and sea level rises. Models suggest that a 0.3o-0.6oC warming should indeed result in a 10 to 25 cm sea-level rise. But other, harder-to-predict, changes also affect the real and apparent sea level, notably snowfall and ice-melt in Greenland and Antarctica and the slow "rebound" of northern continents freed from the weight of ice age glaciers. Almost all recorded mountain glaciers show a retreat over the past century but, as with sea level, this is unlikely to be only a response to changes in greenhouse gases.
The observed global warming trend is larger than the trends that models indicate could be due to natural variability. A key problem in climate change research is that scientists have no direct way of observing what would have happened if humanity had left the climate alone. There is no direct way of comparing the greenhouse "signal" with the background "noise" of natural climate variability. Instead, this background variability can be estimated by running climate change computer models with constant greenhouse gas levels. The results indicate that the warming trend of 0.3o―0.6oC per century is unlikely to be a chance fluctuation. However, indirect evidence from past climates suggests that these models underestimate the size of natural climate variability, so they may be overestimating the significance of the signal.
Climate models omit many sources of variability that could also cause apparent long-term trends.Current model-based estimates of natural variability do not include the effects of volcanic eruptions, which can cool the global climate temporarily by several tenths of a degree. They are also only beginning to include the effects of long―term changes in the power output of the sun. The sun may have been responsible for relatively cool periods during the 16th, 17th, and 19th centuries (the so-called "Little Ice Age") when the northern hemisphere may have been about 0.5oC colder than it is today. Some of the warming over the past century (about 20―30% of it, according to some recent model results) may still be a recovery from that time.
Models can also be used to predict the overall pattern of climate change. Because so many unknown factors may affect the global average temperature, scientists are reluctant to conclude that greenhouse warming has arrived on the evidence of that one number alone. Instead, they look for similarities between the pattern of change emerging in the observations and the pattern projected by climate models.
Several studies have reported increasingly close agreement between model-predicted and observed patterns of temperature change. Studies of surface temperature records show some evidence that the land is warming faster than the oceans. They also show reduced warming in areas affected by sulphate aerosols and in those ocean regions where surface water mixes down, distributing any warming to the ocean depths - all features of the model―predicted pattern. But coverage is incomplete, and observations in different regions (e.g. land vs. sea) are made in different ways. A more consistent, but much shorter, record is provided by air temperatures from meteorological stations. These show a pattern of cooling in the stratosphere (above about 10 km) and warming in the troposphere (lower atmosphere), which is also predicted by climate models (see figure).
The satellite record is still too short to reveal significant trends. The climate has to be observed over several decades before any climate change signal can be distinguished from natural variability. The longest satellite records are still well under 20 years. Models predict that it should not be possible to detect anything in such a short period, so all that can be said about the satellite data for the moment is that they are consistent with climate model projections and with evidence from conventional observations. Satellite data do provide global coverage, which helps to validate models and reduce uncertainties.
The evidence suggests that recent changes are unlikely to be entirely due to known sources of natural variability. The pattern of change seems to point to some human influence on climate similar to that projected by climate models and larger than expected from natural fluctuations. This point is not yet settled, however, mainly because of uncertainty over the ability of current models to simulate natural variability realistically. Nevertheless, it is reassuring for many modelers because it suggests that the models are pointing in roughly the right direction.
Uncertainty about the ability of models to simulate natural climate variability remains a significant problem.As with trends in global mean temperature, scientists must use climate model simulations to assess the probability of getting a certain level of agreement purely by chance between the model and the observed patterns of change. There are many sources of natural variability that these models simulate poorly or not at all, and one of these might be associated with a pattern similar to the greenhouse warming pattern. Thus there is still a wide range of uncertainty about the size and origin of the present signal and about the size of future changes.
The climate system is extremely complex.Consequently, there is no simple way of determining how much the climate will change in response to rising greenhouse gas levels. If the only changes were air and surface temperatures, it would be easy to predict a 1―1.5oC warming by 2100 assuming that current emissions trends continue. But this "direct response" figure (which is less than the current "best guess" of future warming) is almost meaningless because it is physically impossible for the climate system to warm up by over 1oC without any other changes.
Complex computer simulations are therefore essential for understanding climate change. Computers allow scientists to model the many interactions between different components of the climate system. The most detailed projections are based on coupled atmosphere-ocean general circulation models (AOGCMs). These are similar to the models used to predict the weather, in which the physical laws governing the motion of the atmosphere are reduced to systems of equations to be solved on supercomputers. However, climate models must also include equations representing the behaviour of the oceans, land vegetation, and the cryosphere (sea ice, glaciers, and ice caps).
"Positive feedbacks" involving water vapour, snow, and ice may amplify the direct response to greenhouse gas emissions by a factor of two to three. Snow and ice reflect sunlight very effectively. If a small warming melts snow earlier in the year, more energy will be absorbed by the ground exposed underneath it, in turn causing more warming. This is the main reason wintertime northern regions are expected to warm the most. The water vapour feedback is even more important: water vapour is itself a powerful greenhouse gas, and models project that global warming will raise water vapour levels in the lower atmosphere.
Changes in cloud cover, ocean currents, and chemistry and biology, may either amplify or reduce the response.Models generally predict that cloudiness will change in a warmer world, but depending on the type and location of the clouds, this could have various effects. Clouds reflect sunlight, implying that more clouds would have a cooling effect. But most clouds, particularly those at high altitudes, also have an insulating effect: being very cold, they shed energy to space relatively ineffectively, thus helping to keep the planet warm. So the net cloud feedback could go either way. Clouds are the main reason for the large uncertainty about the size of warming under any given emissions scenario.
The speed and timing of climate change strongly depends on how the oceans respond. The uppermost layers of the oceans interact with the atmosphere every year and so are expected to warm along with the earth's surface. But it takes over 40 times as much energy to warm the top 100 m of the ocean as to warm the entire atmosphere by the same amount. With ocean depths reaching several kilometres, the oceans will therefore slow down any atmospheric warming. How much they slow it down depends on how deeply the warming penetrates. The latest climate models are only just beginning to represent the processes which exchange energy between the atmosphere and ocean depths, so this remains an important source of uncertainty.
Climate projections must begin from a stable and realistic simulation of the present-day climate, which is not easy to obtain. Ideally, scientists would like to allow a model to settle down with pre-industrial levels of greenhouse gases and then increase greenhouse gas levels to examine the response. But the inevitable approximations mean that the model generally starts to drift away from the present climate at a rate comparable to, or even larger than, the warming expected due to changing greenhouse gas levels. There are various ways of correcting for this "climate drift" to obtain a stable model climate before starting a climate change experiment. None of these correction schemes is very satisfactory, since they are covering up model errors that might be important for climate change. The size of these corrections is diminishing as models improve, however, which suggests that it may be possible to eliminate them altogether in the relatively near future.
Scientists' ability to verify model projections is often limited by incomplete knowledge of the real climate. The processes that matter for climate change are those that operate on time-scales of decades or more. Detailed observations only exist for a few decades, but scientists can attempt to extend the record back using indirect evidence. This record suggests that model simulations of past climates and natural year-to-year climate fluctuations are improving, although they still have significant shortcomings.
Climate models are scientific tools, not crystal balls. Large climate modelling experiments consume enormous computing resources and are so expensive that each year only a handful of such experiments can be performed world-wide. Then the work involved in interpreting the results of a computer simulation is often greater than the work needed to perform the experiment in the first place. All of this work and expense can give models the aura of truth. But even the most sophisticated models are approximate representations of a very complex system, so they will never be an infallible guide to the future. This said, the level of uncertainty in climate models should not be exaggerated; it is no greater than the uncertainty in the economic models on which many other far-reaching decisions must be based. So think of climate models as sophisticated tools for extending our knowledge of present and past climate into an unexplored future. Since climate change will only happen once, they are the best tool we have.
The earth's climate varies naturally. Each component of this complex system evolves on a different timescale. The atmosphere changes in hours, and its detailed behaviour is impossible to predict beyond a few days. The upper layers of the oceans adjust in the course of a few seasons, while changes in the deep oceans can take centuries. The animal and plant life of the biosphere (which influences rainfall and temperature) normally varies over decades. The cryosphere (snow and ice) is slower still: changes in thick ice sheets take centuries. The geosphere (the solid earth itself) varies slowest of all - mountain-building and continental drift (which influence winds and ocean currents) take place over millions of years.
Past natural climate changes offer vital insights into human-induced climate change. Studies of past climates ("paleoclimatology") give a sense of the scale of future changes projected by climate models. They also provide a crucial check on scientists' understanding of key climate processes and their ability to model them.
Systematic global temperature records are available only since 1860. These include land-based air temperature measurements and sea-surface temperature measurements. Such data need to be checked carefully for any biases that may be introduced by changes in observation methods or sites. For example, many meteorological stations have been located in or near cities. As cities grow, they can have a significant warming effect on the local climate. Such effects must be - and are - taken into account in estimating recent changes in global temperature.
Studies of earlier climates are based on indirect evidence.Changing lake levels, for example, can reveal the past balance between rainfall and evaporation. Tree-rings, coral, ice-caps, or ocean sediments can all preserve information about the past. Using a combination of measurements, models, and "detective work", scientists convert the quantities they can measure (such as the chemical composition of an ice-core sample) into the physical variables they wish to investigate (such as the Antarctic temperature of 100,000 years ago).
The earth's climate has been dominated by ice ages for the past few million years. Ice ages are almost certainly triggered by slow "wobbles" in the earth's axis and its orbit around the sun. These wobbles affect the total amount of energy the planet receives from the sun and in particular its geographic distribution. During an ice age, global temperatures fall by 5oC and ice-sheets advance over much of Europe and North America. Ice ages are separated by warmer "interglacial" periods.
Changes in greenhouse gas concentrations may have helped to amplify ice-age cycles. The small fluctuations in energy arriving from the sun due to the earth's orbital wobbles are not large enough to account for the size of global temperature changes during the ice age cycles. Ice-core samples show that greenhouse gas levels also varied significantly and may have played an important role in amplifying temperature fluctuations.
Reconstructions of past climates can be used as a check on climate model projections.Comparing a model "prediction" of ice-age climate with the evidence from paleoclimatology provides a crucial check on the model's representation of processes relevant for future climate change. But the paleoclimatic evidence can be ambiguous: some sources suggest that, compared with today, tropical seas were some 5oC colder at the peak of the last ice age, while others suggest only 1-2oC. As a result, separating model errors from uncertainties in the evidence can be difficult.
The climate seems to have been remarkably stable since the last ice age ended 10,000 years ago.As far as scientists can tell, global temperatures have varied by less than one degree since the dawn of human civilisation. Against the apparently extreme and sometimes rapid climate fluctuations of the preceding 100,000 years, this stands out as a relatively peaceful interglacial period.
Models predict that the climate could be warmer by the end of the 21st century than it was during any previous inter-glacial period. In a period between two ice ages about 125,000 years ago, much of Europe and Asia appear to have been about 2oC warmer than they are today. However, models are predicting that temperatures could rise by more than 4oC over much of this region during the 21st century if greenhouse gas emissions continue as projected.
Abrupt climate variations in the distant past appear to have been traumatic for life on earth.The earth's biological history is punctuated by so-called "mass extinction events" during which a large fraction of the world's species are wiped out. There are many possible reasons for mass extinctions, but the records suggest that some of these events coincided with relatively abrupt changes in climate - similar in magnitude to the kind of change now forecast for the 21st century. Over the next 100 years we may experience conditions unknown since before the ice ages began many millions of years ago.
Even an immediate and dramatic cut in global greenhouse gas emissions would not fully prevent climate change impacts. The climate system responds to changes in greenhouse gas levels with a time lag, in part because of the oceans' thermal inertia. Past and present emissions have already committed the earth to at least some climate change in the 21st century. Natural ecosystems and human societies will be sensitive to both the magnitude and the rate of this change. Therefore, while controlling emissions is vital, it must be combined with efforts to minimize damage through adaptation.
The most vulnerable ecological and socio-economic systems are those with the greatest sensitivity to climate change and the least ability to adapt. Sensitivity is the degree to which a system will respond to a given change in climate; it measures, for example, how much the composition, structure, and functioning of an ecosystem will respond to a given temperature rise.Adaptability is the degree to which systems can adjust in response to, or in anticipation of, changed conditions.Vulnerability defines the extent to which climate change may damage or harm a system; this depends not only on the system's sensitivity, but on its ability to adapt.
Ecosystems that are already under stress are particularly vulnerable. Most ecosystems are sensitive to humanity's unsustainable management practices and increasing demands for resources. For example, human activities can fragment lightly managed and unmanaged ecosystems, limiting their potential for adapting naturally to climate change. Fragmentation of ecosystems will also complicate human efforts to assist adaptation, for example by creating migration corridors.
Socio-economic systems tend to be more vulnerable in developing countries with weaker economies and institutions.People who live in arid or semi-arid lands, low-lying coastal areas, flood-prone areas, or on small islands are at particular risk. Greater population densities have made some sensitive areas more vulnerable to hazards such as storms, floods, and droughts.
Adapting to climate change can be a spontaneous or planned act. Individuals, businesses, governments, and nature itself will often adapt to climate change impacts without any external help. In many cases, however, people will need to plan how to minimize negative impacts or benefit from positive ones. Planned adaptation can be launched prior to, during, or after the onset of the actual consequences.
Six general strategies are available for adapting to climate change. Measures can be taken in advance toprevent losses, for example by building barriers against sea-level rise. It may be possible toreduce losses to a tolerable level; this could include redesigning crop mixes to ensure a guaranteed minimum yield under even the worst conditions. The burden on those directly affected by climate change can be eased byspreading or sharing losses, perhaps through government disaster relief. Communities can also change a use or activity that is no longer viable, orchange the location of an activity, for example by re-siting a hydro-electric power utility in a place where there is more water. Sometimes it may be best torestore a site, such as an historical monument newly vulnerable to flood damage.
Successful strategies will draw on ideas and advances in law, finance, economics, technology, public education, and training and research. Technological advances often create new options for managed systems such as agriculture and water supply. However, many regions of the world currently have limited access to new technologies and to information. Technology transfer is essential, as is the availability of financial resources. Cultural, educational, managerial, institutional, legal, and regulatory practices are also important to effective adaptation, at both the national and international levels. For example, the ability to incorporate climate change concerns into development plans can help ensure that new investments in infrastructure reflect likely future conditions.
Many adaptation policies would make good sense even without climate change. Present-day climatic variability, including extreme climatic events such as droughts and floods, already causes a great deal of destruction. Greater efforts to adapt to these events could help to reduce damage in the short term, regardless of any longer-term changes in climate.
Crafting adaptation strategies is complicated by uncertainty. It is still not possible to quantify future impacts on any particular system at any particular location. This is because climate change predictions at the regional level are uncertain, current understanding of natural and socio-economic processes is often limited, and most systems are subject to many different interacting stresses.
Detecting early impacts will be difficult, and unexpected changes cannot be ruled out. The unambiguous detection of climate-induced changes in most ecological and socio-economic systems will prove extremely difficult in the coming decades. Knowledge has increased dramatically in recent years, but research and monitoring remain essential for gaining a better understanding of potential impacts and the adaptation strategies needed to deal with them.
Some agricultural regions will be threatened by climate change, while others may benefit. The impact on crop yields and productivity will vary considerably. Added heat stress, shifting monsoons, and drier soils may reduce yields in the tropics and subtropics, whereas longer growing seasons may boost yields in northern Canada and Europe. Projections of regional climate change and the resulting agricultural impacts, however, are still full of uncertainties (as illustrated by the table below).
Climate and agricultural zones are likely to shift towards the poles. Because average temperatures are expected to rise more near the north and south poles than near the equator, the shift in climate zones will be more pronounced at higher latitudes. In the mid-latitude regions (45o to 60o), present temperature zones could shift by 150―550 km. Since each of today's latitudinal climate belts is optimal for particular crops, such shifts could strongly affect agricultural and livestock production. Efforts to shift crops poleward in response could be limited by the inability of soil types in the new climate zones to support intensive agriculture as practiced today in the main producer countries.
Soil moisture will be affected by changing precipitation patterns. Based on a global warming of 1―3.5oC over the next 100 years, climate models project that both evaporation and precipitation will increase, as will the frequency of intense rainfalls. While some regions may become wetter, in others the net effect of an intensified hydrological cycle will be a loss of soil moisture. Some regions that are already drought-prone may suffer longer and more severe dry spells. The models also project seasonal shifts in precipitation patterns: soil moisture will decline in some mid-latitude continental regions during the summer, while rain and snow will probably increase at high latitudes during the winter.
Higher temperatures will influence production patterns. Plant growth and health may benefit from fewer freezes and chills, but some crops may be damaged by higher temperatures, particularly if combined with water shortages. Certain weeds may expand their range into higher-latitude habitats. There is also some evidence that the poleward expansion of insects and plant diseases will add to the risk of crop loss.
More carbon dioxide in the atmosphere could boost productivity. In principle, higher levels of CO2 should stimulate photosynthesis in certain plants. This is particularly true for so-called C3 plants because increased carbon dioxide tends to suppress their photo-respiration, making them more water efficient. C3 plants make up the majority of species globally, especially in cooler and wetter habitats, and include most crop species, such as wheat, rice, barley, cassava and potato. The response of C4 plants would not be as dramatic. C4 plants include such tropical crops as maize, sugar cane, sorghum and millet, which are important for the food security of many developing countries, as well as pasturage and forage grasses. Experiments based on a doubling of CO2 concentrations have confirmed that "CO2 fertilization" can increase mean yields of C3 crops by 30%. This effect could be enhanced or reduced, however, by accompanying changes in temperature, precipitation, pests, and the availability of nutrients.
The productivity of rangelands and pastures would also be affected. For example, livestock would become costlier if agricultural disruption leads to higher grain prices. In general, it seems that intensively managed livestock systems will more easily adapt to climate change than will crop systems. This may not be the case for pastoral systems, however, where communities tend to adopt new methods and technologies more slowly.
The global yield from marine fisheries should remain unchanged by global warming.The principal effects will be felt at the national and local levels as the mix of species changes and people respond by relocating fisheries. These possible local effects could threaten the food security of countries that are highly dependent on fish. In general, some of the positive effects of climate change could include longer growing seasons, lower natural winter mortality, and faster growth rates at higher latitudes. The negative ones could include upsets in established reproductive patterns, migration routes, and ecosystem relationships.
Food security risks are primarily local and national.Studies suggest that global agricultural production could be maintained relative to the expected baseline levels over the next 100 years. However, regional effects would vary widely, and some countries may experience reduced output even if they take measures to adapt. This conclusion takes into account the beneficial effects of CO2 fertilization but not other possible effects of climate change, including changes in agricultural pests and soils.
The most vulnerable people are the landless, poor, and isolated. Poor terms of trade, weak infrastructure, lack of access to technology and information, and armed conflict will make it more difficult for these people to cope with the agricultural consequences of climate change. Many of the world's poorest areas, dependent on isolated agricultural systems in semi-arid and arid regions, face the greatest risk. Many of these at-risk populations live in sub-Saharan Africa; South, East and Southeast Asia; tropical areas of Latin America; and some Pacific island nations.
Effective policies can help to improve food security.The negative effects of climate change can be limited by changes in crops and crop varieties, improved water-management and irrigation systems, adapted planting schedules and tillage practices, and better watershed management and land-use planning. In addition to addressing the physiological response of plants and animals, policies can seek to improve how production and distribution systems cope with fluctuations in yields.
Selected crop studies of future climate change
Region Impact on yields (%)
Maize Wheat
Latin America -61 to an increase ―50 to ―5
Former Soviet Union - ―19 to +41
Europe ―30 to an increase increase or decrease
North America ―55 to +62 ―100 to +234
Africa ―65 to +6 -
South Asia ―65 to ―10 ―61 to +67
Other Asia and Pacific Rim - ―41 to +65
Note: Based on double CO2―equivalent equilibrium scenarios from global climate models.
Source: Intergovernmental Panel on Climate Change, "Summary for Policymakers: Scientific-Technical Analysis of Impacts, Adaptations, and Mitigation of Climate Change", p. 10, in "Climate Change 1995", Vol. 2, Cambridge University Press.
The global average sea level has risen by 10 to 25 cm over the past 100 years. It is likely that much of this rise is related to an increase of 0.3―0.6oC in the lower atmosphere's global average temperature since 1860.
Models project that sea levels will rise another 15 to 95 cm by the year 2100 (with a "best estimate" of 50 cm). This will occur due to the thermal expansion of ocean water and an influx of freshwater from melting glaciers and ice. The projected rise is two to five times faster than the rise experienced over the past 100 years. The rate, magnitude, and direction of sea-level change will vary locally and regionally in response to coastline features, changes in ocean currents, differences in tidal patterns and sea-water density, and vertical movements of the land itself. Sea levels are expected to continue rising for hundreds of years after atmospheric temperatures stabilize.
Coastal zones and small islands are extremely vulnerable. Coasts have been modified and intensively developed in recent decades and thus made even more vulnerable to higher sea levels. Developing countries with their weaker economies and institutions face the gravest risks, but the low-lying coastal zones of developed countries could also be seriously affected. Given the present degree of protection, a sea-level rise of one metre would cause estimated land losses of 0.05% in Uruguay, 1% in Egypt, 6% in the Netherlands, 17.5% in Bangladesh, and up to about 80% for Atoll Majuro in the Marshall Islands.
Flooding and coastal erosion would worsen. Salt-water intrusion will reduce the quality and quantity of freshwater supplies. Higher sea levels could also cause extreme events such as high tides, storm surges, and seismic sea waves (tsunami) to reap more destruction. Flooding due to storm surges already affects some 46 million people in an average year, most of them in developing countries. Studies suggest that this figure could increase to 92 million with a 50 cm sea-level rise, and to 118 million with a one-metre rise.
Sea-level rise could damage key economic sectors . . . A great deal of food is produced in coastal areas, making fisheries, aquaculture, and agriculture particularly vulnerable. Other sectors most at risk are tourism, human settlements, and insurance (which has already suffered record losses recently due to extreme climate events). The expected sea-level rise would inundate much of the world's lowlands, damaging coastal cropland and displacing millions of people from coastal and small-island communities.
. . . and threaten human health. The displacement of flooded communities, particularly those with limited resources, would increase the risk of various infectious, psychological, and other illnesses. Insects and other transmitters of disease could spread to new areas. The disruption of systems for sanitation, storm-water drainage, and sewage disposal would also have health implications.
Valuable coastal ecosystems will be at serious risk. Coastal areas contain some of the world's most diverse and productive ecosystems, including mangrove forests, coral reefs, and sea grasses. Low-lying deltas and coral atolls and reefs are particularly sensitive to changes in the frequency and intensity of rainfall and storms. Coral will generally grow fast enough to keep pace with sea-level rise but may be damaged by warmer sea temperatures.
Ocean ecosystems may also be affected. In addition to higher sea levels, climate change could reduce sea-ice cover and alter ocean circulation patterns, the vertical mixing of waters, and wave patterns. This could have an impact on biological productivity, the availability of nutrients, and the ecological structure and functions of marine ecosystems. Changing temperatures could also cause geographical shifts in biodiversity, particularly in high-latitude regions, where the growing period should increase (assuming light and nutrients remain constant). People would be more affected by changes in fish and other biotic resources, and less by impacts on transport (due to changed currents) and physical resources such as oil and gravel. Finally, any changes in plankton activity could affect the oceans' ability to absorb and store carbon. This could "feedback" into the climate system and moderate or boost climate change.
Various natural forces will influence the impact that higher sea levels will have. Coastal areas are dynamic systems. Sedimentation, physical or biotic defenses (such as coral reefs), and other local conditions will interact with rising sea-water. For example, freshwater supplies in coastal zones will be more or less vulnerable depending on changes in freshwater inflows and the size of the freshwater body. The survival of salt marshes and mangrove forests will depend in part on whether the rate of sedimentation is greater than or less than the rate of local sea-level rise. Sedimentation is more likely to exceed sea-level rise in sediment-rich regions such as Australia, where strong tidal currents redistribute sediments, than in sediment-starved environments such as the Caribbean.
Human activities will also play a role. Roads, buildings, and other infrastructure could limit or affect the natural response of coastal ecosystems to sea-level rise. Pollution, sediment deposits, and land development will influence how coastal waters respond to, and compensate for, climate change impacts.
Many policy options are available for adapting to sea-level rise. Sensitive environmental, economic, social, and cultural values are at stake, and trade-offs may be unavoidable. Until recently, the assessment of possible response strategies focused mainly on protection, and studies have shown that protecting low-lying islands and large delta areas with sea walls and other barriers is likely to be costly. A fuller range of options would include protection (dikes, dune restoration, wetland creation), accommodation (new building codes, protection of threatened ecosystems), and planned retreat (regulations against new coastal development). Other specific examples are dredging ports, strengthening fisheries management, and improving design standards for offshore structures. "Integrated coastal zone management" can offer a portfolio of possible responses from which to choose, including social, cultural, legal, structural, financial, economic, and institutional measures.
Biological diversity- the source of enormous environmental, economic, and cultural value - will be threatened by rapid climate change.A warming of 1―3.5oC over the next 100 years would shift current climate zones poleward by approximately 150―550 km - and vertically by 150―550 m - in mid-latitude regions. The composition and geographic distribution of unmanaged ecosystems will change as individual species respond to new conditions. At the same time, habits will be degraded and fragmented by the combination of climate change, deforestation, and other environmental pressures. Species that cannot adapt quickly enough may become extinct - an irreversible loss.
Forests adapt slowly to changing conditions. Observations, experiments, and models demonstrate that a sustained increase of just 1oC in the global average temperature would affect the functioning and composition of forests. A typical climate change scenario for the 21st century shows a major impact on the species composition of one third of the world's existing forests (varying by region from one seventh to two thirds). Entire forest types may disappear, while new combinations of species, and hence new ecosystems, may be established. Other stresses caused by warming may include more pests, pathogens, and fires. Because higher latitudes are expected to warm more than equatorial ones, boreal forests will be more affected than temperate and tropical forests.
Forests play an important role in the climate system. They are a major reservoir of carbon, containing some 80% of all the carbon stored in land vegetation, and about 40% of the carbon residing in soils. Large quantities of carbon may be emitted into the atmosphere during transitions from one forest type to another because mortality releases carbon faster than growth absorbs it. Forests also directly affect climate on the local, regional, and continental scales by influencing ground temperature, evapo-transpiration, surface roughness, albedo (or reflectivity), cloud formation, and precipitation.
Deserts and arid and semi-arid ecosystems may become more extreme. With few exceptions, deserts are projected to become hotter but not significantly wetter. Higher temperatures could threaten organisms that now exist near their heat-tolerance limits.
Rangelands may experience altered growing seasons. Grasslands support approximately 50% of the world's livestock and are also grazed by wildlife. Shifts in temperatures and precipitation may reshape the boundaries between grasslands, shrublands, forests, and other ecosystems. In tropical regions such changes in the evapo-transpiration cycle could strongly affect productivity and the mix of species.
Mountain regions are already under considerable stress from human activities. The projected declines in mountain glaciers, permafrost, and snow cover will further affect soil stability and hydrological systems (most major river systems start in the mountains). As species and ecosystems are forced to migrate uphill, those whose climatic ranges are already limited to mountain tops may have nowhere to go and become extinct. Agriculture, tourism, hydropower, logging, and other economic activities will also be affected. The food and fuel resources of indigenous populations in many developing countries may be disrupted.
The cryosphere will shrink. Representing nearly 80% of all freshwater, the cryosphere encompasses all of the earth's snow, ice, and permafrost. Frozen water is, of course, highly sensitive to temperature change (a fact that researchers have used for studying past climate changes). Climate models project that mountain glaciers could be reduced by one third to one half over the next 100 years. This in turn will affect nearby ecosystems and communities as well as seasonal river flows and water supplies C which in turn would affect hydropower and agriculture. The landscapes of many high mountain ranges and polar regions would change dramatically. The melting of permafrost could destabilize infrastructure and release additional carbon and methane into the atmosphere. Reduced sea-ice would open certain rivers and coastal areas to navigation for longer seasons. Despite these many striking effects, the Greenland and Antarctic ice sheets are not expected to change much over the next 50―100 years.
Non-tidal wetlands will also be reduced. These open-water and waterlogged areas provide refuge and breeding grounds for many species. They also help to improve water quality and control floods and droughts. Studies from several countries suggest that a warmer climate will contribute to the decline of wetlands through higher evaporation. By altering their hydrological regimes, climate change will influence the biological, biogeochemical, and hydrological functions of these ecosystems, as well as their geographical distribution.
Human actions can help natural ecosystems adapt to climate change.Creating natural migration corridors and assisting particular species to migrate could benefit forest ecosystems. Reforestation and the "integrated management" of fires, pests, and diseases can also contribute. Rangelands could be supported through the active selection of plant species, controls on animal stocking, and new grazing strategies. Wetlands can be restored and even created. Desertified lands may adapt better if drought-tolerant species and better soil conservation practices are encouraged