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global climate change and its cause

An introduction to man-made climate change

Certain gases in the atmosphere, such as carbon dioxide, play a crucial role in determining the earth s climate. Although other factors are important as well, the composition of the atmosphere to a large extent controls our climate. Levels of so-called greenhouse gases are particularly important, because these gases determine how air absorbs and transmits radiant energy.

Atmospheric concentrations of greenhouse gases are rising rapidly, mainly because of human activity. By burning fossil fuels and deforesting the earth, mankind is increasing carbon dioxide levels. Our intensive agriculture and leaky natural-gas lines are major sources of methane. Our industry is emitting chlorofluorocarbons (CFCs). Nitrous oxide levels are also increasing rapidly, for reasons that are less clear. Less than 200 years since we began making major emissions, greenhouse gas concent rations are rising to levels higher than any yet seen while humans have existed on this planet and they will rise much further in the years ahead.

Changes in greenhouse gas concentrations have been associated with dramatic climatic changes in the past. The last time greenhouse gas levels changed as much as they are changing now was when the earth emerged from the most recent ice-age. There is strong evidence that greenhouse gases played a significant role in that post-ice-age warming.

The current increase in greenhouse gases will affect the climate but we don t yet know exactly how. The different components of the earth climate interact on many different time-scales in complex, often chaotic ways. Many of these interactions are still poorly understood. But even if we understood the present climate much better than we do, the future could still hold surprises. We are entering into a new, hitherto unexplored, climatic regime. Because the climate is a non-linear system, we will not be completely sure of the consequences of our actions until after climate change has already occurred. Climate models indicate that one of the main effects of greenhouse gas emissions will be global warming. Assuming that no action is taken to reduce emissions, computer models of the earth s climate indicate that global average surface temperatures will rise by 1.5-4.5 C over the next 100 years. This rise is larger and probably faster than any such change over the past 9,000 years. Climate models are far from perfect, and they rely on projections of future greenhouse gas emissions that are far from certain. But, insofar as a consensus exists in the world scientific community, they are believed to provide the best estimates we have of future climate change. Emission scenarios and model predictions may overstate the risk, but they are equally likely to underestimate it. There is some evidence that this warming has already begun. Average world surface temperatures appear to have risen by 0.3-0.6 C over the past 100 years. But although many climatologists believe that this indicates a real change, the historical temperature record is poor. Moreover, the climate varies naturally, partly because of its chaotic nature, and this observed warming is still just within the range of natural variability. Nevertheless, this warming is broadly in line with what models predict should have resulted from emissions to date. The most plausible explanation is that it is due, at least in part, to mankind s greenhouse gas emissions.

Past greenhouse gas emissions have already committed us to more climate change in the future. The climate does not respond instantly to emissions. Many greenhouse gases remain in the atmosphere for decades after being released, continuing to influence the climate. This built-in delay increases the risks in waiting for more conclusive evidence before acting to reduce emissions.

If no action is taken to reduce greenhouse gas emissions, the consequences for many of the world s societies and ecosystems may be serious. Average sea-levels may rise, which would affect coastal communities through more frequent flooding and increased ground-water salinity. Changes in rainfall patterns and soil moisture levels are probable, but still difficult to predict. Both would have significant implications for agriculture. Not all climate change impacts would be negative, however, and the burden of adapting to them would be spread very unevenly over the world s societies. Those most at risk from climate change are likely to be those least able to adapt. For example, subsistence agricultural societies and many natural ecosystems have evolved over centuries to suit the present climate. Both are likely to find climate change on the scale and speed predicted for the coming decades to be traumatic.

Although uncertainties remain, we know enough to be able to say with confidence that the risk of climate change is genuine and serious.Because we are dealing with a problem without precedent in human history, there is clearly much that we still do not understand about the climate system and our impact on it. But the level of uncertainty in climate models should not be exaggerated. It is probably less than the uncertainty in the economic data and models on which equally far-reaching policy decisions must be based.

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Last revised August 1992 by the Information Unit on Climate Change,** UNEP, Palais des Nations, CH-1211 Geneva 10, Switzerland.

tel: (+41) 22 789-4062** fax: (+41) 22 789-4073

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