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. CFCs and other gases are released by industrial processes, while 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 "thickening 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 1­3.5^oC 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 a discernable human influence on the global climate.

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 15­95 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 150­550 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 may 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 CO₂­doubling to 2100. Emissions would eventually have to fall to about 30% of their current levels for concentrations to stabilize at doubled­CO₂ 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, the Convention seeks to stabilize atmospheric concentrations of greenhouse gases at safe levels. Some 165 countries have become Parties. Developed countries are committed to taking measures aimed at returning their emissions to 1990 levels by the year 2000; they will commit themselves to making further emissions cuts after the year 2000 in a new agreement to be finalized by the end of 1997. Developed countries are also committed to promoting financial and technological transfers to developing countries to help them address climate change. Meanwhile, all Parties are gathering information on their national emissions and developing strategies for adapting to and minimizing climate change.

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 10­30% above baseline trends can be realized over the next 20­30 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.

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Also see

What is Climate Change? What is Biodiversity? What is desertification?