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 ,amongst other things, automobile exhaust fumes and other sources.

Rising levels of greenhouse gases are already changing the climate. By absorbing infrared radiation, these gases control the way natural energy flows through the climate system. In response to humanity's emissions, the climate has started to adjust to a "thicker blanket" of greenhouse gases in order to maintain the balance between energy arriving from the sun and energy escaping back into space. Observations show that global temperatures have risen by about 0.6 ˚C over the 20^th century. There is new and stronger evidence that most of the observed warming over the last 50 years is attributable to human activities.

Climate models predict that the global temperature will rise by about 1.4 – 5.8°C by the year 2100. This change would be much larger than any climate change experienced over at least the last 10,000 years. The projection is based on a wide range of assumptions about the main forces driving future emissions (such as population growth and technological change) but does not reflect any efforts to control emissions due to concerns about climate change. 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 hundreds of years many decades after atmospheric concentrations have stabilized. Meanwhile, there is new and stronger evidence that most of the warming observed over the last 50 years can be attributed to human activities.

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 9 - 88 cm by the year 2100, causing flooding of low-lying areas and other damage. Other effects could include an increase in global precipitation and changes in the severity or frequency of extreme events. Climatic zones could shift poleward and vertically, disrupting forests, deserts, rangelands, and other unmanaged ecosystems. As a result, many will decline or fragment, and individual species couldwill 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 21^st 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. Fortunately, many strategies are available for adapting to the expected effects of climate change.

Stabilizing atmospheric concentrations of greenhouse gases will demand a major effort. Without emissions-control policies motivated by concerns about climate change, atmospheric concentrations of carbon dioxide are expected to rise from today’s 367 parts per million to 490 – 1,260 ppm by the year 2100. This would represent a 75 – 350% increase since the year 1750. Stabilizing concentrations at, for example, 450 ppm would require world-wide emissions to fall below 1990 levels within the next few decades. Given an expanding global 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 185 members, the Convention seeks to stabilize atmospheric concentrations of greenhouse gases at safe levels. It commits all countries to limit their emissions, gather relevant information, develop strategies for adapting to climate change, and cooperate on research and technology. It also requires developed countries to take measures aimed at returning their emissions to 1990 levels.

The Kyoto Protocol would require governments to take even stronger action. In 1997, the Parties to the Convention agreed by consensus that developed countries should accept 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". However, the Protocol has not yet received enough ratifications to enter into force.

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 farmsagriculture. 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.

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.