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1.

Yes, many observations indicate that the world's climate has changed during the 20th century:

The average surface temperature has increased by about 0.6°C (1°F). Snow cover and ice extent have decreased. The sea level has risen by 10 to 20 cm (4 to 8").

Some other important changes include precipitation, cloud cover and extreme temperatures

Some important aspects appear NOT to have changed, like Antarctic sea-ice extent or extreme events such as storms, tornadoes, thunder days, or hail events.

2.

Climate has and will always vary for natural reasons. However, human activities are increasing significantly the concentrations of some gases in the atmosphere, such as greenhouse gases (mainly CO2), which tend to warm the earth surface, and anthropogenic aerosols, which mostly tend to cool it.

Although more research is needed, understanding of climate processes and computer models have improved, leading the IPCC to draw the following conclusion:

Most of the warming over the last 50 years is likely to have been due to man-made activities.

3.

To predict the future climate, several greenhouse gas emission scenarios were developed and fed into computer models

They project for the next century that, without specific policy changes :

global mean temperature should increase by between 1.4 and 5.8°C (2.5 to 10°F). the Northern Hemisphere cover should decrease further, but the Antarctic ice sheet should increase. the sea level should rise by between 9 and 88 cm (3.5" to 35"). other changes should occur, including an increase in some extreme weather events.

After 2100, human induced climate change is projected to persist for many centuries. The sea level should continue rising for thousands of years after the climate has been stabilized.

4.

Regional changes in climate, particularly increases in temperature, have already affected some physical and biological systems.

Both natural and human systems are vulnerable to climate change because of their limited adaptive capacity. This vulnerability varies with geographic location, time, and social, economic and environmental conditions.

Some extreme weather events and the damage, hardship, and death they cause are projected to increase with global warming. There is also a potential for large-scale and possibly irreversible impacts which pose risks that have yet to be reliably quantified; their likelihood is probably very low but is expected to increase with the rate, magnitude, and duration of climate change.

Man will have to adapt to and cope with the climate change consequences that are not prevented by mitigation. Economic losses can be expected, especially in poorest regions; the higher the warming, the greater the losses. Promoting adaptation, sustainable development and equity can be mutually reinforcing.

5.

Projected changes in climate are expected to have both beneficial and adverse effects on water resources, agriculture, natural ecosystems and human health. But the larger the changes in climate the more the adverse effects should dominate.

For instance, some crop and forest productivities could benefit from a small climate change. But for many other natural systems, the adverse effects should be dominant, especially if warming exceeds a few degrees.

Human populations are expected to face increasing flooding and heat waves but reduced cold spells. The geographic range for infectious diseases should increase.

The vulnerability of human populations and natural systems to climate change differs substantially across regions and across populations within regions.

6.

Climate change is a unique, global, long-term problem, involving complex interactions.

There are many technological options for reducing greenhouse gas emissions, some at low or negative cost. Forests and agricultural lands provide significant but not necessarily permanent carbon sinks, which may allow time for other options.

There will be both costs and benefits to reducing greenhouse gases. Most studies project that in 2010, the Kyoto Protocol should cost developed countries a reduction in projected GDP of about 0.2% to 2% without emission trading and about half of that with emission trading.

Greenhouse gas reduction programs need to overcome many barriers. Governments have a wide array of instruments at their disposal. With coordinated actions and international regimes, efficiency and equity should improve.

Further research is required to strengthen future assessments and to reduce uncertainties.

7.

It is not possible to link any particular event definitively to global warming. But as the world warms, more of some types of extreme events are expected, such as heat waves, heavy precipitation, blizzards and droughts. For some other events such as extra-tropical storms, there is little agreement between current predictive models.

8.

Water vapor is by far the most important greenhouse gas. Nevertheless, the man-made increase in other greenhouse gases such as CO2 is expected to induce some additional warming in the coming decades. Warmer air contains more water vapor; this in turn amplifies the man-made warming. Other reactive mechanisms (feedback) could both amplify or reduce this warming.

9.

Ecosystems have a limited capacity to adapt to climate change; some might not be able to cope as they had done in earlier periods and are expected to suffer damages because:

The rate and extent of climate change is expected to be faster and greater than in the past and could exceed nature's maximum adaptation speed; Human activities and pollution have increased the vulnerability of ecosystems.

10.

The earth's average surface temperature has increased by about 0.6 °C (1°F) during last century, and human activities are increasing the levels of greenhouse gases which tends to warm the planet.

How much and how fast temperatures will continue to rise remains uncertain, and the exact impacts of climate change over the 21st century, especially for local regions, remain largely unknown.

However, global warming poses real risks. Because the exact nature and amplitude of these risks remains uncertain, we have to use our best judgment – guided by the current state of science – to determine what the most appropriate response should be.

11.

The earth's climate has changed over the last century (see 1). There is new and stronger evidence that most of the warming observed the last 50 years is attributable to human activities (see 2.3). Evolving computer models (see 2.2.1) are predicting that, because of greenhouse gas emissions, temperatures should continue to rise over the 21st century (see 3), impacting nature and mankind both positively and negatively (see 4. and 5).

The impacts should vary among regions (see 5.3), but they can not yet be predicted accurately, especially for small-scale areas(see 10.3). However, it is expected that:

the more the greenhouse gases are emitted, the higher the tendency for the earth to warm the greater and faster the warming, the more the adverse effects will dominate and the higher the possibility, although probably remote, of large-scale and posibly irreversible impacts

Therefore, although an acceptable level for greenhouse gases has not yet been determined, reducing emissions should reduce the risk of adverse effects. Many options for emission reductions are available (see 6.2); their costs need to be balanced with the risks left for future generations.

12.

This summary is based on the latest Intergovernmental Panel on Climate Change (IPCC) review, the Third Assessment Report (TAR) (More...). It is considered by most scientists as a consensus document and other recent scientific assessments reach similar conclusions

However, some people and organizations disagree with certain IPCC conclusions