Science

“Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice and rising global average sea level.”
(Source: Synthesis Report of the IPCC Fourth Assessment Report) 
 
Uncertainties remain in climate change science, especially regarding the operation of Earth systems over various timeframes and how subsystems interact with feedbacks. In particular, more work is required to understand the nature of possible tipping points in those systems. For now, the evidence suggests we may be within a few years of crossing those tipping points which could disrupt seasonal weather patterns supporting the agricultural activities of half the human population, diminish carbon sinks in the oceans and on land, and destabilize major ice sheets that could introduce unanticipated rates of sea level rise within the 21st century.

But the basic scientific building blocks behind forecasts of widespread and damaging climate change, especially those from the Intergovernmental Panel on Climate Change (IPCC), are irrefutable. Unless action is taken soon to stabilize and then decrease concentrations of greenhouse gases in the atmosphere, these changes will cause widespread damage to ecosystems, natural resources, and economic activities. Such damages could end prosperity in developed countries and threaten human survival in developing countries.

Set up by UNEP and the World Meteorological Organization (WMO), the IPCC brings together the best available science on climate change. In November 2007 the IPCC released its Fourth Assessment Report, comprising four sections: The Physical Science Basis, by Working Group I; Impacts, Adaptation and Vulnerability, by Working Group II; Mitigation of Climate Change, by Working Group III; and an overall Synthesis Report. It took six years to complete the report, which runs to several thousand pages. For this and its other work over the last 20 years, the IPCC was the joint winner of the 2007 Nobel Peace Prize.

The Summary for Policy Makers of the Synthesis Report of the IPCC Fourth Assessment Report highlights:

• Around 20-30 per cent of the plant and animal species assessed are likely to be at increased risk of extinction if global average temperatures exceed 1.5 degrees C to 2.5 degrees C over late 20th century levels.
• There is a likelihood of "irreversible" impacts. For example if temperature increases exceed about 3.5 degrees C, between 40 per cent and 70 per cent of the species assessed might be at increased risk of extinction.
• Increases in sea surface temperatures of about one to three degrees C are projected to result in more "frequent coral bleaching events and widespread mortality".
• There is growing concern over the oceans and seas becoming more acidic as they absorb rising levels of CO2 and the impacts on "marine shell-forming organisms" like coral reefs.
• There is a higher confidence in the risks of extreme weather events and the projected increases in droughts, heatwaves and floods as well as their adverse impacts.
• Concern is growing that the poor and elderly in low-latitude and less-developed areas (including those in dry areas and living on mega-deltas) are likely to suffer most.
• There is high confidence that by mid-century "many semi-arid areas, for example the Mediterranean basin, western United States, southern Africa and northeast Brazil, will suffer a decrease in water resources due to climate change".
• New observations linked with the Greenland and possibly Antarctic ice sheets may mean that the rate of ice loss will increase above previous forecasts.
• There is growing concern that any benefits linked with climate change will be gone after more modest temperature rises.

 Intergovernmental Panel on Climate Change Working Groups' Reports

Global Climate Observing System