'Warming of the climate system is unequivocal': highlights of the fourth IPCC Assessment Report.

• Author: Zhang, Yuwei
• Position: The Intergovernmental Panel on Climate Change - Report

The Intergovernmental Panel on Climate Change (IPCC) was established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) in 1988 to recognize the problem of potential global climate change. IPCC has three Working Groups and a Task Force and continues to provide scientific, technical and socio-economic advice to the world community, in particular to the Parties to the United Nations Framework Convention on Climate Change (UNFCCC).

The reports by the three IPCC Working Groups provide a comprehensive assessment of the current state of knowledge on climate change and contribute to the Panel's Fourth Assessment Report, Climate Change 2007, which is coming out in 2007. With 450 lead authors and 800 contributors, the Assessment Report includes findings from more than 2,500 scientists from over 130 countries, summing up the last six years of research.

Building upon past IPCC assessments, Working Group I reports on progress in understanding the human and natural drivers of climate change. Working Group II reviews the current understanding of the climate change impacts on natural, managed and human systems, as well as the vulnerability of socio-economic and natural systems to climate change. Working Group III focuses on the scientific, technological, environmental and socio-economic aspects of mitigation of climate change and the options for limiting greenhouse gas emissions.

THE PHYSICAL SCIENCE BASIS

Human and Natural Drivers of Climate Change

* Carbon dioxide (C[O.sub.2]) is the most important anthropogenic greenhouse gas (GHG). The global atmospheric concentration of C[O.sub.2] has increased from a pre-industrial value of about 280 to 379 parts per million in 2005, exceeding by far the natural range over the last 650,000 years (180 to 300 ppm), as determined from ice cores. The annual C[O.sub.2] concentration growth rate was larger over the period 1995-2005.

Warming of the climate system is unequivocal. This can be found in evidence of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising sea levels.

* Eleven of the last 12 years of 1995 to 2006 rank among the 12 warmest in the instrumental record of global surface temperature since 1850.

* Observations since 1961 show that the average temperature of the ocean has increased to at least 3,000 metres in depth and has been absorbing more than 80 per cent of the heat added to the climate system; such warming causes seawater to expand and contributes to sea-level rise.

* Mountain glaciers and snow cover have declined on average in both hemispheres. Widespread decreases in glaciers and ice caps have contributed to sea-level rise; the global sea level rose at an average rate of 1.8 mm per year from 1961 to 2003 and the rate was faster over the period 1993-2003.

At continental, regional and ocean basin scales, numerous long-term changes in climate have been observed. These include changes in Arctic temperatures and ice, widespread changes in precipitation, ocean salinity, wind patterns and aspects of extreme weather conditions, such as droughts, heavy precipitation, heatwaves and the intensity of tropical cyclones like hurricanes and typhoons.

* Average Arctic temperature increased at almost twice the global average rate in the past 100 years.

* Temperatures at the top of the permafrost layer in the Arctic have generally increased since the 1980s.

* More intense and longer droughts have been observed over wider areas since the 1970s, particularly in the tropics and subtropics. Changes in sea surface temperatures and wind patterns, and decreased snowpack and snow cover have also been linked to droughts.

* Widespread changes in...