IPCC science summary

Summary of IPCC's 4th Science Report

Thousands of scientists are working to understand climate and its implications. Fortunately every six years the UN’s Intergovernmental Panel of Climate Change publishes a report bringing together the authoritative conclusions of 2500 top climate scientists. The most recent report was published in February 2007. Here are some of its conclusions. Even better, click here to read the whole report. See the links below for further explanations.

What drives climate change?
Direct observations of recent climate change
Historical changes
Understanding climate change
Future changes in climate

What drives climate change?

• Carbon dioxide (CO₂) is the most important human-produced greenhouse gas (Click here for the BBC’s explanation of the Greenhouse Effect)
• The concentration of CO₂ in the atmosphere has grown to 379 parts per million (ppm) in 2005. This is far higher than the natural range over the last 650,000 years (180-300ppm). It is still growing fast at about 1.9ppm per year. [CO₂ graph on p3 of corrected SPM]
• The main source of the increase of CO₂ in the atmosphere is burning fossil fuels (coal, oil, gas). Change in land use is the next most important (eg cutting down rainforest).
• Other greenhouse gases (methane and nitrous oxide) are also far above the pre-industrial average.
• There is very high confidence (at least 9 out of 10 chance) that the overall effect on the planet of human activities has been one of warming since the Industrial Revolution in 1750.

Direct observations of recent climate change

• Many different observations confirm the climate is definitely warming. The rate of warming has increased to 0.13ºC per decade. Urban heat islands have a negligible effect.
• 11 of the last 12 years (1995-2006) are in the top 12 warmest years since surface temperature records began in 1850
• The troposphere is warming similarly to the earth’s surface according to new analysis of balloon and satellite measurements.
• The average temperature of the oceans has increased to depths of at least 3000m. This has absorbed 80% of the climate’s added heat and contributed to sea level rise.
• Widespread decreases in glaciers and ice caps have contributed to sea level rise.
  Annual average Arctic sea ice extent has shrunk by 2.7% per decade. In summer it has shrunk by 7.4% per decade. (Editor: since this was written it has speeded up)
• The temperature of the top layer of Arctic permafrost has increased by 3ºC since the 1980s.
• More intense and longer droughts have been observed over wide areas since the 1970s, especially in the tropics and subtropics.
• Heavy precipitation events (rain and snow) have increased over most land areas.
• Changes in extreme temperature events have been observed over the last 50 years. In particular, hot days, hot nights and heat waves have become more frequent.

Historical changes

• Average Northern Hemisphere temperatures from 1950-2000 were very likely higher than any other 50 year period in the last 500 years, and likely the highest in at least the past 1300 years. Very likely means >90% likelihood. Likely means >66% likelihood.

Understanding climate change

• Most of the observed increase in globally averged temperatures since c1950 is very likely (>90% likelihood) due to the observed increase in human-produced greenhouse gas concentrations.
• Volcanic and human-produced aerosols (small particles) have lessened the effect of the greenhouse gas warming.
• The widespread warming of the atmosphere and ocean, plus ice mass loss, support the conclusion that this is very likely not due only to known natural causes.
• Human-induced warming has been detected in surface and atmospheric temperatures, temperature of the upper several hundred metres of the ocean, and in contributions to sea level rise.
• It is likely there has been significant human-induced warming on all continents except Antarctica in the last 50 years.
• Computer models fit with the observed data if they include human-induced forcing. They do not fit if they exclude human-induce forcing. This suggests that human-induced forcing contributes to current global warming.

Future changes in climate

• By the end of the 21st century, the temperature will rise somewhere between 1.8ºC and 4ºC. The exact amount will depend on the level of CO₂ emissions.
• By the end of the 21st century, sea-level rise will be somewhere between 0.18m and 0.59m.
• Warming tends to reduce the uptake of atmospheric CO₂ by land and ocean. So more CO₂ stays in the atmosphere contributing to warming.
• Increasing CO₂ in the atmosphere will make the oceans more acidic. (This has implications for things that live in the ocean).
• Snow cover is expected to contract. More permafrost will thaw.
• Sea ice will shrink in the Arctic and the Antarctic. Arctic summer sea-ice could even almost disappear after 2050.
• It is very likely that hot extremes, heat waves and heavy precipitation events will become more frequent.
• It is very likely that the Meridional Overturning Circulation (Gulf Stream) will slow during the 21st century. It is very unlikely to stop.
• Past and future human CO₂ emissions will continue to contribute to warming and sea level rise for more than a millennium.

Visit the IPCC website for more data.