Tiempo Climate Cyberlibrary

Fourth Assessment of Climate Science


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The first volume of the Fourth Assessment of climate science and policy by the Intergovernmental Panel on Climate Change was finalized in February 2007. Newswatch editor Mick Kelly reports.

The latest report by the Intergovernmental Panel on Climate Change (IPCC) on the science of climate change concludes that it is "very likely" - a probability of greater than 90 per cent - that the rise in global air temperature since the mid-1900s has been caused by human activity. (For definition of phrases "likely", "very likely", see IPCC guide for authors.)

Data show that the oceans have warmed to a depth of at least 3,000 metres, contributing to sea-level rise. The report predicts that the average world temperature may rise by about three degrees Celsius by the end of the century. Sea level could rise by as much as 59 centimetres over that period, and some projections indicate the complete disappearance of summer sea ice in the Arctic by the year 2100. Heatwaves and periods of heavy rainfall are "very likely" to become more frequent but tropical cyclones, though more intense, may occur less often.

The report, the first volume of the IPCC's Fourth Assessment, was released on February 2nd in Paris, France. "Any notion that we do not know enough to move decisively against climate change has been clearly dispelled," said Yvo de Boer, head of the Secretariat of the United Nations Framework Convention on Climate Change.

"The big message... is the strength of the attribution of the warming to human activities," said Claudia Tebaldi of the National Center for Atmospheric Research in Boulder, Colorado, in the United States. United Nations Secretary-General Ban Ki-moon pointed to the "scientific consensus regarding the quickening and threatening pace of human-induced climate change" and called for the global response "to move much more rapidly as well, and with more determination."

IPCC Chair, Rajendra Pachauri, said that the report contained "significant advances" over the previous 2001 Assessment. Nevertheless, though the overall message is clear, some uncertainties remain in the detail. The role of clouds in reinforcing or offsetting greenhouse warming is not well-established, neither is the future of Antarctica. The report indicates that the Antarctic ice sheet may well remain too cold for widespread surface melting and could gain in mass as snowfall increases. The possibility of net loss cannot, however, be ruled out as dynamical ice discharge might dominate the mass balance.

Major conclusions of the climate science assessment
  • Scientists now have "very high confidence" - at least a nine out of ten chance of being correct - in their understanding of how human activities are causing the world to warm. It is "very likely" that humanity’s emissions of carbon dioxide, methane, nitrous oxide and other greenhouse gases have caused most of the global temperature rise observed since the mid-20th century. It is "likely" that effect of human activity since 1750 is five times greater than the effect of fluctuations in the sun’s output.
  • The world’s average surface temperature has increased by around 0.74°C over the past 100 years (1906-2005). A warming of about 0.2°C is projected for each of the next two decades, with an accelerating transition to a warmer world - an increase of 3°C is expected this century.
  • The best estimates for sea-level rise due to ocean expansion and glacier melt by the end of the century (compared to 1989-1999 levels) have narrowed to 28-58 cm, versus 9-88 cm in the 2001 report, due to improved understanding. However, larger values of up to one metre by 2100 cannot be ruled out if ice sheets continue to melt as temperature rises.
  • Sea ice is projected to shrink in both the Arctic and Antarctic regions. Large areas of the Arctic Ocean could lose year-round ice cover by the end of the 21st century if human emissions reach the higher end of current estimates. The extent of Arctic sea ice has already shrunk by about 2.7 per cent per decade since 1978, with the summer minimum declining by about 7.1 per cent per decade.
  • Snow cover has decreased in most regions, especially in spring. The maximum extent of frozen ground in the winter/spring season decreased by about seven per cent in the Northern Hemisphere over the latter half of the 20th century. The average freezing date for rivers and lakes in the Northern Hemisphere over the past 150 years has arrived later by some 5.8 days per century, while the average break-up date has arrived earlier by 6.5 days per century.
  • It is "very likely" that precipitation will increase at high latitudes and "likely" it will decrease over most subtropical land regions. The pattern of these changes is similar to what has been observed during the 20th century.
  • It is "very likely" that the upward trend in hot extremes and heat waves will continue. The duration and intensity of drought has increased over wider areas since the 1970s, particularly in the tropics and subtropics. The Sahel, the Mediterranean, southern Africa and parts of southern Asia have already become drier during the 20th century.
  • The number of tropical cyclones (typhoons and hurricanes) per year is projected to decline. However, the intensity of these storms is expected to increase, with higher peak wind speeds and more intense precipitation, due to warmer ocean waters.

Adapted from the launch press release

The report sparked a range of comments in the days that followed its release. "The world's scientists have spoken," said Timothy E Wirth of the United Nations Foundation. "It is time now to hear from the world's policy makers. The so-called and long-overstated 'debate' about global warming is now over," he continued. "Faced with this emergency, now is not the time for half measures. It is the time for a revolution, in the true sense of the term," concluded French President Jacques Chirac.

There were dissenting voices. In the United States, Oklahoma Senator James Inhofe described the IPCC assessment as "the corruption of science for political gain." William O'Keefe of the George Marshall Institute said that predictions of a "climate catastrophe in this century are unjustified."

In Lagos, Nigeria, Thompson Ayodele of the Initiative for Public Policy Analysis announced the launch of the Civil Society Coalition on Climate Change to provide "more rational thinking" on the climate issue. "Many of the proposed policies are likely to harm a society like Nigeria more than the climate changes they are intended to control," he said.

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Updated: May 15th 2015