IPCC working group issues strongest confirmation yet of climate change, link to human activity
Some 600 experts from 40 countries comprising Working Group I (WG1) of the Intergovernmental Panel on Climate Change (IPCC) have issued their strongest consensus to date on climate change and its causes. "Warming of the climate system is unequivocal," says the first volume of "Climate Change 2007," also known as the Fourth Assessment Report (AR4), "as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global mean sea level."
"Climate Change 2007: The Physical Science Basis" assesses the current scientific knowledge of the natural and human drivers of climate change, observed changes in climate, the ability of science to attribute changes to different causes, and projections for future climate change.
The ability-and willingness-of the world scientific community to confirm the reality of climate change and its link to human activities is based on significant improvements in research and data gathering since the Third Assessment Report (TAR) in 2001. In the past six years, says AR4, scientific progress has been marked by "large amounts of new and more comprehensive data, more sophisticated analyses of data, improvements in understanding of processes and their simulation in models, and more extensive exploration of uncertainty ranges."
Worldwide, says the report, levels of carbon dioxide (CO2), methane and nitrous oxide have risen significantly as a result of human activities since 1750 and now far exceed pre-industrial levels, as determined from ice cores dating back thousands of years. The global CO2 increases are attributed primarily to fossil fuel use and land use change, with agriculture cited as the leading factor in methane and nitrous oxide increases.
Global concentrations of CO2 have risen from a pre-industrial value of about 280 parts per million (ppm) to 379 ppm in 2005; this is far above the natural range of 180 to 300 ppm over the last 650,000 years, as determined from ice cores, notes the WG1 report. Moreover, it adds, the annual CO2 growth rate over the past ten years was greater than at any time since continuous direct atmospheric measurements began: 1.9 ppm per year between 1995 and 2005, compared to the 1.4 ppm average between 1960 and 2005, although there has been year-to-year variability in growth rates.
At the same time, the global atmospheric concentration of methane has risen from a pre-industrial level of about 715 parts per billion (ppb) to 1,732 ppb in the early 1990s and 1,774 ppb by 2005. This level also far exceeds the historic natural range of 320 to 790 ppb, also based on ice cores. Although growth rates have declined since the early 1990s, the report concludes that the increased concentration is "very likely" due to human activities, mainly agriculture and fossil fuel use, although relative contributions from different types of sources are not well determined.
(In the scientific context, "very likely" represents a cause-effect probability of greater than 90%, based on expert judgement.)
Further reinforcing its point, the WG1 says the improvement in understanding since the TAR relating to human activity-induced warming and cooling influences on climate has led to "very high confidence" that the globally averaged net effect of human activities since 1750 has been one of warming. (Again in the scientific context, "very high confidence" indicates at least a 9 out of 10 change of correctness, based on underlying scientific knowledge.)
The report details direct observations of recent climate change, noting, for example, that 11 of the last 12 years (1995-2006) have ranked among the 12 warmest years since continuous surface temperature record-keeping began (1850). The linear warming trend over the last 50 years (averaging 0.13*C per decade) is nearly twice that for the last 100 years, while the total temperature increase from 1850-1899 to 2001-2005 has averaged 0.76*C.
The report adds that the average atmospheric water vapour content has increase since at least the 1980s over land and ocean as well as in the upper troposphere. This increase is generally consistent with the ability of warmer air to hold extra water vapour.
In addition, observations since 1961 indicate that the average temperature of the world's ocean has risen, with the warmth penetrating to depths of at least 3,000 metres. The ocean has been absorbing over 80% of the heat added to the climate system, causing the seawater to expand and contributing to rising sea levels.
Long-term changes in climate observed have included changes in Arctic temperatures and ice, widespread changes in precipitation amounts, ocean salinity, wind patterns and extreme weather events (e.g. droughts, heavy rainstorms, heat waves and intense tropical cyclones).
Impacts on Canada's far north and Arctic areas have perhaps been the most dramatic: AR4 notes that average Arctic temperatures increased at almost twice the global average rate over the past 100 years, although the report points out that Arctic temperatures have high decade-to-decade variability (a warm period was also observed between 1925 and 1945).
Satellite data have confirmed the shrinkage of Arctic sea ice reported in the TAR. The annual average Arctic sea ice extent has shrunk an average of 2.7% per decade since 1978, with larger decreases in the summer, averaging 7.4% per decade. At the same time, the top layer of Arctic permafrost has become warmer, with temperatures having risen by up to 3*C since the 1980s. The maximum area in the northern hemisphere covered by seasonally frozen ground has decreased by about 7% since 1900, with a decrease in the spring of up to 15%.
These and other impacts are projected to continue, and increase as a warming of about 0.2*C per decade is forecast for a range of SRES scenarios. (The Special Report on Emission Scenarios (SRES) was prepared for the IPCC in 2000, outlining a range of forecast greenhouse gas (GHG) emission levels and projected impacts associated with each.) Even if concentrations of all GHGs and aerosols were kept constant at 2000 levels, there would still be a warming of about 0.1*C per decade.
Continued GHG emissions at or above current rates would cause further warming and lead to changes in the global climate system during the 21st century that would "very likely" (i.e. greater than 90% probability) be even more substantial than those observed in the 20th century. Examples-with likelihoods ranging from "likely" (i.e. over 66% probability) to "virtually certain" (i.e. over 99% probability of occurrence)-include warmer days and fewer cold days over most land areas; warmer and more frequent hot days and nights over most land areas; increasing frequency of warm spells and/or heat waves over most land areas; increasing frequency of heavy precipitation events; larger areas affected by drought; increasing intense tropical cyclone activity; and increased incidence of extreme high sea levels (excluding tsunamis).
As well, snow cover is projected to continue contracting, together with widespread increases in thaw depth over most permafrost regions. Sea ice is forecast to shrink in both the Arctic and Antarctic regions (under all SRES scenarios), with some scenarios anticipating almost complete disappearance of Arctic late-summer ice by the latter part of the 21st century. The report expects past and future CO2 emissions to continue contributing to warming and sea level rise for more than a millennium due to the length of time required to remove this gas from the atmosphere.
The work of the report's authors was supplemented by the participation of more than 620 expert reviewers and a large number of government reviewers. Representatives from 113 governments reviewed and revised the Summary for Policymakers line by line in the week leading up to its adoption (and acceptance of the underlying AR4) on February 2 in Paris. The Summary can be downloaded in English from www.ipcc.ch.
This is the first of three working group reports to be published this year before a final IPCC Synthesis Report is prepared; its adoption and approval are expected to take place at the 27th Session of the IPCC this November in Valencia, Spain. In the meantime, preparation of the Working Group II report, "Impacts, Adaptation and Vulnerability," is in its final stages, with acceptance and approval slated during the WGII meeting this April in Brussels, Belgium. A final draft of the Working Group III "Mitigation of Climate Change" report is due for distribution this month, with its release expected in May following acceptance and approval during the WGIII meeting slated for late April in Bangkok, Thailand.