IPCC Fourth Assessment Report warns of unequivocal warming, abrupt and irreversible impacts
The Intergovernmental Panel on Climate Change (IPCC) has issued its strongest statement to date, warning of increasing air and ocean temperatures worldwide, a warming of the climate system it calls "unequivocal" and an inevitable rise in sea levels that could inundate low-lying areas around the world.
Its Fourth Assessment Report (AR4) concludes that climate change will likely have irreversible, and possibly abrupt, impacts. The warming of the earth's climate systems and rising sea levels can be expected to continue for centuries after greenhouse gas (GHG) emissions are stabilized, due to the time scales associated with climatic processes.
While many of the impacts can be reduced, delayed or avoided through mitigation, the IPCC states that neither mitigation nor adaptation alone will be enough to avoid all climate change impacts. They can, however, complement each other and together can significantly reduce the risks associated with climate change.
AR4 both synthesizes and adds new data to the three IPCC Working Group reports released earlier this year: The Physical Science Basis (February 2007), Impacts, Adaptation and Vulnerability (April 2007) and Mitigation of Climate Change (May 2007). It represents the work of some 800 contributing authors, including 450 lead authors, from over 130 countries, peer-reviewed by more than 2,500 scientific experts.
In presenting the AR4 Summary for Policymakers, IPCC chairman Dr Rajendra Pachauri called attention to climate change impacts extending beyond higher average air and ocean temperatures to other aspects of climate. The frequency of heavy precipitation events has increased over most areas, with precipitation increasing significantly between 1900 and 2005 in eastern parts of North and South America, northern Europe and northern and central Asia.
During this same period, however, precipitation declined in the Sahel, the Mediterranean, southern Africa and parts of southern Asia. And since the 1970s, areas of the world affected by drought have (likely) increased. (In the scientific context of the report, "likely" reflects a 66 to 90% probability that this has occurred.) The report's authors also express higher confidence than in the 2001 Third Assessment Report in projected patterns of warming and other regional-scale impacts such as changes in wind patterns, precipitation, sea ice and extreme weather events.
Of particular concern is the rise in sea level linked to thermal expansion and the melting of glaciers, ice caps and the polar ice sheets. Within a very short period during the latter half of the 20th century, the annual rate of global average sea level rise nearly doubled, from 1.8 mm per year in 1961 to 3.1 mm/yr by 1993. By the end of the 21st century, the IPCC projects the sea level rise at between 18 and 59 centimetres.
In some areas, sea levels could rise by metres, leading to major changes in coastlines and inundation of low-lying areas as well as significant effects, such as erosion and loss of habitat, on river deltas and low-lying islands and harm to vital infrastructure in coastal communities.
Well-documented decreases in snow and ice cover, in both the northern and southern hemispheres, are also consistent with a warming climate, says the report. Satellite data since 1978 show that the annual average Artic sea ice extent has shrunk by an average of 2.7% per decade, with larger decreases in summer, averaging 7.4%. Mountain glaciers and snow cover have been reduced as well.
These temperature increases have been affecting many natural systems worldwide. Changes in snow, ice and frozen ground, for example, have increased the number and size of glacial lakes, resulted in greater ground instability in mountain and other permafrost regions, and altered some Arctic and Antarctic ecosystems as well as come hydrological systems. Increased runoff and earlier spring peak discharge into glacier- and snow-fed rivers have affected the thermal structure and water quality of warming rivers and lakes.
Terrestrial ecosystems have undergone shifts in timing and ranges that are associated with climate warming, on a high to very high confidence scale. (Again in the report's context, this indicates an eight or nine out of ten chance of being correct, based on expert scientific assessment.)
Notable oceanic impacts include acidification and a change in the circulation of the Atlantic Ocean. The uptake of carbon from human activities since 1750 has produced a discernible decrease in the pH of the average global ocean surface. This trend to increasing acidification is forecast to continue through the 21st century, with effects on marine aquatic life that are as yet undetermined but expected to be negative, especially for organisms such as corals and their dependent species.
The meridional overturning circulation of the Atlantic Ocean is projected to slow during the 21st century. While this change is not expected to be sudden, in the longer term it could affect marine ecosystem productivity, fisheries, the ocean's carbon uptake, oxygen levels in the ocean, and terrestrial vegetation.
The report calculates that global GHG emissions have increased by 70% between 1970 and 2004, with annual emissions of carbon dioxide (CO2, the leading GHG) growing by about 80% during the same period. Global increases in CO2 are attributed primarily to fossil fuel use, with land use change making a smaller, but still significant, contribution. The IPCC further notes that atmospheric concentrations of CO2 (379 parts per million) and methane (CH4, 1,774 parts per billion) as measured in 2005 both far exceed pre-industrial levels (i.e. prior to 1750) determined from ice cores dating back thousands of years.
Increases in methane concentrations are linked mainly to agriculture and fossil fuel use, although the report says methane growth rates have declined since the early 1990s, consistent with a stabilizing trend in total emissions. Increased levels of nitrous oxide (N20) are primarily due to agriculture.
The IPCC expresses "high agreement," backed by "much evidence" that with current climate change mitigation policies and related sustainable development practices, global GHG emissions will continue to increase over the next few decades. A Special Report on Emission Scenarios (SRES) issued by the IPCC in 2000 projected an increase in global GHG emissions ranging from 25 to 90% (as CO2 equivalent) between 2000 and 2030. This assumes that fossil fuels will continue as the dominant source in the global energy mix to 2030 and beyond.
The AR4 summary further concludes that continued GHG emissions at or above current rates would cause further warming and induce many changes in the world's climate system during the 21st century. These changes, it says, would very likely (i.e. 90 to 99% probability) be greater than those observed during the 20th century. Even if concentrations of all GHGs and aerosols were kept constant at their 2000 levels, the IPCC projects a further warming of about 0.1*C per decade over the next two decades; temperature forecasts thereafter would vary according to specific emission scenarios.
The IPCC is highly confident that there is a wide array of options for adapting to climate change and that some can be implemented in certain sectors at low cost and/or with high benefit-cost ratios. At the same time, however, more extensive adaptation is required than is currently occurring in order to reduce vulnerability to climate change impacts. There are barriers, limits and costs which are not fully understood. Adaptive capacity is closely linked to social and economic development, but is unevenly distributed across and within societies. The report outlines a variety of adaptation options and strategies across a full range of sectors (e.g. water, agriculture, transportation, energy), accompanied by the underlying policy framework needed and key constraints to, and opportunities for, implementation.
A full range of studies supports high agreement and much evidence of substantial economic potential for the mitigation of global GHG emissions over the coming decades, the IPCC adds, to a level that could offset the projected increase in global emissions or reduce emissions below current levels. The report also includes an outline of selected mitigation technologies and practices, together with policies, measures and instruments shown to be environmentally effective and key implementation constraints/opportunities. These are illustrated for critical sectors, including energy supply, transportation, industry, agriculture and forestry, as well as waste and buildings.
No single technology can provide all of the mitigation potential in any sector, says the IPCC, and the GHG stabilization levels assessed can be achieved through deployment of a portfolio of technologies that are either currently available or likely to be commercialized in the coming decades. The report concludes that an effective carbon price signal could contribute to realizing significant mitigation potential in all sectors.
Neither adaptation nor mitigation alone will be enough to prevent all climate change impacts, but taken together, they can significantly reduce the associated risks. Adaptation will be needed to deal with the impacts of warming that will occur even under the lowest GHG stabilization scenarios, while mitigation can help reduce, delay or even avoid impacts.
Mitigation efforts and investments over the next two to three decades, says the report, will greatly affect opportunities to achieve lower stabilization levels. In order to stabilize global atmospheric GHG levels, emissions would need to peak, then begin dropping. The lower the target stabilization level, the sooner this peak and decline would have to occur. The IPCC warns that delayed emission reductions will seriously constrain the opportunities to achieve lower stabilization levels and increase the risk of more severe climate change impacts.
The AR4 estimates that in 2050, the global average cost of mitigating emissions to stabilize atmospheric GHGs at levels ranging from 445 to 710 ppm would between a 1% gain and a 5.5% loss in global Gross Domestic Product (GDP). This corresponds to slowing the average annual growth in global GDP by less than 0.12 percentage points.
The AR4 Synthesis Report-Summary for Policymakers may be viewed on the IPCC Web site, www.ipcc.ch/index.htm.