by Jack Rosebro
|Comparison of non-mitigation greenhouse gas emissions scenarios with actual reported emissions, 1990-2006. Click to enlarge. Source: Global Carbon Project (2007).|
In an interview last week with The Guardian, Bob Watson, chief scientific adviser to the UK’s Defra (Department for the Environment, Food and Rural Affairs), warned that although his government should continue to work towards limiting the average global temperature increase to no more than 2ºC (3.6ºF) above pre-industrial levels, it should nevertheless prepare to adapt to as much as a 4ºC (7.2ºF) increase.
Watson’s words represent one of the first statements by a ranking scientific adviser to an industrialized country that a global temperature increase of no more than 2ºC—previously regarded by many experts, governments, and international bodies as the definitive “line in the sand” against climate change—may no longer be attainable.
Although a limit of 450ppm CO2 is one of the more ambitious emissions targets proposed by governments and corporations, the Intergovernmental Panel on Climate Change (IPCC) estimates that even at that limit, there is a 20% likelihood that global temperatures will increase by 3.5ºC or more. Watson’s statement has received support from Sir David King, the UK’s former chief scientist.
Even if we get the best possible global agreement to reduce greenhouse gases on any rational basis, you should be preparing for a 20% risk, so I think Bob Watson is quite right. My own feeling is that if we get to a four degree rise, it is quite possible that we would begin to see an runaway increase.—Sir David King
Abrupt Climate Change
The potential for abrupt rather than gradual climate change has been receiving increased attention in recent years, in part because of accelerated changes in warming indicators such as Arctic sea ice, as well as improved data on past abrupt climactic events. In 2005, Defra sponsored the international symposium Avoiding Dangerous Climate Change, at the invitation of then British Prime Minister Tony Blair, at the Met Office in Exeter. A collection of scientific papers presented at the symposium were later published in book form.
In the introduction to that book, the lead authors wrote “...the primary changes in climate and sea level will be relatively slow and steady (albeit much faster than anything previously experienced by mankind). However, superimposed on these trends, there may well be abrupt and possibly irreversible changes that would have far more serious consequences”—in particular, the deterioration of two of the world’s great ice sheets, West Antarctica and Greenland, as well as the destabilization of the North Atlantic Ocean Thermohaline Circulation (THC), which is part of what is known as the “ocean conveyer belt.”.
Among the papers presented at the symposium was a report from the World Wildlife Fund (WWF) that projected a warming of 3.2 to 6.6ºC (5.76 to 11.88ºF) in the Arctic if the rest of the world warmed by 2ºC.
The debate over appropriate greenhouse gas reduction targets has become complicated by the specter of tipping elements—root causes of potentially sudden and catastrophic climate shifts—in response to rising temperatures worldwide. The concept was discussed a decade ago by climatologist Stephen Schneider of Stanford University, who termed such shifts “imaginable surprises.” The phrase “tipping elements”[*], most notably used in last year’s research paper Tipping elements in the Earth’s climate system, reflects increasing concern over the possibility of eight collapsing ecosystems:
- Melting of Arctic sea-ice;
- Decay of the Greenland ice sheet (GIS) ice volume;
- Decay of the West Antarctic ice sheet (WAIS) ice volume;
- Overturning of the Atlantic Ocean thermohaline cycle;
- Increased amplitude of the El Niño - Southern Oscillation (ENSO);
- Reduced rainfall during the Indian Summer Monsoon;
- Dieback of the Amazon rainforests; and
- Dieback of the boreal forest.
Research conducted by the authors of the paper has indicated, for example, that a 3 to 4ºC warming would trigger sufficient lengthening of the dry season to destabilize the Amazon forest in such a way that it would be unlikely to return, even if the warming trend was later reversed. “The fate of the Amazon,” they point out, “may be determined by a complex interplay between direct land-use change and the response of regional precipitation and ENSO to global forcing.” A ninth potential ecosystem shift, the greening of the Sahara-Sahel deserts, represents “a rare example of a beneficial potential tipping element.”
Uncertainty about physical processes that could trigger tipping elements underlies the ability of science to predict their likelihood. “Our short list differs from that of the IPCC,” notes the authors, “because our definition and criteria differ from, and are more explicit than, the IPCC notion of abrupt climate change. The evidence base we use is also slightly different because it encompasses some more recent studies.” Lesser potential tipping elements include the Antarctic Bottom Water, permafrost, marine methane hydrates, ocean anoxia, and Arctic ozone.
Average global temperatures have already increased by around 0.8ºC from 1850 to 2005, according to the UK Meteorological Office, with the warming trend from 1956 to 2005 (approximately 0.13ºC per decade) nearly twice as much as for the 100 years from 1906 to 2005.
In 2006, the Stern Review on the Economics of Climate Change (earlier post) estimated that a 4ºC increase in average global temperatures could result in the decimation of one-fifth to one-half of the world’s species, and 7 million to 300 million people affected by coastal flooding worldwide. Lord Nicholas Stern has remarked in recent months that the review now appears to have significantly underestimated the “speed of climate change.” (Earlier post.)
In April, James Hansen of NASA called for a global target of 350ppm CO2 (earlier post), saying to The Guardian “If you leave us at 450ppm for long enough it will probably melt all the ice—that’s a sea rise of 75 metres. What we have found is that the target we have all been aiming for is a disaster—a guaranteed disaster.”
4ºC and the IPCC Report
One of the most aggressive reference scenarios used by the IPCC to model projected emissions prior to adjustment for mitigation measures, “A1 Fossil Intensive” (A1FI), projects a potential 4ºC rise in global temperatures by 2100 (earlier post). However, that scenario is already being outpaced by actual emissions as reported by governments. In addition, recent research indicates that some of those reports, especially emissions inventories from emerging economies, may be too optimistic (earlier post).
Although the modeling of a particular emissions scenario does not of itself indicate the likelihood of that scenario, 118 out of 177 emissions scenarios evaluated in the IPCC’s 2007 Fourth Assessment Report project a 3.2 to 4.0ºC increase. The report also cautions that non-linear climate feedbacks such as ice cover and carbon cycle changes may well lead to larger uncertainties for greater warming levels.
|Classification of recent greenhouse gas stabilization scenarios according to different stabilization targets. Click to enlarge. Source: IPCC AR4|
In the interview, King also mentioned that the turning point for the UK government with regard to climate change was not the Stern Review, but a 2004 report by the government’s Foresight program, which projected potential costs of defending Britain’s coasts against sea- level rise, as well as the possibility of abandoning some lands to the sea.
“Under every scenario,” wrote the authors of UK Foresight’s Future Flooding report more than four years ago, “our analysis suggests that if current flood-management policies remain unchanged, the risk of flooding and coastal erosion will increase greatly over the next 30 to 100 years.”
[*] The term “tipping elements,” which focuses on ecosystems that could be pushed into qualitatively different states of operation by human activity, is distinguished from the more common term “tipping points, ” which usually describes specific points in time during which disruptive change might occur.
James Randerson, The Guardian: Climate change: Prepare for global temperature rise of 4C, warns top scientist. 7 August 2008
Avoiding Dangerous Climate Change website
Hans Joachim Schellnhuber, editor (2005) Avoiding Dangerous Climate Change
Dr. Mark New, Oxford University (2005) 2°C Is Too Much! Evidence and Implications of Dangerous Climate Change in the Arctic
Lenton et al. (2007) Tipping elements in the Earth’s climate system
Defra (UK) (2005) Foresight Future Flooding executive summary
Global Carbon Project (2007) Recent Carbon Trends and the Global Carbon Budget updated to 2006