Predicting the Limits: Scientific uncertainty in averting disaster
The Intergovernmental Panel on Climate Change's 2007 report recommended that in order to avert long-term and catastrophic climate change global atmospheric CO2 levels should be limited to 450 parts per million (ppm), which should limit average global temperature rise to 3°C by the end of the century. Perhaps most importantly it made the clear statement that recent rate increase in levels of CO2 and other greenhouse gases is a direct result of human activity. Yet the assessment was arguably both optimistic and conservative; it recommended a limit already disputed in the evidence submitted for the report, whilst also recognising that meeting that limit would require significant international action. In spite of the unanimous support from scientists, many of whom put aside reservations about the limits in return for the urgency of producing an unequivocal statement of the problem, the subsequent summit in Bali failed to produce a political commitment to emissions reduction targets. This article looks at the uncertainties behind those limits and why we may already be flirting with disaster.
The current level of atmospheric CO2 is 383ppm. Global CO2 emissions rose an average of 2.2ppm annually between 2001 and 2006, with 2007 seeing an increase in emissions somewhere between 2.2 and 2.6ppm. The uncertainties are due to incomplete knowledge of numerous factors: carbon sources, sinks and cycles (e.g. CO2 negative feedback loops, oceanic cycles, new evidence that trees absorb less CO2 as concentrations increase, release from melting permafrost, wildfires, etc); natural cycles (e.g. atmospheric CO2 increases significantly during El Nino periods); impact of geological processes (volcanic eruptions, etc); and human factors. Fortunately measuring and predicting climate change and its impacts is a rapidly evolving science, but as the measurements become more precise and the models become more accurate the news invariably gets worse. The best available evidence now suggests the rate of CO2 emissions is increasing faster than the worst-case scenario considered by the IPCC in 2007.
In many respects the IPPC report is a brilliant example of scientific rigour, were it not for the political machinations that were barely hidden beneath the surface. The evidence that a ‘business as usual’ scenario would result in sea level rises and ecosystem damage that would be nothing short of catastrophic is not in doubt, nor are many of the strict criteria adopted for selecting studies to be included as evidence. However, critically the report included cut-off dates for completion and publication of results, meaning that the studies included as evidence had to be completed as much as 2-3 years prior to the report. As has been widely publicised this excluded important results from on-going studies, in particular new work using measurements from glaciers and the ice caps that shows they are being lost at a higher rate than previous models have predicted. These findings have major implications for predicting sea level rise and the role of changes in oceanic atmospheric cycles in exacerbating the negative feedback loops that if left unchecked will ultimately lead to runaway climate change.
It is these studies that are producing the most worrying predictions, but they also provide some of the most scientifically sound evidence. They are not conducted merely by using computer models and data from remote sensing (however this is not to doubt the immense value of these models) but are based on field measurements carried out by scientists with hands on experience of the environments they are studying. The problem is that they cost much more money to produce due to the logistics of getting people and equipment out and working in some of the harshest and remote areas of the planet, but it is these areas that are the canaries in the climate change mine, and the canaries have started to stop singing much sooner than some have predicted.
The 450 ppm, 3°C rise scenario was far from being the worst case scenario considered by the IPCC. It did consider evidence that concluded the highest acceptable risk to avert runaway climate change is a 2°C rise, and that runaway climate change is already unavoidable due to ‘banked’ emissions (emissions already produced but whose real effects we have yet to see due to the response rate of natural systems and the effects of negative feedback loops). Current models predict the 2°C limit could be achieved by the 450 ppm limit, but with a minimum 40% risk of failure, or by a 400 ppm limit, with only a 13% minimum risk of failure (this may be as high as 24%). Even this is not the most cautious approach, with other studies recommending a limit of 350 ppm to avert the worst (not all) affects of long term climate change.
Sadly it seems completely unrealistic to demand an immediate halt to the increase in global GHG emissions and an agreement on a roadmap to get back to 350ppm. Such a demand, even if the predictions are accurate, would be used by many as an excuse to admit defeat and hope technology and the market will step in and deliver a magic bullet at some point in the future. But what about a 400ppm, 2°C target? Those 17ppm might just provide enough room for to make a real difference. Although it is already too late to avert some of the impacts of climate change our remaining 17ppm almost certainly gives us enough room to avert a 3°C rise in time to turn away from the edge of the greenhouse cliff, and acting on a 400ppm limit now would provide a reasonable safety net in face of the mounting evidence for the 2°C target.
The clear message from scientists is that the greatest risk is not the scientific uncertainties in their predicted scenarios, it is the very real risk of a lack of commitment to averting even the worst of them.
Keith Baker, Technology and Science Editor, (29/02/08)References:
Carbon Equity, 2007. ‘The Big Melt’ - http://www.carbonequity.info/PDFs/Arctic.pdf
Hansen, J. et al., “Dangerous human-made interference with climate: a GISS model E study,” Atmos. Chem. Phys., 29 March, 2007: 2298.
Hansen, J. et al., “Dangerous human-made interference with climate: a GISS model E study,” Atmos. Chem. Phys., 29 March, 2007: 2298.
Hansen, J. et al., “Climate change and trace gasses,” Philosophical Transactions of The Royal Society A., 18 May, 2007.
Intergovernmental Panel on Climate Change (IPCC), “Working Group 1: The Physical Basis of Climate Change, Summary for Policymakers,” 2007.
Leslie McCarthy, “Research Finds That Earth's Climate is Approaching 'Dangerous' Point,” NASA, www.nasa.gov/centers/goddard/news/.
New Scientist, ‘Recent CO2 rises exceed worst-case scenarios’ -http://environment.newscientist.com/channel/earth/dn11899-recent-cosub2sub-rises-exceed-worstcase-scenarios.html
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