The sensitivity issue

July 29, 2013
An early peek at the next assessment by the Intergovernmental Panel on Climate Change hints at retreat from a vital element of global-warming alarm.

An early peek at the next assessment by the Intergovernmental Panel on Climate Change hints at retreat from a vital element of global-warming alarm. The Economist reported this month that a document seen by its reporters indicates a softening of position by IPCC writers on climate sensitivity. The shift, if confirmed by final versions of the IPCC's Fifth Assessment, would be important news.

IPCC, the United Nations-World Meteorological Organization group assessing the science of human-induced climate change, responded to the Economist report with an important caveat. The Economist, it said, reported a table subject to change by the time the source, a group studying mitigation, submits its findings for approval next April. Climate sensitivity is the realm of a different group, which submits its Fifth Assessment report this September.

Climate sensitivity

The Economist report thus may have been premature. But it usefully focused attention on climate sensitivity, the amount of warming induced by an increment of increase in concentrations of greenhouse gases, especially carbon dioxide. Climate sensitivity should be the central question of global-warming politics, which too often treats it as a scientific given when in fact it is nothing of the sort.

Past IPCC assessments have gauged climate sensitivity as the amount of change, from preindustrial levels, in globally averaged surface temperature associated with a doubling of the CO2 concentration. In its Fourth Assessment, published in 2007, the IPCC projected warming from a rise of CO2 in the atmosphere to a sustained level of about 550 ppm at 2-4.5º C. It called warming of 3º C. a "best estimate."

Moderation of that expectation in the Fifth Assessment would indicate growing uncertainty among scientists about the contribution of the CO2 build-up during the Industrial Age to observed warming. The uncertainty would apply directly to human influence on average surface temperature, since CO2 is the greenhouse gas most closely affiliated with human activity. And it would strengthen questions about mitigation policies centered on difficult and costly cuts in CO2 emissions, mostly via reduced consumption of fossil energy.

These questions are enormously important for the oil and gas industry and for countries wanting to grow economically. The importance to the industry is immediate in the US, where President Barack Obama has hitched energy to climate change and declared the tandem issues to be a priority of his second term in office. In a speech last month, he promised climate change would be a factor in the making of policies ranging from taxation of oil and gas companies to approval of the border crossing of the Keystone XL pipeline. Companies, industry groups, and energy consumers defending their interests in policy contests must, therefore, address climate change, which now underlies every issue involving energy.

What moderation means

Moderation of the IPCC's view of climate sensitivity would benefit the oil and gas industry by weakening the bipolar trap into which extremists force the issue. With regrettable success, advocacy groups portray climate-change politics as a clash between righteous believers in the inevitability of catastrophic warming and unrighteous doubters. If the IPCC does lower the temperature range it projects under an assumed doubling of CO2 in the atmosphere, it will give discussion about a complex issue a measure of the sophistication it deserves but so far lacks. It will be acknowledging at least some of what doubters persistently have argued: that CO2 is not the only cause of observed warming and might not be the main one and that, therefore, the ability of people to regulate temperature globally with sacrificial policies might be quite limited in relation to other influences.

The doubters' propositions are hardly extreme and in no way evil. Advanced in response to policy prescriptions too frequently cloaked in apocalyptic warnings and fabricated urgency, they are, in fact, highly responsible. Policy should derive from what science knows with reasonable certainty about how people affect climate, not from the scariest temperature forecasts, which are the ones mostly likely to prove wrong.