November 18, 2011

THE PAST MONTH hasn’t been good for climate-change skeptics. At a congressional hearing Monday, Richard Muller, a former global-warming skeptic at the University of California, Berkeley, told lawmakers that, after a two-year review of historical world temperature data, he has verified the scientific consensus that the earth is warming — by about 1.6 degrees Fahrenheit over the past 50 years. This is not surprising; as the National Oceanic and Atmospheric Administration reported last year, the warming of the planet, detected in multiple, independent lines of evidence, is “unequivocal.”

Mr. Muller said that exactly how much humans contribute to such warming is difficult to calculate. But, as the Economist pointed out last year, even if the U.N. Intergovernmental Panel on Climate Change (IPCC) is off by a factor of five in its reckoning of the climate’s sensitivity to an eventual doubling of the amount of carbon in the atmosphere, that still leaves only a 50 percent chance of relatively minor temperature change. The developed world and large developing nations, meanwhile, continue to pump immense amounts of carbon into the atmosphere. The Energy Department released an analysis this month concluding that global carbon emissions in 2010 increased by the largest amount ever, to a higher level than the IPCC’s worst-case projection.

What are the consequences? Scientists can’t be sure. An IPCC committee gathered in Kampala, Uganda, this week to review the available science. It concluded that the warming that scientists have detected so far has likely led to higher extreme daily temperatures and high water on coasts. But, despite the rhetoric that emerges every time a hurricane hits the United States, the data is too thin to conclude that global warming has had any effect on aggregate tropical cyclone activity. The IPCC also noted that linking individual weather events to climate change is unreasonable. Natural variability will continue to be a dominant factor in explaining dangerous weather.

Predicting future effects also comes with exceptional uncertainties because of scientific models’ inadequacy to simulate the complex climate system many years out, among other things. Still, the IPCC says, with more warming it is virtually certain that very hot days will get hotter and more frequent; it’s very likely that heatwaves will, too. It’s also likely that heavy rains and snows will occur more often and that tropical cyclone wind speed will increase. Scientists can’t predict, but they also can’t rule out, worse consequences.

Varying amounts of uncertainty are inherent to climate science, but they do not mean humans can dismiss the dangers. Countries should clear-headedly address the risks of a warming world by cutting back on carbon emissions and preparing to adapt, the Kampala report argues.

The U.S. debate on global warming remains fancifully divorced from the scientific discussion. President Obama hardly ever mentions climate change. Republicans’ behavior is much more embarrassing: GOP presidential candidates often dismiss the warnings of experts in favor of conspiracy-drenched denial. The debate should no longer be about whether the world is warming or whether there is reason to act. It must be about how to respond.