October 8, 2013

IF ONE body represents the international scientific consensus on global warming, it is the Intergovernmental Panel on Climate Change (IPCC), a United Nations panel that just released the first portion of its fifth authoritative report on the science. Backers point out that more than 800 authors and 50 editors from dozens of countries have produced a comprehensive and carefully worded analysis of the greatest environmental threat facing the planet. Critics pore over every sentence looking for errors. They even found a few among the thousands of pages in the IPCC’s last report.

But the bottom line remains this: The likelihood that human-induced global warming will have severe effects on humanity is far too high to ignore.

The report’s headline finding is that “it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.” The experts reckon that natural forces and variability might have pushed things one direction or another but not to the degree that human activity has. There is all sorts of evidence. It’s not just that the planet has warmed over the course of many decades, during which people have released massive amounts of greenhouse gases into the atmosphere. Among many other things, there is what scientists have called a “human fingerprint” — a pattern of warming in the troposphere and cooling in the stratosphere that is very likely characteristic of human influence.

Climate models have improved at matching the observed temperature record since the IPCC’s last report. The scientists admit that they still aren’t adept at modeling short time scales, such as 10 or 15 years, during which various factors can make average temperatures spike and dip. But climate change is a long-term effect, and the decades-long trend is what matters.

The authors did not shrink from addressing one of the primary threads that critics have been pulling in their effort to unravel the scientific consensus — the recent flattening of global temperature rise. “Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850,” the IPCC notes. Nevertheless, a detectable slowing of the warming trend between 1998 and 2012 might well have to do with recent volcanic eruptions, which add heat-reflective materials to the atmosphere, a decade-long solar cycle and simple decade-to-decade variability. The scientists also admit that some models might still overestimate the effects of certain human activities on the climate system.

There is a lot more work to do. Scientists still harbor considerable uncertainty about what will happen to low-level clouds in a warming world and about the effect of aerosols, many of which reduce warming but some of which promote it. Considerations like these lead to a wide estimated range for how sensitive the climate is to additional greenhouse gases.

The experts should keep working to refine their picture of the climate system, with the caution and skepticism that good science demands. Meanwhile, America’s leaders should not take the fruit of that skepticism — some continuing uncertainty — as license to continue stalling. Ignoring the real possibility of large increases in global temperature is not wise leadership. It’s wishful thinking.