August 26, 2013

NEXT MONTH, the international arbiter of the scientific consensus on global warming will release its latest evaluation of the state of the research. A few will dismiss the Intergovernmental Panel on Climate Change’s (IPCC) findings as overwrought alarmism. But a draft leaked to reporters last week indicates that, for most people, the report will serve as another stern warning about the risks of continuing to pump carbon dioxide into the air.

The scientists are set to claim that the increasing amount of greenhouse gases that humans have emitted into the atmosphere has almost certainly been the chief driver of the warming of the planet over the past half-century, a finding to which they ascribe 95 percent confidence. That’s the level of likelihood researchers typically consider robust enough to justify drawing very strong conclusions.

The amount of carbon dioxide in the atmosphere, the IPCC notes, has shot up by 40 percent since 1750, with concentrations of the gas now increasing at a faster rate than at any time in the last 22,000 years. The past three decades were probably the hottest in 800 years. Within this century, the draft report reckons, the average world temperature will increase between 2 and 7 degrees Fahrenheit.

The draft is appropriately careful when discussing global warming’s effects. The sea is risingfaster in recent years than before. Climate change probably has caused more extreme weather events, such as heat waves. But, as Reuters points out, the report doesn’t insist on a connection between warming and intense tropical cyclones, for example.

The IPCC admits that it doesn’t have a sure answer to a vexing question: Why has warming slowed a bit in the past decade or so? With medium confidence, the draft suggests that the explanation lies in a mix of natural variations and things such as the oceans absorbing more heat or more volcano debris reflecting sunlight back into space. It’s also possible, the scientists admit, that the planet’s sensitivity to greenhouse emissions is lower than middle-of-the-road projections.

Unless the IPCC’s report changes drastically between now and next month, the bottom-line message will be clear. Some uncertainties are inevitable when humans try to comprehend an incredibly complex climate system. Scientists might not be able to answer some questions for years, until they can look back at what changed after so much carbon dioxide entered the atmosphere so quickly. Those inevitable uncertainties are all the more reason for governments, starting with the United States’, to head off the ample risks of continuing to release huge amounts of greenhouse gases into the air and to set about it with speed and ambition.