Some recent scientific papers have made a splash by claiming that the answer might not be as bad as previously feared. This work â if it holds up â offers the tantalizing possibility that climate change might be slow and limited enough that human society could adapt to it without major trauma.  Several scientists say they see reasons to doubt that these lowball estimates will in fact stand up to critical scrutiny, and a wave of papers offering counterarguments is already in the works. âThe story is not over,â said Chris E. Forest, a climate expert at Pennsylvania State University.  Still, the recent body of evidence â and the political use that climate contrarians are making of it to claim that everything is fine â sheds some light on where we are in our scientific and public understanding of the risks of climate change.  The topic under discussion is a number called âclimate sensitivity.â Finding this number is the holy grail of climate science, because the stakes are so high: The fate of the earth hangs in the balance.  The first to take a serious stab at it was a Swede named Svante Arrhenius, in the late 19th century. After laborious calculations, he declared that if humans doubled the carbon dioxide in the air by burning fossil fuels, the average temperature of the earth would rise by something like nine degrees Fahrenheit, a whopping figure.  He was on the high side, as it turned out. In 1979, after two decades of meticulous measurements had made it clear that the carbon dioxide level was indeed rising, scientists used computers and a much deeper understanding of the climate to calculate a likely range of warming. They found that the response to a doubling of carbon dioxide would not be much below three degrees Fahrenheit, nor was it likely to exceed eight degrees.  In the years since, scientists have been pushing and pulling within that range, trying to settle on a most likely value. Most of those who are expert in climatology subscribe to a best-estimate figure of just over five degrees Fahrenheit.  That may not sound like a particularly scary number to many people â after all, we experience temperature variations of 20 or 30 degrees in a single day. But as an average for the entire planet, five degrees is a huge number.  The ocean, covering 70 percent of the surface, helps bring down the average, but the warming is expected to be higher over land, causing weather extremes like heat waves and torrential rains. And the poles will warm even more, so that the increase in the Arctic could exceed 10 or 15 degrees Fahrenheit. That could cause substantial melting of the polar ice sheets, ultimately flooding the worldâs major coastal cities.  Whatâs new is that several recent papers have offered best estimates for climate sensitivity that are below four degrees Fahrenheit, rather than the previous best estimate of just above five degrees, and they have also suggested that the highest estimates are pretty implausible.  Notice that these recent calculations fall well within the long-accepted range â just on the lower end of it. But the papers have caused considerable excitement among climate-change contrarians.  It is not that they actually agree with the new numbers, mind you. They have long pushed implausibly low estimates of climate sensitivity, below two degrees Fahrenheit in some cases. But they appear to be calculating that any paper with a lowball number is a step in their direction.  James Annan, a mainstream climate scientist working at a Japanese institute, offers a best estimate of four and a half degrees Fahrenheit. When he wrote recently that he thought some of the highest temperature projections could be rejected, skeptics could not contain their enthusiasm.  âThat is what we call a landmark change of course â by one of climatologyâs most renowned warmist scientists,â declared a blogger named Pierre L. Gosselin. âIf even Annan can see it, then the writing is truly emblazoned on the wall.â  But does this sort of claim â that we can all breathe a sigh of relief about climate change â really hold up?  Dr. Annan said in an e-mail that the Intergovernmental Panel on Climate Change, a mainstream body that periodically summarizes climate science, should be bolder about ruling out extreme temperature scenarios, but he still believes global warming is a sufficient threat to warrant changes in human behavior.  He noted that climate skeptics âare desperate to claim that the I.P.C.C. is being unreasonably alarmist, but on the other hand they donât really want to agree with me either, because my views are close enough to the mainstream as to be unacceptable to them.â He added that he finds it âamusing to watch their gyrations as they try to square the circle.â  It will certainly be good news if these recent papers stand up to critical scrutiny, something that will take at least a year or two to figure out. But the need for additional scientific vetting before we accept the lower numbers is not the biggest flaw in the contrarian argument.  Remember, the climate sensitivity number, whatever it turns out to be, applies to a doubling of carbon dioxide.  Given how weak the political response to climate change has been, there is no reason to think that human society is going to stop there. Some experts think the level of the heat-trapping gas could triple or even quadruple before emissions are reined in. Only last week the level of carbon dioxide passed a milestone of 400 parts per million at the flagship monitoring station atop Mauna Loa, in Hawaii, evidence that efforts to control emissions are failing.  Even if climate sensitivity turns out to be on the low end of the range, total emissions may wind up being so excessive as to drive the earth toward dangerous temperature increases.  So if the recent science stands up to critical examination, it could indeed turn into a ray of hope â but only if it is then followed by a broad new push to get the combustion of fossil fuels under control.
While some recent studies suggest that the doubling of carbon dioxide levels will not result in as high an increase in temperature as previously thought, they are not the last word.