TemperatureThe idea that the global average temperature is actually cooling – not warming, as most climate scientists believe – seems to have a fair number of supporters on the Internet.

To confirm that impression, just check out readers’ comments to a typical news story that touches even tangentially on the subject of climate change influenced by greenhouse gases. The “Earth-is-cooling” argument shows up often.

In response to this assertion, there have been recent signs of what may be an accelerating rebuttal of the cooling concept.

One example is an article this week by Seth Borenstein, a national science reporter for the Associated Press. Another is a blog post last week by John Nielsen-Gammon, the Texas state climatologist and a professor of atmospheric sciences at Texas A&M University.

The AP’s Borenstein reported the responses of four statisticians, whom he had asked to review recent years’ temperature numbers – though without being told the numbers represented temperatures:

Have you heard that the world is now cooling instead of warming? You may have seen some news reports on the Internet or heard about it from a provocative new book. Only one problem: It’s not true, according to an analysis of the numbers done by several independent statisticians for The Associated Press.

The case that the Earth might be cooling partly stems from recent weather. Last year was cooler than previous years. It’s been a while since the super-hot years of 1998 and 2005. So is this a longer climate trend or just weather’s normal ups and downs?

In a blind test, the AP gave temperature data to four independent statisticians and asked them to look for trends, without telling them what the numbers represented. The experts found no true temperature declines over time.

“If you look at the data and sort of cherry-pick a micro-trend within a bigger trend, that technique is particularly suspect,” said John Grego, a professor of statistics at the University of South Carolina.

Yet the idea that things are cooling has been repeated in opinion columns, a BBC news story posted on the Drudge Report and in a new book by the authors of the best-seller “Freakonomics.” Last week, a poll by the Pew Research Center found that only 57 percent of Americans now believe there is strong scientific evidence for global warming, down from 77 percent in 2006.

Global warming skeptics base their claims on an unusually hot year in 1998. Since then, they say, temperatures have dropped — thus, a cooling trend. But it’s not that simple.

Since 1998, temperatures have dipped, soared, fallen again and are now rising once more. Records kept by the British meteorological office and satellite data used by climate skeptics still show 1998 as the hottest year. However, data from the National Oceanic and Atmospheric Administration and NASA show 2005 has topped 1998. Published peer-reviewed scientific research generally cites temperatures measured by ground sensors, which are from NOAA, NASA and the British, more than the satellite data.

In his blog on the Houston Chronicle’s Web site, Nielsen-Gammon saw the post-1998 “dipping” of temperatures that Borenstein noted as a reason for the Pew survey’s finding of declining popular belief that a human-caused warming trend is occurring.

He reflected on the poll’s question, “Is there solid evidence that earth is warming, and if so, why?”:

That’s a two-parter.  Both parts depend on the unstated time scale.

Is it warming on an annual time scale? Depends on the year.  It happens that in 2008 temperatures dropped. In 2009 temperatures are rising. So the correct answer, on an annual time scale, is yes. Last year the correct answer would be no. And the correct reason would be natural processes.

Is it warming on a multi-year time scale? Depends on the multi-year. The past few years saw cooling. So the correct answer, at the moment, is no. And the correct reason would be natural processes.

Is it warming on a multi-decadal time scale? Depends on the decades. For the past few decades, all available evidence indicates the correct answer is yes. And the correct reason would be mostly because of human activity, with the exact fraction attributable to human activity yet to be pinned down precisely.

So the Pew question is pretty much meaningless too. And I suspect this is a major reason why the percentage answering “yes” has gone down over the past year: many members of the public are looking at recent history rather than multidecadal history as a guide to the future. I further suspect that if 2010 is a record or near-record year for global temperatures, as I expect it to be due to the combination of global warming and El Nino, the poll numbers will rebound as well.

What question would be meaningful? What question should drive debates about changes in energy policy, international agreements, etc.? I propose the following:

Is there strong evidence that, by the middle of this century, the effect on global temperatures of anthropogenic greenhouse gases is likely to exceed the combined magnitude of the effect of other natural or man-made processes on global temperatures?

(Correct answer: yes.)

– Bill Dawson

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