Tuesday, October 27, 2009

Blind support for global warming

A popular narrative among global warming skeptics is that climate scientists are so hopelessly biased that we just can't trust what they say. Has it been warming or cooling since 1998? Well, say skeptics, we have a bunch of temperature data points, but the trend you pull out of them depends on who you ask. All knowledge is political, and science is no different: climate scientists' predictions of doom, so the theory goes, are colored by their need to justify research grants, or by pressure to conform to the prevailing view.

Ok. Let's pretend that's the case, and that the issue of global warming has become so hopelessly politicized that whenever the subject is brought up, all sides' arguments ought to be mistrusted. Is there a way to objectively analyze the data?

To do so in a way that satisfies skeptics' criteria, the party interpreting the data would need to have no personal stake in the matter. (This of course raises the problem that you're relying on the opinions of non-experts, but remember: to skeptics, having studied an issue gives you a vested interest in it, so the more knowledgeable you are, the more biased, and therefore untrustworthy, are your opinions.) To be even more objective, you'd want to make it a blind study: show only the raw data points with no identifying characteristics.

That's exactly what the AP did (story courtesy of Joe Romm): they chose a group of statisticians (no vested interest in climate science) to analyze de-labeled temperature data. In other words, the statisticians had no idea they were looking at temperatures, and were simply trying to spot trends in a set of valueless numbers. It's the ultimate unbiased examination of temperature.

Much to the dismay of skeptics, the statisticians came to the same conclusion as scientists: the numbers represented an undeniable upward trend, no matter whether you start from 1880, 1979, 1998, or 2005. Only then were they told that the numbers represented temperatures, at which point "upward trend" means "warming, not cooling." Here's the AP story:

The Associated Press sought independent statistical analyses of global temperatures to determine if there is a true cooling of Earth's climate.

The AP contacted University of South Carolina statistics professor John Grego, a longtime reliable statistics source. In addition, the American Statistical Association sent an e-mail request from the AP seeking statisticians willing to examine certain sets of numbers and look for trends without being told what those numbers represented.

Three professors of statistics agreed: David Peterson, retired from Duke University; Mack Shelley, director of public policy and administration at Iowa State University; and Edward Melnick from New York University.

Each was given two spreadsheets, neither of which had any indication they were temperature data.

One spreadsheet was year-by-year global temperature changes from 1880 to 2009, adjusted through most of this year from the National Oceanic and Atmospheric Administration's ground measurements. The other was year-to-year temperature changes from 1979-2009 gathered by scientists at the University of Alabama in Huntsville from atmospheric measurements by satellite.

None of the experts detected a downward, or cooling, trend in the numbers. All saw a distinct upward trend.

This type of blind test is a valid way of seeking statistical help by trying to keep the statisticians' personal beliefs out of any analysis, said Alan Karr, director of the National Institute of Statistical Sciences. But there is a downside from keeping statisticians in the dark because it ties their hands a bit and may make it difficult to determine trends from variation, he said.

After their analysis, all the statisticians were told that the numbers represented temperature changes. All stuck by their assessments.

So if you distrust climate scientists, you don't have to take their word for it: ask a blind statistician.


The only thing you need to know about global warming

A lesson in denial: Rick Cantor videos, global warming deniers, and how to say something without saying anything

Obama speaks on global warming: What you need to know to be certain that global warming is real

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