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Hullabaloo


Saturday, December 21, 2013

 
Tools to destroy your wingnut uncle's climate denialism this holiday

by David Atkins

Salon has a really tremendous article on climate change by John Rennie of Scientific American, listing seven ways to shut down a climate change denier. It's far too rich and dense to justice with excerpts, but I'll just post the first two of his seven points here. Be sure to head over to the full article to read the rest:

Claim 1: Anthropogenic CO2 can’t be changing climate, because CO2 is only a trace gas in the atmosphere and the amount produced by humans is dwarfed by the amount from volcanoes and other natural sources.Water vapor is by far the most important greenhouse gas, so changes in CO2 are irrelevant.

Although CO2 makes up only 0.04 percent of the atmosphere, that small number says nothing about its significance in climate dynamics. Even at that low concentration, CO2 absorbs infrared radiation and acts as a greenhouse gas, as physicist John Tyndall demonstrated in 1859. The chemist Svante Arrhenius went further in 1896 by estimating the impact of CO2 on the climate; after painstaking hand calculations he concluded that doubling its concentration might cause almost 6 degrees Celsius of warming—an answer not much out of line with recent, far more rigorous computations.

Contrary to the contrarians, human activity is by far the largest contributor to the observed increase in atmospheric CO2. According to the U.S. Geological Survey, anthropogenic CO2 amounts to about 30 billion tons annually—more than 130 times as much as volcanoes produce. True, 95 percent of the releases of CO2 to the atmosphere are natural, but natural processes such as plant growth and absorption into the oceans pull the gas back out of the atmosphere and almost precisely offset them, leaving the human additions as a net surplus. Moreover, several sets of experimental measurements, including analyses of the shifting ratio of carbon isotopes in the air, further confirm that fossil-fuel burning and deforestation are the primary reasons that CO2 levels have risen 35 percent since 1832, from 284 parts per million (ppm) to 388 ppm—a remarkable jump to the highest levels seen in millions of years.

Contrarians frequently object that water vapor, not CO2, is the most abundant and powerful greenhouse gas; they insist that climate scientists routinely leave it out of their models. The latter is simply untrue: from Arrhenius on, climatologists have incorporated water vapor into their models. In fact, water vapor is why rising CO2 has such a big effect on climate. CO2 absorbs some wavelengths of infrared that water does not so it independently adds heat to the atmosphere. As the temperature rises, more water vapor enters the atmosphere and multiplies CO2′s greenhouse effect; the IPCC notes that water vapor (pdf) may “approximately double the increase in the greenhouse effect due to the added CO2 alone.”

Nevertheless, within this dynamic, the CO2 remains the main driver (what climatologists call a “forcing”) of the greenhouse effect. As NASA climatologistGavin Schmidt has explained, water vapor enters and leaves the atmosphere much more quickly than CO2, and tends to preserve a fairly constant level of relative humidity, which caps off its greenhouse effect. Climatologists therefore categorize water vapor as a feedback rather than a forcing factor. (Contrarians who don’t see water vapor in climate models are looking for it in the wrong place.)

Because of CO2′s inescapable greenhouse effect, contrarians holding out for a natural explanation for current global warming need to explain why, in their scenarios, CO2 is not compounding the problem.

Claim 2: The alleged “hockey stick” graph of temperatures over the past 1,600 years has been disproved. It doesn’t even acknowledge the existence of a “medieval warm period” around 1000 A.D. that was hotter than today is. Therefore, global warming is a myth.

It is hard to know which is greater: contrarians’ overstatement of the flaws in thehistorical temperature reconstruction from 1998 by Michael E. Mann and his colleagues, or the ultimate insignificance of their argument to the case for climate change.

First, there is not simply one hockey-stick reconstruction of historical temperatures using one set of proxy data. Similar evidence for sharply increasing temperaturesover the past couple of centuries has turned up independently while looking at ice cores, tree rings and other proxies for direct measurements, from many locations. Notwithstanding their differences, they corroborate that Earth has been getting sharply warmer.

A 2006 National Research Council review of the evidence concluded “with a high level of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries”—which is the section of the graph most relevant to current climate trends. The report placed less faith in the reconstructions back to 900 A.D., although it still viewed them as “plausible.” Medieval warm periods in Europe and Asia with temperatures comparable to those seen in the 20th century were therefore similarly plausible but might have been local phenomena: the report noted “the magnitude and geographic extent of the warmth are uncertain.” And a new research paper by Mann and his colleagues seems to confirm that the Medieval Warm Period and the “Little Ice Age” between 1400 and 1700 were both caused by shifts in solar radiance and other natural factors that do not seem to be happening today.

After the NRC review was released, another analysis by four statisticians, called theWegman report, which was not formally peer reviewed, was more critical of the hockey stick paper. But correction of the errors it pointed out did not substantially change the shape of the hockey stick graph. In 2008 Mann and his colleagues issued an updated version of the temperature reconstruction that echoed their earlier findings.

But hypothetically, even if the hockey stick was busted… What of it? The case for anthropogenic global warming originally came from studies of climate mechanics, not from reconstructions of past temperatures seeking a cause. Warnings about current warming trends came out years before Mann’s hockey stick graph. Even if the world were incontrovertibly warmer 1,000 years ago, it would not change the fact that the recent rapid rise in CO2 explains the current episode of warming more credibly than any natural factor does—and that no natural factor seems poised to offset further warming in the years ahead.
The original also contains a bevy of quotes too numerous to reproduce individually here. It's a great refresher for the holidays when your wingnut uncle tells the family that there's no climate change.


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