Monday, August 20th 2012, 6:59 PM EDT
In a recent study published in Procedings of the National Academy of Sciences (PNAS), NASA scientist James Hansen and two colleagues find that whereas “extremely hot” summer weather ”practically did not exist” during 1951-1980, such weather affected between 4% and 13% of the Northern Hemisphere land area during 2006-2011. The researchers infer that human-caused global warming is “loading” the “climate dice” towards extreme heat anomalies. They conclude with a “high degree of confidence” that the 2003 European heat wave, the 2010 Russian heat wave, and the 2011 Texas-Oklahoma drought were a “consequence of global warming” and have (as Hansen put it in a recent op-ed) ”virtually no explanation other than climate change.”
In a recent post, I reviewed studies finding that the aforementioned anomalies were chiefly due to natural variability. In another post, I summarized an analysis by Patrick Michaels and Chip Knappenberger, who conclude that “the 2012 drought conditions, and every other [U.S.] drought that has come before, is the result of natural processes, not human greenhouse gas emissions.”
But what about the very hot weather afflicting much of the U.S. this summer? Greenhouse gas concentrations keep rising, heat spells are bound to become more frequent and severe as the world warms, and the National Oceanic and Atmospheric Administration (NOAA) reports that July 2012 was the hottest July ever in the U.S. instrumental record. Isn’t this summer what greenhouse warming “looks like“? What else could it be?
University of Alabama in Huntsville (UAH) climatologist John Christy addressed these questions last week in a two-part column. In Part 1, Christy argues that U.S. daily mean temperature (TMean) data, on which NOAA based its report, ”do not represent the deep atmosphere where the enhanced greenhouse effect should be detected, so making claims about causes is unwise.” A better measure of the greenhouse effect is daily maximum temperature (TMax), and TMax records set in the 1930s remain unbroken. In Part 2, Christy argues that Hansen’s 10% estimate of the portion of land affected by extreme heat during 2006-2011 shrinks down to 2.9% when anomalies are measured against a longer, more representative climate baseline.
NOAA’s claim that July 2012 was the hottest July ever is based on daily mean temperature (TMean) data. TMean is the average of daytime maximum temperature and nighttime minimum temperature (TMax + TMin/2). Whereas TMax “represents the temperature of a well-mixed lower tropospheric layer, especially in summer,” TMin “can warm over time due to an increase in turbulent mixing” near the surface. Land use changes such as urbanization, agriculture, and forestry tend to disrupt the natural formation of a shallow layer of cool nighttime air. There has been a lot of population growth and development in the U.S. since 1980, the last year of Hansen’s baseline period. Not coincidentally, most of the surface warming in the U.S. during the past three decades has been in TMin rather than TMax (see second graph below).
The point? TMin warming is not primarily due to the accumulation of heat in the deep atmosphere (i.e. the greenhouse effect). Consequently, averaging TMin with TMax produces a composite (TMean) that inflates the appearance of the greenhouse effect.
Christy’s colleague Roy Spencer produced a chart of TMax using the same weather stations as NOAA. Spencer found that July 2012 was very hot, but not as hot as the summers of 1936 and 1934. More importantly, far more all-time TMax records were set in the 1930s than in any recent decade.
Click source to read FULL report from John Christy