heat wave

Post image for John Christy on Summer Heat and James Hansen’s PNAS Study

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.  [click to continue…]

Post image for Historical Perspective on the Recent Heat Wave

Over at World Climate Report, the indefatigable Pat Michaels and Chip Knappenberger review a new study updating National Climate Data Center (NCDC) data on U.S. State climate extremes. I’ll cut right to the chase. The paper, “Evaluating Statewide Climate Extremes for the United States,” published in the Journal of Applied Meteorology and Climatology, finds that far more State-wide all-time-high temperature records were set in the 1930s than in recent decades.

From Pat and Chip’s review:

Despite the 24/7 caterwauling, only two new state records—South Carolina and Georgia—are currently under investigation. And, looking carefully at Shein et al. dataset, there appears to be a remarkable lack of all-time records in recent years. This is particularly striking given the increasing urbanization of the U.S. and the consequent “non climatic” warming that creeps into previously pristine records. . . .

Notice that the vast majority of the all-time records were set more than half a century ago and that there are exceedingly few records set within the past few decades. This is not the picture that you would expect if global warming from greenhouse gas emissions were the dominant forcing of the characteristics of our daily weather. Instead, natural variability is still holding a strong hand.

The chart below shows the number of State heat records and the year in which they were set. (When the same all-time high occurs in two or more years in the same State, each of those years gets a fraction of one point.)