extreme weather

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 Hansen on Extreme Weather — Pat and Chip Respond

Last week, I posted a commentary on NASA scientist James Hansen’s study and op-ed, which attribute recent extreme weather to global climate change. In the op-ed, Hansen stated:

The deadly European heat wave of 2003, the fiery Russian heat wave of 2010 and catastrophic droughts in Texas and Oklahoma last year can each be attributed to climate change. And once the data are gathered in a few weeks’ time, it’s likely that the same will be true for the extremely hot summer the United States is suffering through right now.

My commentary concluded: “Hansen’s sweeping assertion that global warming is the principal cause of the European and Russian heat waves, and the Texas-Oklahoma drought, is not supported by event-specific analysis and is implausible in light of previous research.”

Although Hansen does not explicitly attribute the ongoing U.S. drought to global warming, he does blame global warming for both the 2011 Texas-Oklahoma drought and the current summer heat. And in his study, Hansen states: “With the temperature amplified by global warming and ubiquitous surface heating from elevated greenhouse gas amounts, extreme drought conditions can develop.”

This week on World Climate Report, Pat Michaels and Chip Knappenberger argue that the current U.S. drought “is driven by natural variability not global warming.” Their post (“Hansen Is Wrong“) is concise and layman-friendly. Here I offer an even briefer summary.

A standard measure of drought in the U.S. is the Palmer Drought Severity Index (PDSI), which measures the combined effects of temperature (hotter weather = more soil evaporation) and precipitation (more rainfall = more soil moisture). “The more positive the PDSI values, the wetter conditions are, the more negative the PDSI values, the drier things are.” The PDSI for the past 117 years (1895-2011) shows a small non-significant positive trend (i.e. towards wetter conditions). There is no greenhouse warming signal in this data.

[click to continue…]

Post image for Hansen’s Study: Did Global Warming Cause Recent Extreme Weather Events?

A study by NASA’s James Hansen and two colleagues, published Monday in Proceedings of the National Academy of Sciences (PNAS), finds that during the past 30 years, extreme hot weather has become more frequent and affects a larger area of the world than was the case during the preceding 30 years. Specifically, the study, “Perception of climate change,” reports that:

  • Cool summers occurred one-third of the time during 1951-1980 but occurred only 10% of the time during 1981-2010.
  • Very hot weather affected 0.2% of the land area during 1951-1980 but affected 10% of the land area during 1981-2010.

Hansen is the world’s best known scientist in the climate alarm camp and a leading advocate of aggressive measures to curb fossil-energy use. He and his co-authors are up front about the policy agenda motivating their study. The “notorious variability of local weather and climate from day to day and year to year” is the “great barrier” to “public recognition” of man-made climate change and, thus, to public support for policies requiring “rapid reduction of fossil fuel emissions.” When heat waves or drought strike, the authors want the public to perceive global warming. On Saturday, the Washington Post published an op-ed by Hansen summarizing the study’s results.

Heat waves will become more frequent and severe as the world warms; some areas will become drier, others wetter. Those hypotheses are not controversial.

What the Hansen team concludes, however, is controversial. The researchers contend that the biggest, baddest hot weather extremes of recent years — the 2003 European heat wave, the 2010 Russian heat wave, the 2011 Texas-Oklahoma drought, the ongoing Midwest drought — are a “consequence of global warming” and have “virtually no explanation other than climate change.”

There’s just one small problem. The reseachers do not examine any of those events to assess the relative contributions of natural climate variability and global warming. The study provides no event-specific evidence that the record-setting heat waves or droughts would not have occurred in the absence of warming, or would not have broken records in the absence of warming.  [click to continue…]