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

by Marlo Lewis on August 8, 2012

in Blog, Features

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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. 

The PNAS study (hereafter, “Hansen”) finds that the bell curve showing the distribution of extreme hot weather has steadily moved to the right as the planet has warmed from 1951 to 2011. Events that were once outliers (right hand tail) in 1951-1980 occur with increasing frequency in each subsequent decade, and today’s most extreme events did not occur in the baseline (1951-1980) period.

One question that springs to mind is whether 1951-1980 is an appropriate baseline for assessing trends in extreme weather. Consider the graph at the top of this page, which shows the U.S. Annual Heat Wave Index (source: U.S. Climate Change Research Program). In the U.S., the period of 1951-1980 was not representative or typical of prior decades.

In recent testimony before the Senate, University of Alabama in Hunstville climatologist John Christy made a by-the-numbers case that when data from the 1920s-1940s are included, there is no long-term trend in U.S. extreme heat events. Christy finds that:

  • More state all-time high temperature records were set in the 1930s than in recent decades.
  • More state all-time cold records than hot records were set in the decades since 1960.
  • In a database of 970 weather stations, daily all-time high temperatures occurred more frequently before 1940 than after 1954.
  • The 1930s set twice as many daily maximum temperature records than were set in the 1980s, 1990s, or 2000s.
  • More Midwest daily maximum temperature records were set in the heat waves of 1911 and the 1930s than in the 2012 heat wave.
  • The Palmer Drought Severity Index for the continental U.S. shows considerable interannual variability but no long-term trend from 1900 to the present.
  • The upper Colorado River Basin experienced more frequent multi-decadal droughts in the 19th, 18th, 17th, and 16th centuries than in the 20th century.

Viewed in the context of Christy’s longer datasets, Hansen’s 1951-1980 baseline period looks anomalous, not the following three decades.

Hansen’s own plot of U.S. climate data going back to the 19th century also shows a period of pronounced warmth in the 1930s and 1940s, i.e. prior to his baseline.

Hansen is looking at all Northern hemisphere data whereas Christy is looking just at U.S. data. But the U.S. arguably has the best long-term weather data of any country in the world. What would have been the result had Hansen used only U.S. data and chosen an earlier period as the baseline, say 1925-1954, when there was far less greenhouse ‘forcing’ but many daily high temperature records? It is doubtful his statistical results would be anywhere near as dramatic.

Hansen argues that global warming, not weather patterns associated with drought (La Niña) and heat waves (atmospheric blocking), caused the 2003 European heat wave, the 2010 Russian heat wave, and the 2011 Texas-Oklahoma drought. La Niñas and blocking patterns “have always been common, yet the large areas of extreme warming have come into existence only with global warming.” Therefore, Hansen concludes, today’s extreme anomalies have at least two causes, “specific weather patterns and global warming.”

This is spin, speculation, or ‘trust-me-I’m-the-expert’ assertion. Chase et al. 2006, a team of scientists from Colorado and France, found “nothing unusual” in the 2003 European heat wave that would indicate a change in global climate. Look at the global temperature map included in the study. During June, July, and August 2003, more than half the planet was cooler than the mean temperature from 1979 through 2003. Europe – a tiny fraction of the Earth’s surface – was the only place experiencing high heat. Does it make sense to attribute that local anomaly to global warming?

Figure explanation (courtesy of World Climate Report): 1000–500 mb thickness temperature anomaly for June, July, and August 2003. Green and blue tones indicate below-normal temperature anomalies.

Similarly, a National Oceanic & Atmospheric Administration (NOAA) analysis found that the 2010 Russian heat wave “was mainly due to natural internal atmospheric variability.” The study specifically addressed the question of a possible linkage to anthropogenic climate change:

Despite this strong evidence for a warming planet, greenhouse gas forcing fails to explain the 2010 heat wave over western Russia. The natural process of atmospheric blocking, and the climate impacts induced by such blocking, are the principal cause for this heat wave. It is not known whether, or to what extent, greenhouse gas emissions may affect the frequency or intensity of blocking during summer. It is important to note that observations reveal no trend in a daily frequency of July blocking over the period since 1948, nor is there an appreciable trend in the absolute values of upper tropospheric summertime heights over western Russia for the period since 1900.

The Texas-Oklahoma drought of 2011 was a record breaker. According to NOAA (State of the Climate in 2011, p. 166), “Several climate divisions in Texas, New Mexico, Oklahoma, and Louisiana, as well as the Rio Grande and Texas Gulf Coast river basins, had record low values for the Palmer Hydrological Drought Index in the 117-year record.” For Texas, 2011 was also a year of record heat. However, this correlation is not evidence that global warming was the principal factor. Detection and — more importantly — measurement of the impact of global climate change on the Texas drought requires a long and complicated analysis.

Texas State Climatologist John Nielsen-Gammon conducted a “preliminary analysis” of the role of global warming in the Texas drought. Although far from definitive, it is (to my knowledge) the most detailed and thorough analysis to date.  Nielsen-Gammon examines Texas drought and temperature data, climate modeling studies, and data on natural climate cycles (La Niña/El Niño, Pacific Decadal Oscillation, Atlantic Multi-Decadal Oscillation) to estimate the potential contribution of global warming. Here are some of his findings:

  • Some IPCC AR4 climate models “at one extreme” project precipitation to increase in Texas, while others project a substantial decrease. “The general model consensus is that precipitation is likely to decrease a bit, but it’s not a sure thing.”
  • The model-projected change is “smaller in magnitude than the past observed multi-decade-scale changes,” which indicates that “global warming is not going to be the dominant driver of mean precipitation changes, at least for the next several decades.”
  • From 1895 to 2010, precipitation in Texas increased overall, by more than 10%.
  • There has been no net change in Texas precipitation variability since 1920.
  • Although the 2011 drought was the most severe 1-year Texas drought, it was not the most severe in the instrumental record. That distinction belongs to the 1950-1957 drought. Aside from 2009 and 2011, all the droughts that rank as most severe in at least 1% of the State occurred in 1956 and earlier.
  • Texas summer temperature in 2011 was record-breaking because of the drought rather than the other way around. “This record-setting summer was 5.4 F above average.  The lack of precipitation accounts for 4.0 F, greenhouse gases global warming accounts for another 0.9 F, and the AMO [Atlantic Multi-Decadal Oscillation] accounts for another 0.3 F.  Note that there’s uncertainty with all those numbers, and I have only made the crudest attempts at quantifying the uncertainty.”

 Among Nielson-Gammon’s key conclusions:

So I conclude, based on our current knowledge of the effects of global warming on temperature and precipitation, that Texas would probably have broken the all-time record for summer temperatures this year even without global warming.

This drought was an outlier.  Even without global warming, to the best of my knowledge, it would have been an outlier and a record-setter.

* * *

Until we learn more, it is appropriate to assume that the direct impact of global warming on Texas precipitation interannual variability has been negligible, and that the future variability trend with or without global warming is unknown.

In short, 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.

A concluding comment on what might be called Hansen’s political science is in order. Hansen believes the “great barrier” to aggressive action on climate change is the “notorious variability” of weather and climate at local scales. But the public’s rejection of cap-and-trade, the collapse of the Kyoto-Copenhagen treaty agenda, and the GOP/Tea Party opposition to the Obama administration’s war on affordable energy are only partly related to public “perceptions” of climate change risk. More important is the fact that nobody knows how to run and grow a modern economy with zero-carbon energy.

The Breakthrough Institute develops this thesis in great detail in a collection of posts titled the “Death of Cap-and-Trade.” Because affordable energy is vital to prosperity and much of the world is energy poor, it would be economically ruinous and, thus, politically suicidal to demand that people abandon fossil fuels before cheaper alternative energies are available. But that is exactly what warmistas like Hansen urge the U.S. and other governments to do — lock up vast stores of carbonaceous fuel and penalize fossil energy use before commercially-viable alternatives exist.

As the Breakthrough folks argue, if you’re worried about climate change, then your chief policy goal should be to make alternative energy cheaper than fossil energy. Instead, the global warming movement has attempted to make fossil energy more costly than alternative energy, or to simply mandate the switch to alternative energy regardless of cost. Al Gore’s call in 2008 to “re-power America” with zero-carbon energy within 10 years is epitomizes this folly. More “moderate” variants would only do less harm, less rapidly.


thoman August 8, 2012 at 9:28 pm

Records set in the 1930s are not the same as records set in the 2010s. Records set in the 1930s are compared to the last 35 years, back to 1895 when records were first recorded. From the 2010s to 1895 is 3x as long as records set in the 1930s.

Elliott Friedman August 8, 2012 at 11:47 pm

Great analysis and discussion of salient points.

Marlo Lewis August 16, 2012 at 12:46 am


The records set in the 1930s are records that still stand today. As indicated above, those are “all-time” records within the 117-year period from 1895 to the present.

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