A Defense of Jim Hansen's Climate Conclusions

Dan Miller, an engineer and venture capitalist focused on non-polluting energytechnologies, blogs on climate at ClimatePlace.org and helped NASA climate scientistJames E. Hansen prepare his recent New York Times Op-Ed article, “Game Over for theClimate.” Miller complained about the Dot Earth post in which the climate scientistsMartin Hoerling and Kerry Emanuel separately criticized aspects of Hansen’s piece andwas offered a chance to write the following “Your Dot” post.(For his part, Hansen declined to respond to the criticisms, saying this in an e-mailmessage: “I have several papers well along in the publication process that make clearyour characterizations are far off the mark. The editors prefer, indeed are insistent, that Inot discuss these in blogs. Some scientists may be able to spend their time blogging ande-mailing without a significant impact on their scientific productivity -- Im not one ofthem -- but I do make an effort to make my papers understandable to a wide audience.”)Here’s Miller’s contribution:In Andy Revkins recent DotEarth column about James Hansens New York Times Op-Edpiece, he quoted two scientists who seemed to cast doubt on Dr. Hansens statementsabout climate change and the need to begin reducing carbon dioxide emissionsimmediately.I helped Jim edit his piece and I also assisted him by providing references to the NewYork Times fact checker.Martin Hoerling is quoted in the DotEarth column disputing Jims claim that "The globalwarming signal is now louder than the noise of random weather." Martin stated, "Thevariability of daily temperature over the U.S. is much larger than the anthropogenicwarming signal at the time scales of local weather." The time scales of local weather arehourly to daily, but no one is looking for the global warming signal on that time scale, soIm not quite sure why Martin even mentions that. Weather and climate aredifferent. "Weather is what you get, climate is what you expect" and what we expect is awarmer world. It is quite straightforward to detect the global warming signal from thenoise of local weather over climate time scales (decades). Many studies, including onepartially funded by the Koch brothers, have determined that the global mean temperaturehas increased by about 1.4°F (0.8°C) since the industrial revolution. TheIntergovernmental Panel on Climate Change (IPCC) has stated that warming is"unequivocal" using many lines of evidence. Dr. Hansens own work, described below,shows the global warming signal clearly growing in the last few decades.Martin also seems to dispute Jims claim that projections point to semi-permanentdrought conditions in the Western United States and the semi-arid region from NorthDakota to Texas in the coming decades. Martin says "I am unaware of indications, frommodel projections, for a material decline in mean rainfall. Indeed, that region has seen ageneral increase in rainfall over the long term during most seasons (certainly no materialdecline)." First of all, while Martin may indeed be unaware of studies showing

predictions of drought in the U.S. in the coming decades, it is certainly not because theydont exist. I provided references to the New York Times including a study by Aiguo Daiof the National Center for Atmospheric Research. The press release that accompaniedthe study was titled "Climate Change: Drought May Threaten Much of Globe WithinDecades". Quoting from the press release: "By the 2030s, the results indicated thatsome regions in the United States and overseas could experience particularly severeconditions, with average decadal readings potentially dropping to -4 to -6 in much ofthe central and western United States as well as several regions overseas, and -8 orlower in parts of the Mediterranean." I should note that for the Palmer drought indexthey are using, a reading of -3 is associated with the Dust Bowl of the 1930s. Joe Rommat Climate Progress has an in-depth piece on why Martins statements about droughtpredictions are incorrect.Martin also disputes Jims claim that there is a high probability that the recent Texas andRussian heat waves were not natural events and were indeed caused by global warming.For insight into both the global warming signal and heat wave issues, I suggest taking alook at Jim Hansens recent "Climate Dice" paper (it has already been peer-reviewed andwill be published soon). It is quite remarkable. No models. No predictions. It is simplya statistical analysis of measured temperatures from 1951 to 2011. Here is the key graph:The vertical scale is probability and the horizontal scale is temperature in units of"standard deviations". While specific temperatures are location dependent (e.g.,Anchorage vs. Miami), you can think of +1 standard deviation (or "1-sigma") as being a

"Hot" summer, +2-sigma being a "Very Hot" summer, and +3-sigma being an "ExtremelyHot" summer (I inserted that notation on the graph). Note that this is not based onmodels, etc. Its simply looking at the measured temperature data and doing basicstatistical analysis. Now you can see that the curves from the 1950s, 1960s and 1970spretty much follow a "normal distribution". When you look at the 1950s-1970s "+3-sigma" temperature, its probability of occurring is very low (about one-quater of onepercent). As global warming kicks in, the average temperature (effectively the peak ofthe curve) shifts to the right (warmer) somewhat -- about one standard deviation. Butbecause of the shift caused by global warming, the extremes (more specifically, the areaunder the curve that is +3-sigma or beyond) grows dramatically. What this shows isthat the probability of an extreme heat wave has increased enormously… by about40 times (4000%) in the last 50 years.The shift of the curve is exactly the "signal" you would expect with global warming andit is clear and dramatic. See the IPCC SREX Report (Page 5) for more on temperaturedistributions curves under global warming.The fact that 3-sigma "Extremely Hot" summers have increased in probability fromaround 0.25% to around 10% in 50 years tells you that if you now have a 3-sigma heatevent, then you can say with high confidence that it is most likely caused by the shift ofthe temperature distribution curve, in other words, by global warming. Again, this is notbased on models or predictions, just on a straightforward statistical analysis of measuredtemperature data. You should also note that +4 and +5-sigma events, that almost neveroccurred in the past, are now occurring with a noticeable frequency and these formerly 1-in-10,000 events will occur with even more frequency in the future.People wonder how a mere +0.8°C or even +2°C warming can cause bigproblems. While the average temperature increase may not seem that bad (well, unlessyou dont mind the melting of glaciers and ice sheets), the multi-thousand percentincrease in extreme events will certainly get everyones attention and will cause (andalready has caused) incredible damage and economic harm.So, Jims claims that the global warming signal is now bigger than the noise of naturalweather fluctuation and that recent 3-sigma heat waves can be attributed to globalwarming both are supported by temperature measurements over the past 60 years.Climate scientist Kerry Emanuel also weighed in on Jims Op-Ed. He said "I seeoverstatement on all sides" (referring to Martin and Jim). I wanted to know what Kerrysspecific concerns were about Dr. Hansens Op Ed, so I wrote to him. He responded thathe is in agreement with Dr. Hansens main points, though he feels that Jim should nothave stated his case with such confidence. There is no certainty in science (the strongestlevel of scientific confidence is called a "theory") and Jim did not remind people that risk,by its very nature, entails uncertainty. Kerry also said that he agrees with Jims mainconclusion that we need to begin reducing greenhouse gas emissions now.So, contrary to the impression that readers of DotEarth may have gotten from the post

about Dr. Hansens Op-Ed, (1) there are projections of drought in the U.S. in the comingdecades, (2) the global warming "signal" can clearly be detected over the noise of naturalweather variations, and (3) studies of measured temperature data provide clear evidencethat the recent 3-sigma extreme Texas and Russian heat waves were, with highprobability, caused by global warming.Without a doubt, it is time to begin reducing our greenhouse gas emissions.

A Defense of Jim Hansen's Climate Conclusions

by Andrew Revkin

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