An Expanded Critique of Some Climate Conclusions

Martin P. Hoerling, a federal research meteorologist specializing in climate dynamics,has written the following expansion and defense of his criticism of some assertionsmade in an Op-Ed article on climate change by James E. Hansen of NASA. His initialcriticism was posted on the Dot Earth blog.Several scientific conclusions and claims were expressed in the NYT Op-Ed piece byJ. Hansenwith which I raised concerns and objections on a scientific basis. Whileallowing for imprecision in how nuanced climate change science is sometimescommunicated in such venues, there were nonetheless statements in the HansenNYT piece that drew my attention because they stood contrary to peer-reviewedliterature. Some of these claims could also be tested and/or falsified by simple testsusing data available in the public domain, with examples given below.The Hansen NYT piece asserts:"Over the next several decades, the Western United States and the semi-arid regionfrom North Dakota to Texas will develop semi-permanent drought, with rain, whenit does come, occurring in extreme events with heavy flooding. Economic losseswould be incalculable. More and more of the Midwest would be a dust bowl.California’s Central Valley could no longer be irrigated. Food prices would rise tounprecedented levels."To which I replied:“I am unaware of any projection for "semi-permanent" drought in this time frameover the expansive region of the Central Great Plains. He implies the drought is tobe a phenomena due to lack of rain (except for the brief, and ineffectivedownpours). I am unaware of indications, from model projections, for a materialdecline in mean rainfall. “Supporting Material for My StatementThe peer-reviewed study by Milly et al. titled “Global Pattern of Trends inStreamflow and Water Availability in a Changing Climate”(Nature, 2005) used theIPCC CMIP3simulations to diagnose the relative change in surface runoff for theperiod 2040-2060 compared to 1900-1970. A version of their published Fig. 4 isshown below. Runoff is a holistic indicator of surface water balance that integratesthe effects of changes in mean precipitation and its characteristics, and also changesin temperature via evapotranspiration. These published results indicate noappreciable change in surface water availability over the semi-arid region fromNorth Dakota to Texas, or for the Midwest as a whole, within this ensemble ofCMIP3 simulations. The authors point out the various uncertainties in suchregional scale projections, not the least of which one must also include theuncertainty in the consequences of changes in land cover and land use.

The recently published report by the U.S. Global Change Research Program titled“Global Climate Change Impacts in the United States”provided a synthesis of thecurrent understanding of probable regional climate change impacts. This synthesisdocument states, in the section on Water Resources on pg. 45, “Precipitation andrunoff are likely to increase in the Northeast and Midwest in winter and spring.”Drawn from such works as the Milly et al. study, this synthesis is clearly quitecontrary to the assertion made in the Hansen NYT piece.Regarding flooding that ismentioned in the Hansen piece, it is important not to confuse heavy downpourswith hydrologic flooding on a river basin scale, for instance. The IPCC SpecialReport on Managing the Risks of Extreme Events and Disasters to Advance ClimateChange Adaptation (SREX, 2012http://www.ipcc-wg2.gov/SREX/) states a lowconfidence for both observed and projected changes in the magnitude andfrequency of floods.SummaryThe claim in the Hansen NYT piece that the Midwest would be a dustbowl in comingdecades thus runs contrary to peer reviewed literature and recent assessments bythe U.S. Global Research Program that emerged from the synthesis of currentunderstanding by an expert team of scientists.Figure 1.The relative change in runoff in the twenty-first century expressed as the ensemble (arithmetic)mean of relative change (percentage) in runoff for the period 2041–60, computed as 100 times thedifference between 2041–60 runoff in the SRESA1B experiments and 1900–70 runoff in the 20C3Mexperiments, divided by 1900–70 runoff. Based on Fig. 4 from Milly et al. (2005). [Milly, P, K. Dunne, A.Vecchia, Nature, 438, 2005, doi:10.1038/nature04312]. Left-side illustrates runoff change for drainagebasin scale, and right side for geopolitical state scales.

Regarding observed changes in climate of the Great Plains, I stated:“Indeed, that region (Great Plains) has seen a general increase in rainfall over thelong term, during most seasons (certainly no material decline). Also, for the warmseason when evaporative loss is especially effective, the climate of the central GreatPlains has not become materially warmer (perhaps even cooled) since 1900. Inother words, climate conditions in the growing season of the Central Great Plainsare today not materially different from those existing 100 years ago. Thisobservational fact belies the expectations, from climate simulations, and in truth,our science lacks a good explanation for this discrepancy. “Supporting Material for My StatementThe lack of a warming trend over the central United States during the past century,sometimes called the U.S. “warming hole”, has been especially noted in the peer-reviewed literature (e.g., Kunkel et al. Journal of Climate, 2006, Knutson et al. Journal ofClimate2006). Particularly striking has been a cooling trend in summertime temperaturesat many meteorological observing stations located between the Rocky and AppalachianMountains over the period 1901-2010 (Fig. 2, top). This region of summertime coolinghas generally coincided with a region of summertime mean precipitation increase (Fig. 2,bottom). The data is the monthly Global Historical Climate Network data available athttp://www.ncdc.noaa.gov/ghcnm/v3.phpTo date, these regional patterns are not well understood on physical grounds. This, andother regional examples of climate trends, illustrate the need for a more comprehensiveassessment on the causes of regional and seasonal differences in climate trends thatconsiders multiple possible contributing factors, including atmospheric dynamics andcoupled ocean-atmosphere processes, land surface and biological processes, atmosphericchemistry and aerosols, and human-induced climate change.SummaryThe certainty language expressed in the Hansen NYT piece about the coming dustbowlfate for the Great Plains region and Midwest is contrary to the low confidence of regionalclimate change projections for coming decades as documented in USGCRP and IPCCreports. Not only are various regional patterns of trends that have been observed over thelast century poorly understood, but the projections of regional changes in coming decadesare highly uncertain.

Figure 2. The 1901-2010 trends in summertime (June-August) daily averaged surface temperature(°C/110 yrs; top) and rainfall (% of change over 110 yrs, bottom). Trends are plotted at availablestation sites, using the GHCNv3 data. Cooling (warming) trends shown in blue (red), and increased(decreased) rainfall shown in blue (red).The Hansen NYT piece asserts:"The global warming signal is now louder than the noise of random weather..."To which I replied:“This is patently false. Take temperature over the U.S. as an example. The variabilityof daily temperature over the U.S. is much larger than the anthropogenic warming

signal at the local, weather time scales. Depending on season and location, thedisparity is at least a factor of 5 to 10. I think that a more scientifically justifiablestatement, at least for the U.S. and extratropical land areas is that --- Daily weathernoise continues to drum out the siren call of climate change on local, weatherscales.”Supporting Material for My StatementWeather---as experienced on a daily basis and at any particular location--- is highlyvariable, but the challenge offered in the Hansen NYT piece is that the noise of suchvariations are now being drowned out by the global warming signal. Debating thislatter point should not, of course, be confused with opening a debate about the risein global mean temperatures. The IPCC (2007) has stated that warming of theclimate system is unequivocal. The unequivocal rise in global average temperatureand the attribution that most of this rise is due to the rise in anthropogenicgreenhouse gas concentrations, owes to the small natural variability of global meantemperatures compared to the large magnitude of the human-induced warmingsignal in global averaged temperatures. In other words, for global meanconditions, the signal is much louder than natural variability of globally averagedtemperatures. The science is clear, we know that the planet has warming over thepast century, and we are very confident as to why such warming has (andcontinues) to occur.But that appears not to be the point of the NYT piece, as impliedby the subsequentcontext of Hansen’s statement regarding extreme local weather events. The“random weather” called out in the NYT piece is unlikely meant to refer to thevariability in global mean temperature, but rather to the local conditions weregularly encounter in our own backyards and that swing back and forth across arange of conditions. Thisrange (the random noise, as per Hansen) is in fact notsmaller than the global warming signal, as shown from several lines of evidencebelow.The recent published peer-reviewed study by Hawkins and Sutton (2012,Geophysical Research Letters) diagnoses the so-called “time of emergence” ofclimate signals from the noise of random variability at a local level. Their analysiscompares an estimate of a human-induced change in surface air temperatureagainst an estimate of its natural variability for seasonally averaged data. For asignal-to-noise ratio of two (signal being double the magnitude of the noise), theyfind that the time of emergence is after 2050 for most mid-latitude regions duringcold and warm seasons. The tropics, where noise of temperature variability is lessthan in mid-latitudes, the time of emergence is appreciably earlier.But even that analysis is not quite germane to the Hansen assertion regarding thenoise of random weather. Figure 3 presents an analysis of the intensity of thevariability of daily averaged surface temperature across the United States.

The data is daily temperature at Cooperative observing stations, which is availableat http://www.ncdc.noaa.gov/oa/climate/research/gdcn/gdcn.html. The top panelis the intensity of daily temperature variability during 1901-2010, averaged acrossthe 12 calendar months January-December. On average, daily temperaturevariability is about 5°C over North Dakota, to as small as 2°C over Florida. Therecent update, by NCDC, of the annual mean global mean warming signal is +0.51°C(for 2011 relative to a 20th Century reference). It is thus evident that daily surfacetemperature variability is on order 5 to 10 times greater than the global warmingsignal (see Fig. 3, bottom). Consistent with the published work of Hawkins andSutton, it is obvious that the time of emergence of the global warming signal fromthis weather noise is far in the future under the assumption of continuing globalwarming.SummaryThe global warming signal is much smaller than the typical daily variability of surface airtemperature over the United States. Most of the magnitude of daily weather extremesowes its causes to natural internal fluctuations and not to global warming. A possibleexception could be imagined if global warming were also to increase the variability ofdaily temperatures (and not just increase the mean temperatures), but no compellingevidence to such effects has been shown. While globally averaged temperatures haverisen during the past century, the cause for which is very likely human-induce climatechange, the signal of this change is still barely audible among the loud noise of daily,backyard weather fluctuations.Weather, of course, is more than temperature variability. While this discussion hasinvolved temperature, weather involves rain, storms, winds, severe convection, cloudsamong others. In this regard, it is important to reiterate the statement in IPCC SREX(2012) in their Executive Summary which states that “many weather and climateextremes are the result of natural climate variability”, and that “even if there were noanthropogenic changes in climate, a wide variety of natural and weather extremes wouldstill occur”.

Figure 3.The daily surface temperature variability during 1901-2010 averaged for all months duringJanuary-December (°C, top), and the ratio of that daily variability to the magnitude of the observedglobal warming signal (nondimensional). The variability is the standard deviation of dailytemperature fluctuations calculated for each calendar month, and averaged across all months. Theglobal mean warming signal of +0.51°C is derived from the NCDC analysis of the 2011 annual meanglobal averaged surface temperature departure relative to a 20 th Century climatology (seehttp://www.ncdc.noaa.gov/sotc/global/2011/13)The Hansen NYT piece asserts:" We can say with high confidence that the recent heat waves in Texas and Russia,and the one in Europe in 2003, which killed tens of thousands, were not naturalevents — they were caused by human-induced climate change."To which I replied:“Published scientific studies on the Russian heat wave indicate this claim to befalse. Our own study on the Texas heat wave and drought, submitted today toJournal of Climate, likewise shows that that event was not caused by human-inducedclimate change. These are not de novo events, but upon scientific scrutiny,one findsboth the Russian and Texas extreme events to be part of the physics of what hasdriven variability in those regions over the past century. Not to say that climatechange didn’t contribute to the those cases, but their intensity owes to natural, nothuman, causes.”

Supporting Material for My StatementThe principal conclusion by Dole et al. (2011, Geophysical Research Letters) is thatthe extreme magnitude of the 2010 Russian heat wave was mainly due to internaldynamical processes (associated with atmospheric blocking), and that it was veryunlikely that warming attributable to GHG forcing contributed substantially to theheat waves magnitude. Rahmstorf and Coumou (2011, PNAS) concluded that astrong warming over western Russia (which they attribute primarily to GHGforcing) multiplied the likelihood of a record heat wave. They estimated an 80%probability that the 2010 July heat records in Moscow would not have occurredwithout climate warming. Barriopedro et al. (2011, Science), on the other hand,conclude that the magnitude of the 2010 event was so extreme that despite GHGwarming, the likelihood of an analog over the same region remains fairly low at thistime. This is consistent with estimates in Dole et al., which showed a very lowprobability of an event of this magnitude in 2010, but a rapidly increasing likelihoodof crossing given thresholds in future climate, based on results from CMIP3 modelruns.Toward attempting to reconcile these conclusions, Otto et al (2012, GeophysicalResearch Letters) conclude that there is no substantial contradiction betweenstudies by Dole et al. and Rahmstorf and Coumou, in that the heat wave “can be bothmostly internally generated due to magnitude, and mostly externally-driven interms of occurrence-probability”. Further discussion on this matter including anextensive list of recent published work on the Russian heat wave is available athttp://www.esrl.noaa.gov/psd/csi/events/2010/russianheatwave/index.htmlSummaryAnalysis of various forced model simulations indicates that human influences didnot contribute substantially to the magnitude of the Russian heat wave. Evenaccounting for a possible stronger warming signal, as suggested by Rahmsdorf andCoumou, these were still appreciably smaller than the peak magnitude of the event(which reached 10°C over Moscow during July). Barriapedro et al. (2011) concludethat the magnitude of the 2010 event was so extreme that despite an increase intemperatures due to human climate change, the likelihood of an analog over thesame region remains fairly low until the second half of the 21st century. Theseresults are thus consistent also with the Hawkins and Sutton (2012) resultsregarding the time of emergence of a climate change signal at local scales.

An Expanded Critique of Some Climate Conclusions

by Andrew Revkin

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