Drought and Global Climate Change: An Analysis of Statements by Roger Pielke, Jr.
by John P. Holdren, February 28, 2014
Introduction
In the question and answer period following my February 25, 2014, testimony on the Administration’s Climate Action Plan before the Oversight Subcommittee of the U.S. Senate’s Committee on Environment and Public Works, Senator Jeff Sessions (R-AL) suggested that I had misled the American people with comments I made to reporters on February 13, linking recent severe droughts in the American West to global climate change. To support this proposition, Senator Sessions quoted from testimony before the Environment and Public Works Committee the previous July (2013) by Dr. Roger Pielke, Jr., a University of Colorado political scientist. Specifically, the Senator read the following passages from Dr. Pielke’s written testimony:
It is misleading, and just plain incorrect, to claim that disasters associated with hurricanes,
tornadoes, floods or droughts have increased on climate timescales either in the United
States or globally.
Drought has “for the most part, become shorter, less, frequent, and cover a smaller portion
of the U.S. over the last century.” Globally, “there has been little change in drought over
the past 60 years.”
Footnotes in the testimony attribute the two statements in quotation marks within the second passage to the US Climate Change Science Program’s 2008 report on extremes in North America and a 2012 paper by Sheffield et al. in the journal Nature, respectively.
I replied that the indicated comments by Dr. Pielke, and similar ones attributed by Senator
Sessions to Dr. Roy Spencer of the University of Alabama, were not representative of mainstream views on this topic in the climate-science community; and I promised to provide for the record a more complete response with relevant scientific references.
Dr. Pielke also commented directly, in a number of tweets on February 14 and thereafter, on my February 13 statements to reporters about the California drought, and he elaborated on the tweets for a blog post on The Daily Caller site (also on February 14). In what follows, I will address the relevant statements in those venues, as well. He argued there, specifically, that my statements on drought “directly contradicted scientific reports,” and in support of that assertion, he offered the same statements from his July testimony that were quoted by Senator Sessions (see above). He also added this:
The United Nations Intergovernmental Panel on Climate Change found that there is “not
enough evidence at present to suggest more than low confidence in a global-scale observed trend in drought.”
In the rest of this response, I will show, first, that the indicated quote from the US Climate
Change Science Program (CCSP) about U.S. droughts is missing a crucial adjacent sentence in
the CCSP report, which supports my position about drought in the American West. I will also
show that Dr. Pielke’s statements about global drought trends, while irrelevant to my comments about drought in California and the Colorado River Basin, are seriously misleading, as well, concerning what is actually in the UN Panel’s latest report and what is in the current scientific literature.
Drought trends in the American West
My comments to reporters on February 13, to which Dr. Pielke referred in his February 14 tweet and to which Senator Sessions referred in the February 25 hearing, were provided just ahead of President Obama’s visit to the drought-stricken California Central Valley and were explicitly about the drought situation in California and elsewhere in the West.
That being so, any reference to the CCSP 2008 report in this context should include not just the sentence highlighted in Dr. Pielke’s testimony but also the sentence that follows immediately in the relevant passage from that document and which relates specifically to the American West.
Here are the two sentences in their entirety (http://downloads.globalchange.gov/sap/sap3-
Similarly, long-term trends (1925-2003) of hydrologic droughts based on model derived soil
moisture and runoff show that droughts have, for the most part, become shorter, less
frequent, and cover a smaller portion of the U.S. over the last century (Andreadis and
Lettenmaier, 2006). The main exception is the Southwest and parts of the interior of the
West, where increased temperature has led to rising drought trends (Groisman et al., 2004;
Andreadis and Lettenmaier, 2006).
Linking Drought to Climate Change
In my recent comments about observed and projected increases in drought in the American West, I mentioned four relatively well understood mechanisms by which climate change can play a role in drought. (I have always been careful to note that, scientifically, we cannot say that climate change caused a particular drought, but only that it is expected to increase the frequency, intensity, and duration of drought in some regions―and that such changes are being observed.)
The four mechanisms are:
1. In a warming world, a larger fraction of total precipitation falls in downpours, which means a larger fraction is lost to storm runoff (as opposed to being absorbed in soil).
2. In mountain regions that are warming, as most are, a larger fraction of precipitation falls as rain rather than as snow, which means lower stream flows in spring and summer.
3. What snowpack there is melts earlier in a warming world, further reducing flows later in the year.
4. Where temperatures are higher, losses of water from soil and reservoirs due to evaporation are likewise higher than they would otherwise be.
Regarding the first mechanism, the 2013 report of the IPCC’s Working Group I, The Science
Basis (http://www.climatechange2013.org/images/report/WG1AR5_TS_FINAL.pdf, p 110), deems it “likely” (probability greater than 66%) that an increase in heavy precipitation events is already detectable in observational records since 1950 for more land areas than not, and that further changes in this direction are “likely over many land areas” in the early 21st century and “very likely over most of the mid-latitude land masses” by the late 21st century The second, third, and fourth mechanisms reflect elementary physics and are hardly subject to dispute (but see also additional references provided at the end of this comment).
As I have also noted in recent public comments, additional mechanisms have been identified by which changes in atmospheric circulation patterns that may be a result of global warming could be affecting droughts in the American West. There are some measurements and some analyses suggesting that these mechanisms are operating, but the evidence is less than conclusive, and some respectable analysts attribute the indicated circulation changes to natural variability. The uncertainty about these mechanisms should not be allowed to become a distraction obscuring the more robust understandings about climate change and regional drought summarized above.
Global Drought Patterns
Drought is by nature a regional phenomenon. In a world that is warming on the average, there will be more evaporation and therefore more precipitation; that is, a warming world will also get wetter, on the average. In speaking of global trends in drought, then, the meaningful questions are (a) whether the frequency, intensity, and duration of droughts are changing in most or all of the regions historically prone to drought and (b) whether the total area prone to drought is changing.
Any careful reading of the 2013 IPCC report and other recent scientific literature about on the subject reveals that droughts have been worsening in some regions in recent decades while lessening in other regions, and that the IPCC’s “low confidence” about a global trend relates mainly to the question of total area prone to drought and a lack of sufficient measurements to settle it. Here is the key passage from the Technical Summary from IPCC WGI’s 2013 report
Compelling arguments both for and against significant increases in the land area affected
by drought and/or dryness since the mid-20th century have resulted in a low confidence
assessment of observed and attributable large-scale trends. This is due primarily to a lack
and quality of direct observations, dependencies of inferred trends on the index choice,
geographical inconsistencies in the trends and difficulties in distinguishing decadal scale
variability from long term trends.
And much more at this link: http://www.whitehouse.gov/sites/default/files/microsites/ostp/critique_of_pielke_jr_statements_on_drought.pdf
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