On Andy Revkin’s weblog Dot Earth, he presented the viewpoints of two well-respected scientists in his post
I have reproduced them below followed by my comment
From Martin Hoerling:
In his recent New York Times Op-Ed piece, Jim Hansen asserts:
“Over the next several decades, the Western United States and the semi-arid region from North Dakota to Texas will develop semi-permanent drought, with rain, when it does come, occurring in extreme events with heavy flooding. Economic losses would 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 to unprecedented levels.”
He doesn’t define “several decades,” but a reasonable assumption is that he refers to a period from today through mid-century. I am unaware of any projection for “semi-permanent” drought in this time frame over the expansive region of the Central Great Plains. He implies the drought will be due to a lack of rain (except for the brief, and ineffective downpours). I am unaware of indications, from model projections, for a material decline in mean rainfall. Indeed, that region has seen a general increase in rainfall over the long term during most seasons (certainly no material decline). Also, for the warm season when evaporative loss is especially effective, the climate of the central Great Plains has not become materially warmer (perhaps even cooled) since 1900. In other words, climate conditions in the growing season of the Central Great Plains are today not materially different from those existing 100 years ago. This observational fact belies the expectations from climate simulations and, in truth, our science lacks a good explanation for this discrepancy.
The Hansen piece is policy more than it is science, to be sure, and one can read it for the former. But facts should, and do, matter to some. The vision of a Midwest Dustbowl is a scary one, and the author appears intent to instill fear rather than reason.
The article makes these additional assertions:
“The global warming signal is now louder than the noise of random weather…”
This is patently false. Take temperature over the U.S. as an example. The variability of daily temperature over the U.S. is much larger than the anthropogenic warming signal at the time scales of local weather. Depending on season and location, the disparity is at least a factor of 5 to 10.
I think that a more scientifically justifiable statement, at least for the U.S. and extratropical land areas is that daily weather noise continues to drum out the siren call of climate change on local, weather scales.
Hansen goes on to assert that:
“Extremely hot summers have increased noticeably. 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 natural events — they were caused by human-induced climate change.”
Published scientific studies on the Russian heat wave indicate this claim to be false. Our own study on the Texas heat wave and drought, submitted this week to the Journal of Climate, likewise shows that that event was not caused by human-induced climate change. These are not de novo events, but upon scientific scrutiny, one finds both the Russian and Texas extreme events to be part of the physics of what has driven variability in those regions over the past century. This is not to say that climate change didn’t contribute to those cases, but their intensity owes to natural, not human, causes.
The closing comment by Hansen is then all the more ironic, though not surprising knowing he often writes from passion and not reason:
“The science of the situation is clear — it’s time for the politics to follow. ”
Let me borrow from a recent excellent piece in New Scientist by tornado expert Dr. Harold Brooks regarding the global warming and tornado debate, and state:
“Those who continue to talk in certain terms of how local weather extremes are the result of human climate change are failing to heed all the available evidence.”
From Kerry Emanuel:
I see overstatements on all sides. Extreme weather begets extreme views. On the Russian heat wave, Marty is citing a single paper that claims it had nothing to do with climate change, but there are other papers that purport to demonstrate that events of that magnitude are now three times more likely than before the industrial era.
This is a collision between the fledgling application of the science of extremes and the inexperience we all have in conveying what we do know about this to the public. A complicating factor is the human psychological need to ascribe every unusual event to a cause. Our Puritan forebears ascribed them to sin, while in the 80’s is was fashionable to blame unusual weather on El Niño. Global warming is the latest whipping boy. But even conveying our level of ignorance is hard: Marty’s quotation of Harold Brooks makes it sound as though he is saying that the recent uptick in severe weather had nothing to do with climate change. The truth is that we do not know whether it did or did not; absence of evidence is not evidence of absence.
Regular readers of my work will not be surprised that I align with Emanuel.
My Comment: Andy Revkin (and Kerry Emmanuel) have made the error of seeming to assume that one can proof a negative. Kerry wrote
The truth is that we do not know whether it did or did not; absence of evidence is not evidence of absence.
I have a lot of respect for Kerry (and for Andy) but they are in error in terms of the scientific method. To illustrate (and this example applies to the other extreme events), Martin Hoerling’s text on heat waves could be summarized as a hypothesis:
Human caused changes in heat waves resulting from the addition of CO2 into the atmosphere have not been shown using real world observational data.
The scientific method requires presenting analyses that refute this hypothesis. Kerry wrote [regarding heat waves]
“….there are other papers that purport to demonstrate that events of that magnitude are now three times more likely than before the industrial era.”
with the implication, presumably, that by mentioning the “industrial era” he means the effect on climate of added CO2. However. Kerry provided no citations, and Andy accepted this view without questioning this. At the very least, Kerry should have cited papers that claim to refute the hypothesis that I presented above.
In terms of heat waves and lower tropospheric temperature anomalies, we have published on this issue in our papers
Chase, T.N., K. Wolter, R.A. Pielke Sr., and Ichtiaque Rasool, 2006: Was the 2003 European summer heat wave unusual in a global context? Geophys. Res. Lett., 33, L23709, doi:10.1029/2006GL027470. https://pielkeclimatesci.files.wordpress.com/2009/10/r-310.pdf
Chase, T.N., K. Wolter, R.A. Pielke Sr., and Ichtiaque Rasool, 2008: Reply to comment by W.M. Connolley on ‘‘Was the 2003 European summer heat wave unusual in a global context?’’Geophys. Res. Lett., 35, L02704, doi:10.1029/2007GL031574.
whose findings were confirmed in
Connolley W.M. 2008: Comment on “Was the 2003 European summer heat wave unusual in a global context?” by Thomas N. Chase et al. Geophys. Res. Lett., 35, L02703, doi:10.1029/2007GL031171.
In our Chase et al 2006 study we did report on an upward trend in the number of heat waves as measured from tropospheric temperature anomalies, but we concluded that
“….the increased probability of such extremes with time suggested by Stott et al.  is not yet apparent.”
We concluded that the extreme heat in Europe in 2003 was amplified by precedent and concurrent drought conditions which resulted in even higher temperatures than would have occurred with the same tropospheric temperature anomalies (as a result of less evaporation and transpiration from the surface which would have reduced the warmth both from this loss of sensible heating and from more cloudiness). If added CO2 were the main reason for the heat wave, it would have been at least as unusual in the lower tropsphere, but it was not.
A clear signal, of course, may emerge in the coming years, but, for now at least. both Kerry and Andy have not refuted the hypothesis.
Human caused changes in heat waves resulting from the addition of CO2 into the atmosphere have not been shown using real world observational data
Kerry’s statement that the
absence of evidence is not evidence of absence.
is embodying a fallacy where from the Merriam-Webster Dictionary
it’s fallacious to say that something must exist because science hasn’t proven its nonexistence
Kerry and Andy are misleading readers when they make the statement that “absence of evidence is not evidence of absence.”