There is a press release from Purdue University by Elizabeth K. Gardner and Greg Kline regarding a study by Matthew Huber of Purdue and Steve Sherwood of the University of New South Wales. The press release is titled
The news media have already uncritically picked up this story (e.g. see Climate change could make half the world uninhabitable).
The paper version of this study is
Steven C. Sherwood and Matthew Huber,2010: An adaptability limit to climate change due to heat stress http://www.pnas.org/cgi/doi/10.1073/pnas.0913352107
with the abstract
“Despite the uncertainty in future climate-change impacts, it is often assumed that humans would be able to adapt to any possible warming. Here we argue that heat stress imposes a robust upper limit to such adaptation. Peak heat stress, quantified by the wetbulb temperature TW, is surprisingly similar across diverse climates today. TW never exceeds 31 °C. Any exceedence of 35 °C for extended periods should induce hyperthermia in humans and other mammals, as dissipation of metabolic heat becomes impossible. While this never happens now, it would begin to occur with global-mean warming of about 7 °C, calling the habitability of some regions into question. With 11–12 °C warming, such regions would spread to encompass the majority of the human population as currently distributed. Eventual warmings of 12 °C are possible from fossil fuel burning. One implication is that recent estimates of the costs of unmitigated climate change are too low unless the range of possible warming can somehow be narrowed. Heat stress also may help explain trends in the mammalian fossil record.”
The article is edited by Kerry A. Emanuel, Massachusetts Institute of Technology,
The study has a major fault in that it has not properly assessed the actual behavoir of the atmosphere if such warming occurred in the lower troposphere. Moreover, this is another example of the publication of a paper with predictions that cannot be tested.
I discuss these issues in more depth below.
Excerpts from the news article read
WEST LAFAYETTE, Ind. – Reasonable worst-case scenarios for global warming could lead to deadly temperatures for humans in coming centuries, according to research findings from Purdue University and the University of New South Wales, Australia.
Researchers for the first time have calculated the highest tolerable “wet-bulb” temperature and found that this temperature could be exceeded for the first time in human history in future climate scenarios if greenhouse gas emissions continue unabated.
The researchers calculated that humans and most mammals, which have internal body temperatures near 98.6 degrees Fahrenheit, will experience a potentially lethal level of heat stress at wet-bulb temperature above 95 degrees sustained for six hours or more, said Matthew Huber, the Purdue professor of earth and atmospheric sciences who co-authored the paper that is currently available online and will be published in an upcoming issue of the Proceedings of the National Academy of Sciences.
Wet-bulb temperature is equivalent to what is felt when wet skin is exposed to moving air. It includes temperature and atmospheric humidity and is measured by covering a standard thermometer bulb with a wetted cloth and fully ventilating it.
While the Intergovernmental Panel on Climate Change central estimates of business-as-usual warming by 2100 are seven degrees Fahrenheit, eventual warming of 25 degrees is feasible, he said.
This study was supported by the NSF; i.e.
The National Science Foundation-funded research investigated the long-term implications of sustained greenhouse gas emissions on climate extremes. The team used climate models to compare the peak wet-bulb temperatures to the global temperatures for various climate simulations and found that the peak wet-bulb temperature rises approximately 1 degree Centigrade for every degree Centigrade increase in tropical mean temperature.
This is an example of the paradigm, as written by Dick Lindzen (see) where
“In brief, we have the new paradigm where simulation and programs have replaced theory and observation, where government largely determines the nature of scientific activity……”
The Sherwood and Huber 2010 paper, while it fits Dick’s new paradigm, it also fails the scientifically plausibility test. Can the wet bulb temperature actually reach 95 F (35C) over wide areas as they claim?
To explore this issue, we start with a thermodynamic diagram which relates temperature, wet bulb temperature, and pressure together. One form of this diagram is the skew T/log p which is presented below.
http://ccrc.unh.edu/~stm/AS/Common/Skew_T.JPG [click figure below for a larger, clearer image]
This diagram provides a tool to assess the consequences for the Earth’s atmosphere above the surface, if a 95F (35C) wet bulb temperature were actually achieved.
Values of the wet bulb temperature correspond to wet adiabats in the above figure. For example, if you trace along a wet adiabat to 500 hPa, the temperature that results when this high humidity air ascends to that height can be obtained, as discussed in
Pielke Sr., R.A. 2002: Synoptic Weather Lab Notes. Colorado State University, Department of Atmospheric Science Class Report #1, Final Version, August 20, 2002.
With a value of 35C for the wet bulb temperature, this corresponds to a temperature at 500 hPa of ~14C (~57F).
In the current climate, the values of the temperatures at 500 hPa are almost always between -5C and -45C. Only very locally, such as in the eye wall region of a hurricane, are the 500 hPa temperatures warmer than -5C.
The reason is that deep cumulus convection mixes the atmosphere up to this level and higher such that the moist adiabat is a close approximation of the resulting atmospheric temperature lapse rate. We discuss this role of deep cumulus convection for the higher latitudes in our papers
Chase, T.N., B. Herman, R.A. Pielke Sr., X. Zeng, and M. Leuthold, 2002: A proposed mechanism for the regulation of minimum midtropospheric temperatures in the Arctic. J. Geophys. Res., 107(D14), 10.10291/2001JD001425.
Tsukernik, M., T.N. Chase, M.C. Serreze, R.G. Barry, R. Pielke Sr., B. Herman, and X. Zeng, 2004: On the regulation of minimum mid-tropospheric temperatures in the Arctic. Geophys. Res. Letts., 31, L06112, doi:10.1029/2003GL018831.
Herman, B., M. Barlage, T.N. Chase, and R.A. Pielke Sr., 2008: Update on a proposed mechanism for the regulation of minimum mid-tropospheric and surface temperatures in the Arctic and Antarctic. J. Geophys. Res.-Atmos., 113, D24101, doi:10.1029/2008JD009799.
In the first paper, in the abstract we wrote
We further provide evidence that minimum Arctic midtropospheric temperatures are regulated by moist convective processes and that minimum 500 mbar temperatures are controlled to a large extent by high-latitude sea surface temperatures. The temperature -45C is the expected 500 mbar temperature in an atmosphere regulated by moist adiabatic ascent from a surface temperature of 1–2 below 0 C, the approximate freezing point of seawater.
The same mechanism to regulate 500 hPa temperatures occurs in the tropics with respect to how warm temperatures can become at that level. The sea surface tempertures in the tropics determine how warm the 500 hPa temperature can become, as well as being the source for the water vapor that is needed to elevate the wet bulb temperature.
The IPCC multi-decadal climate model predictions, of the type such as the NCAR Community Atmosphere Model used in the Sherwood and Huber PNAS study, however, are not accurately representing this relationship between the surface and mid-troposphere, as illustrated, for example, by the paper
De-Zheng Sun, Yongqiang Yu, and Tao Zhang, 2009: Tropical Water Vapor and Cloud Feedbacks in Climate Models: A Further Assessment Using Coupled Simulations Journal of Climate, Volume 22, Issue 5 (March 2009) pp. 1287–1304.
I commented on this paper in my post in May 2006
Their study indicates that the IPCC models are overpredicting global warming in response to positive radiative forcing.
Their abstract includes the text (referring to global climate models]
“All models we have examined in this analysis are found to have a weaker negative feedback from the net surface heating than that from observations, indicating that deep convection over the equatorial Pacific in the models has a weaker regulatory effect over the SST in that region.”
This means that for Sherwood and Huber to be correct, the ocean surface temperatures over large areas must warm to over 95F (35C). Since, as reported in their article, the current wet bulb temperatures do not rise above about 31C (F), the ocean over large areas would need to warm to 35C and warmer. However, the resulting deep cumulus convection would not only significantly warm the troposphere (i.e. to at and above 14C at 500 hPa), but produce considerable cloud cover and precipitation.
As discussed in the Sun et al 2009 paper, the couple atmosphere-ocean climate models inaccurately represent the cloud-precipitation feedback, and thus incorrectly overstate the positive feedback with respect to ocean surface warming. The Sherwood and Huber 2010 paper is not a scientifically robust examination of this issue with respect to their claims of the elevation of the wet bulb temperature.
Finally, the PNAS article is yet another example of the publication of model results that are not testable.
The Sherwood and Huber paper is not a proper scientific study as I discussed in my post
I wrote in that post
“What the current publication process has evolved into, at the detriment of proper scientific investigation, are the publication of untested (and often untestable) hypotheses… the policy community is being significantly misinformed about the actual status of our understanding of the climate system and the role of humans within it.”
The Sherwood and Huber paper is just a model sensitivity study, not a verifiable prediction. Moreover, not only is it scientifically flawed, but the dissemination of a press release illusrates that this is really not a science study. The funding of such a study by the National Science Foundation (whose predictions cannot be verified) illustrates another failure by the NSF to properly support climate science.