EOS Paper On The Hottest Spots on Earth Illustrates The Major Role of Landscape on Surface Temperatures

The October 24 2006 issue of EOS had an interesting article entitled “What Are the Hottest Spots on Earth?” (subscription required). The article by D. Mildrexler, M. Zhao and S.W. Running is summarized in EOS as follows,

“The location of the hottest spot on the Earth’s surface has long been a source of interest and curiosity. To identify the hottest places on Earth, this article presents a synthesis of remotely sensed radiometric land surface temperatures. Studying patterns of maximum land surface temperatures across the global land surface and the interaction with vegetation, or the absence of vegetation, can help develop an improved methodology for detecting land cover change and understanding the potential consequences of land cover change. ”

The paper includes section headings of the “Influence of Vegetation on Maximum LST” and “Influence of Irrigation on Maximum LST” [LST=land surface temperature].

This paper, however, while quite interesting in its own right, has significance beyond the specific focus of the article. For instance, with the newly developed capability to assess maximum land surface temperatures, this data should be routinely compared with the site-based air temperature data that is used to construct values of the global average temperature trends.

Of even more importance in this paper, however, is the documentation of the very major role of the presence of vegetation or not at a location. The hottest temperatures were observed in the EOS study at the locations without vegetation. This means that if landscape change in a region results in less vegetation, the maximum surface temperatures are expected to be hotter. If an oasis is developed by irrigation from subsurface water in a desert, the maximum temperatures would be less. The difference in the surface temperatures between the Amazon and the Sahara desert seen in Figure 1 of the Mildrexler et al paper illustrates the dramatic differences that occur. Figure 3 in their paper documents that very large differences (over 25C!) occur between distances of just meters.

Thus, in terms of the diagnosis of climate system heat changes (“global warming”) through the use of 1.5m surface air temperatures, there is a clear confounding issue of what type of landscape occurs at each observing site, and if it has changed during the period of record. The paper “Land use/land cover change effects on temperature trends at U.S. Climateâ€? by R. C. Hale, K. P. Gallo, T. W. Owen, and T. R. Loveland June 3 2006 Geophysical Research Letters has shown that the observed temperatures almost always increase due to landscape change in the data they examined.

The third author of the EOS paper (S. Running) published a paper several years ago (which I was co-author on), that demonstrates the importance of landscape change on surface temperatures. The paper is

Nemani, R.R., S.W. Running, R.A. Pielke, and T.N. Chase, 1996: Global vegetation cover changes from coarse resolution satellite data. J. Geophys. Res., 101, 7157-7162, with the abstract

“Land cover plays a key role in various biophysical processes related to global climate and terrestrial biogeochemistry. Although global land cover has dramatically changed over the last few centuries, until now there has been no consistent way of quantifying the changes globally. In this study we used long-term climate and soils data along with coarse resolution satellite observations to quantify the magnitude and spatial extent of large-scale land cover changes attributable to anthropogenic processes. Differences between potential leaf area index (LAI), derived from climate-soil-leaf area equilibrium, and actual leaf area index obtained from satellite data are used to estimate changes in land cover. Further, changes in LAI between potential and actual conditions are linked to climate by expressing them as possible changes in radiometric surface temperatures (Tr) resulting from changes in surface energy partitioning. As expected, areas with high population densities, such as India, China, and western Europe showed large reductions in LAI. Changes in global land cover expressed as summer, midafternoon Tr, ranged from −8° to +16°C. Deforestation resulted in an increase in Tr, while irrigated agriculture reduced the Tr. Many of the current general circulation models (GCMs) use potential vegetation maps to represent global vegetation. Our results indicate that there are widespread changes in global land cover due to deforestation and agriculture below the resolution of many GCMs, and these changes could have a significant impact on climate. Potential and actual LAI data sets are available for climate modelers at 0.5°×0.5° resolution to study the possible impacts of land cover changes on global temperatures and circulation patterns.”

Clearly, the analysis in the Nemani et al paper should be revisited using the new satellite data.

There is also the issue as to what locations is the hottest in terms of heat content. As shown in the paper

Pielke Sr., R.A., C. Davey, and J. Morgan, 2004: Assessing “global warming” with surface heat content. Eos, 85, No. 21, 210-211,

the diagnosis of heat content requires the measurement of moisture content in addition to the air temperature. In terms of heat content in Joules, the Amazon region, for example, may have hotter conditions than in Iran, Libya, Queensland or elsewhere in desert locations!

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