Comments On Meehl Et Al 2009 On Trends In Record High And Low Temperatures

UPDATE November 18 2009: Bruce Hall has posted another valuable contribution on this subject at

The paper

Gerald A. Meehl, Claudia Tebaldi, Guy Walton, David Easterling, and Larry McDaniel, 2009: The relative increase of record high maximum temperatures compared to record low minimum temperatures in the U.S. Geophysical Research Letters. In press

has already been discussed in several excellent posts by others; e.g. see

My post is to point out that the Meehl et al paper did not investigate and question the spatial representativeness of their results, as well as possible non-climatic effects on the data they have used.  We raised a number of issues of these bias and uncertainties in our paper

Pielke Sr., R.A., C. Davey, D. Niyogi, S. Fall, J. Steinweg-Woods, K. Hubbard, X. Lin, M. Cai, Y.-K. Lim, H. Li, J. Nielsen-Gammon, K. Gallo, R. Hale, R. Mahmood, S. Foster, R.T. McNider, and P. Blanken, 2007: Unresolved issues with the assessment of multi-decadal global land surface temperature trends. J. Geophys. Res., 112, D24S08, doi:10.1029/2006JD008229

which need to be resolved before the Meehl et al study should be assumed to be a robust conclusion.  Why was our multi-authored peer-reviewed study not consulted in preparing their paper? Even if they reject our findings, they should not have ignored the issues we raised, but presented reasons for their rejection. Since I have considerable professional respect for the lead author, Jerry Meehl, I can only assume he (and the other co-authors) were not aware of our paper.

More recent papers raise the issue of attribution, even if these temperature records were spatially representative; e.g. see and see). Land use change clearly has a major effect on extreme temperatures, with urbanization, for example, limiting how cold it becomes at night.

In an earlier paper, we did look at the issue of temperature trends with respect to different temperature thresholds.

Pielke Sr., R.A., T. Stohlgren, L. Schell, W. Parton, N. Doesken, K. Redmond, J. Moeny, T. McKee, and T.G.F. Kittel, 2002: Problems in evaluating regional and local trends in temperature: An example from eastern Colorado, USA. Int. J. Climatol., 22, 421-434.

Our abstract reads

We evaluated long-term trends in average maximum and minimum temperatures, threshold temperatures, and growing season in eastern Colorado, USA, to explore the potential shortcomings of many climate-change studies that either: (1) generalize regional patterns from single stations, single seasons, or a few parameters over short duration from averaging dissimilar stations; or (2) generalize an average regional pattern from coarse-scale general circulation models. Based on 11 weather stations, some trends were weakly regionally consistent with previous studies of night-time temperature warming. Long-term (80 + years) mean minimum temperatures increased significantly (P <0.2) in about half the stations in winter, spring, and autumn and six stations had significant decreases in the number of days per year with temperatures ≤−17.8 °C (≤0 °F). However, spatial and temporal variation in the direction of change was enormous for all the other weather parameters tested, and, in the majority of tests, few stations showed significant trends (even at P <0.2). In summer, four stations had significant increases and three stations had significant decreases in minimum temperatures, producing a strongly mixed regional signal. Trends in maximum temperature varied seasonally and geographically, as did trends in threshold temperature days ≥32.2 °C (≥90 °F) or days ≥37.8 °C (≥100 °F). There was evidence of a subregional cooling in autumn’s maximum temperatures, with five stations showing significant decreasing trends. There were many geographic anomalies where neighbouring weather stations differed greatly in the magnitude of change or where they had significant and opposite trends. We conclude that sub-regional spatial and seasonal variation cannot be ignored when evaluating the direction and magnitude of climate change. It is unlikely that one or a few weather stations are representative of regional climate trends, and equally unlikely that regionally projected climate change from coarse-scale general circulation models will accurately portray trends at sub-regional scales. However, the assessment of a group of stations for consistent more qualitative trends (such as the number of days less than −17.8 °C, such as we found) provides a reasonably robust procedure to evaluate climate trends and variability.”

While our study was for a limited geographic area, the approach of examining each observation site in detail in order to seek to explain each of the observed trends is an approach that should have been adopted in the Meehl et al 2009 study.

For example, the reduction in the number of the coldest days for the stations we examined in Colorado is consistent  with the Meehl et al study. However, the fractional contribution to this temperature change from long-term local and/or region land use change, changes in local and regional atmospheric aerosols, change in cloud cover, large-scale  circulation changes, etc., and from non-climatic siting issues (e.g. see), and as well as from a large-scale radiative effect from human-added greenhouse gases,  needs further quantitative examination.

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