There are two recent papers that claim changes in extreme temperatures. The first paper is
(2008), Global changes in extreme daily temperature since 1950, J. Geophys. Res., 113, D05115, doi:10.1029/2006JD008091.
The abstract reads
“Extreme value analysis of observed daily temperature anomalies from a new quasi-global data set indicates that extreme daily maximum and minimum temperatures (>98.5 or <1.5 percentile) have warmed for most regions since 1950. Changes in extreme anomalous daily temperatures are determined by fitting extreme value distributions with time-varying parameters. Changes in the distribution of anomaly exceedances above a high threshold are found to be statistically significant at the 10% level for most land areas when compared with a time-invariant distribution and with the unforced natural variability produced by a coupled climate model. The largest positive trends in the location parameter of the extreme distribution are found in Canada and Eurasia where daily maximum temperatures have typically warmed by 1 to 3°C since 1950. The total area exhibiting positive trends is significantly greater than can be attributed to unforced natural variability. For most regions, positive trend magnitudes are larger and cover a greater area for daily minimum temperatures than for maximum temperatures. The comparatively small areas of cooling are found to be consistent with unforced natural climate variability. The North Atlantic Oscillation (NAO) is found to have a significant influence on extreme winter daily temperatures for many areas, with a negative NAO of one standard deviation reducing expected extreme winter daily temperatures by 2°C over Eurasia but increasing temperatures over northeastern North America. “
The United Kingdom Meteorological Office has highlighted this study on their website.
The second paper is
Peterson, T. C., X. Zhang, M. Brunet-India, and J. L. Vazquez-Aguirre (2008), Changes in North American Extremes Derived from Daily Weather Data, J. Geophys. Res., doi:10.1029/2007JD009453, in press.
The abstract reads
“Detailed homogeneity assessments of daily weather observing station data from Canada, the United States and Mexico enabled analysis of changes in North American extremes starting in 1950. The approach used a number of indices derived from the daily data, primarily based on the number of days per year that temperature or precipitation observations were above or below percentile thresholds. Station level indices were gridded to produce North American area-averaged time series. The results indicated that the increase in the number of days exceeding the 90th percentile is about the same magnitude as the decrease in the number of days below the 10th percentile. Analysis of extremes farther out on the tails of the distribution (e.g., 95th and 97.5th percentiles) reveals changes very similar to the 90th and 10th percentiles. Annual extreme lowest temperatures are warming faster than annual extreme highest temperatures when the index assessed is the actual temperature but cold and hot extremes are changing about the same when examined on a normalized basis. Based on several measures, heavy precipitation has been increasing over the last half century and the average amount of precipitation falling on days with precipitation has also been increasing. These observed changes since the late 1960s, decrease in cold extremes, increases in warm extremes, and increases in heavy precipitation, are consistent with a warming planet.”
What is wrong with these papers?
They have both ignored the peer reviewed literature that has identified major problems with the use of near surface air temperatures to diagnose global warming!
These studies are summarized in the multi-authored 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.
For example, the data used the Brown et al and Peterson et al papers have warm biases as we show for minimum temperatures in
Lin, X., R.A. Pielke Sr., K.G. Hubbard, K.C. Crawford, M. A. Shafer, and T. Matsui, 2007: An examination of 1997-2007 surface layer temperature trends at two heights in Oklahoma.Geophys. Res. Letts., 34, L24705, doi:10.1029/2007GL031652,
and, when land use change has occurred, the maximum temperatures; e.g. see
Hale,R. C., K. P. Gallo, T. W. Owen, and T. R. Loveland, 2006 Land use/land cover change effects on temperature trends at U.S. Climate Geophysical Research Letters.
Brown et al and Peterson et al also ignore the obvious major siting problems with the surface observing sites, as documented convincingly by Watts up With That. This is a problem that appears to affect all observing sites on land; e.g. see
Since Brown et al and Peterson et al are aware of these conflicting studies (e.g. the above peer reviewed papers were published earlier by the same professional society!), their ignoring them, rather than reporting that work and seeking to refute it (which is after all the scientific method), is a clear example of using science as advocates to promote a particular perspective on the issue of climate change.
This is unfortunate, as these authors have the talent and scientific capabilties to assess the major issues we have raised. Instead they chose to ignore these issues.
These papers should, therefore, be recognized for the biased anaylses that they are and the results dismissed until the authors satisfactorily address the several issues with using surface land surface temperature data to assess long term trends. This includes photographs of the observing sites for the temperature data they used, since, as has been clearly documented, the written descriptions of most of the sites are grossly inadequate.