Comment on “Unresolved issues with the assessment of multidecadal global land surface temperature trends” by Roger A. Pielke Sr. et al.

Abstract

[2] According to Pielke et al. [2007], the similarity of the different global surface temperature trends is unsurprising, in view of an estimated overlap of 90% to 95% in the raw data. Regional temperature trends are considered to be less robust in data-sparse regions such as parts of Africa and the Arctic and Antarctic. [3] Effectively independent analyses of land surface global temperature trends have already been carried out. Peterson et al.’s [1999] subset of rural stations showed very similar trends to those derived from the full GHCN data set. Using a worldwide network of about 270 stations, Parker [2006] obtained very similar trends to those produced by Jones and Moberg [2003] from their full network. The sensitivity of estimated land surface global temperature trends to analysis technique was estimated by Vose et al. [2005]. This paper showed that the Smith and Reynolds [2005] and the Jones and Moberg [2003] analyses yield comparable trends but the Hansen et al. [2001] and K. M. Lugina et al. (Monthly surface air temperature time series area averaged over the 30-degree latitudinal belts of the globe, 1881–2005, in Trends: A Compendium of Data on Global Change, 2006, an online report available at http:// cdiac.ornl.gov/trends/temp/lugina/lugina.html) analyses yield reduced trends in recent decades because of the greater oceanic influence on the former and the damping influence of optimal interpolation on the latter [Trenberth et al., 2007]. The generally good agreement between the global trends of land surface global temperature and the independent sea surface temperature and marine air temperature is also illustrated clearly by Trenberth et al. [2007]. Here we conduct an extra experiment: the CRUTEM3 land surface air temperatures [Jones and Moberg, 2003; Brohan et al., 2006] have been subsampled by taking, in each month, alternate 5 5 areas in alternate 5 latitude belts. Figure 1 shows annual global (Figure 1a) and hemispheric (Figures 1b and 1c) average land surface air temperature anomalies for 1850–2007 based on the full database, with uncertainty estimates as in work by Brohan et al. [2006], along with series based on the two sets of locations in Figure 1d. The subsampled series, despite being based on essentially independent data, lie well within the uncertainty estimates since 1900, and are usually within the uncertainty estimates even in the sparsely sampled 19th century, confirming the robustness of all the series. Not only do these analyses use completely different stations, but any adjustments applied are also independent owing to the distances between the stations that contributed to the two sets of 5 5 areas. In essence, the independence of the station data in the subsampled 5 5 areas cannot be compromised by the homogeneity adjustments reported in earlier papers (see sources given by Jones et al. [1986a, 1986b]), or through the inclusion of many additional adjusted data sets by Jones and Moberg [2003]. This is because adjustments were made to only a limited number of stations ( 20%), and almost all of these adjustments will have been based on stations within the same or surrounding 5 5 areas, not on remoter stations. Furthermore the sum total of the adjustments has a near zero effect on large-scale temperature averages [see Brohan et al., 2006, Figure 4]. [4] The spatial coherence of mapped temperature trends, transcending international borders as illustrated for example in Figures 3.9 and 3.10 of Trenberth et al. [2007], suggests strongly that recent global and regional trends are not severely affected by national biases or incomplete coverage. The latter inference can also be drawn from the relatively small numbers of spatial degrees of freedom estimated by Jones et al. [1997]. Brohan et al. [2006] estimated global and hemispheric incomplete-coverage errors by subsampling complete reanalysis fields; since the mid-20th century the 95% confidence range for decadally smoothed global land surface air temperature anomalies has been about 0.25 C and this is mainly due to incomplete-coverage error [Brohan et al., 2006, Figure 12]. However, the robustness of estimated trends for earlier periods and smaller regions is JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, D05104, doi:10.1029/2008JD010450, 2009 Click Here for Full Article