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Clim Dyn (2008) 31:957971
DOI 10.1007/s00382-008-0392-8
Analysis of the Moberg et al. (2005) hemispheric temperature reconstruction
Anders Moberg Rezwan Mohammad Thorsten Mauritsen
Received: 27 July 2007 / Accepted: 7 March 2008 / Published online: 4 April 2008 Springer-Verlag 2008
Abstract The Moberg et al. (Nature 433(7026):613617, 2005. doi:http://dx.doi.org/10.1038/nature03265
Web End =10.1038/nature03265 ; M05) reconstruction of northern hemisphere temperature variations from proxy data has been criticised; the M05 method may articially inate low-frequency variance relative to reality. We test this assertion by undertaking several pseudoproxy experiments in three climate model simulationsone control run and two forced simulations that include several time-varying radiative forcings. The pseudoproxy series are designed to have the same variance spectra as the real M05 proxies, primarily to mimic the low-resolution character of several series. A simple composite-plus-scale (CPS) method is also analysed. In the CPS case all input data behave like annually resolved proxies. The spectral domain performance of both M05 and CPS is found to be dependent on the noise type and noise level in pseudoproxies, on the variance spectrum of the climate model simulation, and on the degree of data smoothing. CPS performs better than M05 in most investigated cases with the control run, but leads to deated low-
frequency variance in some cases. With M05, low-frequency variance tend to be inated for the control run but not for one of the forced runs and only very slightly with the other forced simulation. Hence, the M05 approach does not routinely inate low-frequency variance. In our experiment, the M05 approach performs better in the spectral domain than CPS when applied to forced climate model simulations. The results underscore the importance of evaluating the variance spectrum of climate reconstructions.
Keywords Paleoclimatology
Temperature reconstruction Pseudoproxies
Northern hemisphere Variance spectrum
1 Introduction
The importance of obtaining knowledge about the climate system and its variability is indisputable, as is evident from the prominent position of global warming issues on the worlds political agenda and in all sorts of news media. One branch of climate science that has received much attention in the last decade concerns the size and nature of large-scale temperature changes in the last one to two millennia. About ten research teams have made efforts to reconstruct large-scale temperature variations in the northern...