An intercomparison of trends in surface air temperature analyses at the global, hemispheric, and grid-box scale

The IPCC Third Assessment Report noted that three major temperature analyses exhibited different warming rates over global land areas since 1976. This paper attempts to explain these trend discrepancies by quantifying the sensitivity of global, hemispheric, and grid-box trends to both the spatial averaging technique and the underlying station network. The Global Historical Climatology Network (GHCN) analysis and the Climatic Research Unit (CRU) analysis have comparable trends when the same approach is used to compute the global time series, and since the mid-1970s the rate of warming in both is as much as a third greater than in the Goddard Institute for Space Studies (GISS) analysis. On the hemispheric scale, GHCN and CRU have similar trends regardless of the gridding approach, whereas GISS again has less warming in recent decades ( particularly in the Southern Hemisphere). GHCN and CRU also exhibit reasonable agreement at the grid-box level during the period 1976-2003.