The problem remains one of determining what part of a given temperature trend is climatically real and what part the result of observational difficulties and of artificial modification of the local environment

data inhomogeneities (such as station moves and closure, record discontinuities, equipment change and changes to the time of observation) are known to have affected records of mean temperature. Modification of the land surface, including urbanization and other economic activity, has been shown to affect local, regional and possibly global meteorology

examined the gridded temperature dataset used by the IPCC

Differences between observed and adjusted trends around the world

Distributions of temperature trends 1979–2002. (top) IPCC surface data. (middle) Satellite
(MSU) tropospheric data. (bottom) Adjusted surface data

our analysis does suggest that nonclimatic effects

add up to a net warming bias at the global level that may explain as much as half the observed land-based warming trend