Surface - based temperature histories of the globe contain a significant warming bias due to the urban heat island effect. (friendsofscience.org)
They display a global warming bias throughout — bolstered by temperature data contaminated by «urban heat island» effects, due to measuring stations being located too close to human heat sources. (townhall.com)
A related criticism is that the decline in used station count has disproportionately removed stations from colder climates and thus introduced a false warming bias to the record. (skepticalscience.com)