To monitor climate change adequately and determine the extent to which anthropogenic influences are contributing to observed climate change, it is critical to have land temperature data of a high standard. In particular, it is important to have temperature data whose changes reflect changes in the climate and not changes in other circumstances under which the temperatures were taken. There are numerous factors that can affect land temperature records. Among the most common are changes in instrumentation, changes in local site condition in situ (through urbanization or for other reasons), site relocations, and changes in observing practices. All have the potential, if uncorrected, to have impacts on temperature records at individual locations similar to or greater than the observed century-scale global warming trend. A number of techniques exist to identify these influences and correct data to take them into account. These have been applied in various ways in climate change analyses and in major data sets used for the assessment of long-term climate change. These techniques are not perfect and numerous uncertainties remain, especially with respect to daily and sub-daily temperature data. Copyright © 2010 John Wiley & Sons, Ltd.
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