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FIGURE 14.57 Global average temperature anomaly (AD for land and sea surface measurements relative to the period from 1961 to 1990 (adapted from IPCC, 1996).

time can introduce an artifact into the data, which, if not corrected, leads to artifact cooling trends.

The five warmest years for which there are surface temperature records have all been since 1990 (Jones et al., 1998), with the most recent year for which there are data (at the time of writing), 1997, being the warmest in the past century (see Kerr, 1998, and references therein). Mann et al. (1998) have used a variety of indirect indicators for temperature (e.g., ice core data; see later) over the past 600 years in the Northern Hemisphere and report that mean annual temperatures for three of the eight years up to and including 1995 are higher than any since 1400 A.D.

An interesting aspect of the surface temperature changes is that in many locations, particularly continental regions, the minimum daily temperature has increased more than the maximum daily temperature (e.g., Hansen et al., 1997d). As a result, the daily temperature range has decreased in these regions. Figure 14.58, for example, shows globally averaged maximum and minimum temperatures, as well as the diurnal temperature range, from f950 to 1993 based on approximately 4100 nonurban stations (Easterling et al., 1997). These are expressed as deviations from the mean for all stations in 5° X 5° latitude-longitude grid boxes during the period from 1961 to 1985. The trend in the maximum temperature is 0.82°C per century, but that in the minimum is larger, 1.79°C per century. As a result, the diurnal temperature range decreases, with a slope of — 0.79°C per century.

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