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2 The Instrumental Record
Pages 29-37

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From page 29...
... · Instrumental temperature records extend back over 250 years in some locations, but only since the late 19th century has there been a sufficient number of observing stations to estimate the average temperature over the Northern Hemisphere or over the entire globe. · Combining instrumental records to calculate large-scale surface tem peratures requires including a sufficient number of instrumental sites with wide geographic distribution to get a representative estimate.
From page 30...
... surface temperature estimates used in climate change studies. The global average temperature is produced as a combination of nearsurface land air temperatures and temperatures of the sea water near the surface (or sea surface temperatures [SSTs]
From page 31...
... The Northern Hemisphere and global estimates exhibit less variability because of the additional influence of SSTs, which have less variability than land air temperatures from year to year, mainly due to the higher heat capacity of the ocean mixed layer compared to the land surface. The evolving pattern of fluctuations is similar in these three large-scale averages because (a)
From page 32...
... most sensitive to mean temperatures during the summer growing season. For comparison, Figure 2-2 shows the annual and summer anomalies of the Northern Hemisphere extratropical land area temperature record.
From page 33...
... . UNCERTAINTIES AND ERRORS ASSOCIATED WITH THE INSTRUMENTAL RECORD Because proxy-based surface temperature reconstructions often depend on either local or large-scale average land air temperatures and/or SSTs, any errors in the instrumental temperature record will reduce the confidence in the reconstructed temperature record.
From page 34...
... 34 SURFACE TEMPERATURE RECONSTRUCTIONS FOR THE LAST 2,000 YEARS FIGURE 2-3 Observed surface temperature trends in degrees Celsius per decade through 2005 beginning in 1870 (top) and beginning in 1950 (bottom)
From page 35...
... One way of overcoming this limitation is to sample much longer time series of synthetic climate variations generated by climate models, but this strategy is compromised by the limited capability of the models to simulate temperature variations on the century-to-century timescale and on spatial scales that represent the highly variable character of the Earth's surface. The studies that have been performed to date suggest that 50­100 geographically dispersed sites are sufficient to replicate the variability in the instrumental record (e.g., Hansen and Lebedev 1987, Karl et al.
From page 36...
... Another issue that arises when interpreting proxy records of surface temperature over the last 2,000 years is the degree to which temperature time series in various latitude belts are representative of the globally averaged temperature. The instrumental record of surface temperature shown in Figure 2-5 is instructive in this respect.
From page 37...
... THE INSTRUMENTAL RECORD 37 FIGURE 2-5 Smoothed zonal mean anomalies of surface temperature (in K) for the observations in each latitude band from 1890 to 1999.


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