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Figure 6.1.
Time series of seasonally averaged temperature anomalies (1880–1999). 
The upper figure shows globally averaged sea surface temperature. The 
lower figure shows globally averaged surface air temperature over land. 
The anomalies are computed as differences from the 1880–1998 mean. 
The first season is December 1879–February 1880, and the last season 
is June–August 1999. The data are based on the Quayle et al. 1999) data set.

The exact magnitude of the temperature trend depends on how the observations are globally averaged. Very different global averaging techniques have been used in various studies. Quayle et al. (1999) created separate globally averaged land and ocean time series using only those grid boxes containing data, and then combined the two series with a 30%/70% weighting, proportional to the global area of land and ocean surfaces. The data used in this approach include satellite-derived sea surface temperatures. Jones et al. (1999) employed a different approach, combining land and ocean in situ data in the same gridded data set, with interpolation into blank grid boxes with at least four neighbors, and then areally averaging the grid boxes into a single time series. In a third approach, Hansen and Lebedeff (1987) produced a global time seriescontinue

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