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and Griffiths, 1997) means that we have fewer land surface in situ observations available in the 1990s than in the 1970s or 1980s.

Using the Comprehensive Ocean/Atmosphere Data Set (COADS; Woodruff et al., 1998), Trenberth et al. (1992) analyzed sources of errors for in situ SSTs. By assessing the variability within 2° longitude by 2° latitude boxes within each month for 1979, they found that individual SST measurements are representative of the monthly mean to within a standard error of ±1.0 °C in the tropics and ±1.2 to 1.4 °C outside the tropics. The standard error is larger in the North Pacific than in the North Atlantic and it is much larger in regions of strong SST gradient, such as in the vicinity of the Gulf Stream, because both within-month temporal variability and the within-2° box spatial variability are enhanced. The total standard error of the monthly mean in each box decreases proportionately to the square root of the number of observations available. The overall noise in SSTs ranges from less than 0.1 °C over the North Atlantic to greater than 0.5 °C over the oceans south of about 35 °S.

In addition to the problems of in situ data, satellite-derived SSTs add another source of uncertainty. While Reynolds and Smith (1994) use optimal interpolation to blend satellite-derived and in situ SSTs, there are biases, particularly in areas with sparse in situ data, that can still occur due to volcanic aerosols (Reynolds and Smith, 1994) and differences between satellite-observed ocean skin temperature compared to in situ observed bulk temperature (Reynolds and Marsico, 1993). The incomplete adjustment of satellite data could decrease the linear trend of globally averaged surface temperature between 1979 and 1999 by up to 0.05 °C/decade (Hurrell and Trenberth, 1999). Also, there are large uncertainties in the location of the sea ice margins in regions of sparse data (Hurrell and Trenberth, 1999).

Efforts to Correct the Problems

Efforts to address these problems started over a century ago. The First International Maritime Conference, held in Brussels in 1853, agreed on the need for international cooperation and adopted a standard set of observational instructions and ship weather-log forms (WMO, 1994). Such efforts have continued through to the present international effort to create the Global Climate Observing System Surface Network (Petersoncontinue