Global and Hemispheric Temperature Trends: Uncertainties Related to Inadequate Spatial Sampling

Abstract

Long-term (50 to 100 years) and short-term (10 to 30 years) global and hemispheric trends of temperature have an inherent unknown error due to incomplete and nonrandom spatial sampling. A number of experiments have been conducted to help quantify the potential magnitude of this error. The analysis includes the errors introduced into the climate record because of both incomplete global coverage and inadequate sampling within those areas presumed to have adequate observatory. In these experiments it is found that the uncertainty in calculating historical temperature trends is dependent upon the pattern of temperature change, the method of treating the effect of nonrandom spatial sampling, and the time and length over which the trend is calculated but is relatively insensitive to the random errors associated with estimating regional-scale (grid cell size) temperature anomalies. Results imply that the errors associated with century-scale trends of temperature are probably an order of magnitude smaller than the observed global warming of nearly 0.5°C per 100 years since the late nineteenth century. The errors in estimates of decadal temperature trends are found to be large relative to century-scale trends and are unreliable during the nineteenth and early twentieth centuries. Even during the recent decade of the 1980s, the area-averaging techniques used in some analyses could be improved by addressing the over-sampling of Northern Hemisphere (especially over land) relative to the rest of the globe. Otherwise, significant positive biases are likely during the 1980s. These biases may have contributed to the reported differences between in situ surface and space-based temperatures during the 1980s. The rather encouraging results with respect to the magnitude of the spatial sampling errors associated with the calculation of long-term trends beginning in the nineteenth century cast a positive light on efforts aimed at extending the proxy and observed global temperature record further back in time, despite limited geographic coverage.