GLOBAL AND HEMISPHERIC TEMPERATURE TRENDS - UNCERTAINTIES RELATED TO INADEQUATE SPATIAL SAMPLING

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 e rror. The analysis includes the errors introduced into the climate rec ord because of both incomplete global coverage and inadequate sampling within those areas presumed to have adequate observations. In these e xperiments 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 relativ ely insensitive to the random errors associated with estimating region al-scale (grid cell size) temperature anomalies. Results imply that th e errors associated with century-scale trends of temperature are proba bly an order of magnitude smaller than the observed global warming of nearly 0.5-degrees-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 n ineteenth and early twentieth centuries. Even during the recent decade of the 1980s, the area-averaging techniques used in some analyses cou ld be improved by addressing the over-sampling of Northern Hemisphere (especially over land) relative to the rest of the globe. Otherwise, s ignificant positive biases are likely during the 1980s. These biases m ay have contributed to the reported differences between in situ surfac e and space-based temperatures during the 1980s. The rather encouragin g 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.