Over the years, a large number of potential problems associated with various ways of assessing the temperatures of diverse types of land cover have been identified; and several of them are briefly described, with an emphasis on demonstrating the great difficulty of obtaining various surface-specific sets of annual near-surface air temperature which - when combined for the earth as a whole - yield a set of yearly mean values that accurately represents the mean nearsurface thermal history of the planet well enough to determine whether or not the earth may be warming at the rate that is typically predicted by climate model assessments of the greenhouse effect of the CO2 emitted to the atmosphere as a consequence of mankind's utilization of fossil fuels, such as coal, gas and oil.
First of all, there are a number of mundane difficulties that are encountered in the course of obtaining numerous histories of accurate surface air temperature measurements and assembling them into an aggregate history of global climate change. These problems include (1) temporal changes in microclimate surrounding temperature measurement sites, such as urbanization, which often go unrecognized or for which insufficient adjustments are made, (2) long-term degradation of the shelters that house the temperature-measuring equipment, such as the shelters' white paint becoming less reflective and their louvers partially obstructed, (3) changes in what is actually being measured, such as true daily maximum and minimum temperatures or temperatures at specified times of day, (4) changes in measurement devices and ways of accessing the data, such as changing from having to open the shelter door to read the temperature, as was done in earlier days, to not having to do so, due to the automatic recording of the data, as has become commonplace in more recent times, (5) general station degradation and many station closures over time, (6) the changing and uneven geographical representation of the surface temperature network, (7) poor attention to careful acquisition of data in many parts of the world, and (8) a number of problems associated with obtaining a correct and geographically complete record of surface air temperature over the 70% of the globe that is covered by oceans. In this summary, however, a group of more esoteric problems will be discussed.
Correia and Safanda (1999) reviewed a set of twenty temperature logs derived from boreholes located at fourteen different sites in mainland Portugal in an attempt to reconstruct a fivecentury surface air temperature history for that part of the world; but little did they realize how difficult the task would be. For starters, seven of the borehole temperature logs were too "noisy" to use; while six displayed evidence of groundwater perturbations and were thus not usable for that reason. Of the remaining seven logs, all depicted little temperature change over the first three centuries of record. Thereafter, however, four of them exhibited warming trends that began about 1800 and peaked around 1940, one showed a warming that peaked in the mid-1800s, another was constant across the entire five centuries, and one actually revealed cooling over the last century. And at this point in time, the two researchers concluded that "the single inversions cannot be interpreted individually."
