This is the first in a series of posts on global temperature records. The problems with surface thermometric records are manifold. Are there more reliable methods for measuring the temperature of the surface and the lower atmosphere?
Let’s face it, global surface temperature histories measured by thermometers are a mess. Recording stations come on- and offline seemingly at random. The time of day when the high and low temperatures for the previous 24 hours are recorded varies, often changing at the same station. This has a demonstrable biasing effect on high or low readings. Local conditions can further bias temperatures. What is the effect of a free-standing tree 100 feet away from a station growing into maturity? And the “urban heat island,” often very crudely accounted for, can artificially warm readings from population centers with as few as 2,500 residents. Neighboring reporting stations can diverge significantly from each other for no known reason.
The list goes on. Historically, temperatures have been recorded by mercury-in-glass thermometers housed in a ventilated white box. But, especially in poorer countries, there’s little financial incentive to keep these boxes the right white, so they may darken over time. That’s guaranteed to make the thermometers read hotter than it actually is. And the transition from glass to electronic thermometers (which read different high temperatures) has hardly been uniform.
Some of these problems are accounted for, and they produce dramatic alterations of original climate records (see here for the oft-noted New York Central Park adjustments) via a process called homogenization. Others, like the problem of station darkening, are not accounted for, even though there’s pretty good evidence that it is artificially warming temperatures in poor tropical nations.