One question I often get on the issue of global warming is Why is there doubt about the temperatures?  Meteorologists have had thermometers for hundreds of years.   The premise of the question is correct.  But creating a global temperature index is, unfortunately, far more complicated than is immediately apparent.  Because most of the earth is covered by water, any world temperature index must include sea surface temperatures.  In the 1940's the method for measuring those temperatures changed and an error was recently discovered, requiring a reexamination of ocean temperature data.   The ocean temperatures are a bit esoteric, so lets take a look at the land temperature measurement problem. From about 1850 to 1990 temperatures across the world were measured with mercurial thermometers contained inside Stevenson screens, whitewashed boxes with louvered sides like the one pictured above in Union City, Missouri, in 1979.  Because of labor costs and t...