Ever Wonder How We Take Our Global Temperature?

Musings from the Chiefio explains it like this:

GIStemp: Goddard Institute for Space Studies, temperature Series.

If we would study global temperature change over time, we need a temperature record over time, and over the globe. GIStemp attempts to create a temperature history with full coverage over time and over space. Unfortunately, the (GHCN – Global Historical Climate Network) data start with one thermometer in Germany: Berlin Tempel in 1701.

Over time, thermometers are added, and they slowly migrate south and to both the new, and old, worlds. Eventually, about 1900 A.D., there are sufficient thermometers on the globe to get a partial idea what is happening. But climate is subject to cyclical changes. Some, like the Pacific Decadal Oscillation, have about a 40 to 60 year full cycle length. Others, like solar cycles that run 178 years, and Bond Events – a 1500 year cycle, are a bit longer. A 100 year record is inadequate to allow for these events.

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That's right, it's estimated, interpolated, or just plain made up. Only after the data are fully cooked, is it used to drive public policy.