CHARLIE MARTIN: Where Does Global Warming Come From?

The essential measurement on which the climate change debate is based is the Global Average Surface Temperature (GAST). Conceptually, this is really pretty simple: you just put a thermometer on every point on the surface of the earth, take continuous readings for an entire year and then divide the sum of all those readings by the number of readings.

Practically, that presents problems: that’s a lot of thermometers and people use the surface of the Earth for doing other things than taking temperature readings. The reality is that the thermometers are anywhere from miles apart to thousands of miles, and they’re not at all uniform. So to estimate the GAST, the temperature measurements are fed into a model, which includes corrections. When I was writing about this in 2013, the difference between GAST and the predictions of GAST was becoming too big to ignore. Climate scientists then looked for reasons and added new corrections — but some of them were controversial.

Those who take the readings decide nothing. Those who model the readings decide everything.