Taking a human’s temperature is easy. Taking a pet’s temperature is similarly straightforward, if a bit rude. Taking a planet’s temperature, on the other hand, is much more of a challenge. The temperature isn’t the same everywhere, so one thermometer won’t get it done. Weather stations on land near population centers are relatively common, but remote areas and the vast oceans also need to be represented.
On top of this geographical span, researchers have to deal with the reality that various issues like equipment changes have to be accounted for to ensure that the data is consistent over a century or more.
A handful of teams around the world separately maintain surface temperature datasets, including NASA, NOAA, the UK Met Office, and the Japan Meteorological Agency. The differences between their results are so small that only climate scientists could find them noteworthy. They all show pretty much exactly the same amount of global warming over time. But this hasn’t stopped conspiratorial critics from claiming that temperature measurements are somehow manipulated to create the appearance of warming where none exists. (These critics never explain how this cabal of scientists got shrinking glaciers, rising sea levels, and migrating species to play along.)