It’s mid-January, which means the jokes about New Year’s resolutions are hopefully fading out along with your seasonal depression. Oh, and NOAA’s and NASA’s final 2019 global temperature analyses have dropped. (No need to get the party hats and noisemakers back out.)
Let’s start with the numbers. Last year comes in as the second warmest on record in almost every dataset. The UK Met Office dataset has it in third place, as does one satellite dataset (though it is a bit out of step with other satellite records). Satellite datasets measure temperatures higher in the atmosphere rather than surface temperatures, so small differences are not uncommon. Surface temperature datasets generally go back to the late 1800s, while satellite datasets begin in 1979.
The biggest piece of context you need to understand these annual updates is the El Niño Southern Oscillation—a see-saw of Pacific Ocean temperatures that pushes the global average a little above or below the long-term trend each year. In an El Niño pattern, warm water from the western equatorial Pacific drifts toward South America. In a La Niña pattern, strong winds hold that warm water back, pulling up deep, cold water along South America. Years in which El Niño dominates tend to have a higher global average surface temperature, while La Niña years are a little cooler.