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When scientists say the Earth has warmed by 1° Celsius, which parts of the planet are being measured?
The parts of the planet being measured are our land and ocean surfaces: what we call global average surface temperatures.
December 2, 2021
In conversations about global warming, we often hear about 1°, 1.5°, or 2° Celsius—as in the Paris Agreement, where countries declared a goal to stop the world from warming any more than 1.5° C. But the world is a huge place, including deep oceans and layers of atmosphere extending many miles into space. It can be a little unclear what is actually being measured when scientists talk about the planet getting warmer.
Measures of global warming refer to our global average surface temperatures—that is, air temperatures taken right at the surface of the sea and land. These temperatures are collected across the world every day, week and month, and are combined to give us a global average. (When you read a sentence like “the world has already warmed about 1° C,” what is usually being looked at is yearly global averages starting from a certain year.)
Today, we get temperature measurements from a variety of tools including thermometers, satellites, buoys sprinkled across the world, and now even crowd-sourced data, explains Dr. Adam Schlosser, Deputy Director for Science Research at the MIT Joint Program on the Science and Policy of Global Change. These temperatures are collected and analyzed independently among different research institutes to construct their own datasets.1 Over the past 130 years, these datasets show a consistent trend of warming, and they also agree closely year by year.
Why do we focus on surface temperatures? Quite frankly—it’s easy to measure this, and humans have been doing so for a long time. Global temperature record keeping is said to have begun around 1880, when temperatures began to be recorded in enough places to get an accurate reading of global temperatures. This gives us almost 150 years of surface temperature data, something we certainly don’t have for the stratosphere or the ocean floor. When you read headlines that say “warmest summer on record,” or “winter and February 2020 end as second warmest on record for the globe,” we are able to determine this through our nearly 150 year record of global average surface temperatures.
There is also a practical reason to pay special attention to surface temperatures. They tell us a lot about climate change in the troposphere, the lowest layer of Earth's atmosphere—where we live, where all planetary life dwells, and where nearly all weather takes place. “If we affect the troposphere on the whole, that affects the biosphere,” says Schlosser, referring to the worldwide ecosystem of all living things. And indeed, we can already observe the impacts of a warming troposphere in both subtle and dramatic ways, including sea level rise, more severe weather and more compounding climate change effects like storm surges and heightened flooding.
This doesn’t mean climate change is only changing temperatures at the Earth’s surface. When you zoom out of the troposphere, things can get murkier. Starting in the mid-1970s, satellite measurements have shown that the stratosphere, the next layer of the atmosphere above the troposphere, is actually cooling. The reasons for this are complex, but in part, it’s a result of the same greenhouse effect that is warming the Earth’s surface: as the atmosphere gets higher and thinner, greenhouse gases tend to radiate heat out into space instead of down toward the Earth.
Thank you to Willa of Montreal, Quebec, for the question. You can submit your own question to Ask MIT Climate here.
1 Two widely used datasets on global temperatures are created in the U.S.: NASA’s GISTEMP dataset and the National Oceanic and Atmospheric Administration’s MLOST dataset. Other prominent datasets include HadCRUT4, jointly created by the University of East Anglia and the UK Met Office Hadley Centre, and one produced by The Japan Meteorological Agency.