2021 was one of Earth's hottest years, but how do scientists know?

Last year was tied with 2018 for the sixth hottest year on record on Earth, according to the National Aeronautics and Space Administration (NASA). The National Oceanic and Atmospheric Administration (NOAA), which did its own study, said 2021 was slightly warmer, pushing 2018 to the Number 7 slot. These are two United States government agencies that have been tracking temperatures for decades.

The year 2021 is part of a trend in which the last eight years have been the eight hottest on record. “We’re seeing this steep and continuing march upward in temperature,” said Lesley Ott, a NASA meteorologist at Goddard Space Flight Center. This information can seem scary. But knowing the facts helps scientists understand climate change and how to lessen its impact.

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Climate change isn’t the only reason to keep track of surface temperatures, which are “the kinds of temperatures we would notice when we walk out the door and say, ‘Oh, it’s cold, I should have grabbed my jacket,’ ” Ott said.

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Starting in the mid-19th century, before NOAA or NASA existed, the Weather Bureau began measuring temperatures across the United States and keeping track of how they changed from year to year. This helped the shipping industry figure out the best times to transport goods to keep people and ships safe. It helped farmers understand the best times to plant and harvest crops. The transportation and farming businesses still rely on this kind of information.

But the way weather data is collected has changed a lot since then. “Back in the day, people were using the same kind of thermometers you’ve seen in science class and recording [temperatures] with paper and pen,” Ott said.

Today there are thousands of digital weather stations in the United States and around the world. The equipment is often at universities and on top of community buildings, where it’s easy for people to access it. (Ott says there’s a push to put more stations in the Amazon rainforest and other places where few people live.)

The stations collect data, sometimes every hour or every few minutes, to get daily minimum and maximum temperatures. Computers record the data. This allows scientists to create models that track trends and a lot of other information.

Scientists don’t measure just the surface temperatures that determine the yearly heat records. Ott said they also measure the temperature of soil, which is important for growing plants. They measure temperatures in the ocean, which are important for plants and animals that live in water.

And they measure temperatures higher in the atmosphere, many miles above Earth’s surface. “That’s really important for things like flying planes, and for being able to predict weather systems,” Ott said. It also makes scientists certain that human-caused climate change is responsible for our current hot surface temperatures. Up there, they can detect traces of the greenhouse gases people emit when they drive cars and burn coal for heat.

Not everyone is convinced that climate change is a big deal. But Ott said anyone can check the temperature data; NOAA and NASA make this available online. “Go to the source!” said Ott. “It’s a big point of pride that we’re doing a lot to try to get the real information out there.”

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LEARN MORE

Check out climate data from the NOAA website at ncdc.noaa.gov/cdo-web and NASA at climate.nasa.gov.

Interested in helping climate scientists? Learn about NASA’s Global Learning and Observations to Benefit the Environment (GLOBE) program, and ask a parent if you can download the GLOBE Observer app (observer.globe.gov/do-globe-observer). Users can contribute pictures of their environment to help NASA monitor how climate change affects vegetation, clouds and more.