“The key thing to take away from all of this is that the long-term trends are pretty much relentlessly up,” said Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies (GISS) in New York. 

For nearly 45 years, a team of scientists at NASA’s GISS has been tracking these rising temperatures and producing records and analyses of the global temperature of Earth. Each month, scientists release a dataset.

The process for producing NASA’s temperature record is rigorous and complex. Here’s a look at how — and why — it is done. 

Science institutions measure local temperature in many ways. On land, thermometers affixed to tens of thousands of weather stations give us precise air temperature readings. Ships and buoys floating in the ocean capture temperatures at the sea surface. These individual measurements tell us the temperature at any one place for that day and time. 

Calculating an average global temperature requires collecting all of those datasets  — from the land and ocean surface — and then applying a special mathematical method to the numbers. 

Climate scientists focus on how temperature has changed over time. For each individual station, they calculate how the temperature has strayed from what is considered normal; these are known as anomalies. Normal, according to NASA GISS scientists, is the temperature average from the 30-year period 1951—1980. Every location is measured against that 20th century baseline. 

Think of it this way. A temperature reading taken on a mountaintop will be cooler than one taken in a nearby valley, just as the temperature under the shade of a tree will be cooler than a reading taken in a sunlight-drenched parking lot just feet away. Temperature can vary significantly over relatively short distances. 

But the temperature change between two nearby locations is remarkably consistent, said Nathan Lenssen, a climate scientist at the Colorado School of Mines and the National Center for Atmospheric Research. “When it’s 2 degrees warmer than normal in Denver, it’s going to be 2 degrees warmer than normal at the top of Bear Peak.” 

That the average temperature change is similar in Baltimore and Philadelphia or in Austin and Fort Worth can be attributed to long, consistent weather patterns, meaning the temperature anomalies of weather stations within an 800-mile distance are highly correlated. This is because large-scale weather systems stretch to this distance.  These correlations were first demonstrated in a 1987 paper published in the Journal of Geophysical Review by James Hansen and Sergei Lebedeff. And it has been well documented since, Schmidt said.   

No. For one thing, scientists must account for the varied spacing of temperature stations. There are fewer weather stations in the Sahara Desert and Antarctica, for example, than in other parts of the world.  

But the fact that temperature anomalies stay consistent over distances means scientists can fill the gaps by making estimates for the areas surrounding individual weather stations. These estimates are weighted in the analysis: the closer a point on a map is to a station, the more weight it gets.  

“That allows us to get coverage of nearly the entire Earth's surface, except with maybe some exceptions, like right on the ice sheets of Antarctica,” Lenssen said.

Scientists at NASA GISS also correct for hotter than normal temperatures that could skew the results. For example, the asphalt and concrete of major roads, uncovered parking lots, and buildings absorb more heat than green spaces. As a result, temperatures in cities are typically higher than those in rural areas. Plus, additional heat is generated from a city’s cars, trucks, factories, and air conditioning units. 

Research shows that the impact of these urban heat islands has a miniscule effect on global temperature — about 100th of a degree, Lenssen said. Picture the size of a city compared to the size of the Pacific Ocean. Even the biggest cities aren’t big enough to make a difference on a global scale.

Still, to ensure that these urban data don't make the average artificially high, scientists remove temperature measurements captured in cities or at airports before calculating averages.  

NASA is one of several institutions that measures and tracks global temperature, along with Europe’s Copernicus Climate Change Center, the U.S. National Oceanic and Atmospheric Administration, Berkeley Earth, and the UK’s Met Office. 

The various organizations use different methods to calculate global temperature and address uncertainties. Still, the resulting estimates are in close agreement. And they match up closely to independent datasets derived from satellites and weather forecast models. 

Take a look at this graph below, which shows global temperature records from the mid-1850s through 2023. (NASA’s global temperature record starts in 1880 because observations based on current methods did not sufficiently cover enough of the planet prior to that time.) You can see how closely the records compare and how they’ve become even more aligned over time. 

The records are supported by evidence contained in natural materials. Tree rings grow wider in warmer, wetter years. Ice cores — chunks of ice drilled from ice sheets or glaciers — hold clues to past temperatures. Combine that with natural evidence from coral, pollen, and cave records, and scientists can piece together records of our planet’s climate dating back millions of years. 

“When we say what’s happening now is special, we’re not just saying that because it’s happening to us,” Schmidt said. “We can say it with that full context of all the other things we’ve seen happen in the past.”  

Temperature records agree, and natural evidence backs it up: Global surface temperature has increased faster since 1970 than in any other 50-year period over at least the past 2,000 years. 

The temperature in any individual year can be influenced by El Niño and La Niña, climate patterns that alternately warm and cool the tropical Pacific Ocean and can have far-reaching effects on weather around the world. This warming and cooling occurs cyclically in response to change in the strength of the trade winds; then, as the ocean changes, it alters rainfall patterns. This can result in severe droughts and intense rain in different parts of the world. 

Scientists study other factors that might influence global temperature, including large volcanic eruptions and reductions in shipping pollution resulting from regulations on emissions. Changes in the amount of small ash or pollution particles in the air —  aerosols — can change how much solar energy gets reflected back into space, slightly cooling or warming the planet. 

While volcanoes and the El Niño Southern Oscillation, or ENSO, may contribute to variability from year to year — the peaks and valleys in the line graph above — they don’t alter the overall trend in temperatures. For example, a strong El Niño that began in spring 2023 likely contributed to the soaring temperatures that year, but the global heat surge continued well after El Niño abated, Schmidt noted. 

Recent evidence suggests that the overall reduction in aerosol pollution, on the other hand, is playing a small role in long-term warming. For more than a century, aerosols released by the burning of fossil fuels have had a cooling effect on the climate by blocking solar radiation. Scientists estimate that this has made the planet about 0.7° F (0.4 °C) cooler than it otherwise would be, according to the 2021 report by the Intergovernmental Panel on Climate Change (IPCC). 

A gradual shift toward cleaner power plants, motor vehicles, and other technologies has reduced harmful particulate matter in the atmosphere, which has improved air quality in ways that help human health. But it’s likely making the planet temporarily a little warmer as a result.  For instance, a 2020 international rule that sharply reduced the amount of sulfur dioxide in shipping fuel will likely also lead to some atmospheric warming, but the magnitude is small and further study is needed.

Much public attention has focused recently on the planet’s arrival or near-arrival at global temperatures about 1.5 degrees Celsius (2.7 degrees Fahrenheit)  above pre-industrial levels. This number is significant because in the 2015 Paris Agreement, 195 nations agreed to limit global warming to below this number, “recognizing that this would significantly reduce the risks and impacts of climate change.”

According to NASA's global temperature analysis, known as GISTEMP, the global temperature for 2024 was 1.28 degrees Celsius (2.30 degrees Fahrenheit) above the agency’s 20th-century baseline (1951—1980.) But NASA records show it was 1.47 degrees Celsius (2.65 degrees Fahrenheit) above pre-industrial levels (1850—1900.) Global average temperatures were at least 1.5 degrees above this pre-industrial average for five months in 2024 and four months in 2023, according to Schmidt.  

Reaching this specific number does not necessarily ensure any specific climate impacts. “Every 10th of a degree counts, and so the impacts of 1.6 are going to be worse than the impacts of 1.5, but they're going to be better than the impacts of 1.7 or 1.8 or 1.9,”  Schmidt said. 

“Still, it’s worrying, we’re seeing the impacts already, and this is what’s going to keep happening until we reduce emissions,” he added.

Why should we care about one or two degrees of warming? After all, temperatures fluctuate by many degrees every day where we live.

The temperatures we experience locally and in short periods can fluctuate significantly due to predictable, cyclical events, such as the rising and setting of the Sun and the change of the seasons, along with hard-to-predict wind and precipitation patterns. 

But the global temperature mainly depends on how much energy the planet receives from the Sun and how much it radiates back into space. The energy coming from the Sun fluctuates very little by year, while the amount of energy radiated by Earth is closely tied to the chemical composition of the atmosphere — particularly the amount of heat-trapping greenhouse gases.

A one-degree global change is significant because it takes a vast amount of heat to warm all of the oceans, the atmosphere, and the land masses by that much.  In the past, a one degree global drop was all it took to plunge North America and Northern Europe into the Little Ice Age. A five-degree drop was enough to bury a large part of North America under a towering mass of ice 20,000 years ago.

For perspective, Schmidt said, consider the Pliocene epoch 2.5 million years ago, which had a global temperature that was 3 degrees Celsius warmer than human pre-industrial levels. At that temperature, ice sheets were significantly smaller, possibly nonexistent, and sea levels were estimated to be dozens of feet higher than they are now. 

“And now we’ve warmed by 1.5 degrees,” Schmidt said. “We are halfway Pliocene-level warmth in just 150 years.” 

The origins of the NASA GISS temperature record can be traced back a half century and a million miles away to a study of planet Venus, a blistering planet that is kept hot by a thick atmosphere composed mostly of carbon dioxide. NASA climate scientist James Hansen was researching the atmosphere on Venus and had designed an instrument to be carried there — it snapped this image of the planet in the 1970s. But what he learned about Venus inspired him soon after to turn his attention to planet Earth where, like Venus, greenhouse gases were building up in the atmosphere. 

“I resigned as principal investigator on our Venus experiment because a planet changing before our eyes is more interesting and important, and its changes will affect all humanity,” Hansen said in this 2012 TED talk.  

The institute published its first global temperature record in 1981, the same year that Hansen became the director of NASA GISS, a position he held until 2013. (Schmidt became director a year later.) This coincided with an article Hansen published in Science Magazine that concluded that a warming of 0.4 degrees Celsius in the 20th century was driven by rising levels of carbon dioxide. 

In that article, Hansen also predicted that the planet would warm in the 1980s and that by the turn of the century, a signal of global warming would begin to emerge from the noise of natural variability. He wrote that the 21st century would see more drought, melting ice sheets, rising sea levels, and the opening of the Northwest Passage. By all accounts, he was right. 

The basic method of calculating global temperature, devised by Hansen and programmed by his colleague Sergej Lebedeff, was first described in that article. Over the years, the method has been refined. When Lebedeff died unexpectedly in 1990, Reto Ruedy took over the project and has since led the technical parts of the NASA GISS temperature analysis. 

Over the years, Ruedy’s team has added new data sources, quality controls, and adjustments to account for urban heat. Sea surface anomalies were added to land-only measurements in 1995, improving the estimates. In the late 1990s, the analysis was made public online, along with interactive web pages, “which allowed anybody to display the data in various ways,” Ruedy said. 

The global temperature analysis is just a small part of the work done at GISS, where scientists also use climate models to assess atmospheric changes on Earth and on other planets and exoplanets. But the temperature analysis gets the most attention, as annual and monthly records continue to be broken. 

When NASA started calculating the global temperature in the 1980s, you could only see the warming signal emerge when looking at the global average temperatures, Schmidt said. But we’re seeing it now increasingly at the regional and local levels. 

“The changes occurring in people’s everyday weather experiences have become abundantly clear,” he said. People are noticing changing weather patterns in storm intensity, more frequent wildfires and flooding, but also in seasonal temperature changes where they live. 

The purpose of tracking global temperature “was to give people a heads up that things were changing,” Schmidt said. “And things have changed.”

Composed by Jenny Marder/ NASA's Earth Science News Team

Information in this piece came from the resources below and interviews with the following sources: Gavin Schmidt, Reto Ruedy, Nathan Lenssen, Peter Jacobs.  

Caption for banner image: This map of Earth in 2024 shows global surface temperature anomalies, or how much warmer or cooler each region of the planet was compared to the average from 1951 to 1980. Normal temperatures are shown in white, higher-than-normal temperatures in red and orange, and lower-than-normal temperatures in blue. An animated version of this map shows global temperature anomalies changing over time, dating back to 1880. Download this visualization from NASA Goddard’s Scientific Visualization Studio: https://svs.gsfc.nasa.gov/5450. Credit: NASA Goddard's Scientific Visualization Studio.

Download this visualization from NASA Goddard’s Scientific Visualization Studio: https://svs.gsfc.nasa.gov/5450. Credit: NASA Goddard's Scientific Visualization Studio.

“AR6 Synthesis Report: Climate Change 2023” Intergovernmental Panel on Climate Change. March 20, 2023.

Carlowicz, Michael. (2023) “World of Change: Global Temperatures.” Earth Observatory. 

“Data Buoy Types” Data Buoy Cooperation Panel. 

“GISS Surface Temperature Analysis (GISTEMP): Frequently Asked Questions.” NASA Goddard Institute for Space Studies.  Last updated: Dec 1, 2023. 

“GISTEMP Surface Temperature Analysis” NASA Goddard Institute for Space Studies. Last updated November 26, 2024. 

Hansen, J.E., et al. “Climate Impact of Increasing Atmospheric Carbon Dioxide.” Science Magazine. Vol 213, Issue 4511, pp. 957-966. August 28, 1981.

Hansen, James, “Why I Must Speak Out About Climate Change.” TedTalks. March 2012. 

Hansen, J.E., and S. Lebedeff, “Global Trends of Measured Surface Air Temperature.” Journal of Geophysical Research, Vol 92, Pages 13345-13372, November 20, 1987.

“Introduction to Synoptic Meteorology” National Oceanic and Atmospheric Administration. Last updated May 16, 2023.

Miller, Kenneth G. et al.,  Ancient Sea Level as Key to the Future, Oceanography, Volume 33, No. 2. Pages 32 - 41.

Schmidt, Gavin. “We Study Climate Change. We Can’t Explain Anything.” The New York Times. Nov. 13, 2024. 

“Paris Climate Agreement,” United Nations. December 12, 2015. 

Younger, Sally. “NASA Analysis Confirms a Year of Monthly Temperature Records.” NASA.gov. June 11. 2024.