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Six Answers to Questions You’re Too Embarrassed to Ask about the Hottest Year on…

Six Answers to Questions You’re Too Embarrassed to Ask about the Hottest Year on…

: Data visualization of global temperature anomalies progressing from 1880 to 2023 mapped onto Earth. The map uses color to represent anomalies, ranging from blue for below average temperatures, white for temperatures at baseline, and yellows ranging through oranges and reds to represent higher and higher than average temperatures. At the beginning of the time series, the map is primarily blues and whites, with a few spots of yellow, indicating that temperatures overall are below the baseline. As time progresses, the colors shift and move, with less and less blue and white and more and more yellow, then orange, and red. By 2023, the map is mostly yellow with lots of orange and red. The Arctic region, Europe, Asia, North America, central South America, and the Antarctic peninsula are all dark red, indicating the highest temperature anomalies. Credit: NASA’s Scientific Visualization StudioALT

Six Answers to Questions You’re Too Embarrassed to Ask about the Hottest Year on Record

You may have seen the news that 2023 was the hottest year in NASA’s record, continuing a trend of warming global temperatures. But have you ever wondered what in the world that actually means and how we know?

We talked to some of our climate scientists to get clarity on what a temperature record is, what happened in 2023, and what we can expect to happen in the future… so you don’t have to!

Graph of carbon dioxide emissions from just before 1960 to present day. The X-axis shows years, with each decade listed. The Y-axis shows parts per million of carbon dioxide in the atmosphere. It starts at 300 and runs to 420 ppm. The line on the graph is a fairly straightforward upward trajectory, starting below 320 ppm in 1960 and running to over 420 ppm in 2023. The line on the graph does spike up and down within each year, showcasing the seasonal cycle of carbon dioxide uptake. However, the spikes are extremely minor compared to the upward trajectory. Credit: NOAAALT

1. Why was 2023 the warmest year on record?

The short answer: Human activities. The release of greenhouse gases like carbon dioxide and methane into the atmosphere trap more heat near Earth’s surface, raising global temperatures. This is responsible for the decades-long warming trend we’re living through.

But this year’s record wasn’t just because of human activities. The last few years, we’ve been experiencing the cooler phase of a natural pattern of Pacific Ocean temperatures called the El Niño Southern Oscillation (ENSO). This phase, known as La Niña, tends to cool temperatures slightly around the world. In mid-2023, we started to shift into the warmer phase, known as El Niño. The shift ENSO brought, combined with overall human-driven warming and other factors we’re continuing to study, pushed 2023 to a new record high temperature.

A climate spiral animation. The chart is circular with the year in the center and months of the year around the outside. There are three concentric circles labeled with measures from negative 2 degrees Fahrenheit to 2 degrees Fahrenheit, with the outer ring being the largest value. As the years count up, a line spirals through the months of the year and around the circle. The line starts with blue hues when temperatures are below average and changes to red and orange hues when temperatures are above average. As the spiral progresses, the lines form a deformed circle that becomes larger and more red, indicating Earth’s warming up to just above 2 degrees Fahrenheit above average. Credit: NASA’s Scientific Visualization StudioALT

2. So will every year be a record now?

Almost certainly not. Although the overall trend in annual temperatures is warmer, there’s some year-to-year variation, like ENSO we mentioned above.

Think about Texas and Minnesota. On the whole, Texas is warmer than Minnesota. But some days, stormy weather could bring cooler temperatures to Texas while Minnesota is suffering through a local heat wave. On those days, the weather in Minnesota could be warmer than the weather in Texas. That doesn’t mean Minnesota is warmer than Texas overall; we’re just experiencing a little short-term variation.

Something similar happens with global annual temperatures. The globe will naturally shift back to La Niña in the next few years, bringing a slight cooling effect. Because of human carbon emissions, current La Niña years will be warmer than La Niña years were in the past, but they’ll likely still be cooler than current El Niño years.

Visualization of Earth, rotating, speckled with tiny dots in various colors, representing surface temperature measurements taken over the course of a year. Most of the land surfaces are heavily covered in red dots, which represent land measurements. Yellow dots create streaks across the ocean, representing measurements taken by ships. Pink dots irregularly scattered across the ocean represent measurements from floating ocean buoys. Orange dots similar across the ocean represent measurements from moored buoys. Green dots, primarily along coasts, represent tidal gauge measurements. Finally, a handful of blue dots represent all other measurement locations. Credit: NASA’s Scientific Visualization StudioALT

3. What do we mean by “on record”?

Technically, NASA’s global temperature record starts in 1880. NASA didn’t exist back then, but temperature data were being collected by sailing ships, weather stations, and scientists in enough places around the world to reconstruct a global average temperature. We use those data and our modern techniques to calculate the average.

We start in 1880, because that’s when thermometers and other instruments became technologically advanced and widespread enough to reliably measure and calculate a global average. Today, we make those calculations based on millions of measurements taken from weather stations and Antarctic research stations on land, and ships and ocean buoys at sea. So, we can confidently say 2023 is the warmest year in the last century and a half.

A line graph of temperatures in the Northern Hemisphere Extratropics, Reconstructed Summer, which is May to August, Temperature. The Y-axis is Temperature Anomaly, running from -2 degrees Celsius to 2 degrees Celsius. The X-axis is Years, from 600 to 2023. A jagged black line runs just around the 0 degree Celsius line, with each year slightly higher or lower than the previous, but none jumping above or below 1 and -1 degrees, until just before the year 2000. Around the year 1900, the jagged line begins to climb upwards, reaching to above 1 degree Celsius. At around the time the temperature starts to climb, a red line, indicating NASA’s temperature record, maps very closely to the black line. At the very end, the red line jumps even higher than the black line, reaching almost to 2 degrees Celsius. Credit: NASA/Peter Jacobs using data from N-TREND / Rob Wilson at University of St. AndrewsALT

However, we actually have a really good idea of what global climate looked like for tens of thousands of years before 1880, relying on other, indirect ways of measuring temperature. We can look at tree rings or cores drilled from ice sheets to reconstruct Earth’s more ancient climate. These measurements affirm that current warming on Earth is happening at an unprecedented speed.

4. Why does a space agency keep a record of Earth’s temperature?

It’s literally our job! When NASA was formed in 1958, our original charter called for “the expansion of human knowledge of phenomena in the atmosphere and space.” Our very first space missions uncovered surprises about Earth, and we’ve been using the vantage point of space to study our home planet ever since. Right now, we have a fleet of more than 20 spacecraft monitoring Earth and its systems.

Why we created our specific surface temperature record – known as GISTEMP – actually starts about 25 million miles away on the planet Venus. In the 1960s and 70s, researchers discovered that a thick atmosphere of clouds and carbon dioxide was responsible for Venus’ scorchingly hot temperatures.

The northern hemisphere of Venus, seen by the Magellan spacecraft. Venus is a burnt yellowish circle against the blackness of space. The planet’s surface has darker and yellow orange mottling and darker crater markings. Credit: NASA/JPLALT

Dr. James Hansen was a scientist at the Goddard Institute for Space Studies in New York, studying Venus. He realized that the greenhouse effect cooking Venus’ surface could happen on Earth, too, especially as human activities were pumping carbon dioxide into our atmosphere.

He started creating computer models to see what would happen to Earth’s climate as more carbon dioxide entered the atmosphere. As he did, he needed a way to check his models – a record of temperatures at Earth’s surface over time, to see if the planet was indeed warming along with increased atmospheric carbon. It was, and is, and NASA’s temperature record was born.

5. If last year was record hot, why wasn’t it very hot where I live?

The temperature record is a global average, so not everywhere on Earth experienced record heat. Local differences in weather patterns can influence individual locations to be hotter or colder than the globe overall, but when we average it out, 2023 was the hottest year.

Just because you didn’t feel record heat this year, doesn’t mean you didn’t experience the effects of a warming climate. 2023 saw a busy Atlantic hurricane season, low Arctic sea ice, raging wildfires in Canada, heat waves in the U.S. and Australia, and more.

Satellite image of smoke over the northeastern United States. The smoke is a light gray, cottony blanket creating an irregular shape over the center of the image. Behind it, the land surface is light browns and greens. Credit: NASA’s Earth ObservatoryALT

And these effects don’t stay in one place. For example, unusually hot and intense fires in Canada sent smoke swirling across the entire North American continent, triggering some of the worst air quality in decades in many American cities. Melting ice at Earth’s poles drives rising sea levels on coasts thousands of miles away.

Zoom in from a globe of Earth, showing warming temperatures in yellows, oranges, and reds. The zoom pushes in on the Arctic, which is primarily dark red, indicating the largest temperature anomalies throughout the region. Credit: NASA’s Scientific Visualization Studio/Katy MersmannALT

6. Speaking of which, why is the Arctic – one of the coldest places on Earth – red on this temperature map?

Our global temperature record doesn’t actually track absolute temperatures. Instead, we track temperature anomalies, which are basically just deviations from the norm. Our baseline is an average of the temperatures from 1951-1980, and we compare how much Earth’s temperature has changed since then. 

Why focus on anomalies, rather than absolutes? Let’s say you want to track if apples these days are generally larger, smaller, or the same size as they were 20 years ago. In other words, you want to track the change over time.

Apples grown in Florida are generally larger than apples grown in Alaska. Like, in real life, how Floridian temperatures are generally much higher than Alaskan temperatures. So how do you track the change in apple sizes from apples grown all over the world while still accounting for their different baseline weights? 

By focusing on the difference within each area rather than the absolute weights. So in our map, the Arctic isn’t red because it’s hotter than Bermuda. It’s red because it’s gotten relatively much warmer than Bermuda has in the same time frame.

Want to learn more about climate change? Dig into the data at

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