Here’s how 2023 became the hottest year on record

This year didn’t just shatter records. It changed the scales. 

Graph after graph tracking this year’s soaring global temperatures reveal that not only were the numbers higher than ever recorded in many places around the world, but the deviation from the norm was also astonishingly large.

“The margins by which records are being broken this year have surprised not just me but [other climate scientists] that I trust, even my very unalarmist friends,” says Doug McNeall of the U.K. Met Office Hadley Centre in Exeter, England.

As of late November, months of sweltering global temperatures easily put 2023 on track to be Earth’s hottest year since record keeping began about 150 years ago. The 12-month period from November 2022 through October 2023 is officially the hottest such period on record — a record that’s likely to be broken in 2024, according to the nonprofit group Climate Central (SN: 11/9/23).

Extreme heat waves baked many regions, which in turn fueled catastrophic wildfires. Ocean heat was off the charts, with global average sea surface temperatures sustaining record highs for much of the year. And in the water surrounding Antarctica, sea ice reached new lows.

These records had the fingerprints of human-caused climate change all over them, according to the international scientific consortium World Weather Attribution. Climate change made July’s extreme heat waves in North America, Southern Europe and North Africa hundreds of times as likely, and another in China about 50 times as likely (SN: 7/25/23). Climate change was also the primary cause behind a brutal winter and early spring heat wave in South America, making that event at least 100 times as likely.

On social media, many climate scientists who posted jaw-dropping screenshots of 2023’s temperature anomalies struggled to find words to explain them.

“Surprising. Astounding. Staggering. Unnerving. Bewildering. Flabbergasting. Disquieting. Gobsmacking. Shocking. Mind boggling,” Ed Hawkins, a climate scientist at the National Centre for Atmospheric Science at the University of Reading in England, wrote on X, formerly known as Twitter, about September’s air temperatures.

Global temperatures smashed records

From January through September, Earth’s average global surface air temperature was about 1.1 degrees Celsius (nearly 2 degrees Fahrenheit) higher than the 20th century average of 14.1° C (57.5° F). 

June, July, August, September and October were each the hottest ever recorded for those months — and September was hotter than an average July from 2001 to 2010. The year isn’t out yet, but temperatures so far suggest 2023 has a greater than 99 percent chance of being the hottest year on record, according to the U.S. National Centers for Environmental Information.

The Southern Hemisphere had a particularly sweltering winter and early spring, with temperatures in August and September soaring above 40° C (104° F) across parts of Brazil, Paraguay, Bolivia and Argentina. In some areas, daytime temperatures were about 20 degrees C (36 degrees F) above normal. Madagascar also had its warmest October on record, with some spots 2.5 degrees C (4.5 degrees F) above average.

The second half of 2023 saw the onset of an El Niño climate pattern, which generally means higher global temperatures, says John Kennedy, a climate scientist with the U.N. World Meteorological Organization. But most El Niño–related warming generally comes the year after an El Niño event, he says, as the heat that’s been accumulating in the eastern equatorial Pacific Ocean gets transported elsewhere. That’s what happened in 2016, previously the hottest year on record (SN: 7/13/23).

Ocean temperatures began reaching new highs long before El Niño kicked in. From late March through October, the world’s average sea surface temperature consistently broke daily records. By July, these temperatures were nearly 1 degree C (about 1.8 degrees F) above average, as marine heat waves racked nearly half of the global ocean, compared with a more typical 10 percent.

Such warm waters are unprecedented in the modern record — and possibly in the last 125,000 years, researchers note (SN: 8/9/23). Ocean life suffered, as the relentless accumulation of all that heat took its toll. Coral reefs, for instance, suffered widespread bleaching across the Gulf of Mexico, the northern Atlantic Ocean, the Caribbean Sea and the eastern Pacific Ocean.

High temps can lead to health problems

Most of the unprecedented temperatures to hit the news were daytime maximums, but the record-breaking heat continued into the night, endangering human health.

On July 6, the city of Adrar, Algeria, faced the hottest night ever recorded in Africa — nighttime temperatures never dipped below 39.6° C (103.3° F). And just after midnight on July 17, a weather station in Death Valley, Calif., recorded a temperature of 48.9° C (about 120° F). If confirmed, that is the highest temperature ever recorded anywhere for that dark hour. 

In most parts of the world, nights have been warming faster than days for decades. That’s a concern because, when nights are hot, the body loses a chance to recover from the heat of the day (SN: 8/6/23). 

Balmier bedtime temperatures also diminish the quantity and quality of sleep. Last year, data scientist Kelton Minor of Columbia University and colleagues published an analysis of billions of sleep-duration measurements from nearly 70 countries. The team estimated that, as of 2017, warmer nights contributed to eroding an average of about 44 hours of slumber from each person every year. 

Extrapolating to this year’s extreme heat, Minor says, “you would expect that this summer, on a global scale, would have eroded probably the most [sleep] in the observational record.”

Extreme heat can also lead to heat stroke, cardiovascular and respiratory diseases, and death. And heat-related deaths have been on the rise for years. 

In many parts of the world — such as Africa, where a prolonged spring heat wave in Madagascar would have been virtually impossible without climate change, according to World Weather Attribution — the number of lives lost to extreme heat is unknown. But an analysis of Eurostat data estimated that in Europe last year there were more than 60,000 heat-related deaths, up from around 40,000 in 2018. Provisional data from U.S. Centers for Disease Control and Prevention indicate that over 1,700 people in the United States died from heat in 2022. That’s more than four times the number of U.S. lives lost to heat just eight years earlier. 

It appears 2023 may have continued the trend. By October, hundreds of heat-related deaths had been reported from multiple counties in the American Southwest, where people sweltered through some of the summer’s highest temperatures. A record-breaking 579 such deaths have so far been reported out of Arizona’s Maricopa County — the fourth most populous county in the United States — up from 386 confirmed fatalities in 2022. Neighboring Pima County reported 175 heat-related deaths this year, up from 58 the previous year.

Part of the problem is that the danger of heat is often underestimated. There “is really low awareness that heat is a killer,” says Kristie Ebi, a climate and health researcher at the University of Washington in Seattle.

Moving forward, it will be crucial to spread greater awareness about the dangers and to invest in more interventions like cooling centers and urban green spaces. “Nobody needs to die and this is not like somebody being caught in a flash flood,” Ebi says. “There is enough known that it’s possible to protect people.”

It was a bad year for wildfires

Hotter nights may have also exacerbated wildfires. “In the past, you would get a drop in temperatures overnight, and that would help abate the wildfire spread,” says climate and atmospheric scientist Danielle Touma, of the University of Texas Institute for Geophysics in Austin. “But more recently, especially during a heat wave, these temperatures have not been dropping as much as they used to,” she adds. “That means that the fire continues to spread overnight.”

This year, heat contributed to an especially bad fire season in the Boreal region, a colossal area that wraps around the Earth just south of the Arctic Circle and contains close to one-third of the world’s forests. The largest intact stretch of this forest lies in Canada, which had its worst fire year on record. 

Hundreds of megafires burned across the country, and some 200,000 people were forced to evacuate in the face of approaching flames. Blazes in Quebec billowed smoke that engulfed the U.S. East Coast and Midwest, turning the skies orange and subjecting millions to hazardous air quality (SN: 6/9/23). As of October, the area burned in Canada surpassed 180,000 square kilometers, an area larger than Greece, more than doubling the previous national record from 1995.

Wildfires contribute to carbon emissions, which intensify global warming. The estimated carbon emissions from the Canadian fires amounted to nearly 410 million metric tons, shattering another record for the country. It’s also more than a quarter of the world’s wildfire emissions this year. 

As a whole, though, 2023’s wildfire emissions didn’t break global records. In fact, wildfire emissions have been decreasing for decades, largely because humans have cleared away many forested areas for agriculture, ultimately decreasing the total area where wildfires could burn (SN: 6/16/23).

Nonetheless, terrifying wildfires scorched many parts of the world. 

In the Northern Hemisphere, summer heat contributed to a wildfire in Greece that became the largest ever recorded in the European Union. In Hawaii, a wildfire fueled in part by drought destroyed much of the town of Lahaina and left at least 99 dead, making it the deadliest U.S. wildfire since 1918. 

Meanwhile, the Southern Hemisphere’s warm winter helped fires spread in many regions including Argentina and the Amazon rainforest. In Australia, an unusual spring heatwave helped the fire season kick off early; by August, around 70 blazes had already been reported out of New South Wales, the country’s most populated state, two months before the official start of the bushfire season in that state.

Antarctic sea ice hit a record low extent

Dwindling sea ice in the Arctic has become a familiar story in recent decades, while the southernmost continent’s sea ice has waxed and waned more erratically. 

But in the last few years, satellite data have shown an uptick in the rate of Antarctic sea ice loss, says climate scientist Mark Serreze, director of the U.S. National Snow and Ice Data Center in Boulder, Colo. 

Then came 2023. Antarctica’s sea ice “just plummeted,” Serreze says. 

The sea ice expanse was at near record-low levels for much of the year (SN: 7/5/23). February, the peak of summer, saw a record low minimum extent. By late July, the height of winter, the sea ice was more than 2.6 million square kilometers below the 1981–2010 average. On September 10, the sea ice extent hit its annual maximum at about 17 million square kilometers. That’s roughly 1 million square kilometers smaller than the previous lowest maximum in 1986. 

These numbers were “far outside anything observed in the 45-year modern satellite record,” Serreze says. 

El Niño and other regional climate patterns probably played a role. Shifting ocean circulation or wind directions could have either packed the ice in or shuttled it farther out to sea. But growing evidence suggests that warmer ocean waters may also be complicit, Serreze says. 

Whatever the case, this year’s trail of shattered records has made it clearer than ever that human-caused climate change is not a problem for tomorrow. “We’re standing in the aftermath of one of the biggest waves in the climate system in recent history,” Minor says, “and we need to also prepare for bigger waves that are approaching.”