Last summer's sweltering heat broke more than city or regional or even national records. In what they call an "alarming finding," scientists say that in the Northern Hemisphere, the summer of 2023 was the hottest in 2,000 years. 

Global data already showed that last summer was the hottest on record. Copernicus, the European Union's climate change observation organization, made that determination  But a new study, published in Nature on Tuesday, looked even further back using both observed and reconstructed temperatures from centuries past. 

They found the heat was "unparalleled," the researchers said. 

According to their findings, the Northern Hemisphere experienced its hottest summer over the past 2,000 years by more than 0.5 degrees Celsius. 

Study co-author Ulf Büntgen, from the University of Cambridge, said in a press release that last year was "exceptionally hot," but that the true extent of that heat is visible when looking back at the historical record. 

"When you look at the long sweep of history, you can see just how dramatic recent global warming is," Büntgen said, "...and this trend will continue unless we reduce greenhouse gas emissions dramatically." 

The study also compared the temperatures of June, July and August in 2023 to those in the same months of 536 CE — the year one historian dubbed "the beginning of one of the worst periods to be alive, if not the worst year," as it launched the coldest decade in millennia due to major volcanic eruptions. The difference from that coldest summer to the recent hottest one was 3.93 degrees Celsius. 

When it comes to climate change, some people argue that the climate is constantly changing, as seen in the cold period that was kickstarted in 536 CE. But lead author Jan Esper, from the Johannes Gutenberg University Mainz in Germany, said that while that's true, it's the continued emission of greenhouse gases that really make a difference. Burning fossil fuels, such as oil and coal, releases a set of gases that trap the sun's heat in the atmosphere, steadily raising average temperatures. When that's paired with natural weather events like El Niño, which occurs when surface temperatures warm up over the Pacific, it only amplifies the impact. 

"We end up with longer and more severe heat waves and extended periods of drought," Esper said. "When you look at the big picture, it shows just how urgent it is that we reduce greenhouse gas emissions immediately." 

Experts have long warned that the world needs to take action to try to limit global warming to 1.5 degrees Celsius compared to pre-industrial times. Beyond that, the impacts of rising temperatures, including more frequent and intense droughts, hurricanes and floods, are expected to substantially worsen and drive global migrations, food scarcity and other issues.

But based on the observational record, researchers in this study found that the Northern Hemisphere may have already surpassed that. They found that temperatures in the hemisphere last summer were 2.07 degrees Celsius warmer than the average temperatures between 1850 and 1900. 

"This alarming finding not only demonstrates that 2023 saw the warmest ever recorded summer across the [Northern Hemisphere] extra-tropics, but also that the 2015 Paris Agreement to constrain warming globally to 1.5 ºC has already been superseded at this limited spatial scale," the study says. 

In their research, the scientists found "inconsistencies" and uncertainties in the baseline temperatures that experts have been using to track temperature rise. Those issues were largely due to a lack of station records in more remote areas of the world and "inadequately sheltered thermometers," researchers said. 

Based on their own studies, they found that it was actually cooler in pre-industrial times than what was thought when accounting for extended cold periods. With that in consideration, they found the difference in temperatures between that time and last summer was even greater, at 2.20 degrees Celsius. 

Researchers did note that their findings are largely based on temperatures in the Northern Hemisphere alone, as data for the Southern Hemisphere was sparse for the time periods they analyzed. They also said that the region responds differently to climate change because oceans are more prevalent in the southern half of the globe.

Despite the inability to develop fully worldwide temperature reconstructions and analysis, the researchers said their study "clearly demonstrates the unparalleled nature of present-day warmth at large spatial scales and reinforces calls for immediate action towards net zero emissions."

The paper comes as the planet continues to see back-to-back months of heat records with deadly consequences. Weather experts have warned that this summer could be just as scorching as the last, with above-normal temperatures expected across the majority of the U.S. 

In:
• Climate Change
• Science

Li Cohen

Li Cohen is a social media producer and trending content writer for CBS News.