If you haven’t gotten in on the cold-plunge trend, now might be a good time. Thanks to global warming, summers are getting hotter and hotter . . . and hotter. According to new research published in the journal Nature, scientists from the University of Cambridge and the Johannes Gutenberg University Mainz found that 2023 was the hottest summer in 2,000 years, or, since the height of the Roman Empire.

The researchers turned to examining tree rings over two millennia to come up with the data. They were able to come to a new temperature baseline by comparing old instrumental data with large-scale tree-ring data for context. The tree-ring data also allowed them to look further back in time for more wide-reaching comparisons.

Stunningly, they found that 2023 wasn’t just slightly hotter, but almost four degrees warmer than the coldest summer during the same 2,000-year period.

“When you look at the long sweep of history, you can see just how dramatic recent global warming is,” study coauthor Ulf Büntgen, a professor in Cambridge’s Department of Geography, said in a news release. “2023 was an exceptionally hot year, and this trend will continue unless we reduce greenhouse gas emissions dramatically.”

According to Oceanic and Atmospheric Research scientists’ data, carbon dioxide, methane, and nitrous oxide emissions didn’t slow down as of last year. Emission levels were not quite as high in recent years including 2023, but still followed the steep increases of the past 10 years.

Greenhouse gases are causing extreme weather to become stronger than ever. On top of the strain of natural disasters, it also creates hotter summers, and 2024 is expected to follow the trend. Forecasters say this year’s Atlantic storm season, due in large part to La Niña, will be one to brace for. Essentially, the combination of greenhouse gases and warmer-than-ever weather is hurricane fuel.

The scientists who took part in the research are troubled by the results, and say that they show just how important it is to take action. “It’s true that the climate is always changing, but the warming in 2023, caused by greenhouse gases, is additionally amplified by El Niño conditions, so we end up with longer and more severe heat waves and extended periods of drought,” Jan Esper, lead author of the study, said in a statement.

Esper added, “When you look at the big picture, it shows just how urgent it is that we reduce greenhouse gas emissions immediately.”