The last 12 months were the hottest on record

The last 12 months were the hottest in 150 years of recordkeeping — and probably in the last 125,000 years — thanks to human-caused climate change, a new report finds.

From November 2022 through October 2023, the planet’s average temperature was about 1.3 degrees Celsius higher than the average temperature from 1850 to 1900, say researchers with the nonprofit group Climate Central. That’s just shy of the 1.5-degree threshold often cited as a benchmark for avoiding irreversible impacts on the climate (SN: 1/11/22).

And over the past year, about 1 in 4 people around the world experienced a climate change–driven heat wave that lasted at least five days, the scientists found.

The report, released on November 9, comes just ahead of the 28th United Nations Climate Change Conference of the Parties, which begins on November 30. That’s intentional, says Andrew Pershing, vice president for science at Climate Central. There is no doubt that fossil fuels are driving most of this heat, and it’s to be hoped that the world’s nations will take note of these findings, he says (SN: 4/4/22).

Global average numbers can be hard to grasp. So the new report also quantifies temperatures that people around the world are actually experiencing day-to-day, and how much those are attributable to climate change, Pershing says.

“We have these super important global numbers such as the 1.5- or 2-degree warming targets, but that isn’t the experience that people on planet Earth have,” he says (SN: 12/17/18). “We wanted to develop a way to really localize that experience … to talk about how climate change influenced that day’s temperatures on any given day anywhere in the planet.”

To that end, the analysis used Climate Central’s Climate Shift Index, or CSI, a system first described by the organization in 2022. CSI is a daily local temperature attribution system that uses a combination of observational data and climate simulations to determine the likelihood that local temperature variations are attributable to climate change.

Extreme heat is a relative term, dependent on both place and time. So, in this report, the researchers considered extreme heat for a given location to be daily temperatures that would have been in the 99th percentile for that place from 1991 to 2020 — temperatures, in other words, that locals would recognize as insanely hot.

Using that index with data from hundreds of countries, states, provinces and major cities, the researchers found that about 90 percent of the world’s population, or 7.3 billion people, experienced at least 10 days of extreme temperatures in the last year that were very strongly affected by climate change.

Those days had a CSI rating of at least 3, indicating that human-caused climate change made those temperatures at least three times as likely. Nearly 3 out of 4 people experienced over a month of those temperatures.

The report also reveals stark inequities in the burden of climate change around the world. Earth’s least developed countries, including many nations in sub-Saharan Africa and in Southeast Asia, had a relatively high average CSI of 2, the report notes, though they have contributed the least amount of fossil fuel emissions.

But climate impacts are also accelerating in many of the world’s richest countries, including the United States. The last 12 months saw brutal heat waves across much of the southern United States. Houston sweltered through a 22-day streak of extreme heat, where each consecutive day topped 38° C (100° Fahrenheit). That was the longest such extreme heat streak of the 700 cities examined with a population of at least 1 million people.

The CSI analysis is similar to the analyses performed by the consortium World Weather Attribution, or WWA, which looks for the fingerprints of human-caused climate change in specific extreme events around the world (SN: 7/25/23).

WWA examined a handful of extreme heat waves over the last 12 months, all of which were strongly attributable to climate change, says Friederike Otto, a climate scientist at Imperial College London who has led many of WWA’s attribution studies.

An immediate reduction of fossil fuels won’t shut off some effects of a century of emissions, including heat absorbed by the oceans or melted glaciers, Otto says. But global temperatures would stop rising, and heat waves would stop getting worse, she says. Keeping the planet below a 1.5-degree warming threshold “is in reach,” she says, “if we want it to be in reach.”

The last 12 months saw the onset of an El Niño climate pattern, which can bring higher global temperatures on top of the longer-term global warming trend (SN: 6/15/23). But the greatest temperature impact from El Niño generally takes a year or so to develop, as the heat gets disseminated around the globe, Pershing says.

The previous 12-month record was set from October 2015 to September 2016, as heat from a strong El Niño event spread around the globe. That record — where the global average was 1.29 degrees Celsius higher than the average preindustrial temperature — was tied earlier this year, for the period ending in September 2023.

That means that, as the El Niño pattern continues to develop into next year, 2024 will probably smash records once again.