The past 12 months were the hottest on record. Some 7.3 billion people worldwide were exposed, for at least 10 days, to temperatures that were heavily influenced by global warming, with one-quarter of people facing dangerous levels of extreme heat over the past 12 months, according to a report by the non-profit organization Climate Central, based in Princeton, New Jersey.

“These impacts are only going to grow as long as we continue to burn coal, oil and natural gas,” says Andrew Pershing, vice-president for science at Climate Central.

Researchers have previously estimated the influence of climate change on specific extreme weather events, a process known as climate attribution. Now, scientists have calculated the impact of human-induced climate change on daily air temperatures in 175 countries and 920 cities from November 2022 to the start of October 2023.

They found that the average global temperature over the past 12 months was 1.32 ºC above that during the pre-industrial baseline period of 1850 to 1900, surpassing the previous record of 1.29 ºC that was set from October 2015 to September 2016 (see ‘Heating planet’). The finding comes as the European Union’s Copernicus Climate Change Service predicted that 2023 will be the hottest calendar year on record, with the average temperature up to October being 1.43 ºC above the pre-industrial average.

Heating planet. Chart showing global mean temperature rising since 1970.

Source: Climate Central

“This is the hottest temperature that our planet has experienced in something like 125,000 years,” says Pershing.

Most of this warming, about 1.28 ºC, results from human-induced climate change, with natural variation caused by processes such as the current ocean-warming El Niño event contributing much less, says climate researcher Friederike Otto at Imperial College London.

By analysing daily air-temperature data and using computational models, the team calculated the effect of climate change on temperatures worldwide using a measure called the Climate Shift Index (CSI). The CSI scale runs from −5 to 5; a value of zero means there is no detectable influence of human-caused climate change on daily temperatures, whereas a positive value indicates how much more likely climate change made the observed temperature. A negative value means climate change made the observed temperature less likely.

The researchers found that 7.3 billion people worldwide were exposed, for at least 10 days, to temperatures that were strongly affected by climate change. In the first half of the past 12 months, tropical regions across South America, Africa and southeast Asia experienced the most days with temperatures that were strongly attributable to climate change, defined as having a CSI value of 3 or higher. These effects were felt even more strongly in the second half of the year-long period.

In Jamaica, the country where global warming was found to have had the greatest impact on daily temperatures, people experienced temperatures that were made over 4.5 times more likely by climate change. Guatemala and Rwanda also experienced temperatures that were made more than four times more likely by climate change.

The researchers also estimated the extent to which 700 cities with populations of at least one million experienced extreme heat over the past 12 months, defined as daily temperatures that are expected to occur less than 1% of the time in that location. They did this by comparing recent temperature data with data collected over a reference period of 1991–2020.

Unbroken heat. Chart showing top ten cities which recorded longest extreme heat streaks in 2023.

Source: Climate Central

The team found that 156 cities in 37 countries experienced five or more consecutive days of extreme heat, with 144 cities experiencing temperatures that were made at least 2 times more likely by climate change. Houston, Texas, had the longest heat streak of 22 days. The next most-affected cities were Jakarta; New Orleans, Louisiana; Tangerang, Indonesia; and Qujing in China, all of whose inhabitants faced at least 16 days of extreme heat in a row (see ‘Unbroken heat’). Worldwide, 1.9 billion people, or 24% of the population, endured five consecutive days of extreme heat.

Extreme heat, along with flooding and droughts, is often deadly and displaces thousands of people. “By continuing to burn fossil fuels the way we do, it’s a massive violation of the really basic human rights of the vast majority of the planet,” says Otto.

El Niño, which is projected to last until at least April 2024, will push temperatures even higher next year, says Pershing.

The study clearly provides robust evidence for the science of climate-change attribution, says climate researcher Cecilia Conde at the National Autonomous University of Mexico in Mexico City.

“This is a really appreciated effort,” says climate researcher Karsten Haustein at Leipzig University in Germany. “It’s great in that this approach can provide continuous updates on the hottest 12 months, not just the hottest [calendar] year, so that hopefully helps to raise awareness of climate change’s impacts each month.”

Joyce Kimutai, a meteorologist at Kenya Meteorological Department in Nairobi, says the analysis underscores the urgent need for countries to take action. She adds that at the United Nations COP28 climate summit later this month, the world needs to make progress on phasing out fossil fuels and implementing the ‘loss and damage’ fund through which rich countries have agreed to help low-income countries cope with the social and physical devastation caused by climate change.