Earth’s hottest 12-month period in recorded history exposed 99% of humanity to extreme heat, study says

November 2022 to October 2023 was the hottest yearlong period on record, according to Climate Central. Scientists warn that more records could fall in 2024.

Earth has just experienced its hottest yearlong period in recorded history, with global temperatures exceeding 2.34 degrees Fahrenheit above pre-industrial levels due to climate change, according to an analysis of international data released by Climate Central.

The Climate Central analysis released Thursday showed that average temperatures between November 2022 and October 2023 were above the 30-year norm in 170 countries, exposing 7.8 billion people – 99% of humanity – to above-average heat.


According to Climate Central's analysis, 5.7 billion people were exposed to at least 30 days of above-average temperatures during the same 12-month period. That included nearly every resident of Japan, Indonesia, the Philippines, Vietnam, Bangladesh, Iran, Egypt, Ethiopia, Nigeria, Italy, France, Spain, the United Kingdom, Brazil, Mexico, and every Caribbean and Central American nation.

Only Iceland and Lesotho, a landlocked country in Africa, experienced cooler-than-average temperatures, according to Climate Central.

Climate Central said its Climate Shift Index determined that the extreme heat was three times more likely because of human-caused climate change. The index is used to reveal how much climate change influences the temperature in any particular location on any given day.


Past 12 months were hottest on record

The previous record for the warmest 12-month period was 2.32 degrees Fahrenheit above the pre-industrial baseline (1850-1900) and was set during the period of October 2015 to September 2016. The 12-month period ending in September 2023 tied that record, which was then broken during the most recent 12-month period between November 2022 and October 2023.


In the table above, the trend line is curving up, which indicates that warming is accelerating.

"The most recent and record-breaking 12-month period is highly consistent with this trend," Climate Central scientists said in a news release. "Rising emissions of heat-trapping gasses are the main driver of the trend, followed by some reduction in aerosol pollutants with cooling effects."

1 in 4 people experienced extreme heat waves

Heat waves are the deadliest of all weather-related hazards.

With this analysis, Climate Central focused on extreme heat, which are temperatures expected to occur less than 1% of the time at a location based on a reference period of 1991-2020. The temperatures would also be dangerous to people who were experiencing them.

Climate Central found that 4.9 billion people (61% of the global population) experienced more than five days of locally extreme heat, which is especially dangerous if it persists over several days. In addition, 2.1 billion (26%) experienced five or more consecutive days of these extremes, and 1.9 billion (24%) experienced a five-day heat streak with a CSI level of 2 or higher, meaning that human-caused climate change made those temperatures at least two times more likely.

Houston sees 22-day extreme heat streak

Climate Central analyzed extreme heat in 700 cities around the world with a population of at least 1 million people.

Among those cities, Houston had the longest extreme heat streak at 22 days. In fact, the U.S. had 12 cities with streaks of five days or longer. Most of those were in the South (Texas, Florida and Louisiana) and the Southwest (Arizona and Nevada).

"No major city on Earth matched Houston’s 22 consecutive days of extreme heat between July 31 and August 21," Climate Central scientists said in a news release.


‘No one is safe’

According to Climate Central, more than 9 in 10 people felt extreme heat that was made much more likely by human-caused climate change. 

"This 12-month record is exactly what we expect from a global climate fueled by carbon pollution," Andrew Pershing, Ph.D., vice president for science at Climate Central, said in a statement. "Records will continue to fall next year, especially as the growing El Niño begins to take hold, exposing billions to unusual heat."

The highest exposures of climate-driven heat were found in the tropics, but every country in the world felt the heat.

"While climate impacts are most acute in developing countries near the equator, seeing climate-fueled streaks of extreme heat in the U.S., India, Japan, and Europe underscores that no one is safe from climate change," Pershing said.

Climate Central's analysis comes a little more than a day after a report from Europe's Earth monitoring agency said 2023 will almost certainly go down as the hottest year on record.