Earth marks 12 straight months of record heat, a bewildering climate change milestone

Another month, another climate record: May 2024 was the twelfth straight month of record-warm temperatures for the planet, European scientists announced Wednesday.

That's the second-longest such streak on record, according to data from the Copernicus Climate Change Service.

“It is shocking but not surprising that we have reached this 12-month streak," said Carlo Buontempo, director of the Copernicus Climate Change Service, in a statement. "While this sequence of record-breaking months will eventually be interrupted, the overall signature of climate change remains and there is no sign in sight of a change in such a trend.”

The new milestone is even more worrisome than the one reached in January, which marked 2023 as the hottest year on record. That meant the calendar year was the hottest overall, with many – but not all – months setting records. Now every month for a year straight has been the hottest ever recorded.

Unfortunately, Earth being hot is nothing new: Our planet has seen over 550 consecutive months with temperatures above the 20th-century average. But this uninterrupted run of all-time temperature records starting in June 2023 is unusual, perplexing and worrying.

Unprecedented heat for 12 months straight

So why has our planet been so darn hot recently? Climate scientists interviewed by USA TODAY say the main driver of the record warmth is still human-caused climate change. But scientists are also somewhat confused and alarmed by the steady drumbeat of records. They often cite a variety of other factors as possible reasons for the supercharged global heat, while admitting that no one is fully sure what is going on.

"We know that there can be changes in global temperature due to other causes, such as the pattern of sea-surface temperatures in the equatorial region of the Pacific Ocean – otherwise known as El Ni?o and La Ni?a – or additional factors like volcanic eruptions, changes in the human and natural emissions of aerosols, and solar activity from our sun," the National Oceanic and Atmospheric Administration's Zachary Labe told USA TODAY in an email.

Some of those factors may come and go, which could give the globe some temporary relief from record heat. But the fundamental problem will remain: "The primary reason for this remarkable stretch of record-breaking warmth around the world is due to human-caused climate change," said Labe.

Here's a look at several factors that have led to a perfect storm of heat in the past year, exacerbating already high temperatures of a warming world. Some are natural cycles in the planet's climate – others are scientific oddities that have perplexed scientists.

El Ni?o boosts global temperatures

El Ni?o, a natural warming of tropical Pacific Ocean water, acts to boost global temperatures as well as changing weather patterns worldwide. The warmth from the recent strong El Ni?o, colloquially called a "super El Ni?o" helped to boost global temperatures in 2023, as the year ended up as the warmest since accurate weather records began in the late 1800s.

Berkeley Earth scientist Zeke Hausfather told USA TODAY via email that "the primary cause of the extreme warmth over the past few months is a large El Ni?o event on top of the longer-term warming of the planet driven by human activities."

"What still remains relatively unexplained is the extreme warmth we saw in the summer and fall of 2023, which occurred before the current El Ni?o event had fully developed," Hausfather said.

Shipping regulations may play a confusing role in warming

An attempt to clean up shipping pollution may have had the confusing, unintended consequence of boosting warming, some experts say.

The contested theory goes something like this: When ships switched to cleaner fuels in 2020, the skies over the oceans got less cloudy, allowing oceans and the atmosphere to warm faster.

"Lowering the sulfur content of marine fuel has weakened the masking effect, effectively giving a boost to warming," online climate news site Carbon Brief reported last year.

If true, that means that some of the longer-term benefits of less-polluting fuels are offset in the short term by hotter oceans, no longer artificially cooled by pollution.

But some experts think this theory is a fantasy, including University of Pennsylvania meteorologist Michael Mann. "The claim that it has to do with new shipping restrictions is absurd and not backed up by mainstream published climate research," Mann told USA TODAY via email.

A huge volcano eruption may still be contributing to record heat

Lingering impacts from the eruption of the Hunga Tonga-Hunga Ha'apai volcano in Tonga in January 2022 may have played a small role in the recent warmth.

An increase in stratospheric water vapor from the explosive South Pacific volcanic eruption in 2022 is a "likely contributor" to the record warmth, meteorologist Michael Lowry wrote on Yale Climate Connections last week.

But Mann thinks the volcanic eruption is also irrelevant as a planet-warming factor. "Recent peer-reviewed work by (Texas A&M's) Andrew Dessler and colleagues shows that if anything, Hunga Tonga had a slight global cooling effect.

"This one’s not that hard. (The reasons for the warmth are) steady, ongoing human-caused warming plus a spike from a big El Ni?o event," Mann said.

Not very satisfying explanations

Still, some scientists remain puzzled as to the level of the extreme warmth of the past year.

"We have a lot of not very satisfying explanations for last year – El Nino behaving weirdly, sulfur in marine fuels, the Tonga eruption, an uptick in the 11-year solar cycle – but even added together they don't really add up to the margins we were setting records by last year," Hausfather told USA TODAY via email.

Labe also said the recent warmth will be an active area of study for scientists this year: "Given that 2023 and now 2024 are data points that are well above the long-term trend line, climate scientists are trying to understand how much these other factors contributed to these specific years.

"This remains an active area of research, and I expect that we will have more answers over the coming year. At this stage, it looks like it was related partially to the fact that all of these components were additively combining to contribute more warming on top of the long-term climate change trend," Labe said.

This article originally appeared on USA TODAY: Global heat records set 12 months in a row, fueled by climate change