A sudden spike in global warmth is so extreme, it’s mysterious

Record warmth is to be expected as greenhouse gases heat up the planet. But a spike in global temperatures observed in September was so much more dramatic than past extremes that some climate scientists said it defies a simple explanation.

A National Oceanic and Atmospheric Administration analysis released Friday further cemented what several other data sets had already affirmed: September was not just the globe’s warmest on record, but its most atypically warm month in nearly two centuries of observations. It was 0.83 degrees above the old record for the month, a staggering departure from what was already extreme.

No single factor – not human-caused global warming, not a burgeoning El Niño weather pattern – can immediately assume credit for such a drastic diversion from anything humans have ever seen before, scientists said. It is so far outside the realm of what has occurred, it creates a new conundrum that will take time for research to unpack.

“It is indeed hard to give a good and informed answer to why this is happening – possibly for the first time,” Gavin Schmidt, a climatologist and director of the NASA Goddard Institute for Space Studies, wrote in an email.

That doesn’t mean scientists don’t have their theories. A number of factors could be at play, each with so-far unknown influences adding up to such an unprecedented spike in average global temperatures. Research in the months and years ahead is expected to parse out the extent to which a host of conditions contributed to an unexpected yet likely record for annual global warmth in 2023.

Some stressed that, while the sudden surge in temperatures is alarming, it shouldn’t be used to assert any broader statements about how climates are changing around the globe.

“While it is important to measure and monitor the year-to-year fluctuations, we should not forget that climate change refers to slow changes on decadal to multi-decadal time scales,” Govindasamy Bala, a professor at the Indian Institute of Science, said in an email.

But others said it could add evidence to fears that the pace of global warming is accelerating.

“That acceleration means that the effects of climate change we are already seeing – extreme heat waves, wildfires, rainfall and sea level rise – will only grow more severe in the coming years,” climate scientist Zeke Hausfather wrote Friday in a New York Times guest column.

The arrival of the infamous El Niño climate pattern in June spurred predictions of a record-warm year, but in 2024.

Its effects on weather around the world stem from the unusual warmth it brings to the surface of the central and eastern Pacific Ocean. In the past, that surge in ocean heat – which transfers into the atmosphere when warm waters evaporate – has allowed annual global average temperatures to rise by 0.2 to 0.4 degrees, often in the calendar year following the El Niño pattern’s typical winter peak.

But Earth has warmed much more than that this year alone. As El Niño has developed in recent months, other oceans have surged to abnormal warmth around the world.

Global ocean surface temperatures hit record temperatures for a sixth consecutive month in September, NOAA said, tying August for the largest sea surface temperature anomaly on record, 1.85 degrees above average.

Across land and sea, global temperatures in September were closer to normal for July.

This year is now all but certain to surpass El Niño-influenced 2016 as Earth’s warmest on record. NOAA on Friday said the odds of a record have increased to more than 99 percent.

Volcanic activity is often associated with cooling the Earth; eruptions release particles that reflect sunlight, providing tiny bits of shade and reducing solar radiation. But when the Hunga Tonga-Hunga Ha’apai undersea volcano erupted in 2022, scientists warned that it could have a warming effect on the planet.

That is because unlike most eruptions, Hunga Tonga sent massive amounts of water vapor high into the atmosphere, and even released it into space.

While it might seem innocuous, water vapor is a potent heat-trapping gas in the atmosphere – so the eruption has likely enhanced the greenhouse effect, said Izidine Pinto, a climatologist at the Royal Netherlands Meteorological Institute.

An ongoing upswing in the sun’s normal cycle of activity means there is even more solar radiation than normal to be trapped in the atmosphere.

At the same time, the absence of another substance in the atmosphere could be increasing global heat, he added: Air pollution.

Like particles of volcanic ash, pollutants such as sulfur dioxide act to block sunlight and, in effect, cool the planet. But those particles have been declining in recent decades, and in recent years, have especially diminished over the oceans. That is thought to be because of new limits on sulfur emissions from shipping liners imposed in 2020.

While those trends are seen as a win for global public health, they are expected to add to global warming over time.

All of those factors add to the biggest one observed over decades: Ever-rising levels of carbon dioxide and other greenhouse gases in the atmosphere, the result of fossil fuel combustion. That has allowed global temperatures to rise by about 0.4 degrees each decade, or more than 2 degrees in all since the Industrial Revolution.

That has raised the floor, and the ceiling, for what effect natural fluctuations can have on planetary warmth.

“On top of this slow trend, there is tremendous interannual variability which could cause wide swings in global mean surface temperatures from one year to the other,” Bala said.

It will take time to disentangle the influences behind the unprecedented spike in warmth, scientists said. A growing field in climate research is exploring the extent to which scientists can attribute human-caused warming and other factors to new extremes in temperatures and precipitation.

“A proper attribution will need time and effort,” Schmidt said, adding that it “may well end up being ambiguous.”