Why has Earth been so unusually hot for the past 2 years? Climate scientists are trying to figure that out

Yes, we're going to talk about another year of warming. You might be tired of the same old tale: another year, another position on the top 10 warmest years, a podium on which we definitely don't want to stand.

But the past two years have been different — and climate scientists don't understand why.

We know that fossil fuels are primarily responsible for Earth's upward-trending temperature and our changing climate. But something else seems to be driving temperatures up, higher than scientists expected or would like.

Last year was 1.48 C warmer globally than the pre-industrial average from 1850 to 1900, beating out 2020's record of 1.25 C, according to the European Union's Copernicus Climate Change Service.

The climate service said in its latest monthly bulletin that 2024 is "virtually certain" to be the warmest year on record. They also believe that this year will be more than 1.55 C warmer.

Almost 200 jurisdictions through the Paris Agreement are aiming to limit their warming to well below 2 C above pre-industrial levels, with a target of 1.5 C, to significantly reduce the impact of climate change.

While that limit seems to have breached, it will only be one year, and the threshold looks at long-term warming, not just annual. And there's a chance we could go back down in the following years, though the global warming trend will continue upwards.

Meanwhile, the U.S. National Oceanic and Atmospheric Administration (NOAA) found in its latest monthly report that January to October's temperature was the warmest in the 175-year record, at 1.28 C above the 1901-2000 average, and that "it is practically certain that 2024 will rank as the warmest year on record."

This isn't what was expected. NOAA's 2023 annual climate report had pegged this year as having only a 32.58 per cent chance of being the warmest on record.

Sure, we had an El Niño event, a natural, cyclical warming in a region of the Pacific Ocean that, coupled with the atmosphere, can cause global temperatures to rise. That explained some of 2023. However, the warmth that we typically see after an El Niño was expected to stick around for the first few months of 2024.

"We're now 11 months going on 12 months after the peak of the El Niño event and global temperatures are still exceptionally high," said Zeke Hausfather, a research scientist at Berkeley Earth, a non-profit climate analysis organization.

There are some factors that can cause some warming, like the sun being at its peak in its 11-year cycle, which we're in now; the reduction of the pollutant sulphur dioxide which would typically reflect the sun's radiation; and the Hunga Tonga–Hunga Ha'apai volcanic eruption in the southern Pacific Ocean in 2022 which put a lot of water vapour into the atmosphere. But Hausfather said those typically contribute a rise of hundredths of a degree, and the timing isn't right.

Hausfather said a few recent studies he's read suggest that El Niño may be acting differently than it has in the past, pushed to stick around longer due to the "triple-dip" La Niña — the opposite of El Niño — that we had from the end of 2020 to 2022.

Ahira Sánchez-Lugo, a physical scientist at NOAA's National Centers for Environmental Information, said the oceans are also playing a big role in the temperatures we've seen in 2023 and 2024.