Global average temperatures may pass a threshold in 2024

When will the annual global average temperature rise by more than 1.5°C above pre-industrial levels for the first time? There have been individual days when the global average temperature has exceeded that threshold, but so far no single year has, on average, been that hot overall. This may change in 2024, when the steady, century-long rise in temperatures driven by greenhouse-gas emissions syncs with a natural cyclical warming pattern for the first time in nearly a decade.

Meteorological agencies collect temperature data from across the globe and throughout the year to determine the annual average global surface temperature. That number, published each January, has been rising since early in the 20th century, but not systematically. The line zigzags (see chart). This is because global warming, driven by greenhouse gases, is happening at the same time as natural variations in the global climate system, which cause some years to be hotter or colder than others.

The largest such hot-and-cold cycle is the El Niño Southern Oscillation (enso), a pattern that begins in and above the waters of the equatorial Pacific and affects the weather in the tropics and beyond. ENSO alternates between three states: La Niña, neutral and El Niño. The two extremes are typically cooler (La Niña) and hotter (El Niño) than average; both bring enhanced probabilities of wild weather extremes.

From mid-2020 to early 2023, ENSO was in a La Niña pattern. As well as exacerbating some remarkable weather events, including record-breaking floods in Pakistan in 2022, this unusually long La Niña temporarily depressed global average temperatures, masking some of the warming caused by industrial emissions. There will be no such reprieve in 2024. In June 2023, ENSO flipped into a much-anticipated El Niño state, which will add to global warming. And this El Niño is forecast to be a strong one, bringing a greater likelihood of extremes.

The last such event was in 2015-16. It brought record-breaking global temperatures in 2016, an annual record that still stands. There are two possibilities. El Niño is an end-of year phenomenon that starts in the later days of the boreal summer and peaks at Christmas and the new year (it was named after Baby Jesus by Peruvian fishermen who noticed the way its warmer Pacific temperatures chased anchovies into deeper, cooler waters). Typically, the year after an El Niño is the record-breaker. But the boreal summer of 2023 brought serious climate fevers in both the oceans and the atmosphere. Starting in July, daily temperatures rose to new heights. As a result, when all the data are in and published in January, it may turn out that 2023 was the hottest year ever. If it was not, then 2024 almost certainly will be.

So will either year’s average exceed the Paris threshold? The Paris agreement talks of a rise in temperatures “above pre-industrial”. Naturally, when the threshold is passed depends on what is used as the pre-industrial average (temperatures are now measured with a precision that is not available from the proxies used to estimate averages before the steam engine). So some predict it will happen in 2024, others that it could take one more El Niño cycle.

Paris signatories will, however, have a little longer before the overshoot of 1.5°C will technically have been reached. The deal refers to a vaguely defined long-term average, taken over several years. So there will be a few more ups and downs before that average exceeds the threshold. Not many, though—climate models suggest the game will be up in the 2030s. ■

Catherine Brahic, Environment editor, The Economist