Weather Bee | What a forecast for 1.5°C warming in 2024 means

New climate data from the C3S shows that 2024 is almost certain to be the first year with temperatures exceeding the pre-industrial average by more than 1.5°C

The climate summary for October published by the Copernicus Climate Change Service (C3S) on November 7 has made a grim (although expected) forecast for 2024. Not only is it almost certain now that 2024 will beat 2023’s record to become the new warmest year on record, but it is also almost certain that 2024 will be the first calendar year warmer than the pre-industrial average by more than 1.5°C in the ERA5 global temperature dataset that C3S publishes. Any breaches of the 1.5°C warming threshold at the global level are keenly watched because the Paris Agreement set the goal of keeping long-term global warming under that threshold. Therefore, it is useful to unpack the forecast made by C3S.

Scientists have weather models that can forecast temperatures with some accuracy only up to a few weeks, and they are very accurate for only a few days ahead. So, how are they certain what global temperatures will average up to December? (HT File Photo)(HT_PRINT)

Why is the forecast for 2024 almost certainly breaching the 1.5°C threshold?

Scientists have weather models that can forecast temperatures with some accuracy only up to a few weeks, and they are very accurate for only a few days ahead. So, how are they certain what global temperatures will average up to December?

A careful reading of the C3S forecast will show that the scientists are not making global temperature forecasts with certainty for November and December. The trend they are almost certain about is that the January-December average this year will be the highest ever and that this average will breach the 1.5°C warming threshold. Predicting this does not need any big calculations because of the trends in temperatures we have seen so far.

Here is how.

The 12-month running average deviation in global temperatures (from the pre-industrial average) has been above the 1.5°C threshold since January this year. This 12-month average was 1.62°C at the end of October. For the January-December average to drop below 1.5°C, warming in the next two months needs to average less than 1.06°C. That means the two-month average deviation needs to cool down 0.535°C from the 1.595°C average deviation for September-October. That is a pace of cooling/warming planet Earth has never seen in the ERA5 dataset, which begins in January 1940. The most two-month averages have changed in two months since 1940 is by 0.410°C. This was in 1977 when the average deviation for December 1976 and January 1977 was 0.34°C compared to the -0.07°C average deviation for October-November 1976.

As the trends described above suggest, a breach of the 1.5°C threshold will not be surprising at all, statistically speaking. However, there is more than just statistics to support the prediction made by C3S. For example, the latest model-based forecast has decreased the probability of the October-December period averaging La Nina conditions to under 50%, less than the 53% forecast for the season averaging neutral. La Nina is a periodic cooling of at least 0.5°C (compared to a 30-year base period updated every five years) of the sea surface temperatures (SSTs) in the equatorial Pacific Ocean, which generally has a cooling effect on average global temperature. On the other hand, as the first chart shows, neutral conditions (deviations of SSTs under 0.5°C) after May have not been enough to decrease deviations in global temperature so far.

How bad is the breach of the 1.5°C threshold for the Paris Agreement goal?

While deviation in global average temperature has breached the 1.5°C threshold in individual months before (three months in 2016, two months in 2020, and six months in 2023), 2024 is the first time this has happened consistently for a very long period: all months except July, when the deviation was 1.48°C. 2024 is also the first time, 12-month running averages have breached the 1.5°C threshold (in all months up to October). As suggested in the previous section, this has happened despite neutral conditions in the Pacific after May suggesting that long-term warming has a significant role to play in this. This is why the breach in 2024 is bad news for the Paris Agreement goal.

Deviation of global average temperature from 1851-1900 mean(Abhishek Jha)

To be sure, a one-year period is not usually considered “long-term” for climate. So, what is a long-term period for climate? There is no consensus on this (and the Paris Agreement does not specify this either), but, usually, scientists use ten to fifty-year averages when talking about climate. The 10-year average warming will be at least 1.28°C if C3S forecasts come true (C3S expects the year to average at least 1.55°C) and the 50-year average will be at least 0.78°C.

As the chart above suggests, temperatures appear to change more slowly on longer time scales. However, it would be a mistake to assume that the expected consequences of a world warmer by 1.5°C are far by looking at the distance from the threshold in the 50-year average. The chart also shows that all averages are inching up faster now than earlier, which means that the past rate of warming is no benchmark for future rates of warming. Moreover, as the string of climate disasters around the globe in 2023 and 2024 show (attribution studies have found global warming’s hand in a number of them), the weather does not need the 50-year average to reach 1.5°C to turn deadly.

Abhishek Jha, HT’s senior data journalist, analyses one big weather trend in the context of the ongoing climate crisis every week, using weather data from ground and satellite observations spanning decades.