Why have the sea surface temperature suddenly risen in 2023/24? 🌊 Is it true that climate models cannot simulate such SST jumps? What is common to such jumps? How will SSTs evolve over the next months and years? Are we in uncharted territory? More from our recent study in Nature is here👇
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The jump in SSTs in 2023/24 has come as a surprise to many. The annual averaged SSTs from April 2023 to March 2024 have exceeded the previous record over the same period by 0.25°C. Never before has an SST record been broken by such a margin.
The question was raised if we have a blind spot, if we missed something in the climate system, if we have a blind spot in our understanding. It was also suggested that warming has accelerated more than the already estimated acceleration caused by increasing emissions over the last decades. In our study, we now show that the record-shattering jumps in SSTs might just have been a very rare extreme event. We base this finding on two main methods. First, we estimated the past variability, autocorrelation, and trend of SSTs over the past. Based on these characteristics, we constructed synthetic timeseries of 100 million years. We then counted the jumps in SSTs of at least 0.25°C. We found that we can expect such jumps every 500 years. However, uncertainties are large due to the uncertainties of the underlying characteristics.
We also looked into climate models. More precisely, we collected results from 270 different climate simulations from state-of-the-art models. We looked at the years 2000 to 2040 as they represent roughly the period when the observed jump in 2023/24 occurred. We have thus 11,070 years of data. In these simulations, we find indeed 11 jumps in SSTs that are larger than 0.25°C. Such jumps thus occur around every 1000 years. This is twice as large as estimated from observations but uncertainties are large in both estimates and are not significantly different.
Overall, the fact that models simulate such jumps at a rate that is in agreement with the observation-based estimate (within the respective uncertainties) largely increases our confidence in these models. It also means that we do not necessarily have a blind spot in our understanding. We can also show that the jump in SSTs would not have been possible without global warming. Such a jump needs the variability of temperatures but also the long-term warming trend. Without the warming trend, no such jump can occur. We also looked at common characteristics between the jumps in the models and found that a jump in SSTs of at least 0.25°C can only occur if there is an El-Niño event. In most cases, the Pacific and Atlantic Ocean were also very high.
Our study does not assess the potential influence of aerosols. However, we show that a rapid reduction of aerosols is not necessary for such an event to occur. It would also be interesting to now look in the models if they have other common factors, such as a reduction in cloud cover. We further looked how SSTs evolve in the climate models after such large jumps in SSTs. In all 11 events, the SSTs stopped being record-breaking in the summer of the year after the event started. This is in line with observed temperatures that stopped to be record-breaking in July 2024. Furthermore, SSTs in all model simulations return to the expected long-term warming trend in the years after the jump in SSTs. Thus, the models show no unexpected accelerated warming. However, warming of course accelerates in an expected way in models with increasing emissions over the past. In 8 of the 11 simulations, SSTs return to pre-jump levels by September in the 2nd year after the event. Only in 3 simulations SSTs never return to pre-jump levels but increase slower over the next 5 to 10 years to rejoin the expected long-term warming trend. The 3 models that don't simulate a return to pre-jump level are so-called 'hot models' that warmed faster than observed over the past. As these warm to fast, we expect SSTs to return to pre-jump levels by September 2025. If this is not the case, climate sensitivity might be higher than thought. Thus, so far all signs indicate that the jump in SSTs was a very rare but not unexpected heatwave and not an unexpected additional acceleration of global warming. The next years will tell us more. Overall, we thus can trust our models and should use them as much as possible to better understand such extreme events and their impacts in the climate system. A big thanks to all the co-authors: @linusvogt.bsky.social, Friedrich Burger, @froeltho.bsky.social and Thomas Stocker. Also a big thanks to the support from @unibern.bsky.social and @snsf-ch.bsky.social A link to access the paper is here: rdcu.be
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