Nature Paper Confirms What I've Been Saying For Months: 'Extremely Unlikely That El Nino Caused 2023 Warmth'

A paper has just been published in Nature:

https://www.nature.com/articles/s41612-024-00582-9

September 2023 was the warmest September on record globally by a record margin of 0.5 °C. Here we show that such a record-breaking margin is an extremely rare event in the latest generation of climate models, making it highly unlikely (p ~ 1%) that internal climate variability combined with the steady increase in greenhouse gas forcing could explain it. Our results call for further analysis of the impact of other external forcings on the global climate in 2023.

September 2023 was the warmest September globally, with the highest temperature anomaly of any month in any year since 1940 in the ERA5 dataset¹. September 2023 also broke the previous monthly record by an exceptionally large margin: the previous record, set in 2020, was broken by 0.5 °C (Fig. 1). This is the largest margin by which the previous monthly record has been broken, in any month, in the entire ERA5 dataset.

Time series of September global mean temperature in 1940–2023 based on ERA5 reanalysis. Black circles indicate previous record-breaking Septembers before 2023. The temperatures are given in anomalies with respect to the 1991–2020 period.

In addition to September 2023, June, July, and August were also by far the warmest on record globally, with large margins. However, September had the largest margin of these months and is therefore the subject of this communication.

October, November and December proved just as remarkable. The entire year was the warmest year on record because of it.

We argue that internal climate variability alone is unlikely to explain the unusually large margin by which September’s record was broken. To illustrate this, we consider simulations from three climate model ensembles: Coupled Model Intercomparison Project 6² (CMIP6), the 100-member Max Planck Institute Grand Ensemble³ (MPI-GE), and the 100 member Community Earth System Model version 2⁴ (CESM2-LE). These are well-established models known for their reliable simulations of both internal climate variability, such as the El Niño-Southern Oscillation, and the forced response to greenhouse gas forcing.

The observed margin from September 2023 (0.5 °C) is shown as a black dashed line in Fig. 2. As can be seen, the observation falls in the far right tails of the model simulated margins. In CMIP6 (Fig. 2a), only three of the 5166 model-simulated margins exceed the observed margin, corresponding to the 99.94th percentile of the model distribution. In MPI-GE (Fig. 2b) the observation is completely outside the distribution, and in CESM2-LE (Fig. 2c) there is only one margin higher than the observation, meaning that the corresponding percentile is 99.95%.

For comparison, we also briefly examined the probability of the observed record margin of 0.47 °C in February 2016. In this case, we obtained p-values of 0.115 for CMIP6, 0.078 for MPI-GE and 0.141 for CESM2-LE. The margin observed in February was therefore about an order of magnitude more likely than the one observed in September, and thus more likely to be due to internal variability alone. However, it is worth noting that in February 2016, super El Niño had just peaked, when its impact on global temperature was near maximum. This is not the case for September 2023. [Exactly what I’ve been saying for months!]

The most plausible explanation for the model-observation discrepancy in September 2023 would be that the observed combination of forced warming and internal variability is so rare that it does not occur in the models. The strengthening El Niño following a triple La Niña event observed in 2020–2022 has occurred only a few times since 1950, and not earlier in the 21st century⁶. However, large ensemble models are designed to capture such rare climate anomalies, and we found that no member in MPI-GE and only one member in CESM2-LE simulated temperature jumps as large as the one observed in September 2023.

It wasn’t Hausfather’s ‘weird El Nino’ or simply because El Nino followed hot on the heels of a triple dip La Nina either. No, these explanations just don’t cut it. So, what does cut it?

Probably not solar forcing:

It is also worth noting that increased solar activity may have contributed to the record margin in September 2023. However, solar forcing is included in CMIP6 models⁷, so while it may have added a few hundredths of a degree to the record margin, it is unlikely that increased solar activity contributed to the model-observation discrepancy, although the solar cycle 25 may have risen slightly faster than the estimate prescribed in the scenario.

That leaves:

Since the state-of-the-art climate models cannot generally reproduce the observed margin, we argue that it is highly unlikely (p ~ 1%) that internal climate variability alone would have caused the large increase in global mean temperature in September 2023. It is therefore likely that other external forcings such as (1) the Raikoke and Hunga Tonga volcanic eruptions^8,9 and (2) the removal of sulphur pollution from ships¹⁰ have contributed to the observed temperature anomaly.

Blimey. Eliminate the disappearing ship aerosol emissions explanation and they might actually get there! But they are still underestimating the magnitude of the effect. Either that, or an additional driver of extreme temperature in 2023 (north Atlantic surface temperatures?) which the authors have not accounted for is jointly responsible for the extraordinary warmth.

The combined effects of the volcanic eruptions and the reduction of sulphate emissions from global shipping may plausibly have caused a temperature increase of 0.07–0.15 °C between September 2020 and 2023.

This is all rather worrying though. Why has it taken establishment science months to catch up with what was bleedin’ obvious (to me at least) from the word go? We’ve had at least 6 months of professional scientists (on both sides of the fence) plus various trusted commentators in the media and alt media, plus the ridiculous Met Office blathering about El Nino and/or ‘accelerating’ global warming. Enough of the nonsense.