2023: The year that broke climate science

The past two years have been quite abnormally warm – a lot warmer than scientists expected. They had thought that 2023 would be a warm year, though not a record breaker. What happened in 2023 to make it the warmest year by a huge margin lies far outside the expectations of the models and forecasts. The only other year that climate models erred so spectacularly was 1992, which was understandable as Mount Pinatubo erupted and put a huge amount of sulphur dioxide in the stratosphere, cooling the planet. Scientists cannot explain 2023, and to compound matters 2024 was even warmer – the first year above 1.5 degrees globally.

They have identified many possible effects. One is that El Niño is behaving abnormally, citing as evidence that we've been in an unusually extended La Niña period since the end of 2020, with a triple-dip La Niña between 2020 and the start of 2023. Because we had such a long La Niña perhaps a more extreme heat outpouring occurred in 2023?

That’s not all. There has been a big change in the sulphur content of shipping fuels following a 2020 decision by the International Maritime Organization mandating an 80% reduction, thereby decreasing its sunlight-reflecting effect. Several studies find it resulted in a moderate reduction of only about 0.05°C. Then there is the 2022 eruption of the Hunga Tonga volcano, which was unusual because it occurred underwater and deposited a lot of water vapour in the upper atmosphere in addition to sulphur dioxide. Water vapour warms the planet, but sulphur has the opposite effect, and scientists don’t know how they balance out, but think it’s about 0.02°C – another small effect.

And then there is the Sun, so often dismissed by climate scientists.  The current solar cycle has been stronger than predicted, so this might account for a couple of hundredths of a degree.

Strangely, some climate scientists think that if you juggle all these uncertain factors, and add in some judiciously chosen level of natural climatic variation, then 2024 can be explained…just. I don’t think so. This approach is inadequate and smacks of wishful thinking. As for 2023, that’s still inexplicable. 

Recently a paper in Geophysical Research Letters has suggested that 2023 was not such an unusual year, being quoted as a 1-in-6,000 year, even if due to chance, thereby emphasising its human influence. Scientists from the US, Spain and Italy put forward an argument that it could have happened by chance once every 75 years or so.

Changing cloud cover might also have played a part. Clouds have always been a problem for climate models. Do they warm or cool the atmosphere, and what is the effect of cloud altitude on these factors? Observationally there has been a significant decline in the reflectivity of clouds over the last 10 years or so. No one knows why or how significant it is, as we only have satellite cloud data going back to 1970.

Should we be concerned about our climatic ignorance? The argument is often made that we should have confidence because the basics of climate science are robust; we’ve been aware that CO2 is a greenhouse gas since the 1870s.

True, but the Earth’s climate system is not simple and, as all scientists know, basic effects combine to produce complex systems. Whilst, for example, the radiative transfer components in the computer code of climate models may be relatively straightforward, their effects when manifested in the real world can be anything but. Studying what has happened to global temperatures since 2023 is showing just that.

Uncertainties in climate science are growing. In the past some scientists have painted a too-capable picture of it when it was challenged. But as the data increasingly shows us, reality is not as understandable as we thought. A guileless defence of it, as happened during the so-called “blog wars” of a decade ago, will not serve us well.