Mystery solved – that's where the mysterious heat came from

 

Extreme weather
Mystery solved – where the "unexplained" heat came from

Jonathan Jeppsson

Dig manager and climate columnist

This is a commentary text. Analysis and positions are the writer's.

Published 12.35

Quick version

• Scientists have identified a decrease in low-lying clouds as a key factor behind the record-high temperatures in 2023.
• Three possible reasons for the decrease in clouds include natural variations, reduced aerosol emissions and the effects of global warming on cloud formation.
• The decrease in cloud cover could lead to warming exceeding the critical 1.5-degree target earlier than expected, which would require immediate climate action.

The number of low-lying clouds fell sharply last year, according to the researchers. Photo: Alberto Pezzali / AP

For months, scientists have been trying to understand why temperatures reached record highs in 2023.

Now they believe they have found the answer: the clouds disappeared.

Three reasons could be behind it – two of which could be decisive for the future.

It was those 0.2 degrees that climate scientists could not quite understand.

Temperature record have been broken again and again in both 2023 and 2024. Climate change and the weather phenomenon El Niño have been partly to blame – but none of these factors have been able to fully explain the unusually high heat, which has sent average temperatures up around 1.5 degrees above pre-industrial levels

But now a team of researchers at the Alfred Wegener Institute believe they have found an explanation – and it could have great significance for the future.

It is about a decreasing so-called albedo effect, that is, the planet's ability to reflect sunlight back into space, and it is about clouds. The Earth's albedo has been steadily decreasing since the 1970s, mainly due to melting polar ice caps.

According to Helge Goessling and his colleagues at the Alfred Wegener Institute in Germany, a record low albedo effect was measured in 2023. And they also found that the number of low-lying clouds had dropped sharply last year. The change was particularly noticeable over the world's oceans and especially marked over the Atlantic, where many of the most extraordinary temperature anomalies were measured during the year.

But then the question remained: why?

What caused the low-lying clouds to disappear?
 

To put it somewhat simply, one could say that there are three alternative causes, three doors that are left ajar.

Behind the first lies a completely natural explanation, a variation in the climate that is highly temporary. That is the hopeful path – that everything that is happening around us now is the result of forces we cannot influence and that we are not the cause of.

Behind the next door is the explanation that it could be due to a global reduction in aerosols, i.e. sulfur particles, which contribute to cloud formation. It is clear that aerosol emissions have decreased in recent years, due to changes in regulations for marine fuel, among other things.

A third door to open, which is considerably more serious and scary, is that the cause may be that global warming has begun to change how cloud formation behaves.

That alternative is the one we have the most reason to try to understand – because it has profound consequences for the future. If the lack of clouds is due to feedback effects in the climate, the process could accelerate in the coming years and push temperatures higher than we previously thought.

We have every reason to pay extra attention to what we find difficult to predict, even if it may feel unrealistic.

Because cloud formation is truly the blind spot of climate research. Clouds have long been one of the biggest uncertainties when scientists calculate climate effects – they can both shade the Earth and retain heat, and it is difficult to translate small scales into larger events. Cloud dynamics are shrouded in obscurity. 

But warning signs have nevertheless begun to appear in recent years.

NASA scientist Norman Loeb showed, for example, that the sharp increase in global temperatures since 2013 coincided with a decrease in cloud cover over the oceans.

Other researchers have reported fewer low clouds in the tropics in warmer years. In his 2016 study, climate scientist Tapio Schneider discovered something very unpleasant – that climate models that included this connection in their calculations predicted faster global warming.

A few years ago, it made headlines when the same Schneider claimed that global cloud cover could have a ”tipping point”, beyond which cloud formations would simply become unstable, break up – and disappear altogether. (However, in that case, the amount of carbon dioxide in the atmosphere would have to rise to significantly higher levels than today, levels that we are nowhere near).

But if it turns out that it is warming itself that is the cause of the “unexplained” warming, as Helge Goessling at the Alfred Wegener Institute believes, it leaves us with grim news – we should expect even more intense warming in the future.

Temperatures could become permanent above the 1.5-degree mark sooner than expected. At the same time, our measures to adapt to future extreme weather would need to be significantly more far-reaching.

“It is only now that the signal seems to emerge from the noise,” some researchers say in a new study in the journal Science.

The changes are small, but the trend could be huge. “This would indicate a cloud feedback that is out of control,” says Björn Stevens, a climate scientist at the Max Planck Institute.

We have three doors in front of us – two we must avoid entering at all costs.

Now we stand on the threshold: what will be our next step?