Three reasons for the heat - and an unknown factor

 Three reaons for the heat – and a mystery  

Jonathan Jeppsson

Dig manager and climate columnist

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

Updated 19.05 | Published 19.01

 

These days, Southern Europe is hit by the first extreme heat of the year. Photo: Yorgos Karahalis / AP

A heat wave is now spreading across southern Europe - there is a risk that Paris will experience 40 degrees next week.

But what is it due to?

Here are three reasons for the heat - and an unknown factor. 

Quick version

• The heat wave in southern Europe is caused by heat domes where warm air is trapped under high pressure, which may have been caused by a weaker jet stream affected by the warming of the Arctic.
• Sea temperatures in the Mediterranean are unusually high, amplifying the warming effect and intensifying the heat wave in the region.
• Reduced aerosols in the atmosphere, especially sulfur particles, have led to unexpectedly high temperatures in western and central Europe, as climate models underestimated their effect.

1. Heat domes

So-called “heat domes” are a weather phenomenon that occurs when warm air is trapped under a high pressure that becomes trapped, and this is what is happening in in southern Europe right now.

The reason for this, according to some scientists, is that the jet stream, which drives our weather systems, has become weaker and more wavy. This creates pockets where heat, cold and rainy weather can get stuck and can stay there for much longer than before.

 

Extreme heat has settled over Spain and large parts of the Mediterranean region. Photo: Metdesk

The jet stream is driven by the temperature difference between the Arctic and lower latitudes – when the Arktic warms up, the difference decreases and the stream also slows down and begins to meander like a river.

During the year, renowned climate scientist Michael Mann published a study showing that there has been a “threefold increase” in such events over the past 70 years – something he believes is closely linked to the increased warming of the Arctic.

“It is entirely possible that this trend will continue towards a fourfold increase and so on, as long as we continue to warm the planet with carbon dioxide emissions,” Mann told The Hill.

 

The Mediterranean Sea is already warmer than normal. The image shows anomalies in the surface temperature, where the darkest parts mean a water temperature of more than 5 degrees above normal. Photo: European Union/Copernicus Marine Service Data

2. Warm seas

As early as May, the Atlantic Ocean off Western Europe and in the western Mediterranean Sea began to store heat, which had already reached unusually high levels.

Data from the EU's weather service Copernicus shows that parts of the Mediterranean Sea are already more than five degrees warmer than usual at this time of year – a very large deviation from normal.

The sea has absorbed the heat for a long time, which is then released back with the potential to amplify the warming effect. According to information in the  Washington Post, the strong heat wave in the Mediterranean Sea – a recurring phenomenon in recent years – is further amplifying the heat in the region.

The image shows the change in average summer temperatures from 1980 to 2022. Photo: Institute For Atmospheric And Climate Science Eth Zurich

3. Aerosols

In large parts of western and central Europe, summer temperatures have risen three times faster times faster than the global average since 1980.

The remarkable thing is that this change came as a complete surprise – the climate models missed it completely. A new studyi published in Nature last year showed this.

According to the report authors, the problem is that the models do not include anything about the effect of reduced emissions of aerosols, i.e. sulfur particles in the atmosphere. Changes to marine fuel regulations, which have led to a drastic drop in sulphur particles, have sparked an intense debate that has raged in recent years – aerosols reflect sunlight and can therefore hide already-existing warming. What are the consequences?

Well, the fact that the temperature for heat waves is constantly underestimated in western and central Europe – both now and in the future. Future heat waves could be significantly worse than climate models are allowed to predict.

 

Severe flooding in Germany. Photo: Anna Tärnhuvud

4. The unknown factor

It has been shown that warming has accelerated over the past decade. But the exact cause of this is still shrouded in mystery. What can be observed is that the Earth's so-called energy imbalance has increased sharply - less and less sunlight is reflected back into space. Is it because the ice sheets have shrunk? Is it due to the reduction in the aerosols just mentioned? Or does it have to do with changes in cloud formation - which would be very ominous - or is it a feedback effect that we have not yet been able to identify?

- Something is missing in the models, but we do not know what it is, says Thorsten Mauritsen, professor at the Department of Meteorology at Stockholm University.

Severe fires hit Los Angeles in December. Photo: Eric Thayer / AP

5. The future

Although the heat is now extreme in Spain, there is no evidence that we have seen the worst – quite the opposite. The number of summer days has increased by 36 in the past50 years and the risk is imminent that this trend will continue. By 2050, the combination of reduced rainfall, heat and drought could be on the way to transforming large parts of Spain's climate into a steppe-like state.

 

Drought follows in the heat's wake. Photo: Björn Larsson Rosvall/TT

Just now the climate issue was an abstraction, perhaps, some claimed, it wasn't even real.

Now it is spreading its tentacles among us.

By the end of the century, there is even a risk that three-quarters of the "Garden of Europe" Spain will resemble a  desert.