As Europe recovers from a heatwave with temperatures impressive even by Australian standards, there's one big issue on everyone's lips — climate change.

But attributing an event to climate change is a complicated business, especially when the jet stream is involved.

How has the 'omega' jet stream influenced events and was climate change to blame?

First it requires working out what went on.

So, what did go on?

France officially recorded its all-time highest temperature on record, reaching 45.9 degrees Celsius in the southern region of Gallargues-le-Montueux on Friday. The previous record of was 44.1C, dating back to the infamous heatwave of August 2003.

June records fell widely, Germany and the Czech Republic set new temperature records and more than half of Switzerland's stations broke records for the month.

To get the whole picture you need to consider something we don't deal with often in Australia — the jet stream.

The jet stream is a river of air that goes around the upper latitudes of the globe. ( ABC Weather: Kate Doyle )

The northern jet stream is a band of air that flows like a river around the arctic from west to east — roughly keeping cold air above it and warmer air below.

There is a southern jet stream that goes around Antarctica, but it rarely reaches far enough north to affect our weather in Australia as directly as it does in Europe.

"The position of the jet stream has a major influence on the UK and Europe's weather," according to Nicola Maxey, senior press officer from the UK's Met Office.

"The jet stream formed an omega-shaped wave last week which resulted in warm, dry air from Africa being pulled north, pushing temperatures higher than normal."

She said the omega pattern, when the jet stream is shaped like the capital of the Greek letter omega, leads to high pressure at the surface with relatively low pressure to the east and west.

An approximation of the jet stream position last Friday based off of the 250hPa winds. ( ABC Weather: Kate Doyle )

Dr Peter Bissolli from the Deutscher Wetterdienst (Germany's meteorological service) said the omega bend in the jet stream was the primary cause of last week's heatwave.

He said the omega pattern was a combination of three causes:

Warm air flow from the south

Warm air flow from the south High pressure which causes subsidence of air, the descending air warms

High pressure which causes subsidence of air, the descending air warms Long sunshine duration because of high pressure and thus higher solar radiation which warms the earth and the air above.

But for Dr David Salas, head of the Climate Research Group within Meteo-France (France's meteorological service), the heatwave was more about the effects closer to the ground.

"[The heatwave] was triggered by a very warm air mass transported from Sahara and Spain to France by a southerly flow between a low-pressure system over the near-Atlantic and a high over France and Central Europe," Dr Salas said.

"Subsequently, as is often the case, the [already] warm air mass has warmed further due to the very sunny conditions over France.

"Changes in the sinuosity of the jet stream clearly explains cold waves over many regions of the north hemisphere mid-latitudes, but concerning the recent heat wave, I'm not sure the jet-stream plays a role."



Is the jet stream changing?

The jet stream is the focus of ongoing research.

"Some people say that meridional phases of the jet stream occur more frequently with climate change because the poles are warming faster than the tropics," according to Dr Bissolli.

'Meridional' describes north south movement or a wiggly jet stream. 'Zonal' flow describes a relatively straight jet stream.

"The mechanism behind it is not completely understood and it is not proved for a longer time duration," said Dr Bissolli.

As the arctic warms and the jet streams weakens, its flow is expected to become more meridional or 'wiggly'. Adapted Mason 2013. ( ABC Weather: Kate Doyle )

Dr Jennifer Francis, senior scientist at the Woods Hole Research Centre USA, is researching how the jet stream is being affected by climate change.

"Evidence is accumulating that suggests large northward swings in the jet stream, like the one that's is causing this heat wave, will occur more often in connection with a rapidly warming Arctic," Dr Francis said.

The theory is that as the Arctic warms, the difference in temperature above and below the jet stream will lessen, causing the jet stream to slow and its flow to become more wiggly. Similar to how a slow flowing river has wider meanders than a fast one.

This would result in the jet stream swinging north more often to bring unusually high temperatures as well as swinging south to bring unusually cold temperatures.

Dr Salas is less convinced.

"It seems that the jet stream is getting more sinuous in winter. Note however that results are not extremely robust for winter, and I don't know about results for summer," Dr Salas said.

"So I would not focus on the jet stream, just climate change."

Then there is the underlying warming

"Climate change causes a warming. Regardless if there is a relation between climate change and omega patterns or not, it enhances warming situations and record high temperatures are more likely with climate change than without," Dr Bissolli said.

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Ms Maxey said that without a specific attribution study, it was not possible to categorically link this spell of hot weather to man-made climate change.

Climate change attribution studies use highly complex computer models to compare the world that is, with what it would have been without human-induced greenhouse gasses.

These can then say how much more or less likely an event would be with human-induced climate change.

According to Ms Maxey, an attribution study completed after the 2018 heatwave in the UK showed that the probability of a similar event was now 30 times more likely than in 1750, when the atmospheric concentration of greenhouse gases was far lower prior to the industrial revolution.

"UK climate projects show that with little or no mitigation to counteract climate change, the probability of a heatwave event like we saw last summer in the UK increases to around 50 per cent by 2050," Ms Maxey said.

A study after the 2003 heatwave in France, which killed an estimated 15,000 people, suggested it was twice as likely due to human influence. Another suggested that even in a future with limited warming, summers like 2003 would be expected 59 per cent of the time.

Official attribution studies for this event are just commencing.

But if I was in Europe, with the traditionally warmest two months of summer still to go, I wouldn't wait to invest in an energy-efficient fan.