Met Office: last month tied for UK's hottest September on record - with climate change a key culprit

The record-breakingly hot month would have been 'virtually impossible' without human-induced climate change
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Last month has tied for first place as the UK's warmest September on record - after a climate change-driven heatwave saw a warmer than usual start to autumn.

The average temperature across the UK in September 2023 was 15.2C, the Met Office has announced, putting it at the same level as 2006’s record-breaking figure - with records going back all the way to 1884. This comes right on the back of the world's hottest summer on record.

England and Wales also both had their respective warmest Septembers on record, with England’s provisional average of 16.7C topping its previous record of 16.5C in 2006. Wales’ 15.6C also beat its 2006 figure of 15.2C.

For Northern Ireland, September 2023 was the joint-warmest on record with an average mean temperature of 14.2C, equal to both 2006 and 2021’s figures. Scotland provisionally had its third warmest September on record, with an average mean temperature of 12.8C. Only 2021 and 2006 were warmer for the country.

A cyclist rides along a past at Hackney Marshes in north east London during the September heatwave (Photo by JUSTIN TALLIS/AFP via Getty Images)A cyclist rides along a past at Hackney Marshes in north east London during the September heatwave (Photo by JUSTIN TALLIS/AFP via Getty Images)
A cyclist rides along a past at Hackney Marshes in north east London during the September heatwave (Photo by JUSTIN TALLIS/AFP via Getty Images)

Met Office science manager Mark McCarthy said September’s temperature records were heavily driven by how significantly warm the first half of the month was - with much of the UK experiencing days of heatwave conditions.

"Not only did September have the hottest day of the year – something that has only happened on four previous occasions in our observations – but it also had seven consecutive days where temperatures were above 30C somewhere in the UK, which had never happened in this month in Met Office observations," he said.

September's warm start was influenced by high pressure across Europe, which helped to draw warmer air over the UK at the start of the month, Mr McCarthy said. Once this subsided, there were periods of winds and rain, as well as Storm Agnes later in the month, "yet often a south westerly flow resulting in above average temperatures but also humid conditions and plenty of rain".

According to a Met Office rapid attribution study, which looks at the likelihood of temperature thresholds being reached with and without manmade greenhouse gas emissions, this September's average temperature would be virtually impossible without human-induced climate change.

With global temperatures rising by around 1.1C since the industrial revolution, there’s now a 3% chance in a given year of the mean September temperature reaching or exceeding 15.2C in the UK.

Met Office senior scientist Jennifer Pirret, part of the weather and climate extremes impact Team, said: “September 2023’s temperature was substantially influenced by climate change and our attribution study shows how this figure would have been practically impossible in a climate without human-induced greenhouse gas emissions. 

“While a 3% chance of these September temperatures occurring in the current climate might seem like a small amount, in a pre-industrial climate these temperatures would have been nearly impossible for the UK in September," she said.

"It shows how climate change is moving the dial on mean temperatures.  This September’s temperatures are still unlikely and needed the right combination of large-scale conditions and weather patterns, but climate change has helped to make it possible."

As global temperatures continue to rise as a result of human greenhouse gas emissions, Ms Pirret said we could expect these chances to increase over the coming decades, "though the natural year-to-year variability of the UK climate will obviously continue to play a role.”

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