It keeps the world on course for its hottest year ever - expected to be 1.4C warmer than before the industrial era.

The new record is just the latest to be shattered this year - with new highs set in June, July and August.

Scientists are pointing the finger primarily at climate change, and warned of worse to come. But they also put it down to a warm weather pattern called El Nino, and natural changes in the weather.

The Copernicus Climate Change Service (C3S) confirmed that, in September, average surface air temperatures globally reached 16.38C (61.484F).

The figure is a significant 0.5C higher than the previous warmest ever September in 2020, and is the biggest jump in temperature seen since at least 1940.

The margin has astonished scientists.

"It's huge," said Professor Ed Hawkins, climate scientist at Reading University. "We shouldn't be breaking records by this amount."

Piers Forster - the interim chair of the government's Climate Change Committee but speaking in his capacity as climate change professor at Leeds University - said variations between months each year are usually quite small.

"Therefore, breaking the previous September record by a whopping 0.5C is crazy and shows something really bizarre is going on in the oceans," he said.

Greenhouse gases have been warming not just the atmosphere but also the deep ocean, especially in the Atlantic, added Prof Forster, and now a change in ocean circulation is "causing some of that heat from the deeper ocean to resurface and bite us".

Dr Samantha Burgess, deputy director of C3S, said the "unprecedented temperatures" for September broke records by "an extraordinary amount".

With two months until the next global climate talks, COP28 in Dubai in December, the "urgency for ambitious climate action has never been more critical", added Dr Burgess.

The sea was hot, too

Today's findings are based on billions of measurements from satellites, ships, aircraft and weather stations around the world.

They confirmed that temperatures on the ocean surface had also soared in September, reaching the second highest ever, behind only August 2023.

Scientists have also been alarmed by the record low sea ice cover in Antarctica, which C3S confirmed has continued into September.

It is fuelling worries that climate change is finally catching up with the continent once thought to be relatively shielded.

Why were September and this summer so hot globally?

Part of the heat has come from a natural switch in the tropical east Pacific around July from a cool El Nina phase to a warm El Nino one, influencing temperatures around the world.

But while a number of factors are at play, including natural variability from year to year, scientists believe the main cause is climate change.

Prof Forster said: "We think it's caused by a combination of factors, principally greenhouse gas emissions being at an all-time high." But he said there may be other things going on "that we don't fully understand yet".

Global temperatures have warmed by around 1.1C since the pre-industrial era due to human activity, according to UN's Intergovernmental Panel on Climate Change.

"What we're seeing this year is there is a bump on top of that," said Prof Hawkins, which could be "due to other factors such as El Nino or other weather patterns".

"But the largest component by far is the fact that we have added so much greenhouse gases in the atmosphere from burning fossil fuels."

Prof Hawkins said month after month of record heat was not necessarily a sign the climate crisis was accelerating, but a taste of the future under continued global warming.

Extreme weather events this summer - like fires in the Mediterranean, floods in New York or heatwaves in the UK - will become "more frequent and sadly become normal", he added.

He said climate change is exacerbating flooding because warmer air can hold more water, meaning rainfall is heavier.

It also makes naturally occurring heatwaves even hotter, and it fuels the hot and dry conditions that allow wildfires to spread rapidly, even if the fires were sparked by something unrelated, he added.

October mini-heatwave

The unseasonably warm weather is set to continue this weekend, with an "Indian summer" predicted by one forecaster.

The maximum and minimum average temperatures for the UK in October are 13.8C and 6.4C respectively - and 14.7C and 7.5C for southern England.

The Met Office said temperatures could hit highs of 27C (80.6F) this weekend as "unseasonably warm air" moves across the UK from the south - although it does not expect the record of 29.9C in Gravesend, Kent, in 2011, to be broken.

The forecaster said: "For many further south it will be a fine, dry day on Saturday with temperatures getting to highs of around 25C, which is unusually warm for the time of year.

"By Sunday, it'll be a more widely dry and sunny day for many. Highs or around 26C or even 27C in isolated spots is possible."

The Met Office long-range forecast for October states: "Temperatures are expected to be close to or slightly above average through the period.

"Some more dry, settled periods, are likely until the middle of the month.

"It is expected that temperatures will generally be above average for October."

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