British summers are likely to reach temperatures of above 40C on a "regular basis" even if we limit global warming to the internationally recognised target of 1.5C, leading meteorologists have warned.

It comes as the UK is already experiencing increasingly extreme weather, with 2020 the third warmest, fifth wettest and eighth sunniest year on record – the first ever to fall into the top 10 for all three variables.

That makes December 2019 to February 2020 the fifth warmest winter on record, while the temperature last summer was 0.4C above average at 14.8C.

Early August 2020 saw temperatures hit 34C on six consecutive days, with five "tropical nights" where the mercury did not drop below 20C, making it one of the most significant heatwaves to affect southern England in the past 60 years, the report's authors said.

Comparing data from the Central England Temperature series, which dates back to 1772, the research found the early 21st century has been 0.5C to 1C warmer than 1901 to 2000 and 0.5 to 1.5C warmer than 1801 to 1900.

Chief executive of the Royal Meteorological Society, Professor Liz Bentley, said the world was already seeing extreme heat as a result of warming of 1.1C to 1.2C above pre-industrial levels.

"If you take that up by another 0.3C, these [heatwaves] are just going to become much more intense – we're likely to see 40C in the UK although we have never seen those kinds of temperatures [before]," she said.

"As we hit 1.5C of global warming, that's going to not just become something that we see once or twice, it'll start to become something that we see on a much more regular basis."

Mike Kendon, climate scientist at the Met Office and lead author of the report, said the figures indicated a new normal for the UK.

"In seven out of the last 10 years, we've seen temperatures of 34C in the UK compared to seven out of the previous 50 years before that," he said.

"So this is an indication of the fact that our baseline of our climate is changing and what we regard as normal is changing."

Mr Kendon warned human-induced global warming will last "for a very, very long time to come", adding scientists at the Met Office's National Climate Information Centre had been "blown away" by the extreme heat of 49.6C seen on the west coast of Canada in recent weeks.

"An event like that would basically be pretty much impossible without the influence of man-made warming, that's obviously a very severe impact," he said.

As well as being warmer, since 2009 the UK has recorded its wettest February in 2020, its wettest April and June in 2012, the wettest November in 2009 and the wettest December in 2015, out of a series going back to 1862.

Last year logged two of the three wettest days on record – 15 February and 3 October 2020 – from a dataset of 47,000 days.

The 21st century has also seen the UK's wettest and second wettest winters in 2014 and 2016 respectively.

Changes in temperature and rainfall patterns are increasingly impacting the natural world, with first leaf dates in 2020 recorded an average of 10.4 days earlier than the 1999 to 2019 baseline across a range of common British shrub and tree species.

Leaves are also falling earlier, with the end-of-season bare tree dates for 2020 coming 4.3 days earlier than the baseline across the same species, the report said.

Chief executive of the Woodland Trust, Dr Darren Moorcroft, said the research can indicate if interlinked species are potentially becoming "out of sync with each other in the natural environment".

He said it could indicate future breakdowns within food chains, leading to some species struggling to survive.

The world's next major climate conference, COP26, will be held in Glasgow in November.

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