The physical and mental health of more than 12 million people in the UK is under threat from the growing impacts of climate change, including blistering heatwaves and devastating flooding, a major new study today warns. The report – published by the Climate Coalition alongside experts that the Priestly International Centre for Climate – estimates a proportion of the population equivalent to the number of people living in London and Manchester combined could be vulnerable to intensifying weather events made more likely by the changing climate.

Last year was Britain’s third-hottest on record, causing some 2,500 heat-related deaths – a number that could double by the 2050s unless action is taken said a report by The Climate Coalition and the Priestley International Centre for Climate. Failure to act swiftly on climate change “will spell disaster not only for our natural world but for public health”, said Clara Goldsmith, campaigns director at The Climate Coalition, which includes more than 140 organisations.

Britain faces a particular health risk from heatwaves due to its large elderly and vulnerable population, said the report, noting that heat-related deaths among the over-65s had risen 21% between 2004 and 2018. The report said flooding related to extreme weather was an increased risk for mental health, citing research that found one in three people reported having post-traumatic stress disorder (PTSD) after their homes flooded. But climate change adaptation could also reap major health benefits, from reducing the number of early deaths by encouraging more active means of transport such as cycling to improving air quality by fostering renewable energy sources. “Our mental and physical health are clearly linked to the health of the one place we all call home: our planet,” Tanya Steele, chief executive at World Wildlife Fund, a member of The Climate Coalition, said in a written statement. “To show true global leadership at this year’s climate summit, the UK government must take more ambitious steps to reach our net-zero targets,” she said, referring to upcoming U.N. climate talks in Glasgow later this year.