According to a report, people's health is being damaged today by climate change effects ranging from deadly heatwaves in Europe to rising dengue fever in tropics.
It has said that billions of hours of farm work have been lost during high temperatures and global warming has damaged the ability to grow crops.
The Lancet Countdown on Health and Climate Change was produced by 150 experts from 27 universities and institutions including the World Health Organization and the World Bank.
"The findings are clear and the stakes could not be higher," said Tedros Adhanom Ghebreyesus, WHO Director-General. "We can not delay action on climate change. We can not sleep over this health emergency any longer. "
The report sets out the impacts of global warming on health in stark terms. "A rapidly changing climate has dire implications for every aspect of human life, exposing vulnerable populations to extremes of weather, altering patterns of infectious disease, and compromising food security, safe drinking water and clean air," it said.
Nick Watt, executive director of the Lancet Countdown, said: "These are not things happening in 2050, but are things we already see today. We think of these as the canary in ironically the coalmine. "
On Tuesday, the UN said the move to reduce carbon emissions must be tripled to avoid catastrophic warming. International climate change negotiations were due to resume on Monday in Poland.
The Lancet report said the lack of progress "threatens both human lives and the viability of the national health systems they depend on, with the potential to overwhelm health services."
A survey in the report of the leaders of nearly 500 global cities found half expected of their public health infrastructure to be seriously compromised by climate change, meaning systemic failures like shutdown of hospitals.
A warmwave in Europe this summer was associated with hundreds of premature deaths in the UK alone. The MPs said in July that the UK was "woefully unprepared" for heatwaves.
The Lancet report states that populations in Europe and the Eastern Mediterranean are at higher risk than those in Africa and South-East Asia, due to the high proportion of vulnerable and elderly people living in cities.
The report says 157 million more vulnerable people were exposed to heatwaves in 2017 than in 2000. Hot conditions directly damage the health through heat stroke, but dehydration and exacerbating conditions such as heart disease is also very dangerous. Heat also worsens air pollution and mental health problems.
Prof Kristie Ebi, of the University of Washington, said: "Increased mortality in extreme heatwaves is happening now [but] There is abundant evidence that communities are not prepared for the ongoing heatwave frequency, intensity and duration. "
The Lancet report said 153bn hours of work were lost in 2017 due to extreme heat, 80% of it in agriculture. Almost half of the losses were in India, equivalent to 7% of its total working population, while China lost its equivalent of 1.4% of its employees. "This has led to huge losses for national economies and household budgets," said Prof Joacim Rocklöv of Umeå University in Sweden.
Relatively small changes in temperature and rainfall can cause large changes in the transmission of infectious diseases spreading through water and mosquitoes. The ability of the dengue fever virus to be transmitted – its "vectorial capacity" – reached a record high in 2016, according to the report, 10% above the 1950s baseline. The risk of cholera risk was also rising in regions such as the Baltic states where the sea was rapidly warming up.
Doctors not involved in the report said they presented convincing evidence. "It is clear that climate change is directly affecting our health," said Howard Frumkin, head of the Wellcome Trust's Our Planet, Our Health Program. "All sectors must prioritize action on climate change if we are to significantly reduce potentially devastating effects on our planet and our health, affecting generations to come."
Prof. Paul Ekins, of University College London, said that the health benefits of tackling climate change have long been underestimated, with just 5% of funding for adaptation to global warming being spent on health.
"These benefits are enormous, near-term and affect our health immediately," Ekins said. "If you factor in these benefits, cutting emissions to [keep the temperature rise below] 1.5C is going to be a net benefit to humanity in monetary terms. "
The Lancet report noted some promising trends, such as the phasing out of coal and the growth of electric cars.
Prof. Hilary Graham, of the University of York and part of the Lancet Countdown team, said that linking health and climate change could help spur further action. "Health is what people feel." It makes a direct connection with their lives and the lives of people they care about like their children and grandchildren. "