Climate change has caused severe harm to human health since the year 2000 by stoking more heat waves, the spread of some mosquito-borne diseases and under-nutrition as crops fail, scientists say.

Scant action to slow global warming over the past 25 years has jeopardised “human life and livelihoods”, they write in a The Lancet medical journal.

“The human symptoms of climate change are unequivocal and potentially irreversible,” says the report, entitled Lancet Countdown and drawn up by 24 groups, including universities, the World Bank and the World Health Organisation (WHO).

Many governments are now trying to cut their greenhouse gas emissions under the 2015 Paris climate agreement, though US President Donald Trump has weakened the pact by saying the United States, the world’s second biggest greenhouse gas polluter after China, will pull out.

“This (report) is a huge wake-up call,” Christiana Figueres, chair of the Lancet Countdown’s high-level advisory board and the United Nations’ climate chief at the Paris summit, told Reuters. “The impacts of climate change are here and now.”

Among its findings, the report said an additional 125 million vulnerable people had been exposed to heat waves each year from 2000 to 2016, with the elderly especially at risk.

Labour productivity among farm workers fell by 5.3 per cent since the year 2000, mainly because sweltering conditions sapped the strength of workers in nations from India to Brazil.

The report, based on 40 indicators of climate and health, said climate change seemed to be making it easier for mosquitoes to spread dengue fever, which infects up to 100 million people a year.

The number of undernourished people in 30 countries across Africa and Asia rose to 422 million in 2016 from 398 million in 1990, it said.

But despite the overall gloom, Anthony Costello, a director at WHO and co-chair of the Lancet Countdown study, said there were “significant glimmers of hope” in the situation.

The number of weather-related disasters such as hurricanes and floods rose 46 per cent since 2000, but the number of deaths remained stable, suggesting that societies were improving protection measures against environmental catastrophes.

The Lancet Countdown study did not estimate the total number of deaths from climate change. The WHO has previously estimated there could be 250,000 extra deaths a year between 2030 and 2050 because of climate change.

– AAP