Extreme heat is threatening the world’s food systems, with farmers unable to work outside, livestock experiencing stress and crop yields falling, putting the livelihoods of more than a billion people in peril, the UN has warned.

Experts said food supply in some areas was being “pushed to the brink” by increasingly common and severe heatwaves, on land and at sea, in a major report written jointly by the Food and Agriculture Organization (FAO) and the World Meteorological Organization (WMO).

Farmers could find it impossible to work safely for as many as 250 days of the year – more than two-thirds of the time – in already hot regions including much of India and south Asia, tropical sub-Saharan Africa and swathes of Central and South America.

Livestock are already experiencing an increase in mortality rates, as heat stress begins for common species at about 25C. Extreme heat reduces yields from dairy cows and cuts the fat and protein content of milk. Pigs and chickens are unable to sweat and, as temperatures rise, face digestive tract breakdowns, organ failure and cardiovascular shock.

Yields begin to decline at temperatures above 30C for most agricultural crops, with damage including weakened cell walls and the production of toxins. The yields of maize in some areas have declined by about 10%. Wheat has fallen by nearly as much, and is projected to decline further as temperatures rise to more than 1.5C above preindustrial levels.

Ocean heatwaves are also killing fish, as heat reduces the level of dissolved oxygen in the water, leading to mass decline in populations.

Much more could be done to warn farmers, as heatwaves are often predictable, according to the report published on Wednesday. Weather forecasts and mobile phone communications could be used to alert farmers when extreme weather is expected.

Richard Waite, the director of agriculture initiatives at the World Resources Institute thinktank, who was not involved in the report, said it was crucial to start adapting to rising temperatures now, by giving farmers the tools, knowhow and early warnings to help them anticipate and protect against extreme weather.

“Without adaptation, extreme heat will cut crop and livestock yields, forcing more land into agriculture to maintain food production. That would drive even higher emissions from land use change, which in turn would make climate impacts on agriculture even worse,” he said. “What’s needed is the opposite: scaling solutions that help farmers maintain and sustainably increase productivity, even in a changing climate, so we can break that vicious cycle rather than reinforce it.”

Morgan Ody, a small-scale farmer and the general coordinator of La Via Campesina, a global organisation of food and land workers and small farmers, said the lives of working people were increasingly at risk. “Farmers, agricultural workers and small-scale fisherfolk – especially women and elderly people among them – whose livelihoods depend on daily work in fields, rivers and oceans, are highly vulnerable to extreme heat, which also threatens their health and lives. These extreme weather events are driven in large part by industrial monocultures and livestock systems that emit large amounts of greenhouse gases,” he said.

Ody called for compensation for such workers for the losses they experience from extreme weather, debt relief and public investment in adaptive measures, as well as rules on worker safety that would limit how long workers in fields and on boats could be exposed to high temperatures and force employers to provide shade, rest and water. In the longer term, he called for the replacement of intensive farming with more nature-friendly methods.

Modern industrialised food systems rely on a narrow range of staple crops, and highly specialised systems that are dependent on inputs such as fertiliser. That makes them highly vulnerable and less able to cope with shocks, such as extreme heat, according to Molly Anderson, professor of food studies at Middlebury College in Vermont, who was not involved with the report.

Anderson called for the development of a more diverse food system, better equipped to withstand shocks, and a reversal of trends in intensive agriculture that have robbed farms of trees, shade and mixtures of crops and livestock.

She said: “The risk of simultaneous crop failures from extreme heat could ripple through food prices, supply chains and economies. Adaptation has limits – the only durable response is to tackle fossil fuels, accelerate the shift to renewable energy, and invest massively in adaptation.”

Tim Lang, emeritus professor of food policy at the University of London, said though the worst effects would be felt in already hot countries, temperate regions and developed countries could not ignore the impacts. “The acceleration of climate uncertainties poses dire challenges for food growers worldwide,” he said.

“The British Isles are not immune to the effects. Places we’ve got food from will dry up. Land use here will be changed. Water dependencies are exposed. Crops that started off well will fail to thrive. Productivity will be disrupted. Regular patterns of growing and consuming will be forced to change. Anyone who thinks climate change won’t affect us should think again.”