Climate change is set to push up food prices, increase hunger and reduce the quality of food, campaigners warn ahead of the publication of a key scientific review of global warming.
With one in eight people in the world already going hungry, the pressure of rising temperatures and extreme weather events on production means more people will not be able to afford enough to eat, Oxfam warns.
The number of people at risk of hunger could rise by 10 to 20 per cent by 2050 compared to a world where the climate was not changing, undoing efforts to tackle the problem, the aid agency says.
Oxfam has previously estimated that the average price of staple foods is likely to more than double in the next 20 years, compared to 2010 prices, with up to half the increase down to climate change.
In a report published ahead of a major global assessment of climate change science that is due out on Friday, Oxfam says yields are already being hit by global warming and in some of the poorest parts of the world could be reduced by 10 to 20 per cent by 2050.
The aid agency points to research which found that global yields of maize over the past three decades were 3.8 per cent below what they would have been without climate change and wheat was 5.5 per cent lower than it would otherwise be.
Food production would be negatively affected in most parts of the world by rising temperatures and shifting weather patterns, while extremes such as droughts and floods will damage crops, kill livestock and affect distribution of food.
Recent agricultural disasters, such as the 2012 US drought in the Midwest which reduced maize crops by a quarter and contributed to global prices rising by 40 per cent, are likely to become more frequent, it says.
In the UK, 2012 was the second wettest year on record, and the wettest ever in England, causing UK wheat yields to fall to their lowest levels in 20 years, and the country had to import 2.5 million tonnes of wheat, the report says.
Extreme heat is also likely to reduce nutrients in crops, affect livestock growth and productivity and increase crop contamination by poisonous fungal mycotoxins, the report warns.
In a separate report, Unicef UK warns that two million children already die a year because they could not get enough to eat and a further 165 million suffered from malnutrition, a situation which is going to worsen due to climate change.
There are also likely to be indirect problems caused by climate change hitting food production, such as the loss of agricultural income in poor areas, which could lead to children being taken out of school and increased migration which puts them at risk of exploitation.
Unicef also estimates that for every year for the next decade, 175 million children will be affected by sudden climate-related disasters.
"Just as the evidence of man-made climate change is becoming stronger, so too is our understanding of how it hits people, especially around hunger," Tim Gore, head of policy for Oxfam's Grow campaign, said.
"We've long known that climate change will mean lost crops, but increasingly we're seeing its impacts through higher food prices, lower earnings, more health problems and lower quality food too."
"We want a world in which everyone enjoys the right to enough affordable and nutritious food and we cannot allow climate change to throw us off course.
"Leaders listening to the latest findings from climate scientists this week must remember that a hot world is a hungry world.
"They must take urgent action to slash emissions and direct more resources to building a sustainable food system."
The Intergovernmental Panel on Climate Change is publishing its fifth review of the science of climate change this week, six years after its last report.
The new report from the international body tasked with assessing climate science, which draws on thousands of studies, is expected to show scientists are increasingly sure that humans have caused the majority of warming which has been seen since 1950.
Leaked drafts of the report show they have found warming is unequivocal, and while there has been slowing down of warming in the past 15 years, scientists are confident it is temporary and warming will continue without action to cut emissions.