UN:Global warming is expected to make the planet a bit greener by spurring plant growth, but crops may wilt beyond mid-century if temperatures keep rising, according to a draft UN report.
Scientists have long disputed the extent to which higher temperatures might help or hamper plants - and farmers.
Plants absorb carbon dioxide, the main greenhouse gas, as they grow, and release it when they rot.
"Global agricultural production potential is likely to increase with increases in global average temperature up to about three degrees Celsius, but above this it is very likely to decrease," the draft said.
Plants in tropical and dry regions from Africa to Asia are set to suffer from even a small rise in temperatures, risking increased levels of hunger, linked to other threats such as desertification, drought and floods. But some plants in temperate regions, such as parts of Europe or North and South America, could grow more in a slightly warmer world, according to the draft.
A 79-page technical summary, a copy of which was obtained by Reuters,will be released in Brussels on April 6th after a final review as part of a report based on the work of 2,500 scientists to guide governments in combating warming.
The first part of the report, by the Intergovernmental Panel on Climate Change (IPCC), projected as a "best estimate" that temperatures, stoked by emissions led by burning fossil fuels, would rise 1.8 to 4 degrees Celsius this century.
Plants now absorb more carbon than they release, "but this is likely to peak before mid-century and then tend towards a net carbon source before 2100", without accounting for other effects such as deforestation, the draft said.
"In temperate regions, moderate warming benefits cereal crops and pasture yields, but even slight warming decreases yields in seasonally dry and tropical regions," it said.
"Further warming has increasingly negative impacts in all regions," it added. In South America, for instance, rice yields are expected to fall by the 2020s, while soybean yields could rise in temperate zones.
The report warns that warming could worsen water and food shortages in some regions, especially in developing nations least able to cope. Rising sea levels could threaten coasts.
There are also risks that projected changes in extreme climate events could have "significant consequences on food and forestry production, and food insecurity" .
"Growth will probably increase a little bit," said Anders Portin, senior vice-president of the Finnish Forestry Industry Federation. But he said climate change was harmful overall.