PARIS - UN scientists are set to deliver their darkest report yet on the impacts of climate change, pointing to a future stalked by floods, drought, conflict and economic damage if carbon emissions go untamed.
A draft of their report, seen by AFP, is part of a massive overview by the Intergovernmental Panel on Climate Change (IPCC), likely to shape policies and climate talks for years to come.
Scientists and government representatives will meet in Yokohama, Japan, from Tuesday to hammer out a 29-page summary. It will be unveiled with the full report on March 31.
"We have a lot clearer picture of impacts and their consequences... including the implications for security," said Chris Field of the United States' Carnegie Institution, who headed the probe.
The work comes six months after the first volume in the long-awaited Fifth Assessment Report declared scientists were more certain than ever that humans caused global warming.
It predicted global temperatures would rise 0.3-4.8 degrees Celsius (0.5-8.6 degrees Fahrenheit) this century, adding to roughly 0.7 C since the Industrial Revolution. Seas will creep up by 26-82 centimetres (10.4-32.8 inches) by 2100.
The draft warns costs will spiral with each additional degree, although it is hard to forecast by how much.
Warming of 2.5 C over pre-industrial times -- 0.5 C more than the UN's target -- may cost 0.2-2.0 per cent of global annual income, a figure that could amount to hundreds of billions of dollars each year.
"The assessments that we can do at the moment probably still underestimate the actual impacts of future climate change," said Jacob Schewe of the Potsdam Institute for Climate Impact Research (PIK) in Germany, who was not involved in the IPCC drafting.
Many scientists concurred, he said, that recent heatwaves and floods were evidence of climate change already on the march -- and a harbinger of a future in which once-freakish weather events become much less rare.
Among the perils listed in the draft are these:
FLOODING: Rising greenhouse-gas emissions will "significantly" boost the risk of floods, with Europe and Asia particularly exposed. In the highest warming scenarios of untamed greenhouse gas emissions, three times as many people will be exposed to severe river flooding as with lower warming.
DROUGHT: For every 1 C (1.8 F) rise in temperature, another seven per cent of the world's population will see renewable water resources decline by a fifth.
RISING SEAS: If no measures are taken, "hundreds of millions" of coastal dwellers will be displaced by 2100. Small-island states and East, Southeast and South Asia will be the biggest land-losers.
HUNGER: Average yields of wheat, rice and corn may fall by two per cent per decade, while demand for crops is likely to rise by up to 14 per cent by 2050 as Earth's population grows. The crunch will hit poor, tropical countries worst.
SPECIES LOSS: A "large fraction" of land and freshwater species may risk extinction, their habitat destroyed by climate change.