Global warming will disrupt food supplies, slow world economic growth and may already be causing irreversible damage to nature, according to a UN report due this week that will put pressure on governments to act.
A 29-page draft by the Intergovernmental Panel on Climate Change (IPCC) will also outline many ways to adapt to rising temperatures, more heatwaves, floods and rising seas.
"The scientific reasoning for reducing emissions and adapting to climate change is becoming far more compelling," Rajendra Pachauri, chair of the IPCC, told Reuters in Beijing.
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Scientists and more than 100 governments will meet in Japan from March 25-29 to edit and approve the report. It will guide policies in the run-up to a UN summit in Paris in 2015 meant to decide a deal to curb rising greenhouse gas emissions.
The 29-page draft projects risks such as food and water shortages and extinctions of animals and plants. Crop yields would range from unchanged to a fall of up to two per cent a decade, compared to a world without warming, it says.
And some natural systems may face risks of "abrupt or drastic changes" that could mean irreversible shifts, such as a runaway melt of Greenland or a drying of the Amazon rainforest.
It said there were "early warning signs that both coral reef and Arctic systems are already experiencing irreversible regime shifts". Corals are at risk in warmer seas and the Arctic region is thawing fast.
Climate change will hit growth. Warming of 2.5 C above pre-industrial levels could mean "global aggregate economic losses between 0.2 and 2.0 per cent of income", it says.
Almost 200 governments have agreed to limit warming to less than 2 C above pre-industrial times, mainly by curbing emissions from burning fossil fuels.
Temperatures have already risen by about 0.8 C.
"A wide range of impacts from climate change are already happening," said Chris Field of Stanford University and a co-chair of the IPCC report. "Risks are much greater with more warming than less warming."
"And it doesn't require 100 percent certainty before you have creative options for moving forwards ... there are compelling adaptation options," he told Reuters by telephone.
The report points to options such as improved planning for disasters such as hurricanes or flooding, efforts to breed drought- or flood-resistant crops, measures to save water and energy or wider use of insurance.
Field said the IPCC will have to take account of thousands of comments since the draft was leaked to a climate sceptic's website late last year.
And the findings will be under scrutiny, especially after the previous IPCC assessment in 2007 wrongly projected that Himalayan glaciers might all melt by 2035, affecting water supplies for millions of people from China to India.
This time, a sub-chapter projects Himalayan ice will range from a 2 percent gain to a 29 percent loss by 2035. "It is virtually certain that these projections are more reliable than an earlier erroneous assessment," it says.
The study is the second part of a mammoth three-part report.
The first, in September, raised the probability that human activities, rather than natural variations, are the main cause of warming since 1950 to at least 95 per cent from 90 in 2007.
But many people in big emitting nations are unconvinced.
Only 40 per cent of Americans and 39 per cent of Chinese view climate change as a major threat, according to a Pew Research Center survey of 39 nations in 2013.
A third instalment will show solutions to climate change such as more renewable energy.