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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Report could save Earth
Scientists and more than 100 governments will meet in Japan from 25 to 29 March 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 on 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 2% 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.
Warning signs in coral reefs
It said there were "early warning signs that both coral reefand 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% degrees Celsius above pre-industrial levels could mean "global aggregate economic losses between 0.2% and 2.0% of income", it says.
Almost 200 governments have agreed to limit warming to less than 2.0 degrees Celsius above pre-industrial times, mainly by curbing emissions from burning fossil fuels.
Temperatures have already risen by about 0.8 Celsius (1.4F).
Dying corals: An aerial view of the soon-to-be extinct coral reefs in the Maldives islands. They are threatened by global warming and rising sea levels.
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Compelling adaptation options
"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% 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.
Findings under scrutiny
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.
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Dry land: Cattle graze on grassland that remains dry and brown at the height of the rainy season on February 15, 2014 south of Bakersfield, California. Now in its third straight year of unprecedented drought, California is experiencing its driest year on record, dating back 119 years.
Are humans the only cause?
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% from 90 in 2007.
But many people in big emitting nations are unconvinced.
Only 40% of Americans and 39% of Chinese view climate change as a major threat, according to a Pew Research Centre survey of 39 nations in 2013.
A third instalment, due in Berlin in mid-March, will show solutions to climate change such as more renewable energy.
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