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, said in Beijing.
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 U.N. 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 2 percent 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 degrees Celsius above pre-industrial levels could mean "global aggregate economic losses between 0.2 and 2.0 percent 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).
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.