The world is on course for a "catastrophic" temperature rise this century, the United Nations said Thursday as it confirmed that 2020 rivalled 2016 as the hottest year on record.
The relentless pace of climate change is destroying lives, said Secretary-General Antonio Guterres as the UN's World Meteorological Organization said 2011-2020 had been the warmest decade recorded.
The UN weather agency said the warmest three years on record were 2016, 2019 and 2020, and the differences between them in average global temperatures were
It said the average global temperature in 2020 was about 14.9 degrees Celsius -- a figure 1.2 C above the pre-industrial (1850-1900) level.
The 2015 Paris Agreement on climate change calls for capping global warming at well below 2 C above the pre-industrial level, while countries will pursue efforts to limit the increase to 1.5 C.
The WMO believes there is at least a one in five chance of the average global temperature temporarily exceeding the 1.5 C mark by 2024.
"The confirmation by the WMO that 2020 was one of the warmest years on record is yet another stark reminder of the relentless pace of climate change, which is destroying lives and livelihoods across our planet," said UN chief Guterres.
"We are headed for a catastrophic temperature rise of 3-5 C this century. Making peace with nature is the defining task of the 21st century. It must be the top priority."
Forests and other land ecosystems today absorb 30 percent of humanity's CO2 pollution, but rapid global warming could transform these natural 'sinks' into carbon 'sources' within a few decades, opening another daunting front in the fight against climate change, alarmed researchers have said.
Climate sceptics often describe CO2 as "plant food", suggesting that increased greenhouse gas emissions will be offset by a massive upsurge in plant growth.
But the new study shows that beyond a certain temperature threshold -- which varies according to region and species -- the capacity of plants to absorb CO2 declines.
Under current greenhouse gas emission trends, plants across half the globe's terrestrial ecosystem could start to release carbon into the atmosphere faster than they sequester it by the end of the century, researchers reported this week in Science Advances.
Ecosystems that store the most CO2 -- especially tropical and boreal forests -- could lose more than 45 percent of their capacity as carbon sponges by mid-century, a team led by Katharyn Duffy from Northern Arizona University found.
"Anticipated higher temperatures associated with elevated CO2 could degrade land carbon uptake," said the study. Failure to take this into account leads to a "gross overestimation" of the role Earth's vegetation might play in reducing global warming, the researchers warned.