PARIS • Limiting global warming to 2 deg C will not prevent destructive and deadly climate impacts, as once hoped, dozens of experts concluded in a score of scientific studies.
A world that heats up by 2 deg C - long regarded as the temperature ceiling for a climate-safe planet - could see mass displacement due to rising seas, a drop in per capita income, regional shortages of food and fresh water, and the loss of animal and plant species at an accelerated speed.
Poor and emerging countries of Asia, Africa and Latin America will be hit hardest, according to the studies published yesterday in the British Royal Society's Philosophical Transactions.
"We are detecting large changes in climate impacts for a 2C (2 deg C) world, and so should take steps to avoid this," said lead editor Dann Mitchell, an assistant professor at the University of Bristol.
The 197-nation Paris climate treaty, inked in 2015, vows to halt warming at "well under" 2 deg C compared with mid-19th century levels, and "pursue efforts" to cap the rise at 1.5 deg C.
With only 1 deg C of warming so far, the Earth has seen a crescendo of droughts, heatwaves and storms ramped up by rising seas.
Voluntary national pledges made under the Paris pact to cut CO2 emissions, if fulfilled, would yield a 3 deg C warming at best.
The treaty also requires that - by the end of the century - humanity stop adding more greenhouse gases to the atmosphere than oceans and forests can absorb, a threshold known as "net zero emissions".
How fast we get to a 2C world is critical, Dr Mitchell said. "If it only takes a couple of decades, we will be in trouble because we won't have time to adapt to the climate."
Among the conclusions found in the new studies:
•Economic growth: Researchers led by Dr Felix Pretis, an economist at the University of Oxford, predict that 2 deg C of global warming will see gross domestic product per person drop, on average, 13 per cent by 2100. A 2 deg C world will also "show significant negative impact on the rates of economic growth". Under a 1.5 deg C scenario, growth projections "are near indistinguishable from current conditions".
•Rising seas: Under a 2 deg C scenario, oceans rise about 0.5m over the course of the 21st century, but well over 1m by 2300. That would take the ocean hundreds, if not thousands, of years to "fully respond" - bad news for 500 million people living in "highly vulnerable" low-lying deltas, mainly in Asia, along with some 400 million people in coastal cities, many of which are already sinking.
•Food, water stress: Two degrees of warming would spare humanity much misery compared with the current trajectory, but would still lead to increased droughts, flooding, heatwaves and the disruption of weather patterns. Some regions will be hit worse than others, as will countries with rainfall-dependent agriculture. The countries that show "the greatest increase in vulnerability to food insecurity are Oman, India, Bangladesh, Saudi Arabia and Brazil".
"We can still keep temperatures well below 2 degrees," said geosystem science professor Myles Allen at the University of Oxford, a co-author on several of the studies.
But doing so requires that "we start now and reduce emissions steadily to zero in the second half of the century," he added.