Temperature rises as a result of global warming could eventually be double what has been projected by climate models, according to an international team of researchers from 17 countries.
Sea levels could also rise by six metres or more even if the world does meet the 2 degree target of the Paris accord.
The findings, published last week in Nature Geoscience, were based on observations of evidence from three warm periods in the past 3.5m years in which global temperatures were 0.5-2 degrees above the pre-industrial temperatures of the 19th century.
The researchers say they increase the urgency with which countries need to address their emissions.
The scientists used a range of measurements to piece together the impacts of past climatic changes to examine how a warmer earth would appear once the climate has stabilised.
They found sustained warming of one to two degrees had been accompanied by substantial reductions of the Greenland and Antarctic ice sheets and sea level rises of at least six metres – several metres higher than what current climate models predict could occur by 2100.
“During that time, the temperatures were much warmer than what our models are predicting and the sea levels were much higher,” said Katrin Meissner from the University of New South Wales’s Climate Change Research Centre and one of the study’s lead authors.
She said the effects today would mean populous urban areas around the world and entire countries in the Pacific would be underwater.
“Two degrees can seem very benign when you see it on paper but the consequences are quite bad and ecosystems change dramatically.”
Meissner said potential changes even at two degrees of warming were underestimated in climate models that focused on the near term.
“Climate models appear to be trustworthy for small changes, such as for low-emission scenarios over short periods, say over the next few decades out to 2100,” she said. “But as the change gets larger or more persistent ... it appears they underestimate climate change.”
The researchers looked at three documented warm periods, the Holocene thermal maximum, which occurred 5,000 to 9,000 years ago, the last interglacial, which occurred 116,000 to 129,000 years ago, and the mid-Pliocene warm period, which occurred 3m to 3.3 m years ago.
In the case of the first two periods examined, the climate changes were caused by changes in the earth’s orbit. The mid-Pliocene event was the result of atmospheric carbon dioxide concentrations that were at similar levels to what they are today.
In each case, the planet had warmed at a much slower rate than it is warming at today as a result of rising greenhouse gas emissions caused by humans.
“Observations of past warming periods suggest that a number of amplifying mechanisms, which are poorly represented in climate models, increase long-term warming beyond climate model projections,” Prof Hubertus Fischer of the University of Bern, one of the study’s lead authors.
“This suggests the carbon budget to avoid 2°C of global warming may be far smaller than estimated, leaving very little margin for error to meet the Paris targets.”