The Earth may end up being twice as warm as projected by climate models, even if the world meets the target of limiting global warming to under two degrees Celsius, a study has found.
The study, published in the journal Nature Geoscience, showed that sea levels may rise six metres or more even if Paris climate goals are met.
The findings are based on observational evidence from three warm periods over the past 3.5 million years when the world was 0.5-2 degree Celsius warmer than the pre-industrial temperatures of the 19th Century.
The research also revealed how large areas of the polar ice caps could collapse and significant changes to ecosystems could see the Sahara Desert become green and the edges of tropical forests turn into fire dominated savanna.
"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," said Hubertus Fischer from the University of Bern in Switzerland.
"This suggests the carbon budget to avoid 2 degree Celsius of global warming may be far smaller than estimated, leaving a very little margin for error to meet the Paris targets," said Fischer.
To get their results, the researchers looked at three of the best-documented warm periods, the Holocene thermal maximum (5,000-9,000 years ago), the last interglacial (129,000-116,000 years ago) and the mid-Pliocene warm period (3.3-3 million years ago).
The warming of the first two periods was caused by predictable changes in the Earth's orbit, while the mid-Pliocene event was the result of atmospheric carbon dioxide concentrations that were 350-450ppm - much the same as today.
Combining a wide range of measurements from ice cores, sediment layers, fossil records, dating using atomic isotopes and a host of other established paleoclimate methods, the researchers pieced together the impact of these climatic changes.
In combination, these periods give strong evidence of how a warmer Earth would appear once the climate had stabilised. By contrast, today our planet is warming much faster than any of these periods as human-caused carbon dioxide emissions continue to grow.
Even if our emissions stopped today, it would take centuries to millennia to reach equilibrium.
The changes to the Earth under these past conditions were profound - there were substantial retreats of the Antarctic and Greenland ice sheets and as a consequence sea-levels rose by at least six metres; marine plankton ranges shifted reorganising entire marine ecosystems; the Sahara became greener and forest species shifted 200 km towards the poles, as did tundra; high altitude species declined, temperate tropical forests were reduced and in Mediterranean areas fire-maintained vegetation dominated.
"Even with just 2 degree Celsius of warming - and potentially just 1.5 degree Celsius - significant impacts on the Earth system are profound," said Alan Mix of Oregon State University in the US.
"We can expect that sea-level rise could become unstoppable for millennia, impacting much of the world's population, infrastructure and economic activity," said Mix.