The Earth could be twice as warm as projected by climate models, even if the world reaches the goal of limiting global warming to below two degrees Celsius, a study has found.
The study, published in the journal Nature Geoscience, showed that sea levels can rise to six meters or more even when reaching the climate targets of Paris.
The results are based on observations from three warm periods of the last 3.5 million years, when the world was 0.5-2 degrees Celsius warmer than the pre-industrial temperatures of the 19th century.
Research also showed how large areas of polar ice caps could collapse and significant changes in ecosystems could turn the Sahara desert green and turn the edges of tropical forests into fire-dominated savanna.
"Observations from past warming periods suggest that a number of amplification mechanisms that are poorly represented in climate models increase long-term warming beyond climate model projections," said Hubertus Fischer of the University of Bern, Switzerland.
"This suggests that the carbon budget to avoid 2 degrees Celsius of global warming is much lower than estimated, so there is very little margin for error to reach the Paris targets," Fischer said.
To obtain their results, 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).
The warming of the first two periods was caused by predictable changes in Earth orbit, while the mean Pliocene event was the result of atmospheric carbon dioxide concentrations that were 350-450 ppm – as much as today.
The researchers combined a variety of measurements from ice cores, sediment layers, fossils, atomic isotopic dating, and a variety of other established paleoclimate methods, and collated the effects of these climatic changes.
In combination, these periods provide strong evidence of what a warmer earth would look like once the climate had stabilized. In contrast, our planet now warms much faster than any of these periods, as man-made carbon dioxide emissions continue to rise.
Even if our emissions came to a standstill today, it would take centuries to milliseconds to reach equilibrium.
Earth's changes under these past conditions were profound – there were significant retreats of the Antarctic and Greenland ice sheets, and as a result, sea levels rose at least six meters; marine plankton areas shifted and reorganized whole marine ecosystems; The Sahara became greener and the forest species moved 200 km towards the poles, as well as the tundra. The altitude of the species decreased, the temperate tropical forests were reduced and in the Mediterranean areas dominated by the fire-protected vegetation.
"Even with only 2 degrees Celsius warming – and possibly only 1.5 degrees Celsius – the impact on the Earth system is significant," said Alan Mix of Oregon State University in the US.
"We can expect that sea-level rise has been unstoppable for millennia and would affect much of the world's population, infrastructure and economy," said Mix.
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