Paris, Sep 26 Our planet may grow intolerably hot even if greenhouse gases in the atmosphere remain at current levels, according to the first two-million-year reconstruction of surface temperatures, published today.
"Stabilisation at today's greenhouse gas levels may aready commit Earth to an eventual total warming of five degrees Celsius (nine degrees Fahrenheit) over the next few millennia," said a study in the peer-reviewed science journal Nature.
This was the middle of a predicted warming range of 3 degrees Celsius, (5.4 F) to 7 degrees Celsius (12.6). Even 3 degrees Celsius would, in the long-run, unleash a maelstrom of climate change impacts including storm surges engorged by rising seas, deadly heat waves, and severe flooding, said the study.
The UN's Intergovernmental Panel on Climate Change (IPCC) has said that current atmospheric concentrations of the main greenhouse gas CO2 - just over 400 parts per million (ppm) - would, over the next century, push average global temperatures 2 to 2.4 degrees Celsius above the pre-industrial era benchmark.
The IPCC had concluded that global warming of 2 degrees Celsius was a relatively safe limit for humanity for most regions. But a recent crescendo of climate-enhanced extreme weather pushed world leaders to inscribe an even more stringent temperature cap of "well under two degrees" in the Paris Agreement inked by 195 nations in December.
The planet has already heated up 1 degree Celsius (1.8 F) above the pre-industrial benchmark, and could see its first year at 1.5 degrees Celsius within a decade, scientists reported at a conference in Oxford last week.
The new study, by palaeoclimatologist Carolyn Snyder of Stanford University's Interdisciplinary Program in Environment and Resources, is the first to piece together a continuous record of average surface temperatures stretching back two million years.
Some parts of Earth's climate history have been relatively easy to reconstruct: there is broad agreement, for example, on carbon dioxide levels, sea surface temperatures and sea level going back hundreds of thousands - sometimes millions - of years.
But evidence of the change in air temperatures has been harder to come by. In what a climate expert not involved in the study called "an original approach", Snyder extracted 20,000 bits of data from 59 ocean sediment cores, to build a temperature timeline at 1,000-year intervals.