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 on Monday.
"Stabilisation at today's greenhouse gas levels may already 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 C (5.4 F) to 7 C (12.6). Even 3 C 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 C above the pre-industrial era benchmark.
The IPCC had concluded that global warming of 2 C 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.
No more ice sheets
The planet has already heated up 1.0 C (1.8 F) above the pre-industrial benchmark, and could see its first year at 1.5 C 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.
She then used climate models to infer wider trends.
The result agreed with a well-established link between global temperature and atmospheric greenhouse gas concentrations, especially over the last 800,000 years of cyclical ice ages, occurring roughly every 100,000 years.
The new data suggests that a doubling of CO2 levels in the atmosphere would drive global temperatures up by 9 C, an increase that would melt away ice sheets and raise sea levels by dozens of metres.
This is considerably higher than most estimates. Researchers not involved in the study also cautioned that it relied on numerous assumptions that may turn out to be wrong. Extrapolating land temperatures based on what's going on in the oceans, for example, is rife with uncertainty, they said.
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