Earth locked into path to hit hottest mark in over 2 million years, study indicates

The Japan Times , Tuesday, September 27, 2016
Correspondent :
WASHINGTON/PARIS – A new study paints a picture of an Earth that is warmer than it has been in about 120,000 years, and is locked into eventually hitting its hottest mark in more than 2 million years.

As part of her doctoral dissertation at Stanford University, Carolyn Snyder, now a climate policy official at the U.S. Environmental Protection Agency, created a continuous 2 million year temperature record, much longer than a previous 22,000 year record. Snyder’s temperature reconstruction, published Monday in the journal Nature, doesn’t estimate temperature for a single year, but averages 5,000-year time periods going back a couple million years.

Snyder based her reconstruction on 61 different sea surface temperature proxies from across the globe, such as ratios between magnesium and calcium, species makeup and acidity. But the further the study goes back in time, especially after half a million years, the fewer of those proxies are available, making the estimates less certain, she said.

These are rough estimates with large margins of errors, she said. But she also found that the temperature changes correlated well to carbon dioxide levels.

Temperatures averaged out over the most recent 5,000 years — which includes the last 125 years or so of industrial emissions of heat-trapping gases — are generally warmer than they have been since about 120,000 years ago or so, Snyder found. And two interglacial time periods, the one 120,000 years ago and another just about 2 million years ago, were the warmest Snyder tracked. They were about 3.6 degrees (2 degrees Celsius) warmer than the current 5,000-year average.

With the link to carbon dioxide levels and taking into account other factors and past trends, Snyder calculated how much warming can be expected in the future.

Snyder said if climate factors are the same as in the past — and that’s a big if — Earth is already committed to another 7 degrees or so (about 4 degrees Celsius) of warming over the next few thousand years.

“This is based on what happened in the past,” Snyder said. “In the past it wasn’t humans messing with the atmosphere.”

Scientists give various reasons for past changes in carbon dioxide and heat levels, including regular slight shifts in Earth’s orbital tilt.

Four outside scientists praised the study’s tracking of past temperatures, with caveats about how less certain it is as it gets deeper in the past. Jeremy Shakun of Boston College said “Snyder’s work is a great contribution and future work should build on it.”

But many of the same scientists said Snyder’s estimate of future warming seems too high. Shakun called it unrealistic and not matching historical time periods of similar carbon dioxide levels.

A fifth scientist, Michael Mann of Pennsylvania State University, called the study provocative and interesting but said he remains skeptical until more research confirms it. He found the future temperature calculations “so much higher than prevailing estimates that one has to consider it somewhat of an outlier.”

Our planet may grow intolerably hot even if greenhouse gases in the atmosphere remain at current levels, according to the first 2-million-year reconstruction of surface temperatures, published on Monday.

“Stabilization at today’s greenhouse gas levels may already commit Earth to an eventual total warming of 5 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 U.N.’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 preindustrial 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 2 degrees” in the Paris Agreement inked by 195 nations in December.

The planet has already heated up 1.0 C (1.8 F) above the preindustrial 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 Snyder of Stanford’s Interdisciplinary Program in Environment and Resources, is the first to piece together a continuous record of average surface temperatures stretching back 2 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 meters.

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.

 
SOURCE : http://www.japantimes.co.jp/news/2016/09/27/world/science-health-world/earth-locked-path-hit-hottest-mark-2-million-years-study-indicates/#.V-ofMmx97IW
 


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