Yahoo News 13 Jul 09;
PARIS (AFP) – A runaway spurt of global warming 55 million years ago turned Earth into a hothouse but how this happened remains worryingly unclear, scientists said on Monday.
Previous research into this period, called the Palaeocene-Eocene Thermal Maximum, or PETM, estimates the planet's surface temperature blasted upwards by between five and nine degrees Celsius (nine and 16.2 degrees Fahrenheit) in just a few thousand years.
The Arctic Ocean warmed to 23 C (73 F), or about the temperature of a lukewarm bath.
How PETM happened is unclear but climatologists are eager to find out, as this could shed light on aspects of global warming today.
What seems clear is that a huge amount of heat-trapping "greenhouse" gases -- natural, as opposed to man-made -- were disgorged in a very short time.
The theorised sources include volcanic activity and the sudden release of methane hydrates in the ocean.
A trio of Earth scientists, led by Richard Zeebe of the University of Hawaii, try to account for the carbon that was spewed out during PETM.
They believe that levels of atmospheric carbon dioxide (CO2) rose by 70 percent during PETM's main phase to reach 1,700 parts per million (ppm), attaining a concentration of between four and five times that of today.
But all this CO2 can only account for between one and 3.5 C (1.8-6.3 F) of PETM's warming if the models for climate sensitivity are right, the team found.
There must have been some other factor that stoked temperatures higher.
Even though there are big differences between Earth's geology and ice cover then and now, the findings are relevant as they highlight the risk of hidden mechanisms that add dramatically to warming, says the paper.
Some of these so-called "positive feedbacks" are already known.
For instance, when a patch of Arctic sea ice melts, this exposes the uncovered sea to sunlight, depriving it of a bright, reflective layer.
That causes the sea to warm, which leads to the loss of more ice, which in turn helps the sea to warm, and so on.
But these "feedbacks" are poorly understood and some scientists believe there could be others still to be identified.
"Our results imply a fundamental gap in our understanding about the amplitude of global warming associated with large and abrupt climate perturbations," warns Zeebe's team.
"This gap needs to be filled to confidently predict future climate change."
After the big warm-up, the planet eventually cooled around 100,000 years later, but not before there had been a mass extinction, paving the way to the biodiversity that is familiar to us today.
Man-made global warming, driven mainly by the burning of oil, gas and coal, has amounted to around 0.8 C (1.12 F) over the past century.
Last week in L'Aquila, Italy, the Group of Eight (G8) industrialised countries and other economies that together account for 80 percent of greenhouse-gas emissions pledged to try to limit overall warming to 2 C (3.6 F) over pre-industrial times.
Past warming shows gaps in climate knowledge: study
David Fogarty, Reuters 15 Jul 09;
SINGAPORE (Reuters) - A dramatic warming of the planet 55 million years ago cannot be solely explained by a surge in carbon dioxide levels, a study shows, highlighting gaps in scientists' understanding of impacts from rapid climate change.
During an event called the Palaeocene-Eocene Thermal Maximum, global temperatures rose between 5 and 9 degrees Celsius within several thousand years. The world at that time was already warmer than now with no surface ice.
"We now believe that the CO2 did not cause all the warming, that there were additional factors," said Richard Zeebe, an oceanographer with the University of Hawaii at Manoa.
"There may have been an initial trigger," he told Reuters on Wednesday from Hawaii. This could be a deep ocean warming that caused a catastrophic release of methane from hydrate deposits under the seabed.
Methane is a potent greenhouse gas but much of it is oxidised into CO2 when it is released from hydrate deposits.
Zeebe and his colleagues estimated the amount of CO2 released during the Palaeocene-Eocene event by studying sediment cores from seabeds around the globe. Their study is published in the latest issue of Nature Geoscience.
FUTURE WARMING
They estimated about 3 trillion tons of carbon (11 trillion tons of CO2) was released over several thousand years from the methane deposits, leading to a 70 percent rise in atmospheric CO2 levels from pre-event levels.
But Zeebe said this could only explain a 1 to 3.5 degree Celsius rise in temperatures, adding that a commonly accepted scientific range for a doubling of CO2 is between 1.5 and 4.5 degrees Celsius.
This meant other factors must have been at work to drive up temperatures between 5 and 9 degrees Celsius.
"If this additional warming which we do not really understand, was caused as a response to the CO2 warming, then there is a chance that also a future warming could be more intense than people anticipate right now," Zeebe said.
He said the study suggested there could be atmospheric or ocean processes as yet unknown or poorly understood that might have accelerated the warming. Possibilities could be changes in ocean currents, a much larger release of methane or even greater impacts from higher CO2 levels than currently thought.
At present, CO2 levels have already risen from 280 parts per million to nearly 390 ppm since the Industrial Revolution and could exceed a 70 percent increase during this century, a rate much faster than the Palaeocene-Eocene event, Zeebe said.
While this would cause initial effects, much worse could follow in the coming decades and centuries as the oceans, land and atmosphere tried to deal with the higher CO2 levels, he said.
"The carbon that we put into the atmosphere right now is going to stay there for a very long time. Much of it will stay there for tens of thousands of years."
(Editing by Sugita Katyal)
