warming in past
About 55 million years ago, a period
of global warming began abruptly and lasted about 100,000 years. Current theory
has linked this Late Paleocene Thermal Maximum (LPTM) to a vast release of frozen
methane from beneath the sea floor. The subsequent increase in atmospheric methane
led to the warming. Now, climatologists are studying the mechanism and
results of this abrupt warming period in Earth’s past to better understand what
the current global heating could mean for the planet’s future.
“An event like the LPTM is an example of surprises that have happened and that are conceivable perhaps in the future in a relatively similar climate to the one we’re moving toward,” says Gavin Schmidt, a researcher at NASA’s Goddard Institute for Space Studies and at Columbia University. Last December, at the American Geophysical Union fall meeting in San Francisco, a NASA research team led by Schmidt presented findings explaining how a release of methane hydrates — methane gas frozen beneath the sea floor — warmed Earth by up to 7 degrees Celsius during the LPTM.
At the onset of the LPTM, marine and terrestrial carbon isotope values dropped a significant amount. “If you do the math and try to work out how much carbon must have been released into the system to cause such a large overall change, it turns out to be a very large amount of very depleted carbon,” Schmidt says. The only terrestrial source, he says, could have been methane hydrates.
But, he adds, “there are a couple of little twists to this story.” Methane generally oxidizes into carbon dioxide in 12 years. “If you calculate the amount of carbon dioxide that you would get, it doesn’t seem to be enough to cause the warming that you see in oxygen-18 records,” he says. And climate proxies do not show much evidence of change either. “And so, you have a slight conundrum,” he says, in that the radiative forcing from the converted methane is not enough to account for the climate response.
So Schmidt’s team decided to model the methane release that initiated the LPTM based on atmospheric chemistry studies that show that the more methane in the atmosphere, the longer it sticks around. Instead of 12 years, it may remain in the atmosphere for hundreds of years. Methane has 20 times the heat-trapping power of carbon dioxide in the atmosphere. “So, it turns out that if you can get methane to stick around, you get much more bang for your carbon molecule,” he says. The subsequent warming is much larger than what you would get from the same amount of carbon from carbon dioxide.
After putting the hypothetical release of methane and its new feedback into the models, “we got warming in the high latitudes of about 5 to 7 degrees, and warming in tropics of about 2 degrees, which matches almost exactly what people had seen in the paleoclimate record. So what you have is a forcing that’s hypothesized, the modeling that kind of does all the arithmetic and then a modeled response which matches the observed response,” Schmidt says.
The LPTM temperatures from the methane modeling closely match those predicted by current models that double atmospheric carbon dioxide, he adds. “So you have a period 55 million years ago where an extra amount of carbon comes into the system, there’s warming and it takes about 100,000 years to go away.” And Earth’s current and future climate, he says, could follow a quite similar pattern.
Jim Zachos, a paleoclimatologist at the University of California, Santa Cruz, is working to constrain the timing and magnitude of the LPTM based on the observed response from individual foraminifera (forams). His study has found a rapid transition of the carbon isotopes in the forams on the order of hundreds of years. “So it’s consistent with the idea that you have this methane eruption event,” he says.
At the peak warming, Zachos saw temperatures of 20 degrees Celsius at polar latitudes — an overall warming of 8 degrees Celsius. He says that if the methane were converted to carbon dioxide, the amount of warming based on current models and measurements would not be enough to explain the amount of warming they see during the LPTM, particularly at high latitudes. He says it is likely “the methane stayed as methane long enough to have a climate impact.”
If the warming was as abrupt as expected, Zachos adds, it might have slowed down or shut off the ocean’s thermohaline circulation. His research team is currently speculating this potential change in ocean circulation and looking at how the carbon signal from atmospheric methane propagates down in the ocean. Zachos adds that the warming during the LPTM “in some ways is very similar to what we’re seeing. It just takes a while to propagate in the deep sea.”
Lisa M. Pinsker
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