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Sudden
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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