The Earth may be able to recover from rising carbon
dioxide emissions faster than previously thought, according to evidence from a
prehistoric event analyzed by a Purdue Univ.-led team.
When faced with high levels of atmospheric carbon dioxide
and rising temperatures 56 million years ago, the Earth increased its ability
to pull carbon from the air. This led to a recovery that was quicker than
anticipated by many models of the carbon cycle—though still on the order of
tens of thousands of years, said Gabriel Bowen, the associate professor of
earth and atmospheric sciences who led the study.
“We found that more than half of the added carbon
dioxide was pulled from the atmosphere within 30,000 to 40,000 years, which is
one-third of the time span previously thought,” said Bowen, who also is a
member of the Purdue
“We still don’t know exactly where this carbon went, but the evidence
suggests it was a much more dynamic response than traditional models
Bowen worked with James Zachos, a professor of earth and
planetary sciences at the Univ. of California, Santa
Cruz, to study the end of the Palaeocene-Eocene
Thermal Maximum, an approximately 170,000-year-long
period of global warming that has many features in common with the world’s
current situation, he said.
“During this prehistoric event billions of tons of
carbon was released into the ocean, atmosphere, and biosphere, causing warming
of about 5 degrees Celsius,” Bowen said. “This is a good analog for
the carbon being released from fossil fuels today.”
Scientists have known of this prehistoric event for 20
years, but how the system recovered and returned to normal atmospheric levels
has remained a mystery.
Bowen and Zachos examined samples of marine and terrestrial
sediments deposited throughout the event. The team measured the levels of two
different types of carbon atoms, the isotopes carbon-12 and carbon-13. The
ratio of these isotopes changes as carbon dioxide is drawn from or added to the
atmosphere during the growth or decay of organic matter.
Plants prefer carbon-12 during photosynthesis, and when
they accelerate their uptake of carbon dioxide it shifts the carbon isotope
ratio in the atmosphere. This shift is then reflected in the carbon isotopes present
in rock minerals formed by reactions involving atmospheric carbon dioxide,
“The rate of the carbon isotope change in rock
minerals tells us how rapidly the carbon dioxide was pulled from the
atmosphere,” he said. “We can see the fluxes of carbon dioxide in to
and out of the atmosphere. At the beginning of the event we see a shift
indicating that a lot of organic-derived carbon dioxide had been added to the
atmosphere, and at the end of the event we see a shift indicating that a lot of
carbon dioxide was taken up as organic carbon and thus removed from the
A paper detailing the team’s National Science
Foundation-funded work was published in Nature
It had been thought that a slow and fairly constant
recovery began soon after excess carbon entered the atmosphere and that the
weathering of rocks, called silicate weathering, dictated the timing of the
Atmospheric carbon dioxide that reacts with silicon-based
minerals in rocks is pulled from the air and captured in the end product of the
reaction. This mechanism has a fairly direct correlation with the amount of
carbon dioxide in the atmosphere and occurs relatively slowly, Bowen said.
The changes Bowen and Zachos found during the
Palaeocene-Eocene Thermal Maximum went beyond the effects expected from
silicate weathering, he said.
“It seems there was actually a long period of higher
levels of atmospheric carbon dioxide followed by a short and rapid recovery to
normal levels,” he said. “During the recovery, the rate at which
carbon was pulled from the atmosphere was an order of magnitude greater than
the slow drawdown of carbon expected from silicate weathering alone.”
A rapid growth of the biosphere, with a spread of forests,
plants and carbon-rich soils to take in the excess carbon dioxide, could
explain the quick recovery, Bowen said.
“Expansion of the biosphere is one plausible
mechanism for the rapid recovery, but in order to take up this much carbon in
forests and soils there must have first been a massive depletion of these
carbon stocks,” he said. “We don’t currently know where all the
carbon that caused this event came from, and our results suggest the troubling
possibility that widespread decay or burning of large parts of the continental
biosphere may have been involved.”
Release from a different source, such as volcanoes or sea
floor sediments, may have started the event, he said.
“The release of carbon from the biosphere may have
occurred as a positive feedback to the warming,” Bowen said. “The
forests may have dried out, which can lead to die off and forest fires. If we
take the Earth’s future climate to a place where that feedback starts to happen
we could see accelerated rates of climate change.”
The team continues to work on new models of the carbon
cycle and is also investigating changes in the water cycle during the
Palaeocene-Eocene Thermal Maximum.
“We need to figure out where the carbon went all
those years ago to know where it could go in the future,” he said.
“These findings show that the Earth’s response is much more dynamic than
we thought and highlight the importance of feedback loops in the carbon cycle.”