- Date: September 20, 2017
- Source: Massachusetts Institute of Technology
- Summary: Scientists have analyzed significant changes in the carbon cycle over the last 540 million years, including the five mass extinction events. They have identified 'thresholds of catastrophe' in the carbon cycle that, if exceeded, would lead to an unstable environment, and ultimately, mass extinction.
- In the past 540 million years, the Earth has endured five mass extinction events, each involving processes that upended the normal cycling of carbon through the atmosphere and oceans. These globally fatal perturbations in carbon each unfolded over thousands to millions of years, and are coincident with the widespread extermination of marine species around the world. Read more at: https://danelite.blogspot.com/
The question for many scientists is whether the carbon cycle is now
experiencing a significant jolt that could tip the planet toward a sixth
mass extinction. In the modern era, carbon dioxide emissions have risen
steadily since the 19th century, but deciphering whether this recent
spike in carbon could lead to mass extinction has been challenging.
That's mainly because it's difficult to relate ancient carbon anomalies,
occurring over thousands to millions of years, to today's disruptions,
which have taken place over just a little more than a century.
Now Daniel Rothman, professor of geophysics in the MIT Department of
Earth, Atmospheric and Planetary Sciences and co-director of MIT's
Lorenz Center, has analyzed significant changes in the carbon cycle over
the last 540 million years, including the five mass extinction events.
He has identified "thresholds of catastrophe" in the carbon cycle that,
if exceeded, would lead to an unstable environment, and ultimately, mass
extinction. Read
more at: https://danelite.blogspot.com/
In a paper published in Science Advances, he proposes that
mass extinction occurs if one of two thresholds are crossed: For changes
in the carbon cycle that occur over long timescales, extinctions will
follow if those changes occur at rates faster than global ecosystems can
adapt. For carbon perturbations that take place over shorter
timescales, the pace of carbon-cycle changes will not matter; instead,
the size or magnitude of the change will determine the likelihood of an
extinction event.
Taking this reasoning forward in time, Rothman predicts that, given
the recent rise in carbon dioxide emissions over a relatively short
timescale, a sixth extinction will depend on whether a critical amount
of carbon is added to the oceans. That amount, he calculates, is about
310 gigatons, which he estimates to be roughly equivalent to the amount
of carbon that human activities will have added to the world's oceans by
the year 2100.
Does this mean that mass extinction will soon follow at the turn of
the century? Rothman says it would take some time -- about 10,000 years
-- for such ecological disasters to play out. However, he says that by
2100 the world may have tipped into "unknown territory."
"This is not saying that disaster occurs the next day," Rothman says.
"It's saying that, if left unchecked, the carbon cycle would move into a
realm which would be no longer stable, and would behave in a way that
would be difficult to predict. In the geologic past, this type of
behavior is associated with mass extinction."
History follows theory
Rothman had previously done work on the end-Permian extinction, the
most severe extinction in Earth's history, in which a massive pulse of
carbon through the Earth's system was involved in wiping out more than
95 percent of marine species worldwide. Since then, conversations with
colleagues spurred him to consider the likelihood of a sixth extinction,
raising an essential question: Read
more at: https://danelite.blogspot.com/
"How can you really compare these great events in the geologic past,
which occur over such vast timescales, to what's going on today, which
is centuries at the longest?" Rothman says. "So I sat down one summer
day and tried to think about how one might go about this
systematically."
He eventually derived a simple mathematical formula based on basic
physical principles that relates the critical rate and magnitude of
change in the carbon cycle to the timescale that separates fast from
slow change. He hypothesized that this formula should predict whether
mass extinction, or some other sort of global catastrophe, should occur.
Rothman then asked whether history followed his hypothesis. By
searching through hundreds of published geochemistry papers, he
identified 31 events in the last 542 million years in which a
significant change occurred in Earth's carbon cycle. For each event,
including the five mass extinctions, Rothman noted the change in carbon,
expressed in the geochemical record as a change in the relative
abundance of two isotopes, carbon-12 and carbon-13. He also noted the
duration of time over which the changes occurred.
He then devised a mathematical transformation to convert these
quantities into the total mass of carbon that was added to the oceans
during each event. Finally, he plotted both the mass and timescale of
each event.
"It became evident that there was a characteristic rate of change
that the system basically didn't like to go past," Rothman says.
In other words, he observed a common threshold that most of the 31
events appeared to stay under. While these events involved significant
changes in carbon, they were relatively benign -- not enough to
destabilize the system toward catastrophe. In contrast, four of the five
mass extinction events lay over the threshold, with the most severe
end-Permian extinction being the farthest over the line.
"Then it became a question of figuring out what it meant," Rothman says.
A hidden leak
Upon further analysis, Rothman found that the critical rate for
catastrophe is related to a hidden process within the Earth's natural
carbon cycle. The cycle is essentially a loop between photosynthesis and
respiration. Normally, there is a "leak" in the cycle, in which a small
amount of organic carbon sinks to the ocean bottom and, over time, is
buried as sediment and sequestered from the rest of the carbon cycle.
Rothman found that the critical rate was equivalent to the rate of
excess production of carbon dioxide that would result from plugging the
leak. Any additional carbon dioxide injected into the cycle could not be
described by the loop itself. One or more other processes would instead
have taken the carbon cycle into unstable territory. Read
more at: https://danelite.blogspot.com/
He then determined that the critical rate applies only beyond the
timescale at which the marine carbon cycle can re-establish its
equilibrium after it is disturbed. Today, this timescale is about 10,000
years. For much shorter events, the critical threshold is no longer
tied to the rate at which carbon is added to the oceans but instead to
the carbon's total mass. Both scenarios would leave an excess of carbon
circulating through the oceans and atmosphere, likely resulting in
global warming and ocean acidification.
The century's the limit
From the critical rate and the equilibrium timescale, Rothman
calculated the critical mass of carbon for the modern day to be about
310 gigatons.
He then compared his prediction to the total amount of carbon added
to the Earth's oceans by the year 2100, as projected in the most recent
report of the Intergovernmental Panel on Climate Change. The IPCC
projections consider four possible pathways for carbon dioxide
emissions, ranging from one associated with stringent policies to limit
carbon dioxide emissions, to another related to the high range of
scenarios with no limitations.
The best-case scenario projects that humans will add 300 gigatons of
carbon to the oceans by 2100, while more than 500 gigatons will be added
under the worst-case scenario, far exceeding the critical threshold. In
all scenarios, Rothman shows that by 2100, the carbon cycle will either
be close to or well beyond the threshold for catastrophe.
"There should be ways of pulling back [emissions of carbon dioxide],"
Rothman says. "But this work points out reasons why we need to be
careful, and it gives more reasons for studying the past to inform the
present." Read
more at: https://danelite.blogspot.com/
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