http://www.nature.com/climate/2008/0812/full/climate.2008.122.html
This had slipped me by, it should (idealistically) impact heavily upon
policy and carbon frameworks.
News Feature
Nature Reports Climate Change
Published online: 20 November 2008 | doi:10.1038/climate.2008.122
Carbon is forever
*Carbon dioxide emissions and their associated warming could linger for
millennia, according to some climate scientists. Mason Inman looks at
why the fallout from burning fossil fuels could last far longer than
expected.*
Carbon is forever
Distant future: our continued use of fossil fuels could leave a CO_2
legacy that lasts millennia, says climatologist David Archer
123RF.COM/PAUL MOORE
After our fossil fuel blow-out, how long will the CO_2 hangover last?
And what about the global fever that comes along with it? These sound
like simple questions, but the answers are complex — and not well
understood or appreciated outside a small group of climate scientists.
Popular books on climate change — even those written by scientists — if
they mention the lifetime of CO_2 at all, typically say it lasts "a
century or more"^1
<http://www.nature.com/climate/2008/0812/full/climate.2008.122.html#B1>
or "more than a hundred years".
"That's complete nonsense," says Ken Caldeira of the Carnegie
Institution for Science in Stanford, California. It doesn't help that
the summaries in the Intergovernmental Panel on Climate Change (IPCC)
reports have confused the issue, allege Caldeira and colleagues in an
upcoming paper in /Annual Reviews of Earth and Planetary Sciences/^2
<http://www.nature.com/climate/2008/0812/full/climate.2008.122.html#B2>
. Now he and a few other climate scientists are trying to spread the
word that human-generated CO_2 , and the warming it brings, will linger
far into the future — unless we take heroic measures to pull the gas out
of the air.
University of Chicago oceanographer David Archer, who led the study with
Caldeira and others, is credited with doing more than anyone to show how
long CO_2 from fossil fuels will last in the atmosphere. As he puts it
in his new book /The Long Thaw/, "The lifetime of fossil fuel CO_2 in
the atmosphere is a few centuries, plus 25 percent that lasts
essentially forever. The next time you fill your tank, reflect upon
this"^3
<http://www.nature.com/climate/2008/0812/full/climate.2008.122.html#B3> .
"The climatic impacts of releasing fossil fuel CO_2 to the atmosphere
will last longer than Stonehenge," Archer writes. "Longer than time
capsules, longer than nuclear waste, far longer than the age of human
civilization so far."
The effects of carbon dioxide on the atmosphere drop off so slowly that
unless we kick our "fossil fuel addiction", to use George W. Bush's
phrase, we could force Earth out of its regular pattern of freezes and
thaws that has lasted for more than a million years. "If the entire coal
reserves were used," Archer writes, "then glaciation could be delayed
for half a million years."
Cloudy reports
"The longevity of CO_2 in the atmosphere is probably the least well
understood part of the global warming issue," says paleoclimatologist
Peter Fawcett of the University of New Mexico. "And it's not because it
isn't well documented in the IPCC report. It is, but it is buried under
a lot of other material."
It doesn't help, though, that past reports from the UN panel of climate
experts have made misleading statements about the lifetime of CO_2 ,
argue Archer, Caldeira and colleagues. The first assessment report, in
1990, said that CO_2 's lifetime is 50 to 200 years. The reports in 1995
and 2001 revised this down to 5 to 200 years. Because the oceans suck up
huge amounts of the gas each year, the average CO_2 molecule does spend
about 5 years in the atmosphere. But the oceans also release much of
that CO_2 back to the air, such that man-made emissions keep the
atmosphere's CO_2 levels elevated for millennia. Even as CO_2 levels
drop, temperatures take longer to fall, according to recent studies.
"The climatic impacts of releasing fossil fuel CO_2 to the atmosphere
will last longer than Stonehenge, longer than time capsules, longer than
nuclear waste, far longer than the age of human civilization so far."
David Archer
Earlier reports from the panel did include caveats such as "No single
lifetime can be defined for CO_2 because of the different rates of
uptake by different removal processes." The IPCC's latest assessment,
however, avoids the problems of earlier reports by including similar
caveats while simply refusing to give a numeric estimate of the lifetime
for carbon dioxide. Contributing author Richard Betts of the UK Met
Office Hadley Centre says the panel made this change in recognition of
the fact that "the lifetime estimates cited in previous reports had been
potentially misleading, or at least open to misinterpretation."
Instead of pinning an absolute value on the atmospheric lifetime of CO_2
, the 2007 report describes its gradual dissipation over time, saying,
"About 50% of a CO_2 increase will be removed from the atmosphere within
30 years, and a further 30% will be removed within a few centuries. The
remaining 20% may stay in the atmosphere for many thousands of years."
But if cumulative emissions are high, the portion remaining in the
atmosphere could be higher than this, models suggest. Overall, Caldeira
argues, "the whole issue of our long-term commitment to climate change
has not really ever been adequately addressed by the IPCC."
The lasting effects of CO_2 also have big implications for energy
policies, argues James Hansen, director of NASA's Goddard Institute of
Space Studies. "Because of this long CO_2 lifetime, we cannot solve the
climate problem by slowing down emissions by 20% or 50% or even 80%. It
does not matter much whether the CO_2 is emitted this year, next year,
or several years from now," he wrote in a letter this August. "Instead
... we must identify a portion of the fossil fuels that will be left in
the ground, or captured upon emission and put back into the ground."
Slow on the uptake
Unlike other human-generated greenhouse gases, CO_2 gets taken up by a
variety of different processes, some fast and some slow. This is what
makes it so hard to pin a single number, or even a range, on CO_2 's
lifetime. The majority of the CO_2 we emit will be soaked up by the
ocean over a few hundred years, first being absorbed into the surface
waters, and eventually into deeper waters, according to a long-term
climate model run by Archer. Though the ocean is vast, the surface
waters can absorb only so much CO_2 , and currents have to bring up
fresh water from the deep before the ocean can swallow more. Then, on a
much longer timescale of several thousand years, most of the remaining
CO_2 gets taken up as the gas dissolves into the ocean and reacts with
chalk in ocean sediments. But this process would never soak up enough
CO_2 to return atmospheric levels to what they were before
industrialization, shows oceanographer Toby Tyrrell of the UK's National
Oceanography Centre, Southampton, in a recent paper^4
<http://www.nature.com/climate/2008/0812/full/climate.2008.122.html#B4> .
Finally, the slowest process of all is rock weathering, during which
atmospheric CO_2 reacts with water to form a weak acid that dissolves
rocks. It's thought that this creates minerals such as magnesium
carbonate that lock away the greenhouse gas. But according to
simulations by Archer and others, it would take hundreds of thousands of
years for these processes to bring CO_2 levels back to pre-industrial
values (Fig. 1
<http://www.nature.com/climate/2008/0812/full/climate.2008.122.html#f1>).
Figure 1: Long lifetime.
Figure 1 : Long lifetime. Unfortunately we are unable to provide
accessible alternative text for this. If you require assistance to
access this image, or to obtain a text description, please contact
[log in to unmask]
Model simulation of atmospheric CO_2 concentration for 40,000 years
following after a large CO_2 release from combustion of fossil fuels.
Different fractions of the released gas recover on different timescales.
Reproduced from /The Long Thaw/^3
<http://www.nature.com/climate/2008/0812/full/climate.2008.122.html#B3> .
Full figure and legend (18 KB)
<http://www.nature.com/climate/2008/0812/fig_tab/climate.2008.122_F1.html>
Several long-term climate models, though their details differ, all agree
that anthropogenic CO_2 takes an enormously long time to dissipate. If
all recoverable fossil fuels were burnt up using today's technologies,
after 1,000 years the air would still hold around a third to a half of
the CO_2 emissions. "For practical purposes, 500 to 1000 years is
'forever,'" as Hansen and colleagues put it. In this time, civilizations
can rise and fall, and the Greenland and West Antarctic ice sheets could
melt substantially, raising sea levels enough to transform the face of
the planet.
New stable state
The warming from our CO_2 emissions would last effectively forever, too.
A recent study by Caldeira and Damon Matthews of Concordia University in
Montreal found that regardless of how much fossil fuel we burn, once we
stop, within a few decades the planet will settle at a new, higher
temperature^5
<http://www.nature.com/climate/2008/0812/full/climate.2008.122.html#B5>
. As Caldeira explains, "It just increases for a few decades and then
stays there" for at least 500 years — the length of time they ran their
model. "That was not at all the result I was expecting," he says.
But this was not some peculiarity of their model, as the same behaviour
shows up in an extremely simplified model of the climate^6
<http://www.nature.com/climate/2008/0812/full/climate.2008.122.html#B6>
— the only difference between the models being the final temperature of
the planet. Archer and Victor Brovkin of the Potsdam Institute for
Climate Impact Research in Germany found much the same result from much
longer-term simulations^6
<http://www.nature.com/climate/2008/0812/full/climate.2008.122.html#B6>
. Their model shows that whether we emit a lot or a little bit of CO_2 ,
temperatures will quickly rise and plateau, dropping by only about 1 °C
over 12,000 years.
"The longevity of CO_2 in the atmosphere is probably the least well
understood part of the global warming issue."
Peter Fawcett
Because of changes in the Earth's orbit, ice sheets might start to grow
from the poles in a few thousand years — but there's a good chance our
greenhouse gas emissions already may prevent that, Archer argues. Even
with the amount of CO_2 emitted so far, another ice age will almost
certainly start in about 50,000 years. But if we burn all remaining
fossil fuels, it could be more than half a million years before the
Earth has another ice age, Archer says.
The long-term effects of our emissions might seem far removed. But as
Tyrrell says, "It is a little bit scary, if you think about all the
concerns we have about radioactive wastes produced by nuclear power. The
potential impacts from emitting CO_2 to the atmosphere are even longer
than that." But there's still hope for avoiding these long-term effects
if technologies that are now on the drawing board can be scaled up
affordably. "If civilization was able to develop ways of scrubbing CO_2
out of the atmosphere," Tyrrell says, "it's possible you could reverse
this CO_2 hangover."
Top of page
<http://www.nature.com/climate/2008/0812/full/climate.2008.122.html#top>
References
1. Flannery, T. The Weather Makers: The History and Future Impact of
Climate Change 162 (Atlantic Monthly Press, New York, 2005).
2. Archer, D. /et al/. Ann. Rev. Earth Pl. Sc. (in the press).
3. Archer, D. The Long Thaw: How Humans Are Changing the Next 100,000
Years of Earth's Climate (Princeton Univ. Press, 2008).
4. Tyrrell, T., Shepherd, J. G. & Castle, S. Tellus 59, 664–672,
doi:10.1111/j.1600-0889.2007.00290.x (2007).
5. Matthews, H. D. & Caldeira, K. Geophys. Res. Lett. 35, L04705,
doi:10.1029/2007GL032388 (2008).
6. Archer, D. & Brovkin, V. Climatic Change 90, 283–297 (2008).
/Mason Inman is a freelance science writer currently based in Pakistan/.
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