C02 Levels Indicate Earth May Be Entering a New Pliocene Era
Comparing Earth's current warming to the Pliocene
The early Pliocene period might be the best analog for the warmer world scientists expect in the not-too-distant future.
Christian Science Monitor, Jan. 8, 2010
About
4.5 million years ago, during the early Pliocene period (3 to 5 million
years ago), temperatures on Earth were some 3 to 4 degrees C (5.4 to
7.2 degrees F.) higher in the tropics, and perhaps 10 degrees C (18
degrees F.) warmer near the poles.
To
get that much warming, current climate models have to pump up
atmospheric carbon dioxide levels to between 500 and 600 parts per
million — about twice the preindustrial level of 280 ppm.
We're
currently around 387 ppm. And, given the lack of progress so far toward
curbing fossil-fuel emissions, we'll be fortunate to stabilize
atmospheric concentrations at 450 ppm.
Scientists
are therefore quite interested in what the world looked like during the
early Pliocene, and why. At least in terms of atmospheric CO2
concentrations, that seems to be where we're headed.
A
new paper in Nature Geoscience concludes that we may, in fact, already
be there. According to current climate models, with each doubling of
CO2, Earth warms around 3 degrees C (5.4 degrees F.). Scientists have
deduced, therefore, that atmospheric CO2 concentrations during the
Pliocene must have been between 500 and 600 ppm.
But
according to this study, the fossil evidence doesn't support that
assumption. Carbon dioxide concentrations in the early Pliocene, as
inferred from several sources, were more likely between 365 and 415
ppm. We're already well within that range.
The
implications are (at least) two-fold: 1. It takes a lot less CO2 to
warm Earth than previously thought. 2. Earth's climate may be more
sensitive to increases in CO2 than current climate models assume.
"[T]he
Earth-system climate sensitivity has been significantly higher over the
past five million years than estimated from fast feedbacks alone,"
write the authors.
Antarctic
ice cores allow us to directly measure earth's air going back 800,000
years. Beyond that, we have to infer atmospheric CO2 concentrations
from other sources.
The
authors of this paper looked at isotopic signatures in organic
compounds called alkenones. Alkenones are produced by phytoplankton,
photosynthetic ocean-dwelling organisms with unpronounceable names like
coccolithophorids and prymnesiophytes.
These
organisms, like all photosynthesizers, take the carbon atom from CO2 in
the air, freeing the two oxygen atoms. Scientists can infer from the
ratio of carbon carbon 13 to carbon 12 isotopes in their tissues — and
the alkenone that remains behind in ocean sediments — what the
concentration of CO2 was in the ocean's surface layers.
There
are some caveats with this approach, as an accompanying article points
out. But temperature reconstructions from at least one other source —
the ratio of boron to calcium in fossilized shells — support the basic
finding: Atmospheric carbon dioxide concentrations during the early
Pliocene were significantly lower than previously assumed. They were
just 90 to 125 ppm more than preindustrial levels of 280 ppm — in other
words, about where we are now.
What
do these findings mean? They indicate that the Pliocene might be the
best analog for the world in the not-too-distant future. They also
imply that our climate models, which account for short-term feedbacks
like water vapor and sea-ice formation, but don't include feedback
cycles that take place on a longer time scales — receding ice sheets
and vegetation changes, for example — significantly underestimate CO2's
impact on Earth's climate.
As
models improve and scientists have more proxy reconstructions of
paleoclimate at their fingertips, this seems to be a recurring theme.
Last
year,scientists looked at the Paleocene-Eocene Thermal Maximum (PETM) —
a spike in temperatures that occurred about 55 million years ago. The
PETM was warmer than the Pliocene, and, on average, about 7 degrees C
warmer than today. Palm trees grew in Antarctica and alligators
inhabited swamps above the Arctic Circle.
In
models, these researchers also couldn't reproduce the warmer PETM
conditions by increasing just CO2. When they put in the CO2 they know
existed, they got only about half the warming they know occurred.
As
a result, they concluded that Earth's climate was much more sensitive
to changes in atmospheric CO2 concentration than was commonly assumed.
They faulted as-yet-unidentified positive feedbacks other than carbon
dioxide for this added warmth. One of their more intriguing conjectures
was the probability of a greater number of swamps in a warmer world.
Swamps produce lots of methane, and methane is more than 20 times more
effective at trapping heat than CO2.
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