A new study suggests that the rate of global warming from
doubling of atmospheric carbon dioxide may be less than the most dire estimates
of some previous studies—and, in fact, may be less severe than projected by the
Intergovernmental Panel on Climate Change report in 2007.
Authors of the study, which was funded by the National
Science Foundation’s Paleoclimate Program and published online in Science, say that global warming is real
and that increases in atmospheric carbon dioxide will have multiple serious
impacts.
However, the most Draconian projections of temperature increases
from the doubling of carbon dioxide are unlikely.
“Many previous climate sensitivity studies have looked at
the past only from 1850 through today, and not fully integrated paleoclimate
date, especially on a global scale,” says Andreas Schmittner, an Oregon State
University researcher and
lead author on the Science article. “When you reconstruct sea and land surface temperatures from the peak of the last
Ice Age 21,000 years ago—which is referred to as the Last Glacial Maximum—and
compare it with climate model simulations of that period, you get a much
different picture.”
“If these paleoclimatic constraints apply to the future, as
predicted by our model, the results imply less probability of extreme climatic
change than previously thought,” Schmittner adds.
Scientists have struggled for years trying to quantify “climate sensitivity”—which is how the Earth will respond to projected
increases of atmospheric carbon dioxide. The 2007 IPCC report estimated that
the air near the surface of the Earth would warm on average by 2 to 4.5 C with
a doubling of atmospheric carbon dioxide from pre-industrial standards. The
mean, or “expected value” increase in the IPCC estimates was 3.0 degrees; most
climate model studies use the doubling of carbon as a basic index.
Some previous studies have claimed the impacts could be much
more severe—as much as 10 degrees or higher with a doubling of carbon dioxide—although
these projections come with an acknowledged low probability. Studies based on
data going back only to 1850 are affected by large uncertainties in the effects
of dust and other small particles in the air that reflect sunlight and can
influence clouds, known as “aerosol forcing,” or by the absorption of heat by
the oceans, the researchers say.
To lower the degree of uncertainty, Schmittner and his
colleagues used a climate model with more data and found that there are
constraints that preclude very high levels of climate sensitivity.
The researchers compiled land and ocean surface temperature
reconstructions from the Last Glacial Maximum and created a global map of those
temperatures. During this time, atmospheric carbon dioxide was about a third
less than before the Industrial Revolution, and levels of methane and nitrous
oxide were much lower. Because much of the northern latitudes were covered in
ice and snow, sea levels were lower, the climate was drier (less precipitation),
and there was more dust in the air.
All these factor, which contributed to cooling the Earth’s
surface, were included in their climate model simulations.
The new data changed the assessment of climate models in
many ways, says Schmittner, an associate professor in OSU’s College of Earth,
Ocean, and Atmospheric Sciences. The researchers’ reconstruction of
temperatures has greater spatial coverage and showed less cooling during the
Ice Age than most previous studies.
High sensitivity climate models—more than 6 degrees—suggest
that the low levels of atmospheric carbon dioxide during the Last Glacial
Maximum would result in a “runaway effect” that would have left the Earth
completely ice-covered.
“Clearly, that didn’t happen,” Schmittner says. “Though the
Earth then was covered by much more ice and snow than it is today, the ice
sheets didn’t extend beyond latitudes of about 40 degrees, and the tropics and
subtropics were largely ice-free—except at high altitudes. These
high-sensitivity models overestimate cooling.”
On the other hand, models with low climate sensitivity—less
than 1.3 degrees—underestimate the cooling almost everywhere at the Last
Glacial Maximum, the researchers say. The closest match, with a much lower
degree of uncertainty than most other studies, suggests climate sensitivity is
about 2.4 degrees.
However, uncertainty levels may be underestimated because
the model simulations did not take into account uncertainties arising from how
cloud changes reflect sunlight, Schmittner says.
Reconstructing sea and land surface temperatures from 21,000
years ago is a complex task involving the examination of ices cores, bore
holes, fossils of marine and terrestrial organisms, seafloor sediments, and
other factors. Sediment cores, for example, contain different biological
assemblages found in different temperature regimes and can be used to infer
past temperatures based on analogs in modern ocean conditions.
“When we first looked at the paleoclimatic data, I was
struck by the small cooling of the ocean,” Schmittner says. “On average, the
ocean was only about two degrees (Celsius) cooler than it is today, yet the planet
was completely different—huge ice sheets over North America and northern
Europe, more sea ice and snow, different vegetation, lower sea levels, and more
dust in the air.”
“It shows that even very small changes in the ocean’s
surface temperature can have an enormous impact elsewhere, particularly over
land areas at mid- to high-latitudes,” he adds.
Schmittner says continued unabated fossil fuel use could
lead to similar warming of the sea surface as reconstruction shows happened
between the Last Glacial Maximum and today.
“Hence, drastic changes over land can be expected,” he says. “However, our study implies that we still have time to prevent that from
happening, if we make a concerted effort to change course soon.”