[Vision2020] NOAA: New Study Shows Climate Change Largely Irreversible

[Ted Moffett]

http://www.noaanews.noaa.gov/stories2009/20090126_climate.html

New Study Shows Climate Change Largely Irreversible

January 26, 2009

A new scientific study led by the National Oceanic and Atmospheric
Administration reaches a powerful conclusion about the climate change caused
by future increases of carbon dioxide:  to a large extent, there's no going
back.

The pioneering study, led by NOAA senior scientist Susan Solomon, shows how
changes in surface temperature, rainfall, and sea level are largely
irreversible for more than 1,000 years after carbon dioxide (CO2) emissions
are completely stopped. The findings appear during the week of January 26 in
the Proceedings of the National Academy of Sciences.

"Our study convinced us that current choices regarding carbon dioxide
emissions will have legacies that will irreversibly change the planet," said
Solomon, who is based at NOAA's Earth System Research
Laboratory<http://www.esrl.noaa.gov/>in Boulder, Colo.

"It has long been known that some of the carbon dioxide emitted by human
activities stays in the atmosphere for thousands of years," Solomon said.
"But the new study advances the understanding of how this affects the
climate system."

The study examines the consequences of allowing CO2 to build up to several
different peak levels beyond present-day concentrations of 385 parts per
million and then completely halting the emissions after the peak. The
authors found that the scientific evidence is strong enough to quantify some
irreversible climate impacts, including rainfall changes in certain key
regions, and global sea level rise.

If CO2 is allowed to peak at 450-600 parts per million, the results would
include persistent decreases in dry-season rainfall that are comparable to
the 1930s North American Dust Bowl in zones including southern Europe,
northern Africa, southwestern North America, southern Africa and western
Australia.

The study notes that decreases in rainfall that last not just for a few
decades but over centuries are expected to have a range of impacts that
differ by region. Such regional impacts include decreasing human water
supplies, increased fire frequency, ecosystem change and expanded deserts.
Dry-season wheat and maize agriculture in regions of rain-fed farming, such
as Africa, would also be affected.

Climate impacts were less severe at lower peak levels. But at all levels
added carbon dioxide and its climate effects linger because of the ocean.

"In the long run, both carbon dioxide loss and heat transfer depend on the
same physics of deep-ocean mixing. The two work against each other to keep
temperatures almost constant for more than a thousand years, and that makes
carbon dioxide unique among the major climate gases," said Solomon.

The scientists emphasize that increases in CO2 that occur in this century
"lock in" sea level rise that would slowly follow in the next 1,000
years. Considering just the expansion of warming ocean waters—without
melting glaciers and polar ice sheets—the authors find that the irreversible
global average sea level rise by the year 3000 would be at least 1.3–3.2
feet (0.4–1.0 meter) if CO2 peaks at 600 parts per million, and double that
amount if CO2 peaks at 1,000 parts per million.

"Additional contributions to sea level rise from the melting of glaciers and
polar ice sheets are too uncertain to quantify in the same way," said
Solomon. "They could be even larger but we just don't have the same level of
knowledge about those terms. We presented the minimum sea level rise that we
can expect from well-understood physics, and we were surprised that it was
so large."

Rising sea levels would cause "…irreversible commitments to future changes
in the geography of the Earth, since many coastal and island features would
ultimately become submerged," the authors write.

Geoengineering to remove carbon dioxide from the atmosphere was not
considered in the study. "Ideas about taking the carbon dioxide away after
the world puts it in have been proposed, but right now those are very
speculative," said Solomon.

The authors relied on measurements as well as many different models to
support the understanding of their results. They focused on drying of
particular regions and on thermal expansion of the ocean because
observations suggest that humans are contributing to changes that have
already been measured.

Besides Solomon, the study's authors are Gian-Kasper Plattner and Reto
Knutti of ETH Zurich, Switzerland, and Pierre Friedlingstein of Institut
Pierre Simon Laplace, Gif-Sur-Yvette, France.
NOAA understands and predicts changes in the Earth's environment, from the
depths of the ocean to the surface of the sun, and conserves and manages our
coastal and marine resources.
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Vision2020 Post: Ted Moffett
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