New Study: Climate Already in Danger Zone

Yale University
Public release date: 7-Nov-2008

Contact: Janet Rettig Emanuel

Revised theory suggests carbon dioxide
levels already in danger zone

New Haven, Conn. – If climate disasters are to be averted,
atmospheric carbon dioxide (CO2) must be reduced below the levels
that already exist today, according to a study published in Open
Atmospheric Science Journal by a group of 10 scientists from the
United States, the United Kingdom and France.

The authors, who include two Yale scientists, assert that to maintain
a planet similar to that on which civilization developed, an optimum
CO2 level would be less than 350 ppm – a dramatic change from most
previous studies, which suggested a danger level for CO2 is likely to
be 450 ppm or higher. Atmospheric CO2 is currently 385 parts per
million (ppm) and is increasing by about 2 ppm each year from the
burning of fossil fuels (coal, oil, and gas) and from the burning of

“This work and other recent publications suggest that we have reached
CO2 levels that compromise the stability of the polar ice sheets,”
said author Mark Pagani, Yale professor of geology and geophysics.
“How fast ice sheets and sea level will respond are still poorly
understood, but given the potential size of the disaster, I think
it’s best not to learn this lesson firsthand.”

The statement is based on improved data on the Earth’s climate
history and ongoing observations of change, especially in the polar
regions. The authors use evidence of how the Earth responded to past
changes of CO2 along with more recent patterns of climate changes to
show that atmospheric CO2 has already entered a danger zone.

According to the study, coal is the largest source of atmospheric CO2
and the one that would be most practical to eliminate. Oil resources
already may be about half depleted, depending upon the magnitude of
undiscovered reserves, and it is still not practical to capture CO2
emerging from vehicle tailpipes, the way it can be with coal-burning
facilities, note the scientists. Coal, on the other hand, has larger
reserves, and the authors conclude that “the only realistic way to
sharply curtail CO2 emissions is phase out coal use except where CO2
is captured and sequestered.”

In their model, with coal emissions phased out between 2010 and 2030,
atmospheric CO2 would peak at 400-425 ppm and then slowly decline.
The authors maintain that the peak CO2 level reached would depend on
the accuracy of oil and gas reserve estimates and whether the most
difficult to extract oil and gas is left in the ground.

The authors suggest that reforestation of degraded land and improved
agricultural practices that retain soil carbon could lower
atmospheric CO2 by as much as 50 ppm. They also dismiss the notion of
“geo-engineering” solutions, noting that the price of artificially
removing 50 ppm of CO2 from the air would be about $20 trillion.

While they note the task of moving toward an era beyond fossil fuels
is Herculean, the authors conclude that it is feasible when compared
with the efforts that went into World War II and that “the greatest
danger is continued ignorance and denial, which could make tragic
consequences unavoidable.”

“There is a bright side to this conclusion” said lead author James
Hansen of Columbia University, “Following a path that leads to a
lower CO2 amount, we can alleviate a number of problems that had
begun to seem inevitable, such as increased storm intensities,
expanded desertification, loss of coral reefs, and loss of mountain
glaciers that supply fresh water to hundreds of millions of people.”


In addition to Hansen and Pagani, authors of the paper are Robert
Berner from Yale University; Makiko Sato and Pushker Kharecha from
the NASA/Goddard Institute for Space Studies and Columbia University
Earth Institute; David Beerling from the University of Sheffield, UK;
Valerie Masson-Delmotte from CEA-CNRS-Universite de Versaille, France
Maureen Raymo from Boston University; Dana Royer from Wesleyan
University and James C. Zachos from the University of California at
Santa Cruz.

Citation: Open Atmospheric Science Journal, Volume 2, 217-231 (2008)

Mark Pagani
geology and geophysics
Robert Berner
Open Atmospheric Science Journal


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