Hansen et al sport their tinfoil hats and sandwich boards

Carbon Dioxide Levels Already In Danger Zone, Revised Theory Shows

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 forests.

"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."
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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)