Their new study is markedly different from consensus projections that carbon dioxide levels would become critical later this century. They publish their new estimates in the current edition of the Open Atmospheric Science Journal.
"There is a bright side to this conclusion," says lead author James E. Hansen, director of the Goddard Institute for Space Studies, part of Columbia University's Earth Institute. ""By following a path that leads to lower CO2, we can alleviate a number of problems that had begun to seem inevitable."
Hansen said these include expanding desertification, reduced food harvests, increased storm intensities, loss of coral reefs, and the disappearance of mountain glaciers that supply water to hundreds of millions of people.
The scientists say now that CO2 needs to be reduced to the level under which human civilization developed until the industrial age—about 350 parts per million (ppm)—to keep current warming trends from moving rapidly upward in coming years. They say the current level is currently at 385 ppm, and rising about 2 ppm each year, mainly due to the burning of fossil fuels and incineration of forests. As a result, global temperatures have been creeping upward.
The authors say that they have improved data on past climate changes, and the pace at which earth is changing now, especially in the polar regions, and those new data contributed to their conclusion. Among other things, ongoing observations of fast-melting ice masses that previously helped reflect solar radiation, and the release of stored-up "greenhouse" gases from warming soils and ocean waters, show that feedback processes previously thought to move slowly can occur within decades, not millennia, and thus warm the world further. Once CO2 gas is released, a large fraction of it stays in the air for hundreds of years.
The group say that current atmospheric CO2 could be reduced if emissions from coal, the largest contributor, are largely phased out by 2030. Use of unconventional fossil-fuel sources such as tar sands also would have to be minimized, they say. They predict that oil use will probably decline anyway as reserves shrink.
"Geoengineering" solutions that would remove CO2 from the air have been proposed by others, but the group is skeptical; they estimate that artificially removing 50ppm of CO2 from the atmosphere would cost at least $20 trillion, or twice the current U.S. national debt. They suggest that reforestation of degraded land and use of more natural fertilizers could draw down CO2 by a similar amount.
"Humanity today, collectively, must face the uncomfortable fact that industrial civilization itself has become the principal driver of global climate," says the paper. "The greatest danger is continued ignorance and denial, which would make tragic consequences unavoidable."
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