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Target Atmospheric CO2: Where Should Humanity Aim?

Humanity must find a path to reduced atmospheric carbon dioxide, to less than the amount in the air today, if climate disasters are to be averted, according to a study recently published in Open Atmospheric Science Journal by a group of ten scientists from the United States, the United Kingdom and France. They argue that such a path is feasible, but requires a prompt moratorium on new coal use that does not capture CO2 and phase-out of existing coal emissions by 2030.

"There is a bright side to this conclusion" according to the Goddard Institute for Space Studies' James Hansen, lead author on the study, "by 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."

Atmospheric carbon dioxide is already 385 parts per million (ppm) and it is increasing by about 2 ppm each year as a result of the burning of fossil fuels (coal, oil, and gas), with a smaller contribution from burning of forests. The authors use evidence of how the Earth responded to past changes of CO2 and on-going climate changes to show that atmospheric CO2 has already entered the dangerous zone.

The authors suggest that global policies should have an initial target for atmospheric CO2 of 350 ppm. They note that the optimum CO2 level for maintaining a planet similar to that on which civilization developed is likely to be less than 350 ppm, but a 350 ppm target already reveals that dramatic policy changes are needed urgently. By the time such fundamental changes are achieved, knowledge will exist to help fine-tune the target CO2.

Bar chart of emissions

Figure 1, at right: Fossil fuel and net land-use CO2 emissions (purple), and potential fossil fuel emissions (light blue). Fossil fuel reserve estimates of EIA (Energy Information Administration), IPCC (Intergovernmental Panel on Climate Change) and WEC (World Energy Council) differ as shown. Figure available as large GIF or PDF.

The first figure illustrates geophysical constraints that dictate essential policy actions. Coal is the largest source of atmospheric CO2 and it is the source that would be practical to eliminate. Oil resources may be already about half depleted, depending upon the magnitude of undiscovered reserves, and it is impractical to capture CO2 emerging from vehicle tailpipes. 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."

The second figure shows that if coal emissions were thus phased out between 2010 and 2030, and if emissions from unconventional fossil fuels such as tar shale were minimized, atmospheric CO2 would peak at 400-425 ppm and then slowly decline. The peak CO2 amount would depend upon whether the smaller oil and gas reserve estimates of IPCC or the optimistic estimates of EIA are more accurate. The authors note that even if the large EIA reserve estimates are valid, peak CO2 could be kept close to 400 ppm if the most difficult to extract oil and gas is left in the ground via a rising price on carbon emissions that discourages remote exploration and environmental regulations that place some areas off-limits.

Bar chart of emissions

Figure 2, at right: Atmospheric CO2 if coal emissions are phased out linearly between 2010 and 2030, calculated using a version of the Bern carbon cycle model. Figure available as large GIF or PDF.

The authors discredit the notion of "geo-engineering" solutions, noting that with present cost estimates the price of artificially removing 50 ppm of CO2 from the air would be about $20 trillion. They suggest instead that improved agricultural and forestry practices offer a more natural way to draw down CO2, noting that reforestation of degraded land and improved agricultural practices that retain soil carbon could draw down atmospheric CO2 by as much as 50 ppm. Additional significant CO2 reduction could be achieved by using carbon-negative biofuels to replace liquid fossil fuels and phasing out emissions from natural gas-fired power plants, according to the authors. They find that a combination of these approaches could bring CO2 back to 350 ppm well before the end of the century.

The conclusion that humanity must aim for a CO2 amount less than the current amount is a dramatic change from most previous studies, which suggested that the dangerous level of CO2 was likely to be 450 ppm or higher. The change is caused by realization that 'slow' feedback processes, such as ice melt and release of greenhouse gases by the soil and ocean in a warming climate, can occur on the time scale of decades and centuries. This realization stems from both improving data on the Earth's climate history and ongoing observations of change, especially in the polar regions.

The authors conclude that "humanity today, collectively, must face the uncomfortable fact that industrial civilization itself has become the principal driver of global climate." Specifically, they say that humanity "must begin now to move toward the era beyond fossil fuels", and "the most difficult task, phase-out over the next 20-25 years of coal use that does not capture CO2, is Herculean, yet feasible when compared with the efforts that went into World War II. The stakes, for all life on the planet, surpass those of any previous crisis. The greatest danger is continued ignorance and denial, which could make tragic consequences unavoidable."


Hansen, J., Mki. Sato, P. Kharecha, D. Beerling, R. Berner, V. Masson-Delmotte, M. Pagani, M. Raymo, D.L. Royer, and J.C. Zachos, 2008: Target atmospheric CO2: Where should humanity aim? Open Atmos. Sci. J., 2, 217-231, doi:10.2174/1874282300802010217.

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Media Contact

Leslie McCarthy, NASA Goddard Institute for Space Studies, New York, N.Y., 212-678-5507,