Study Suggests Atmospheric CO2 Already in “Dangerous Zone”; Calls for Urgent Reduction Below Today’s Levels With 350 ppm as Initial Target
|Top: Fossil-fuel CO2 emissions with coal phase-out by 2030. Bottom: Resulting atmospheric CO2. Click to enlarge. Source: Hansen 2008|
In an analysis using paleoclimate data to show how the Earth has responded to past changes of CO2, a group of ten scientists from the United States, the UK and France, conclude that the present global mean atmospheric concentration of CO2 (385 ppm and increasing about 2 ppm per year) is already in the “dangerous zone” in terms of long-term climate change.
Averting climate disasters, they argue in an open-access paper published in the journal Open Atmospheric Science Journal, requires the reduction of CO2 this century to less than the current amount via prompt policy changes. The authors argue that such reductions are feasible, but requires a moratorium on any new coal use that does not capture CO2 as the phase out of existing coal emissions by 2030. Dr. James Hansen is the lead author of the study.
The authors advocate an initial target concentration of 350 ppm, noting that it might have to be lower once more studies and analysis are done. Most other current studies have settled on a concentration of 450 ppm as being capable of preventing the worst effects of climate change.
Our current analysis suggests that humanity must aim for an even lower level of GHGs. Paleoclimate data and ongoing global changes indicate that ‘slow’ climate feedback processes not included in most climate models, such as ice sheet disintegration, vegetation migration, and GHG release from soils, tundra or ocean sediments, may begin to come into play on time scales as short as centuries or less. Rapid on-going climate changes and realization that Earth is out of energy balance, implying that more warming is ‘in the pipeline’, add urgency to investigation of the dangerous level of GHGs.
...Our estimated history of CO2 through the Cenozoic Era provides a sobering perspective for assessing an appropriate target for future CO2 levels. A CO2 amount of order 450 ppm or larger, if long maintained, would push Earth toward the ice-free state. Although ocean and ice sheet inertia limit the rate of climate change, such a CO2 level likely would cause the passing of climate tipping points and initiate dynamic responses that could be out of humanity’s control.—Hansen et al. (2008)
Because a large fraction of fossil-fuel generated CO2 emissions stays in the atmosphere a long time (25% of it remaining airborne for several centuries), a moderate delay of fossil fuel use will not appreciably reduce long-term human-made climate change, according to the authors.
Preservation of a climate resembling that to which humanity is accustomed, the climate of the Holocene, requires that most remaining fossil fuel carbon is never emitted to the atmosphere.—Hansen et al. (2008)
The only realistic way to sharply curtail CO2 emissions is to phase out coal use except where it is captured and sequestered, they argue. Even with that, however, CO2 would remain above 350 ppm for more than two centuries.
A practical global strategy almost surely requires a rising global price on CO2 emissions and phase-out of coal use except for cases where the CO2 is captured and sequestered. The carbon price should eliminate use of unconventional fossil fuels, unless, as is unlikely, the CO2 can be captured. A reward system for improved agricultural and forestry practices that sequester carbon could remove the current CO2 overshoot. With simultaneous policies to reduce non-CO2 greenhouse gases, it appears still feasible to avert catastrophic climate change.
Present policies, with continued construction of coalfired power plants without CO2 capture, suggest that decision-makers do not appreciate the gravity of the situation. We must begin to move now toward the era beyond fossil fuels. Continued growth of greenhouse gas emissions, for just another decade, practically eliminates the possibility of near-term return of atmospheric composition beneath the tipping level for catastrophic effects.
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 et al. (2008)
James Hansen, Makiko Sato, Pushker Kharecha, David Beerling, Robert Berner, Valerie Masson-Delmotte, Mark Pagani, Maureen Raymo, Dana L. Royer, James C. Zachos (2008) Target Atmospheric CO2: Where Should Humanity Aim? The Open Atmospheric Science Journal Vol. 2 doi: 10.2174/1874282300802010217