With increasing calls from many scientists, e.g. Jim Hansen and Rajendra Pachauri,, to seek to stabilize atmospheric carbon dioxide concentrations at 350 ppm, down from the current levels of 387-390ppm, some analysts have been crunching the numbers to determine what emissions trajectory could effectuate this, as well as the potential costs of such a dramatic transformation. The researchers at Greenhouse Development Rights, have released a very good new brief in this context, A 350 ppm Emergency Pathway, a short reading for students with some excellent graphs of potential emissions trajectories over the course of the next few centuries and the potential implications of these scenarios for atmospheric concentrations of carbon dioxide.

Among the key findings of the memo:

• To emphasize the truly daunting task of achieving a pathway to 350 ppm, the International Energy Agency’s business-as-usual projection by 2030 for carbon dioxide emissions is eight times higher than they would be in the 350 ppm pathway;
• The emissions pathway proposed by the G8, peaking at 2020, with emissions halved by 2020, is much less stringent than what’s necessary to ensure that temperatures do not rise 2C above pre-industrial levels, and is 3x the emissions permissible under the 350 pathway set forth in the memo;
• To effectuate a 350 ppm pathway, emissions must peak by 2011 and decline at a rapid pace fo 10% per decade for several decades. If the peak is delayed only four years, the subsequent decline would have to be nearly 20% per year;
• Even if developed countries commit themselves to a very strenuous reduction fo emissions of 50% between 2020, and eliminate emissions by 2050, this would consume 180 gigatons of the carbon dioxide budget for a 350 pathway, leaving developing countries only 240 gigatons.
  • This would require emissions in the South to fall by 50% by 2020, and continue to drop by 10% for the next three decades, with eventual eimination.
• The cost of a 350 ppm remains speculative, but one study pegs it at 3-5% of GWP

Of course, the authors of this memo made the choice to focus solely on reductions of carbon dioxide emissions. Given recent evidence that a focus on several short-lived GHGs not regulated under Kyoto (including black carbon and HFCS, see, e.g. Molina, et., Reducing Abrupt Climate Change Risk Using the Montreal Protocol and other Regulatory Actions to Complement Cuts in CO2 Emissions, PNAS Early Edition, October 12, 2009) could help us avoid dangerous anthropogenic climate change while permitting a more gradual reduction in carbon dioxide emissions, one wonders why a more comprehensive approach to emissions policies was not undertaken, especially since a focus on short-lived GHG reductions may prove more politically viable. It might be good to assign this memo in conjunction with the Molina et al. article to help students explore various options to achieve long-term objectives.

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