This January 2009 paper published in the Proceedings of the National Academy of Sciences provides strong support for insisting that, we could not afford to allow CO2 to PEAK anywhere near 450ppm, even briefly, let alone STABILIZING at 450ppm (which policy discussions have been aiming for but can't even achieve that[1]). With a peak of 450–600 ppm CO2 followed by complete cessation of emissions (and ignoring the effects of other greenhouse gases), the authors predict 1,000 years of irreversible dry-season rainfall reductions in several regions worldwide comparable to those of the "dust bowl" era, and inexorable sea level rise (which still did not model the rise that will be caused by melting ice cover, which dwarfs the heat expansion sea level rise that is modeled, by far).

According to James Hansen, director of NASA's Goddard Institute for Space Studies, and 9 other international top scientists, Earth was nearly ice-free (and sea level was 75 meters higher than today) in the past until CO2 fell to 450 ± 100 ppm, when Antarctica glaciated 35 million years ago. We are passing 450 ppm in the opposite direction within decades! He and others tell us that the safe upper limit of atmospheric CO2 concentration is between 300-350 ppm, possibly close to 300 ppm, the level never exceeded for at least 800,000 years in the past, and possibly much longer, even during the peak warmth through many glacial-interglacial cycles. Staying above 350 ppm (we are currently at 387 ppm) for a prolonged period risks climate feedback loops to proceed beyond human control and lead to irreversible warming.

So let's run some numbers. The long term (quasi-equilibrium) level of CO2 is determined by the peak level (~40% of the peak excess above pre-industrial level of 280 ppm is still retained after 1000 years), even if emission completely stops immediately after the peak - an impossibility in itself. Peaking at 450 ppm means at best that CO2 will return to barely below 350ppm after 1000 years [280+40%*(450-280) = 348ppm], but permafrost melting and ice sheets disintegration would likely have occured long before then, and caused much more warming, which the PNAS paper (like all others before it) did not model (too complicated). Neither did they take into consideration the dramatic reduction of CO2 uptake observed in the Southern Ocean in recent years, as well as increased respiration (consequently less uptake) of warmer soil and biosphere, and the increased tree mortality and higher frequency of forest fires due to dryer climate, all of which will slow down the CO2 concentration drop even further.

Hansen et al. already pointed out in their target CO2 paper, that we must keep peak CO2 at about 400 ppm if we are to retain the possibility of a drawdown of CO2 beneath the 350 ppm level by the end of century using methods that are more-or-less “natural”, i.e., by adopting agricultural and forestry practices that sequester carbon. The pre-requisite for this to be possible, is that the most carbon-intensive and most abundant fossil fuel - coal, be phased out by 2030, unless the carbon is captured and stored (a technology that is not commercially feasible yet for at least more than a decade, possibly much longer). Likewise, it also requires that lower grades of other fossil fuels such as tar sand, which are harder to extract, generating higher emissions, and more polluting due to the highly sensitive natural environments they are found in, must be left in the ground. They also assumed that worldwide deforestation will stop by 2010 (currently deforestation accounts for 12-25% of annual anthropogenic greenhouse gas emissions)! They suggest that "more rapid drawdown could be provided by CO2 capture at power plants fueled by gas and biofuel. Low-input high-diversity biofuels grown on degraded or marginal lands, with associated biochar production, could accelerate CO2 drawdown, but the nature of a biofuel approach must be carefully designed." This combined with aggressive reforrestation and soil management practices provides the potential to return below 350ppm by mid-century. Of course, if we want to return to 300ppm in such a short time frame, which is more prudent to do, we will need to stop emissions from all other fossil fuels (as well as petrochemical fertilizers, factory farm feedlot methane releases, etc.) even faster.

[1] A Comparison of Legislative Climate Change Targets in the 110th Congress, Chart 2. Produced by the World Resources Institute.