The Economist’s survey of climate change describes the challenge of stabilizing greenhouse gas concentrations as follows:

The concentration of CO2 in the air has risen from 280ppm before the industrial revolution to around 380ppm now, and the IPCC reckons that if emissions continue to grow at their current rate, by 2100 this will have risen to around 800ppm. Depending on population changes, economic growth and political will, this could be adjusted to somewhere between 540ppm and 970ppm. The prospect of anything much above 550ppm makes scientists nervous.

But a close examination of research in this area does appear to lend anything but pessimism to the notion that stabilization at 550 ppm is even possible. Forget about 500 or 450.

By contrast, the Economist suggests some optimism for reaching a 550 ppm target. My reading of the Economist survey on climate change suggests that this optimism may be the result of its confusion between stabilizing emissions reductions with stabilization of atmospheric concentrations of carbon dioxide — a common error in discussions of climate change. This distinction is important because it can lead one to dramatically underestimate the magnitude of the challenge represented by achieving stabilization at levels such as 450, 500, or 550 ppm carbon dioxide.

Indeed, it seems that this misplaced optimism has led the Economist to conclude, “The technological and economic aspects of the problem are, thus, not quite as challenging as many imagine. The real difficulty is political.” This line of thinking is the same as that presented in the IPCC’s Working Group III, but it is not at all reflective of a consensus. For instance, the IPCC’s conclusion that climate change is not a technological but a political challenge was strongly criticized by Hoffert et al. (2002) as reflecting a “misperception of technological readiness” and they conclude that “although regulation can play a role, the fossil fuel greenhouse effect is an energy problem that cannot be simply regulated away.”

A closer look at the studies referred to by the Economist in its survey on prospects for stabilization of carbon dioxide concentrations is a somewhat sobering exercise. The Economist writes,

If an answer is to be found, it lies in using a combination of economics and a broad range of technologies. Robert Socolow, an economist at Princeton University, offers an encouraging way of thinking about this. His “stabilisation wedges” show how different ways of cutting emissions can be used incrementally to lower the trajectory from a steep and frightening path towards a horizontal one that stabilises emissions at their current level.

What Socolow has proposed is an approach to getting a start on the challenge of stabilizing greenhouse gas emissions at a level that might make stabilization still feasible, not an answer to the challenge of stabilization. A closer look at Socolow’s work suggests less reason for optimism than reported by the Economist.

Socolow suggests that under business-as-usual carbon dioxide emissions will continue to increase at a rate of 1.5% per year. This growth rate would result in an additional 525 gigatons of carbon (GtC) being added to the atmosphere by 2054, and at that time an annual rate of emissions of 15.0 GtC. Socolow argues that to eventually achieve stabilization at 550 ppm requires that the annual rate of emissions 2004-2054 not exceed an annual average of 7.0 GtC. Perhaps the simplest way to think about this is that under Socolow’s assumptions the emissions of carbon dioxide by 2054 would need to be reduced by about 53%. Socolow’s approach is valuable in that it has proposed a wide range of approaches that in some combination might feasibly make some progress toward a reduction of 53% in emissions by 2054.

But even assuming the tremendous achievement of a 53% reduction in carbon dioxide emissions would not be enough to stabilize carbon dioxide emissions at 550 ppm. It is absolutely essential to recognize that constant emissions at the present level will not lead to a stabilization of emissions concentrations in the atmosphere. As Pierre Friedlingstein and Susan Solomon wrote in PNAS last year,

It is worth recalling that constant emissions will lead to a linear increase in atmospheric CO2, not to stabilization. Atmospheric CO2 stabilization can be reached only with an emission scenario that eventually drops to zero.

By “zero” this means net of carbon dioxide “sinks.” If the oceans and land serve as “sinks” (i.e., they have a net uptake of carbon dioxide) then in order for the atmosphere concentrations to reach stabilization, then human emissions need not be zero but cannot exceed the net “sink.” This is described by Socolow et al. (2004) on pp. 14-15 of this paper in PDF). Socolow et al. suggest that the sink level is about 2.5 GtC per year, although they acknowledge that the figure is highly uncertain. Thus, under Socolow et al.’s sink estimate, in order to achieve stabilization of atmospheric concentrations at below a doubling of pre-industrial levels requires that over the period 2054-2104 annual emissions must be reduced by 4.5 GtC, from the 7 GtC that their scenario has for 2054. This rate of reduction corresponds to about a 2% annual decrease in carbon dioxide emissions.

Based on this scenario, we can then determine the aggregate carbon dioxide emissions implied over 2054-2104, which are about 225 GtC. Thus under Socolow’s assumptions, over the period 2004-2104, to achieve stabilization at 550 ppm requires that total emissions not exceed more than 658 GtC (i.e., 433 + 225, for the figure of 433 GtC allowed 2004-2054 see Table S1 in Pacala and Socolow’s SOM). Under Socolow’s business-as-usual, the total carbon dioxide emissions 2004-2104 are about 1630 GtC. Thus, over this period there needs to be a reduction in total emissions of about 60%.

According to the U.S. Energy Information Agency’s figures for global carbon dioxide emissions 2004 and 2005 saw about 7.04 and 7.25 GtC of carbon dioxide and 2006 is estimated at about 7.45 GtC. Adding these up results in about 21.7 GtC. Subtracting this from Socolow’s allowable 658 GtC 2004-2104 results in about 636 GtC. One way to think about this is that the world has a carbon dioxide emissions budget of 636 GtC to “spend” by 2104. Under business-as-usual (i.e., EIA estimates to 2030, Socolow’s growth of 1.5% after) this level of aggregate emissions will be exceeded by about 2057. Faster growth rates would of course reach that point faster.

Presumably, those who say that we have no more than ten years to get started on this challenge probably recognize that under business-as-usual by 2015 about an additional 100 GtC will be emitted into the atmosphere, drawing down Socolow’s allowable “budget” to 536 GtC to be “spent” over the following 88 years.

For my part, I fail to see any meaningful difference between 636 GtC to be spent over 98 years (or an average of 6.5 GtC/year) and 536 GtC to be spent over 88 years (6.1 GtC/year). If we can reduce carbon dioxide emissions from the business-as-usual average level of about 16.2 GtC/year 2004-2104 to an average of 6.5 GtC year over that period, then surely squeezing out an additional 0.4 GtC year would not be a show stopper.

Let me suggest another possibility. Under the assumptions presented here (i.e., from Socolow’s recent work) stabilization at 550 is not in the cards. If indeed it is true that waiting ten years is too late, then one has no choice but to conclude that starting immediately is too late as well. This is likely to be an unwelcomed and unacceptable conclusion to many, I know.

What would this conclusion mean for climate policy? Here are a few thoughts.

1. Serious thought and research needs to be given to the prospect of stabilization levels much higher that currently being discussed. What are their policy implications for mitigation and adaptation?

2. The EU, for instance, needs to move discussion beyond its fantasy of stabilization at 450 ppm (see Richard Tol on this here).

3. If stabilization at higher than 550 ppm is determined to be “dangerous interference” in the climate system, then the Framework Convention on climate change needs to be renegotiated from the bottom up. Specifically, its Article 2 needs to be recognized as no longer relevant, and no longer an effective guide to action.

4. Much, much more attention needs to be given to adaptation and its role in climate policy.

5. To continue prospects for successful mitigation policy in the face of the reality that mitigation cannot achieve the goals once set for it will require renewed attention to no-regrets policies.

6. Those who say that abandoning a 550 ppm (or lower) target represents “giving up” or “throwing in the towel” will be setting the stage for a backlash when it inevitably becomes inescapable that those targets are not going to be achieved. At some point policy must be grounded in reality.

7. The longer advocates of mitigation continue to hold unrealistic goals for mitigation policies, the longer it will be before realistic policies are being discussed with a greater chance for policy success.

There are of course a lot of assumptions in the above discussion of Socolow’s work, though I have tried to select those most favorable to stabilization. And there are of course many studies of stabilization paths and scenarios (e.g., as cited by IPCC WGIII). Perhaps this broader literature leads to different conclusions than those presented here. I would be interested in hearing from anyone with a substaintive case to be made for why prospects for stabilization at 550 ppm are more optimistic than the gloomy picture painted here.