Wedge Issue: Climate Researchers Debate an Iconic Climate Study

Driving a wedge. A 2004 Science study identified seven emissions wedges that would have to be eliminated to stabilize emissions by 2054, but new analysis says it will take more.

Princeton University/Carbon Mitigation Initiative

In 2004, Science published a climate change study—now widely known as the "wedges" paper—that drew attention to the question of how to tackle the challenge of mitigating carbon pollution in the 21st century. The paper called for seven massive campaigns that would each avoid the emissions of 25 billion tons of carbon by 2054, stabilizing annual emissions at 2004 levels. The authors dubbed each chunk of avoided emissions a wedge, after the shape formed on a graph illustrating the idea. It was an audacious goal, the authors wrote, but one achievable with existing techniques such as building new nuclear plants and preventing tropical deforestation.

A new analysis of the wedges approach released this week, however, suggests that things could be much more difficult. Stabilizing and then phasing out emissions by 2060 could require the elimination of 19 to 31 wedges, says the study, which appeared in Environmental Research Letters.

A story on the paper and the issues it explores appears in this week's issue of Science, and on 17 January I'll be hosting a ScienceLIVE chat devoted to the issue.

The following are edited excerpts from extensive e-mails that I received from Robert Socolow, an energy expert at Princeton University who co-authored the original wedges paper, and Steven Davis, an earth systems scientist at the University of California, Irvine, who is the lead author of the new update. The sometimes feisty and poignant exchanges offer additional insight into how the two scientists are thinking about this complex and difficult problem, and into some of the points of debate raised by the two studies.

One main issue discussed in the correspondence is how to view the needed "transformation" of the global energy system: Does transformation mean we can make a major dent in emissions by just scaling up existing technologies, or do we need some profound breakthroughs?


The heart of your argument is what the four of you, collectively, mean by "transformation." You evidently think that to discuss transformation in [your] paper would have been a distraction. My view is that the problem of climate change is too urgent to disconnect what you have to say about transformation from the case that it is necessary.


We do not offer specifics of what exactly the transformation we call for would look like. That was intentional because it was just not within the scope of this paper to perform the sort of comprehensive energy technology assessment that would be required to be more specific. However, I imagine an ideal resembling the hypothetical that we modeled in our paper on infrastructure—where no new emitting infrastructure is built and all new energy infrastructure is carbon-free.


As for "transformation," I would not use that word for a world "where no new emitting infrastructure is built and all new energy infrastructure is carbon-free." Is there really any daylight between that view and the world with N stabilization wedges (then seven, now nine) Steve [Pacala, Socolow's co-author] and I have been describing? Marty [Hoffert, one of Davis's co-authors, was quoted in] a Scientific American article about Plan B. I take him seriously. I want his ideas to have a chance; if they work, we will be ahead of the game. I am thinking about wind turbines in the jetstream, for starters.


I do not view transformation as a pejorative and it carries no apocalyptic connotations for me. My dictionary says "a thorough or dramatic change in form or appearance." I would not hesitate to call modern information technology a transformation.

I see value in an aspirational goal that may or may not be operationally practical. Our government strives for full employment—where everyone who wants a job has one, but that's of course unattainable. Rob and I agree that all our climate targets are arbitrary, but I for one would find it just a bit easier to sleep at night if the goal emphasized ELIMINATING CO2 emissions—which would actually stabilize the climate—rather than a goal that wants only to hold emissions constant for the rest of my life and most of my kids' lives.

Maybe [you] and I are talking past each other, but it seems like there is a real difference between us on this. I do not want us to sacrifice all other goals in order to stabilize the climate. But I have little doubt that scientists (and for that matter environmentalists and human rights advocates and every other sort of stakeholder) will be assessing and balancing our mitigation actions against other socioeconomic and political goals every step of the way. I see the greatest risk of failure to solve climate change as simply not trying hard enough.

The two researchers also differed on what the ultimate target of society's efforts to tackle the climate and energy challenge should be. Some of the discussion revolves around the goal, adopted by nations at the 2009 climate summit in Copenhagen, of limiting the global average temperature increase to 2°C.


You argue that we should set a more modest target than 2 degrees [of warming] so as not to discourage or provoke anyone, and reserve the option of tighter constraints if climate science (or impacts) show a need. There is the question of whether the primary purpose of a target is to motivate mitigation efforts or to minimize harm (whether from climate change, incurred mitigation costs, or geoengineered disasters). Your justifications for a target of stable emissions for 50 years seem to imply that you are first and foremost worried with discouraging action or engendering socioeconomic backlash. While these are of course real possibilities, a target justified by such concerns lacks moral force, which will perniciously undermine the difficult work of cutting emissions. Most people I know value highly the well-being of future generations and openly admire hard work performed in the face of daunting challenges and despite catastrophic setbacks. As I see it, we are far better off setting the right target—no matter how difficult—and depending upon these better angels of our nature, which can only be bolstered by an understanding of the tremendous inertia and existential stakes of climate change.


I worry more than most of those who are pushing for two degrees about what will go wrong. I truly believe that if the aspirational goal of two degrees were to become an operational goal, we would do very unfortunate things, unless we were very thoughtful, more thoughtful than human beings are capable of being. It is too singlemindeded, and its pursuit has too high a chance of being monomaniacal. The geoengineers are salivating and can't wait to be called on, the breeder reactor people likewise (Hansen's favorite for a while), and Lord knows what would happen to biodiversity if we decided that the single highest use for land was to store carbon, period.

I am first of all minimizing harm, not motivating action. The Hippocratic Oath is primary. In a modern version, the doctor pledges "to avoid the twin perils of overtreatment and therapeutic nihilism."


If stopping climate change is likened to dieting to lose weight, [you] are saying that if we can just replace enough cookies with lettuce that we stop *gaining* weight, then we'll be ready to lose weight steadily a year or two from now. Now I like to eat, but I'm far more likely to celebrate a couple long years of willpower with an extra cookie or two than by eating fewer and fewer cookies.

Unless and until carbon-free energy technologies are cheaper than coal, I think constraining emissions—like dieting for a lot of people—will be a constant battle. A goal of stabilizing current emissions is tantamount to long-term business certainty for the fossil fuel industry. If we know we can't stabilize the climate while emitting CO2, why set goals that secure that industry? It's like starting a diet by buying a new cookie jar.


The diet analogy is a good one for thinking about aspirational and operational goals. You could commit yourself to losing a pound a day or a pound a week. Which one makes sense depends on your current weight and other factors. There is art in all of this.

You seem to be missing that we are talking about a grown-up who has been gaining weight week after week, not someone with a stable weight who wants to weigh less. Getting a person gaining weight every week to adopt the goal of keeping his weight constant is a fine first step.

You will need to take away the cookie jar.