Climate Study Highlights Wedge Issue
Eli Kintisch, Science
Eight years ago, Steven Davis was a 26-year-old graduate student when he heard an energy scientist give an inspiring talk about tackling the global climate challenge. The speaker, Robert Socolow of Princeton University, had just co-authored a plan for “Stabilization Wedges: Solving the Climate Problem for the Next 50 Years with Current Technologies,” as the title put it (Science, 13 August 2004, p. 968). Humanity could stabilize rapidly rising annual carbon emissions at 2004 levels by 2054, Socolow argued, if it embarked on seven massive campaigns that would each prevent 25 billion tons of carbon emissions over 5 decades; the options included building a fleet of nuclear reactors and ending tropical deforestation. Socolow and Princeton ecologist Stephen Pacala dubbed each campaign a “wedge,” after the angular shapes formed on a graph used to illustrate the concept.
Now, Davis, an earth systems scientist on the faculty at University of California, Irvine, has offered a provocative update to the wedges saga. In a paper published this week, he and three colleagues call for a more audacious plan, and calculate that at least a whopping 19 wedges—and perhaps as many as 31—will be needed to stabilize and then phase out carbon pollution. Beyond the 2004 plan’s emphasis on expanding the use of current technologies, “fundamental and disruptive transformation of the global energy system” is needed to avoid devastating climate change, the authors warn in Environmental Research Letters.
The original wedges paper was very influential, shaping policy in President George W. Bush’s administration and getting cited by scientists more than 1200 times. In part, that’s because it offered hope, Davis says. “Each wedge is incredibly ambitious, but by breaking up the problem it made me feel we could do it.”
Yet the iconic paper also proved controversial. Critics argued that Pacala and Socolow proposed too few wedges to achieve their stated goals, and gave short shrift to the need for fundamental research into breakthrough energy technologies. Subsequent findings suggested that their initial target—capping atmospheric concentrations of carbon dioxide at 500 parts per million by midcentury—wasn’t aggressive enough to avoid dangerous warming.
Davis’s sobering reassessment utilizes new emissions data, better understanding of climate risks, and more refined computer models of the global climate cycle. It also reflects the failure of governments to enact policies to limit global emissions. “We haven’t acted and things are getting worse,” he says.
Everywhere Davis and his co-authors looked, they found more wedges needed. The 2004 proposal, for instance, estimated it would take seven wedges to simply keep yearly carbon emissions—then 7 billion tons—stable for 50 years. Now, however, annual emissions have risen to 10 billion tons, and the new study estimates that just maintaining that rate will require two additional wedges. That makes nine.
An additional 12 wedges might be needed, Davis and colleagues estimate, if the global economy doesn’t “decarbonize” in the way the original study expected. Economic forecasters back then predicted that economic and technological changes—such as a shift from oil to natural gas—would slowly reduce the amount of carbon emitted per unit of economic output. Instead, the opposite has happened, driven mostly by Chinese and Indian fossil fuel use. As a result, up to 12 “hidden” wedges could be needed “just to stay at what we thought was business as usual,” Davis says.
Finally, the authors estimate that an additional 10 wedges will be needed to actually phase out carbon pollution and avoid major temperature increases. The original paper called for maintaining stable annual emissions until 2054 and then subsequently cutting them by 2% annually. But a climate model used by Davis’s team suggests that approach would raise global temperatures by 1.52° to 2.32°C above preindustrial levels by midcentury. So “holding emissions constant for 50 years will not prevent us from exceeding 2°,” Davis predicts, citing the warming limit that nations agreed upon at the Copenhagen climate summit in 2009. The goal should be zero emissions in 50 years, the authors say. The bottom line: It could take 19 to 31 wedges to reach that goal.
Socolow doesn’t dispute the new paper’s arithmetic or its daunting scientific conclusions. But he says it’s time to move “beyond accounting discussions” to actual policy and action. Eight years since wedges first joined the climate lexicon, humanity is nowhere near achieving even a single one, he and others emphasize. (Socolow does point to the recent expansion of natural gas use, however, as a rare sign of progress.)
The new calculation also “raises, implicitly, the intriguing question of how to choose among goals,” Socolow adds. The original study, he says, focused on “difficult but achievable goals” so as to balance the potential risks of warming against the potential risks posed by some of the solutions. For example, if an expansion of nuclear power is done recklessly, he says, it could proliferate weapons material. The latest update highlights the need for making a careful “balance of investments” in both short- and long-term solutions, says climate specialist Daniel Schrag of Harvard University. “New technologies really are needed,” he says. And he and others agree that one simple assertion from the 2004 paper still holds true: “The choice today is between action and delay.”
