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[ Climate Change Mitigation: Caught Between a Wedge and a Hard Place ]

New study concludes that addressing climate change will require “fundamental and disruptive transformation of the global energy system.”

Wedges 101

Since the publication in 2004 of the watershed paper by Princeton University’s Steve Pacala and Rob Socolow, the discussion of how to mitigate climate change has been dominated by the concept of wedges.

Before Pacala and Socolow’s paper much of the debate about lowering greenhouse gas emissions to avoid dangerous climate change tended to focus on trying to find a “silver bullet”: a single, optimum technology (e.g., nuclear, renewables, carbon capture and storage, efficiency) that could be deployed to get us from today’s carbon-glutinous energy system to the carbon-free energy system of the future. (See related story: “Climate Scientist Fears His ‘Wedges’ Made It Seem Too Easy“)

Pacala and Socolow argued that we should be using buckshot instead of a bullet; that is, instead of finding any one single low-carbon technology to deploy, we should be taking a more diversified approach. In this approach we can think of any single technology that would lower carbon emissions as an “emissions stabilization wedge,” which, if deployed, could by itself avoid a modest one gigaton of carbon emissions per year. (We currently emit about nine gigatons of carbon per year.) While no single one-gigaton wedge would be adequate to avoid dangerous climate change, a combination of stabilization wedges could be.


Since the 2004 publication of Pacala and Socolow’s seminal wedges paper, the number of wedges needed to reduce business-as-usual emissions to a level that would stabilize the climate has increased. In the original paper it was argued that seven wedges were necessary as in this figure. A new paper argues that we now need 19 to stabilize emissions and another 12 to eliminate emissions. (Figure is from EPA and was reproduced from Pacala and Socolow [2004].)

At the time of their paper, Pacala and Socolow identified 15 possible stabilization wedges that were ready for deployment, and concluded that deploying just seven would be sufficient to stabilize carbon dioxide (CO2) emissions at their current rate and would keep CO2 concentrations below 500 parts per million through 2050. Beyond that, continued climate mitigation would require that emissions be radically lowered and approach zero by the end of the century.*

Pacala and Socolow concluded on an optimistic note: “Humanity can solve the carbon and climate problem in the first half of this century simply by scaling up what we already know how to do.” In other words, a combination of seven technologies already in existence would start us on the right path.

Wedges Revisited … and Again

In a 2011 article published in the Bulletin of the Atomic Scientists Socolow revisited the wedge strategy and reaffirmed that with one change, the strategy was still viable — our inaction since 2004 meant that now nine wedges (instead of seven) would be needed to stabilize emissions. (See my previous post.)

Writing in the journal Environmental Research Letters, Steve Davis of the University of California at Irvine and co-authors take a fresh look at the wedges notion from the perspective of 2012 and find that even nine will not be adequate. In introducing their study, the authors note that while Pacala and Socolow’s premise was that stabilizing emissions at 2004 levels would be sufficient to meet the target of 500 parts per million, emissions have increased over the past seven years from about seven gigatons a year to about nine gigatons a year. What that means is that it is no longer a matter of stabilizing emissions, but of lowering them to meet the target of 500 parts per million.

Moreover, Davis et al point out, Pacala and Socolow punted on the real challenge of avoiding dangerous climate change: not just stabilizing emissions but taking them to zero. And not just doing that, but doing it in a world with increasing demands for energy — energy that in a business-as-usual scenario would be mostly generated by fossil fuels.

Again, More Wedges Needed

Taking that growth in energy demand into account, the authors estimate that we will need not just seven or nine but 31 stabilization wedges. Fortunately, about 12 of those should come more or less free of charge: it is estimated that increases in carbon efficiency as the developing world industrializes should shave off about 12 gigatons of global carbon emissions per year. An additional nine wedges will be needed to stabilize emissions at today’s level of nine gigatons of carbon per year and another 10 wedges to take us down to zero emissions.

While Pacala and Socolow called the solution to the climate problem a “heroic challenge,” they still thought it was possible because we had the basic technological know-how. Davis et al are far less sanguine. While not suggesting that we throw in the towel, they argue that major new innovations and technological advances will be needed and needed in short order:

“The emission reductions required by current targets, let alone a complete phase-out of emissions, demand fundamental, disruptive changes in the global energy system over the next
50 years. Depending on what sort of fossil-fuel infrastructure is replaced and neglecting
any emissions produced to build and maintain the new infrastructure (see, e.g. [43]), a
single wedge represents 0.7–1.4 terawatts (TW) of carbon-free energy (or an equivalent
decrease in demand for fossil energy)… few would dispute that extensive innovation of
technologies will be necessary to afford many terawatts of carbon-free energy and
reductions in energy demand.”

Sounds daunting but consider what the world was like 50 years ago in 1963. A lot has happened since — it could happen again, this time to energy.

End Notes

* A note about CO2 and climate stabilization: Pacala and Socolow’s 2004 paper limited the rise in CO2 to 500 parts per million to avoid dangerous climate change. More recently the international community (for example the Copenhagen Accord) adopted a target of 450 parts per million. However, some believe this is still too high and that the target should be closer to 350 parts per million. Others opine that we may have waited too long to meet either a 350 or 450 parts per million target.

A note about other greenhouse gases: If the scenario laid out by Davis et al isn’t daunting enough, consider that fact that there are other greenhouse gases in the atmosphere besides CO2 (e.g., methane, nitrous oxide, and HFCs). Factoring their contributions into the climate equation is equivalent to having an atmospheric CO2 concentration of about 450 parts per million — the threshold that many believe will lead to dangerous climate change. The saving factor — at least for now — is that atmospheric particulate matter from air pollution cools the planet enough to more or less cancel out the warming from the non-CO2 greenhouse gases. And so you can sort of think of air pollution as a pseudo-stabilizing wedge. But air pollution is a bad thing that kills people. It’s something we want to get rid of. But getting rid of air pollution means losing that pseudo wedge. And that is what’s known as being caught between a wedge and a hard place.

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