Quick Read

Grab a Wedge of Carbon

Chemistry textbooks taught us to work with carbon and carbon dioxide in units of grams, moles, and parts per million. Over the next 50 years, however, the most important unit may be the “wedge,” or 25 billion tons of carbon.

Cheese comes in wedges. So do pie and pizza. But a wedge of carbon?

That’s the unit of measurement being used at the Carbon Mitigation Initiative (CMI), a joint project of Princeton University, BP, and the Ford Motor Co. In recent articles in Science and Scientific American, CMI’s co-directors Stephen Pacala and Robert Socolow introduced the wedge concept to focus discussion on one of the most important global problems facing humanity today—the rising greenhouse-gas emissions that are leading to global climate change.

The authors project that, if we continue on our current course for the next 50 years, atmospheric carbon dioxide (CO2 ) will reach a level that’s double the pre-industrial level. Although scientists are still working out the exact implications of a doubling—from warmer average temperatures and increasingly severe weather patterns to rising sea levels and shrinking ecosystems—the evidence is mounting that the changes could be both dramatic and disastrous. For an introduction to the issues, I recommend that you watch Al Gore’s 2006 documentary, An Inconvenient Truth.

Can we prevent this doubling? What will it take in terms of new technology and economic restructuring? In terms of lifestyle changes and shifts in public opinion? In terms of international cooperation and political will? These are huge and complex questions, and the all-too-human temptation is to postpone action, while pointing optimistically to a revolutionary solution in the distant future.

The Pacala and Socolow articles, however, offer an alternative, arguing that “a portfolio of technologies now exists to meet the world’s energy needs over the next 50 years and limit atmospheric carbon dioxide to a trajectory that avoids a doubling of the pre-industrial concentration.” As summarized in their graphs we can prevent this doubling, beginning right now, by implementing mitigation technologies that will keep carbon out of the atmosphere.

Two possible emissions scenarios define the “stabilization triangle.” The emissions-doubling path (black dotted line) is a reference that extends historical trends and falls in the middle of the field of most estimates of future carbon emissions. We can prevent a doubling by keeping emissions flat for the next 50 years (orange line).

Two possible emissions scenarios define the “stabilization triangle.” The emissions-doubling path (black dotted line) is a reference that extends historical trends and falls in the middle of the field of most estimates of future carbon emissions. We can prevent a doubling by keeping emissions flat for the next 50 years (orange line).


Keeping emissions flat will require the world to “fill in” the seven wedges of the stabilization triangle.

Keeping emissions flat will require the world to “fill in” the seven wedges of the stabilization triangle.



What’s a “Wedge”?

To represent the cumulative impact of these mitigation technologies, the authors introduce the concept of a “stabilization triangle” with seven “stabilization wedges.” According to their definition, “a wedge represents an activity that reduces emissions to the atmosphere that starts at zero today and increases linearly until it accounts for 1 GtC/year [gigaton of carbon per year] of reduced carbon emissions in 50 years. It thus represents a cumulative total of 25 GtC of reduced emissions over 50 years.”

According to Sally Benson, the carbon sequestration program leader at Lawrence Berkeley National Laboratory, the Pacala–Socolow approach provides an “extraordinarily useful framework.” Benson says, “Up until the time that it was published, lots of people viewed that there were going to be one or two technologies that would really make the big difference. The very valuable thing that they did was say, ‘No, it’s going to be a whole suite of things, and every one of them is going to be challenging. Therefore, we’d better make sure that we’re investing and keeping open the possibilities across the board.’”

Follow the Carbon Atom

If a whole portfolio of technologies will be required to mitigate the carbon-emission problem, will chemistry and chemists play a role in any of those technologies? “Frankly, I can’t imagine a discipline that’s more central to this problem than chemistry,” says Socolow. “For me, the way to think about this problem is to follow the carbon atom. Wasn’t it Deep Throat in All the President’s Men who told Woodward and Bernstein to ‘follow the money’? Well, if you’re following the carbon atom, you’re doing chemistry, whether it’s geochemistry or other kinds of chemistry.”

To calibrate the objectivity of this statement about the critical role of chemistry, it’s helpful to know that Socolow is not a chemist. He received his Ph.D. in theoretical high-energy physics at Harvard, and his faculty position at Princeton is in the department of mechanical and aerospace engineering.

The Pacala–Socolow approach lists 15 technologies that could each provide at least one of the seven stabilization wedges. Examples include (among others): solar electricity; nuclear electricity; biofuels; carbon capture and storage; and increased energy efficiency in the areas of transportation, heating, or electricity. The CMI Web site provides details about each of the 15 technologies.

All these technologies bring with them a series of technological challenges, economic implications, social changes, and political issues. Choosing among the alternatives to find a set of seven technologies, while balancing all these factors, will involve every sector of our society, not just scientists and engineers. To illustrate the sort of public debate that will be necessary, the CMI Web site offers materials for a team-based “stabilization wedge game” that drives home the scale of the carbon mitigation challenge and the tradeoffs involved in planning climate policy.

Show Me the Chemistry

Looking through the list of potential mitigation technologies, a chemist can easily see that chemistry is indeed the central science for this issue. Many of the most active areas of chemistry research and development today are, in fact, addressing these needs.

Here are just three examples of how chemistry can help solve the greenhouse-gas problem:

  • Green chemistry. Although “green chemistry” isn’t listed as one of the mitigation technologies, it’s an enabling science and technology for many of the listed technologies. Through its 12 guiding principles (see http://chemistry.org/greenchemistryinstitute), green chemistry emphasizes atom economy, energy efficiency, and renewable feedstocks. Its environmental and economic benefits often include reduced emissions of greenhouse gases such as organic solvents and CO2 .
  • Biofuels. As discussed in the Winter 2007 issue of Chemistry, biodiesel and cellulosic ethanol from biomass can offer significant mitigation of CO2 emissions. It’s important to note that today’s most common biofuel—alcohol derived from corn grain—doesn’t offer as large a reduction of greenhouse-gas emissions as will newer technologies under development.
  • Carbon capture and storage. Also called “carbon sequestration,” this approach involves capturing CO2 while generating energy at coal–electric or natural-gas power plants and then injecting and storing it permanently underground. Benson says, “The level of interest and engagement in this area has increased dramatically over the past five years. You go to a conference now, and you see that half of the people there are new to the community.” In 2003, the U.S. Department of Energy established seven regional carbon-sequestration partnerships to foster cooperation among industry, academe, and the Federal government. The goal of the partnerships is to determine the most suitable technologies, regulations, and infrastructure needs for carbon capture and storage in different areas of the country.

The Invisible Wedge—Educating the Public

Although an increasing number of chemists will be directly involved in research and innovation for carbon-mitigation technologies, most will continue working in other important areas of chemical research, from biotechnology to nanotechnology, from the chemistry classroom to the pharmaceutical laboratory. Does this mean that most chemists don’t really have a role to play in carbon mitigation?

Absolutely not! One of the most important needs today is for an improved public understanding of basic chemistry concepts. Although “public understanding” is not directly listed by Pacala and Socolow as a carbon-mitigation technology, none of the listed technologies can succeed without it.

Benson points out that the general public is hearing and reading more and more every day about global warming and greenhouse gases. However, she says, “I think there’s still a long way to go to increase general knowledge about the basic scientific issues and what we’ll need to do to remedy the situation.” For example, how many members of the general public know what it means to be “carbon-neutral” or understand the difference between carbon and CO2 ?

Socolow, who has extensive interactions with journalists and the public, says, “One of the problems with this field is that some people are talking about costs per ton of carbon and others are talking about costs per ton of CO2 . It’s the most common mistake that journalists make.”

If the distinction between these units isn’t understood, the calculations and estimates quickly become confusing. “The conversation becomes hopeless in about five minutes,” says Socolow. “This is a job for high school and college chemistry teachers.”

The subject of carbon mitigation offers a wealth of good examples for teaching chemistry at all levels, from middle school to graduate school. Examples include pH and buffering (the interaction of CO2 , carbonic acid, the ocean, and the global carbon cycle), spectroscopy (the interaction of various greenhouse gases and infrared radiation), and states of matter (CO2 injected and stored underground is “supercritical”).

Chemistry—the Central Science

Near the end of An Inconvenient Truth, after he presents the evidence that global warming is indeed occurring, Gore says, “If we accept that this problem is real, maybe it’s just too big to do anything about. There are a lot of people who go straight from denial to despair, without pausing on the intermediate step of doing something about the problem.” At this point in the film, Gore introduces the Pacala–Socolow strategy, saying, “We already know everything we need to know to effectively address this problem. We’ve got to do a lot of things, not just one.”

Gore’s words are directed at an audience of the general public, but his words are especially relevant for us chemists. We occupy a central place in the sciences, and we can also occupy a central place in that space “between denial and despair.” We have the knowledge to do something about the problem, either by working directly on carbon-mitigation technologies or by helping the public understand the underlying chemistry.

So, what are you waiting for? Pick a wedge and get to work.

For further information

  • Carbon Mitigation Initiative, www.princeton.edu/~cmi
  • Chemistry of the Environment, Second Edition, Thomas G. Spiro and William M. Stigliani, Prentice Hall, 2003
  • “A Plan to Keep Carbon in Check,” R. Socolow and S. Pacala, Scientific American; Sept 2006, Vol. 295, Issue 3, pp 50–57
  • “Stabilization Wedges: Solving the Climate Problem for the Next 50 Years with Current Technologies,” S. Pacala and R. Socolow, Science, Aug 13, 2004, Vol. 305, pp 968–972

Randy Wedin (ACS ’77) writes from Wayzata, MN. After spending a decade working for the ACS and as a Congressional Science Fellow, he launched a freelance writing business, Wedin Communications, in 1992.