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Stabilization wedges (Rev #2, changes)

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Stephen Pacala and Robert Socolow have sketched a flexible plan for tackling the global warming problem:

They list 15 measures, each of which could reduce carbon emissions by 1 billion tons per year by 2057. They believe global warming would be manageable, though still a serious problem, if 12 of these measures were carried out by that time. They call these measures stabilization wedges, thanks to their appearance in a chart that illustrates their effects.


Here are the 15 stabilization wedges:

The problem to be solved

Pacala and Socolow wrote this now-famous paper in 2004. Back then we were emitting about 6.2 gigatons of carbon per year, there were 375 ppm of carbon dioxide in the atmosphere, and many proposals to limit global warming urged that we keep the concentration below 500 ppm. Their paper outlined some strategies for keeping it below 500 ppm.

They estimated that to do this, it would be enough to hold emissions flat at 7 gigatons of carbon per year for 50 years, and then lower it to nothing. On the other hand, in a “business as usual” scenario, they estimate the emissions would ramp up to 14 gigatons per year by 2054. That’s 7 too many.

So, to keep emissions flat it would be enough to find 7 ways to reduce carbon emissions, each one of which ramps up linearly to the point of reducing carbon emissions by 1 gigaton/year in 2054. They called these stabilization wedges, because if you graph them, they look like wedges:

Despite the above graph, they don’t really think carbon emissions will increase linearly in a business-as-usual scenario. This is just a deliberate simplification on their part. They also show this supposedly more accurate graph:

They say the top curve is “a representative business as usual emissions path” for global carbon emissions in the form of CO2 from fossil fuel combustion and cement manufacture, assuming 1.5% per year growth starting from 7.0 GtC/year in 2004. Note this ignores carbon emissions from deforestation, other greenhouse gases, etc. This curve is growing exponentially, not linearly.

Similarly, the bottom curve isn’t flat: it slopes down. They say the bottom curve is a “CO2 emissions path consistent with atmospheric CO2 stabilization at 500 ppm by 2125 akin to the Wigley, Richels, and Edmonds (WRE) family of stabilization curves described in [11], modified as described in Section 1 of the SOM text.”

Here reference [11] is:

  • T. M. L. Wigley, in The Carbon Cycle, eds. T. M. L. Wigley and D. S. Schimel, Cambridge U. Press, Cambridge, 2000, pp. 258–276.

and the “SOM text” is the supporting online material for their paper, which unfortunately doesn’t seem to be available for free.

The proposed solutions

Their paper listed 15 possible stabilization wedges, each one with the potential to reduce carbon emissions by 1 gigaton/year by 2054. This is a nice way to start thinking about a very big problem, so many people have adopted it and modified it and criticized it in various ways, which I hope to discuss later.

Before listing their stabilization wedges, we should emphasize: stabilizing emissions at 7 gigatons is not enough to stay below 500 ppm forever! Carbon dioxide stays in the atmosphere a very long time. So, as Pacala and Socolow note:

Stabilization at any level requires that net emissions do not simply remain constant, but eventually drop to zero. For example, in one simple model that begins with the stabilization triangle but looks beyond 2054, 500-ppm stabilization is achieved by 50 years of flat emissions, followed by a linear decline of about two-thirds in the following 50 years, and a very slow decline thereafter that matches the declining ocean sink. To develop the revolutionary technologies required for such large emissions reductions in the second half of the century, enhanced research and development would have to begin immediately.

What’s the “declining ocean sink”? Right now the ocean is absorbing a lot of CO2, temporarily saving us from the full brunt of our carbon emissions — while coral reefs, shellfish and certain forms of plankton suffer from increased acidity. But this won’t go on forever; the ocean has limited capacity.

Pacala and Socolow consider several categories of stabilization wedges:

  • efficiency and conservation
  • shifting from coal to gas
  • carbon capture and storage
  • nuclear fission
  • renewable energy sources
  • forests and agriculture

Here are all 15 stabilization wedges:


Here’s We a shall summary describe of what Pacala and Socolow analyze claim: these in turn.

  • If we adopt 12 of these measures, we could lower our carbon emissions from the current figure of 8 billion tons per year to 4 billion tons per year by 2057. This might mean 450 parts per million of CO2 in the atmosphere by this time, and a global temperature rise of 2 °C. With this, we could still expect coastal flooding that affects millions of people per year. Cereal crop yields will tend to decrease in low latitudes. And up to 30% of species might face the risk of extinction, with most coral reefs being bleached. But this is the “good” scenario.

  • If adopt only 8 of the measures, we could hold our carbon emissions constant at the current figure of 8 billion tons per year. This might mean 525 ppm of CO2 in the atmosphere, and a global temperature rise of 3 °C. With this, we can expect the widespread death of coral reefs. We can also expect the bad consequences listed above, and: 30% of coastal wetlands being lost, with most ecosystems becoming carbon sources as permafrost thaws and vegetation burns or rots.

  • If we adopt none of the measures, we can expect carbon emissions to double by 2057, to 16 billion tons per year. This might mean 800 ppm of CO2 in the atmosphere, and a global temperature rise of 5 °C. With this, we can expect that more than 40% of species will face extinction. We can also expect the bad consequences listed above, and cereal crop yields decreasing in some mid- to high-latitude regions.

Efficiency and conservation

  1. Efficient vehicles: increase the fuel economy for 2 billion cars from 30 to 60 miles per gallon. Or, for those of you who don’t have the incredible good luck of living in the USA: increasing it from 13 to 26 kilometers per liter. When they wrote their paper, there were 500 million cars on the planet. They expected that by 2054 this number would quadruple. When they wrote their paper, average fuel efficiency was 13 kilometers/liter. To achieve this wedge, we’d need that to double.

  2. Reduced use of vehicles: decrease car travel for 2 billion 30-mpg cars from 10,000 to 5000 miles per year. In other words: decreasing the average travel from 16,000 to 8000 kilometers per year. (Clearly this wedge and the previous one are not additive: if we do them both, we don’t save 2 gigatons of carbon per year.)

  3. Efficient buildings: cut carbon emissions by one-fourth in buildings and appliances. This could be done by following “known and established approaches” to energy efficient space heating and cooling, water heating, lighting, and refrigeration. Half the potential savings are in the buildings in developing countries.

  4. Efficient coal plants: raise the efficiency of coal power plants to 60%. In 2004, when they wrote their paper, “coal plants, operating on average at 32% efficiency, produced about one fourth of all carbon emissions: 1.7 GtC/year out of 6.2 GtC/year.” They expected coal power plants to double their output by 2054. To achieve this wedge, we’d need their average efficiency to reach 60%.

category: action