The Azimuth Project
Stabilization wedges (Rev #8)



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.


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:


We shall describe and analyze these in turn.

Wedges 1-4: efficiency and conservation

\7.\ 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.

  1. 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.)

  2. 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.

  3. 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%.

Wedge 5: Substituting gas for coal

  1. Substituting natural gas for coal. Carbon emissions per unit of electricity are about half as large from natural gas power plants as from coal plants. Assume that the capacity factor of the average baseload coal plant in 2054 has increased to 90% and that its efficiency has improved to 50%. Because 700 GW of such plants emit carbon


Wedges 1 and 2

Number of cars

The emissions model assumes a four-fold increase in the number of cars in 50 year: is this reasonable? It seems a plausible back-of-the-envelope estimate. Firstly, Worldometers suggests that China and India have lots of “capacity” if they reach a sufficiently high income:

China became the world’s third-largest car market in 2006, as car sales in China soared by nearly 40% to 4.1 million units. China should become the world’s second-largest car market by 2010, as low vehicle penetration, rising incomes, greater credit availability and falling car prices lift sales past those of Japan. Furthermore, vehicle penetration in China stands at only 24 vehicles per 1,000 people, compared with 749 vehicles per 1,000 people in the mature markets of the G7.

Secondly, a four-fold increase in 50 years is about 2.8 percent net increase (i.e. new cars being added minus old cars being removed) per year. Is this too high, too low or about right? According to Worldometers, world car production rose 6.45% in 2006, and in China it rose 40% that year. But in 2008 they report a decline in world car production, and they projected a further decline in 2009. The average world increase in car production from 1999 to 2009, based on their projection for 2009, was 3.2% per year.

Higher efficiency of cars

More fuel efficient vehicles already exist. See, e.g. Most and least fuel efficient cars. The main question is how to urge the consumer to use these cars? One possible solution is higher taxation: both a tax on gasoline, and one on inefficient cars. Related to taxation is the “natural” rise of oil product prices when oil becomes more scarce.

Nevertheless, there is a problem of attitude, that is that large cars are often a status symbol and drivers may be willing to pay more for driving these cars, to promote their social status.

In addition, the Jevons paradox may come into play: that is, simply making cars more energy efficient might lead to more cars purchased or more distance traveled. However, because cars are a status symbol, people voluntarily buying energy efficient cars may also voluntarily try to reduce, or keep equal, their distance traveled. On the other hand, people buying energy efficient cars only to reduce taxation, may use the savings in their budget to travel more.

Another danger of taxation may be that when the tax becomes an income for the government, the government may become less determined to use the tax for what its purpose is for: to reduce the number of inefficient cars.

Finally, it would be reassuring to check that people do respond to taxation in the way the economic model predicts. The evidence in the UK suggests that people don’t respond to tax “incentives” in the smooth way. The already high (by global standards) tax on petrol increased by 8.7% between 2000 and 2010, at a time when incomes and inflation in the UK weren’t increasing much. Over the same time period private car miles driven (admittedly extrapolated from sampling, so possible issues) increased by 4.5 percent per driver (i.e., this isn’t due to there being more drivers) in the same time. Maybe the taxation has suppressed even large rises in miles driven, but at first glance this is not what theoretical economists argue should happen in the presence of rising taxes. Sources:

Another option is to impose stronger emission norms for cars. In this case, energy inefficient cars can neither be produced nor bought.

Less distance traveled

Taxation could also be useful for reducing the distance traveled. In addition, for large distances (> 2020 km) other means of transportation are more energy-efficient than cars. For small distances, transportation by foot and bicycle is also more environmentally friendly. References to be added

Wedge 4: Efficiency of coal plants

The Rankine cycle is the cycle used in steam turbine power plants. The overwhelming majority of the world’s electric power is produced with this cycle. Since the cycle’s working fluid, water, changes from liquid to vapor and back during the cycle, their efficiencies depend on the thermodynamic properties of water.

The thermal efficiency of modern steam turbine plants with reheat cycles can reach 47%, and in combined cycle plants it can approach 60%. An example of the latter is given by the GE H system. There are programs that attempts to develop plants with these and even higher efficiences by 2015, e.g. Vision 21.

Can you give us more information on similar programs?

See also:

category: action