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Stabilization wedges


In 2004 Stephen Pacala and Robert Socolow sketched a flexible plan for tackling the global warming problem for the next 50 years using only present technologies:

As they write:

The debate in the current literature about stabilizing atmospheric CO2 at less than a doubling of the preindustrial concentration has led to needless confusion about current options for mitigation. On one side, the Intergovernmental Panel on Climate Change (IPCC) has claimed that “technologies that exist in operation or pilot stage today” are sufficient to follow a less-than-doubling trajectory “over the next hundred years or more”. On the other side, a recent review in Science asserts that the IPCC claim demonstrates “misperceptions of technological readiness” and calls for “revolutionary changes” in mitigation technology, such as fusion, space-based solar electricity, and artificial photosynthesis. We agree that fundamental research is vital to develop the revolutionary mitigation strategies needed in the second half of this century and beyond. But it is important not to become beguiled by the possibility of revolutionary technology. 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.

They listed 15 measures, each of which could reduce carbon emissions by 1 billion tons per year by 2054. They claimed that global warming would be manageable, though still a serious problem, if 7 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.

In 2011, Socolow said the number of wedges required to hold carbon emissions constant for the next 50 years (i.e., until 2061) had gone up to 9:



The problem to be solved

Pacala and Socolow wrote this now-famous paper in 2004. Back then we were emitting about 6.2 gigatonnes 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 gigatonnes 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 gigatonnes 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 gigatonne/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 gigatonne/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 gigatonnes 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

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 gigatonnes 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%.

Wedge 5: shifting from coal to gas

5) Shifting from coal to natural gas: replace 1400 gigawatts coal-burning power plants with gas-burning plants. Natural gas puts out half as much CO2 as coal does when you burn them to make a given amount of electricity. After all, it’s mainly methane, which is made from hydrogen as well as carbon. Suppose by 2054 we have coal power plants working at 90% of capacity with an efficiency of 50%. 700 gigawatts worth of coal plants like this emit 1 gigatonne of carbon per year. So, we can reduce carbon emissions by one ‘wedge’ if we replace 1400 gigawatts of such plants with gas-burning plants.

That is four times the 2004 worldwide total of gas-burning plants.

Wedges 6-8: carbon capture and storage

6) Capturing CO2 at power plants: 800 GW of coal plants or 1600 GW of gas plants . Carbon capture and storage at power plants can stop about 90% of the carbon from reaching the atmosphere, so we can get a wedge by doing this for 800 GW of baseload coal plants or 1600 GW of baseload gas plants by 2054. One way to do carbon capture and storage is to make hydrogen and CO2, burn the hydrogen in a power plant, and inject the CO2 into the ground. So, from one viewpoint, building a wedge’s worth of carbon capture and storage would resemble a tenfold expansion of the plants that were manufacturing hydrogen in 2004. But it would also require multiplying by 100 the existing amount of CO2 injected into the ground.

7) Capturing CO2 at plants that make hydrogen for fuel: 250 megatons/year made from coal, or twice as much made from gas. You’ve probably heard people dream of a hydrogen economy?. But it takes energy to make hydrogen. One way is to copy wedge 6, but then ship the hydrogen off for use as fuel instead of burning it to make electricity at power plants. To capture a wedge’s worth of carbon this way, we’d have to make 250 megatons of hydrogen per year from coal, or 500 megatons per year from natural gas. This would require a substantial scale-up from the 2004 total of 40 megatons of hydrogen manufactured by all methods. There would also be the task of building the infrastructure for a hydrogen economy. The challenge of injecting CO2 into the ground would be the same as in wedge 6.

8) Capturing CO2 at plants that turn coal into synthetic fuels: 1.8 teraliters of synfuels per year. As the world starts running out of oil, people may start turning coal into synfuels, via a process called coal liquefaction. Of course burning these synfuels will release carbon. But suppose only half of the carbon entering a synfuels plant leaves as fuel, while the other half can be captured as CO2 and injected underground. Then we can capture a wedge’s worth of CO2 from plants that produce 1.8 teraliters of synfuels per year. For comparison, total yearly world oil production in 2004 was 4.7 teraliters

Wedge 9: nuclear power

9) Replacing 700 gigawatts of coal-fired power plans with nuclear power. As Pacala and Socolow already argued in wedge 5), replacing 700 gigawatts of efficient coal-fired power plants with some carbon-neutral form of power would keep us from burning one gigatonne of carbon per year. To do this with nuclear power would require 700 gigawatts of nuclear power plants running at 90% capacity (just as assumed for the coal plants). The means doubling the world production of nuclear power. The global pace of nuclear power plant construction from 1975 to 1990 could do this! But of course, there’s still a downside: we can only substantially boost the use of nuclear power if people become confident about all aspects of its safety.

Wedges 10-13: renewable energy

10) Replacing 700 gigawatts of coal-fired power plants by 2000 gigawatts of peak wind power. Wind power is intermittent: Pacala and Socolow estimate that the ‘peak’ capacity (the amount you get under ideal circumstances) is about 3 times the ‘baseload’ capacity (the amount you can count on). So, to save a gigatonne of carbon per year by replacing 700 gigawatts of coal-fired power plants, we need roughly 2000 gigawatts of peak wind power. Wind power was growing at about 30% per year when they wrote their paper, and it had reached a world total of 40 gigawatts. So, getting to 2000 gigawatts would mean multiplying the world production of wind power by a factor of 50.

The wind turbines would ‘occupy’ about 30 million hectares, or about 30-45 square meters per person — some on land and some offshore. But because windmills are widely spaced, land with windmills can have multiple uses. To increase something by a factor of 50 in 50 years, it’s enough to maintain an annual growth rate of slightly more than 8%.

11) Replacing 700 gigawatts of coal-fired power plants by 2000 gigawatts of peak photovoltaic solar power. Solar power is also intermittent. Pacala and Socolow estimate that to save a gigatonne of carbon per year, we need 2000 gigawatts of peak photovoltaic solar power to replace coal. Like wind, photovoltaic solar was growing at 30% per year when Pacala and Socolow wrote their paper. However, only 3 gigawatts had been installed worldwide. So, getting to 2000 gigawatts would require multiplying the world production of photovoltaic solar power by a factor of 700.

In terms of land, this would take about 2 million hectares, or 2-3 square meters per person. To increase photovoltaic power by a factor of 700 in 50 years, it’s enough to maintain a 14% annual growth rate.

12) Using 4000 gigawatts of peak wind power to generate hydrogen for powering automobiles. Renewable energy can be used to produce hydrogen for vehicle fuel. 4000 gigawatts of peak wind power, for example, used in high-efficiency fuel-cell cars, could keep us from burning a gigatonne of carbon each year in the form of gasoline or diesel fuel. Unfortunately, this is twice as much wind power as we’d need in wedge 10, where we use wind to eliminate the need for burning some coal. Why? Gasoline and diesel have less carbon per unit of energy than coal does.

13) Making 5.4 gigaliters of bioethanol per day to replace gasoline. Fossil fuels can also be replaced by biofuels such as ethanol. To save a gigatonne per year of carbon, we could make 5.4 gigaliters per day of ethanol as a replacement for gasoline — provided the process of making this ethanol didn’t burn any fossil fuels! Doing this would require multiplying the world production of bioethanol by a factor of 50. It would require 250 million hectares committed to high-yield plantations, or 250-375 square meters per person. That’s an area equal to about one-sixth of the world’s cropland. An even larger area would be required to the extent that the biofuels require fossil-fuel inputs. Clearly this could cut into the land used for growing food.

Wedge 14 and 15

14) Stop deforestation, start reforestation. They say we could stop half a gigatonne of carbon emissions per if we completely stopped clear-cutting tropical forests over 50 years, instead of just halving the rate at which they’re getting cut down. For another half gigatonne, plant 250 million hectares of new forests in the tropics, or 400 million hectares in the temperate zone!

To get a sense of the magnitude here, note that current areas of tropical and temperate forests are 1500 and 700 million hectares, respectively.

Pacala and Socolow also say that another half gigatonne of carbon emissions could be prevented by created by establishing approximately 300 million hectares of plantations on nonforested land.

15) Soil management. When forest or grassland is converted to cropland, up to one-half of the soil carbon gets converted to CO2, mainly because tilling increases the rate of decomposition by aerating undecomposed organic matter. Over the course of history, they claim, 55 gigatonnes of carbon has gone into the atmosphere this way. That’s the equivalent of two wedges. (Note that one wedge, ramping up linearly to 1 gigatonne/year for 50 years, adds up to 25 gigatonnes of carbon by 2054.)

However, good agricultural practices like no-till farming can reverse these losses — also reduce erosion! By 1995, these practices had been adopted on 110 million of the world’s 1600 million hectares of cropland. If this could be extended to all cropland, accompanied by a verification program that enforces practices that actually work as advertised, somewhere between half and one gigatonne of carbon per year could be stored in this way. So: maybe half a wedge, maybe a whole wedge!


As a general remark, it seems a little unrealistic to only list technologies that are currently viable on large scales when we’re talking about what we’ll be doing 50 years in the future. But the point that Pacala and Socolow want to make is that keeping emissions flat is possible with current technologies only.

Wedges 1 and 2: vehicles

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. However, this quantity is only indirectly related to the growth in number of cars on the road.)

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 rebound effect 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. For some empirical data, see:

Abstract: We estimate the rebound effect for motor vehicles, by which improved fuel efficiency causes additional travel, using a pooled cross section of US states for 1966-2001. Our model accounts for endogenous changes in fuel efficiency, distinguishes between autocorrelation and lagged effects, includes a measure of the stringency of fuel-economy standards, and allows the rebound effect to vary with income, urbanization, and the fuel cost of driving. At sample averages of variables, our simultaneous-equations estimates of the short- and long-run rebound effect are 4.5% and 22.2%. But rising real income caused it to diminish substantially over the period, aided by falling fuel prices. With variables at 1997-2001 levels, our estimates are only 2.2% and 10.7%, considerably smaller than values typically assumed for policy analysis. With income at the 1997 – 2001 level and fuel prices at the sample average, the estimates are 3.1% and 15.3%, respectively.

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.

We need references here.

As trend indicators, calculated risk has a graph of US vehicle miles travelled shows US miles driven has begun increasing again.

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 we find more information on similar programs?

See also:

Wedge 5 and 6: shifting from coal to gas and carbon capture

Wedge 5 and 6 would certainly lower the amount of carbon released into the atmosphere, but there are some problems.

Wedge 5: how can a shift from coal to gas be enforced? Some Western countries are currently burning less coal voluntarily, but instead they are shipping it China ( International Herald Tribune , 23/11/10). An idea could be to pay countries to temporarily stop mining coal.

Wedge 6: there is some discussion about the feasibility of carbon capture and storage.

These issues are discussed into more detail on carbon capture and storage.

Wedge 7: hydrogen economy

So far, there have been no comments on wedge 7, but there are also ideas for a methanol economy.

Some thoughts on hydrogen as an energy carrier: outlook for 2010, 2030 and 2050 from the year that Pacala and Sokolow published their paper on the stabilization wedges:

Wedge 8: coal into synfuels

Pacala and Socolow say 1 gigatonne carbon/year is the flow of carbon in 24 million barrels/day, or 1.4 teraliters/year. They assume the same value for synfuels and allow for imperfect capture, which leads them to conclude that carbon capture at synfuels plants producing 1.8 teraliters/year of synfuel can catch 1 GtC/year. Does this calculation make sense? If we’re catching just half the carbon, and 1 GtC/year = 1.4 teraliters oil/year, don’t we need to generate at least twice that — 2.8 teraliters synfuel/year — to catch wedge’s worth of carbon?

It’s also unclear what percentage of the carbon you can actually capture while turning coal into synfuels. Can we really capture half of it?

There’s also another funny feature of this last wedge. If we assume people are already committed to making synfuels from coal, then it may be true they will emit less carbon if they use carbon capture and storage as part of the manufacturing process. But compared to making electricity or hydrogen as in wedges 6 and 7, turning coal into synfuels seems bound to emit more carbon, even with the help of carbon capture and storage.

In general, it only makes sense to talk about how much carbon emission some action prevents when we compare it to some alternative action. That’s pretty obvious, but it gets a bit confusing when some of Pacala and Socolow’s wedges look like plausible alternatives to other ones.

Wedge 9: nuclear power

From a technological point of view this wedge could be achieved rather easily, but there is (at least in some Western countries) a strong opposition against nuclear power (a “culture war”). Nevertheless, as a replacement of coal-fired power plants, the external costs of nuclear power should be compared to those of burning coal.

Wedge 10: wind power

A problematic issue with wedges 10-13 is that if burning coal will keep on providing the cheapest form of energy (neglecting externalities) it will be difficult to replace it with other, more expensive, forms of energy.

Regarding Pacala and Socolow’s calculation for wind power: it does not suffice to account for intermittency by equating baseload capacity to one third of nominal peak capacity, one needs to supplement wind farms with e.g. pumped storage systems to account for variabilities such as lulls. See Capacity factor of wind power for a more information.

2009 reassessment

As mentioned, to use wind power to reduce our coal burning by 1 gigatonne of carbon per year, we would need to increase the amount of peak wind power worldwide by a factor of 50, starting from its 2004 level.

This sounds difficult, but to grow by a factor of 50 over 50 years, wind power would only need to grow at an average annual rate of 8.3%. And according to the Renewables 2010 Global Status Report (page 16), the average annual growth rate over the five-year period from the end of 2004 to 2009 was much higher than this: namely, 27%.

According to the same source, peak wind power capacity reached 159 gigawatts in 2009. So at that time, reaching Pacala and Socolow’s goal of 2000 gigawatts required multiplying the world production of wind power by a factor of 12.5. To reach this goal by 2054 required an average annual growth rate of only 5.8%.

2011 reassessment

By 2011, peak wind power capacity reached 238 gigawatts. So at this time, reaching Pacala and Socolow’s goal of 2000 gigawatts by 2054 would require multiplying wind power by a factor of 8.4. This would require a 5% average annual growth of wind power over the next 43 years. The growth rate was 20%, but slowing.

2012 reassessment

By 2012, peak wind power capacity reached 283 gigawatts. So at this time, reaching Pacala and Socolow’s goal of 2000 gigawatts by 2054 would require multiplying wind power by a factor of 7.1. This would require a 4.8% average annual growth of wind power over the next 42 years. The growth rate was 19%, but slowing.

Wedge 11: solar power

Most of the discussion on the blog about solar panels was focused on their average power output and the amount of land needed, etc., instead of whether the necessary growth rate for solar power is feasible. For more information and specific critiques of technologies, see e.g. wind power, solar power and biofuels.

2009 reassessment

As mentioned, to use photovoltaic solar power to reduce our coal burning by 1 gigatonne of carbon per year, we would need to increase the amount of peak photovoltaic solar power worldwide to 2000 gigawatts. This required increasing it by a factor of 700 starting from its 2004 level. This sounds difficult, but to grow by a factor of 700 over 50 years, it would only need to grow at an average annual rate of 14%. And according to Renewables 2010 (page 15), the average annual growth rate over the five-year period from the end of 2004 to 2009 was much higher than this: namely, 60%.

According to the same source, by 2009 peak photovoltaic power reached 24-25 gigawatts worldwide. (Of this, only 21 gigawatts were grid-connected.) So, at this time, reaching Pacala and Socolow’s goal of 2000 gigawatts would require multiplying the world’s peak photovoltaic solar power by a factor of 80. To reach this goal by 2054 would require an average annual growth rate of 10.3%.

2011 reassessment

By 2011, peak solar photovoltaic capacity reached 71 gigawatts. So at this time, reaching Pacala and Socolow’s goal of 2000 gigawatts by 2054 would require multiplying solar power by a factor of 30. This would require an 8% average annual growth rate over the next 43 years. In 2011 the rate was 75%.

2012 reassessment

By 2012, peak solar photovoltaic capacity reached 100 gigawatts. So at this time, reaching Pacala and Socolow’s goal of 2000 gigawatts by 2054 would require multiplying solar power by a factor of 20. This would require an 7.4% average annual growth rate over the next 42 years. In 2012 the rate was 41%, but slowing.

Wedge 12: wind for hydrogen-powered vehicles

It is remarkable that Pacala and Socolow opt for hydrogen vehicles instead of EVs/PHEVs as the former are less efficient and more expensive. The following argument leads to the estimate that a hydrogen powered vehicle fleet would require ~1.5 to 3 times as much renewable generation as an electric version.

Wedge 14: Stop deforestation, start reforestation

The UN’s REDD+ program is a plan to reduce emissions from deforestation and forest degradation. There is some controversy, see e.g. the Wikipedia article. There’s also a lot of argument about just how much long-term impact on atmospheric CO2 a standing forest has.

In any case, forest monitoring is possible by remote sensing, see e.g.

Pacala’s re-evaluation

In 2008 Pacala re-evaluated his paper with Socolow in the light of new evidence. He said of the original paper:

The message was a very positive one: “gee, we can solve this problem: there are lots of ways to solve it, and lots of ways for the marketplace to solve it.”

Based on new evidence, he said:

It’s at least possible that we’ve already let this thing go too far, and that the biosphere may start to fall apart on us, even if we do all this. We may have to fall back on some sort of dramatic Plan B. We have to stay vigilant as a species.

You can watch his talk here:

The discussion of recent evidence begins at 10:58, and at 33:34 he brings up a slide summarizing the reasons he is more pessimistic. What follows is a summary of a few key points.

Emissions targets

Pacala begins by reviewing different carbon emissions targets.

The old scientific view, circa 1998: if we could keep the CO2 from doubling from its preindustrial level of 280 parts per million, that would count as a success. Namely, most of the ‘monsters behind the door’ would not come out: continental ice sheets falling into the sea and swamping coastal cities, the collapse of the Atlantic ocean circulation, a drought in the Sahel region of Africa, etcetera.

Many experts say we’d be lucky to get away with CO2 merely doubling. At current burn rates we’ll double it by 2050, and quadruple it by the end of this century. We’ve got enough fossil fuels to send it to seven times its preindustrial levels.

Doubling it would take us to 560 parts per million. A lot of people think that’s too high to be safe. But going for lower levels gets harder:

  • In Pacala and Socolow’s original paper, they talked about keeping CO2 below 500 ppm. This would require keeping CO2 emissions constant until 2050. This could be achieved by a radical decarbonization of the economies of rich countries, while allowing carbon emissions in poor countries to grow almost freely until that time.

  • For a long time the IPCC and many organizations advocated keeping CO2 below 450 ppm. This would require cutting CO2 emissions by 50% by 2050, which could be achieved by a radical decarbonization in rich countries, and moderate decarbonization in poor countries.

  • But by 2008 the IPCC and many groups wanted a cap of 2°C global warming, or keeping CO2 below 430 ppm. This would mean cutting CO2 emissions by 80% by 2050, which would require a radical decarbonization in both rich and poor countries.

The difference here is what poor people have to do. The rich countries need to radically cut carbon emissions in all these scenarios. In the USA, the Lieberman-Warner bill would have forced the complete decarbonization of the economy by 2050.

Then, Pacala spoke about 3 things that make him nervous:

Faster emissions growth

A 2007 paper by Canadell et al pointed out that starting in 2000, fossil fuel emissions started growing at 3% per year instead of the earlier figure of 1.5%. This could be due to China’s industrialization. Will this keep up in years to come? If so, the original Pacala-Socolow plan won’t work.

On the Azimuth Blog, Nathan Urban gives an update on this issue:

The recession did cut into the global emissions growth rate, as described in Friedlingstein et al. (2010) (considering emissions through 2009). The 3%/year acceleration is largely due to China, from what I’ve read. They can’t keep that growth up forever, but they don’t have to in order to require more wedges.

The ocean sink

Each year fossil fuel burning puts about 8 gigatons of carbon in the atmosphere. The ocean absorbs about 2 gigatons and the land absorbs about 2, leaving about 4 gigatons in the atmosphere.

However, as CO2 emissions rise, the oceanic CO2 sink has been growing less than anticipated. This seems to be due to a change in wind patterns, itself a consequence of global warming.

On the Azimuth Blog, Nathan Urban gives an update on this issue:

The story on the ocean carbon sink, particularly the wind-driven mixing in the Southern Ocean, is still ambiguous, as is the story on terrestrial CO2 fertilization. For the ocean sink, you could start with Le Quéré et al. (2007) and scan forward through the papers that cite it.

The land sink

As the CO2 levels go up, people expected plants to grow better and suck up more CO2. In the third IPCC report, models predicted that by 2050, plants will be drawing down 6 gigatonnes more carbon per year than they do now! The fourth IPCC report was similar.

This is huge: remember that in 2011 we emitted about 10.4 gigatonnes. Indeed, this effect, called CO2 fertilization, could be the difference between the land being a big carbon sink and a big carbon source. Why a carbon source? For one thing, without the plants sucking up CO2, temperatures will rise faster, and the Amazon rainforest may start to die, and permafrost in the Arctic may release more greenhouse gases (especially methane) as it melts.

In a simulation run by Pacala, where he deliberately assumed that plants fail to suck up more carbon dioxide, these effects happened and the biosphere dumped a huge amount of extra CO2 into the atmosphere: the equivalent of 26 stabilization wedges.

So, plans based on the IPCC models are essentially counting on plants to save us from ourselves.

But is there any reason to think plants might not suck up CO2 at the predicted rates? Maybe. First, people have actually grown forests in doubled CO2 conditions to see how much faster plants grow then. But the classic experiment along these lines used young trees. In 2005, Körner et al did an experiment using mature trees… and they didn’t see them growing any faster!

Second, models in the third IPCC report assumed that as plants grew faster, they’d have no trouble getting all the nitrogen they need. But Hungate et al have argued otherwise. On the other hand, Alexander Barron discovered that some tropical plants were unexpectedly good at ramping up the rate at which they grab ahold of nitrogen from the atmosphere. But on the third hand, that only applies to the tropics. And on the fourth hand—a complicated problem like this requires one of those Indian gods with lots of hands—nitrogen isn’t the only limiting factor to worry about: there’s also phosphorus, for example.

Pacala goes on and discusses even more complicating factors. But his main point is simple. The details of CO2 fertilization matter a lot. It could make the difference between their original plan being roughly good enough… and being nowhere near good enough!

On the Azimuth Blog, Martin Gisser points out that some recent research on CO2 fertilization looks worrisome:

From satellite observations, Zhao & Running (2010) estimate a 0.55 Gt (ca. 1%) decline in global terrestrial NPP (net primary production) from 2000 to 2009. Between 1982 and 1999 the increase was up to 6%.

This paper suggests that from 1997 to 2006 the Normalized Difference Vegetation Index or NDVI, a measure of the amount of vegetation, has been decreasing:

  • Shilong Piao, Xuhui Wang, Philitppe Ciais, Biao Zhuz, Tao Wang, and Jiu Liu, Changes in satellite-derived vegetation growth trend in temperate and boreal Eurasia from 1982 to 2006, Global Change Biology, preview 31 March 2011.

Abstract (…) although a statistically significant positive trend of average growing season NDVI is observed (0.5×10 30.5 \times 10^{-3} per year, P=0.03P = 0.03) during the entire study period, there are two distinct periods with opposite trends in growing season NDVI. Growing season NDVI has first significantly increased from 1982 to 1997 (1.8×10 31.8 \times 10^{-3} per year, P<0.001P \lt 0.001 ), and then decreased from 1997 to 2006 (1.3×10 3-1.3 \times 10^{-3} per year, P=0.055P = 0.055). (…)

Also see this article:

reporting on this paper:

  • Scott J. Goetz, Andrew G. Bunn, Gregory J. Fiske, R. A. Houghton, Satellite-observed photosynthetic trends across boreal North America associated with climate and fire disturbance, PNAS September 20, 2005 vol. 102.

For parts of the region, growth has not changed (gray), but in interior Alaska and a wide swath of Canada, growth has declined (brown). Only in the far north, regions of tundra, has growth increased (green).

  • Jofre Carnicera, Marta Colla, Miquel Ninyerolac, Xavier Ponsd, Gerardo Sáncheze, Josep Peñuelasa, Widespread crown condition decline, food web disruption, and amplified tree mortality with increased climate change-type drought, PNAS January 25, 2011 vol. 108.

Abstract. Climate change is progressively increasing severe drought events in the Northern Hemisphere, causing regional tree die-off events and contributing to the global reduction of the carbon sink efficiency of forests. (…) Here we report a generalized increase in crown defoliation in southern European forests occurring during 1987–2007. Forest tree species have consistently and significantly altered their crown leaf structures, with increased percentages of defoliation in the drier parts of their distributions in response to increased water deficit. We assessed (…) Our results reveal a complex geographical mosaic of species-specific responses to climate change–driven drought pressures on the Iberian Peninsula, with an overwhelmingly predominant trend toward increased drought damage.

Socolow’s reaffirmation

In 2011, Socolow said the number of wedges required to hold carbon emissions constant for the next 50 years (i.e., until 2061) had gone up from 7 to 9:


Additional wedges

We may consider additional wedges beyond those envisioned by Pacala and Socolow. Some are listed in The Full Global Warming Solution. Here are more:

  • According to the United Nations, in 2006 the total carbon footprint of livestock was 7.1 gigatonnes CO2 equivalent per year. Dividing by 3.6666…, that’s 1.93 gigatonnes of carbon equivalent per year. This is almost two wedges. Conversely, a vegetarian lifestyle lowers emissions by about 1200 kg CO2 equivalent per year. We can estimate that if enough people go vegetarian (or, equivalently, a larger amount of people that eats fewer meat), that would be the equivalent of reducing carbon emissions by one gigatonne per year.


There is a series of posts on the Azimuth Blog:

category: action