Climate stabilization wedge

The approach presents global warming as a problem which can be attacked using commercially available technologies to reduce CO₂ emissions. Selecting a set of mitigation strategies to create a stabilization triangle is a planning framework for identifying possible interventions for the reduction of emissions. The objective is to stabilize CO
₂ concentrations under 500 ppm over fifty years, by choosing strategies for mitigation as represented by wedges. Each wedge represents 25 gigatons of avoided carbon emissions over a fifty-year period.

This approach to presenting complex information about mitigation was introduced in 2004 by Princeton University researchers Stephen Pacala and Robert H. Socolow. In 2004, Pacala and Socolow estimated that seven wedges worth of improvements would be needed to mitigate climate change by 2054, if serious actions were taken immediately. In 2011, they increased their estimate from seven to nine wedges, given the ongoing increase in emissions since the original 2004 paper. Some researchers have estimated that the number of wedges needed to mitigate climate change may be much higher. A 2020 update by the Carbon Mitigation Initiative projected mitigation efforts to 2060 with eight wedges. Both the urgency and the difficulty of achieving substantial changes are emphasized.

In 2021, a summary of progress towards 16 climate mitigation strategies, expressed in terms of stabilization wedges, reported that while some areas have seen substantial progress (e.g. improving vehicle efficiency) others have seen little or negative progress (e.g. loss of tropical tree cover in Asia and Africa). Net progress as of 2021 is estimated to be only about 1.5 ± 0.9 of the originally estimated seven wedges.

Concept

A stabilization wedge is an action which incrementally reduces projected emissions. The name is derived from the triangular shape of the gap between reduced and unreduced emissions trajectories when graphed over time. For example, a reduction in electricity demand due to increased efficiency means that less electricity needs to be generated and thus fewer emissions need to be produced. The term originates in the Stabilization Wedge Game.

As a reference unit, a stabilization wedge is equal to the following examples of mitigation initiatives: deployment of two hundred thousand 10 MW wind turbines; completely halting the deforestation and planting of 300 million hectares of trees; the increase in the average energy efficiency of all the world's buildings by 25 percent; or the installation of carbon capture and storage facilities in 800 large coal-fired power plants. Pacala and Socolow proposed in their work, Stabilization Wedges, that seven wedges are required to be delivered by 2050 – at current technologies – to make a significant impact on the mitigation of climate change. There are, however, sources that estimate the need for 14 wedges because Pacala and Socolow's proposal would only stabilize carbon dioxide emissions at current levels but not the atmospheric concentration, which is increasing by more than 2 ppm/year. In 2011, Socolow revised their earlier estimate to nine.

Scenario

Emissions of CO
₂ and other greenhouse gases have been increasing ever since the Industrial Revolution. If the trend continues to hold, emissions will double by 2055. To prevent the worst consequences of global warming, scientists recommend freezing and reducing net global emissions immediately.

Stabilization triangle

If global emissions of CO
₂ are graphed for the next 50 years, the difference between the business as usual scenario and the flat path forms a triangle. This triangle is known as the stabilization triangle. Pacala and Socolow initially divided this hypothetical triangle into seven stabilization wedges, which represent different initiatives that must be taken to reduce emissions. When speaking of different strategies to reduce emissions, the language "to reduce one wedge's worth," is often employed. Reducing a projected fourteen gigatons of carbon emissions into seven wedges in a stabilization triangle makes the task easier to conceptualize.

"A wedge represents an activity that reduces emissions to the atmosphere that starts at zero today and increases to 1 giga tonne per year of reduced carbon emissions in 50 years, a cumulative total of 25 giga tonnes of emission reduction over 50 years."

Wedge strategies

Pacala and Socolow originally presented the wedges concept in Science in 2004. In that paper they identified fifteen different wedge strategies. Regarding the specific number, Socolow says that he and Pacala didn't include all of the possibilities, but that "It was a matter of rhetoric to stop at 15. And exhaustion. There was nothing magic about 15."

Wedges tend to fall into three categories: dealing with energy demand, dealing with energy supply, and dealing with CO
₂ capture and storage. Each wedge represents a billion metric tons of carbon per year. As of 2020 the CMI website presents the fifteen strategies and groups them into nine categories, as follows:

1. Efficiency
2. Fuel switching
3. Carbon capture and storage
4. Forest and agricultural soils
5. Nuclear
6. Wind
7. Solar
8. Biomass fuels
9. Natural sinks

In addition to limiting emissions of CO
₂, such changes offer public health cobenefits such as reduced air pollution, enhanced fitness, and regulation of infectious disease.

Like many other discussions of global climate change, the majority of Pacala and Socolow's wedges focus on improvements in energy efficiency. A couple address limiting consumption, and none consider population reduction. Yet economic and demographic growth have been identified as fundamental drivers of global climate change. Of the fifteen wedges developed by Pacala and Socolow, only one—halving the number of miles driven by the world's automobile fleet—might be considered a "demand reduction" wedge. Other researchers have discussed the potential for a range of mitigation strategies including modified diet and population growth rate.