The Wedge Game Developed by: Rob Socolow and Steve Pacala Carbon Mitigation Initiative Princeton University www.princeton.edu/~cmi

Historical emissions profile 2055 2005 14 7 Billions of Tonnes of Carbon Emitted per Year 1955 Historical emissions 1.9  2105 Science, Vol 305, Issue 5686, 968-972, 13 Aug. 2004 Scientific American, Special Issue, pp. 50-57, Sept. 2006 Carbon emissions from fossil fuel burning have grown rapidly since WWII, from less than 2 billion metric tons in 1955 to nearly 7 billion in 2005. (Metric tons are sometimes abbreviated as tonnes.)

Stabilization Triangle The stabilization triangle Billions of Tonnes of Carbon Emitted per Year 14 Currently projected path Stabilization Triangle Interim Goal 500 - 550ppm in 50 years. Steep decline beyond Flat path O 7 Historical emissions These emissions are projected to double in the next 50 years, keeping the world on course to more than triple the atmosphere’s carbon dioxide (CO2) concentration from its pre-industrial level. Models suggest that at much higher concentration levels, this path (dashed black line) could lead to significant global warming by the end of this century, along with decreased crop yields, increased threats to human health, and more frequent extreme weather events. In contrast, if emissions can be kept flat over the next 50 years (orange line), we can steer a safer course. The flat path, followed by emissions reductions later in the century, is predicted to limit CO2 rise to less than a doubling and skirt the worst predicted consequences of climate change. 1955 2005 2055 2105

Wedges O 14 BtC/y Historical 7 BtC/y emissions Seven “wedges” 2055 2005 14 7 Billion of Tonnes of Carbon Emitted per Year 1955 Currently projected path Flat path Historical emissions 2105 14 BtC/y Seven “wedges” O 7 BtC/y Keeping emissions flat for 50 years will require trimming projected carbon output by roughly 7 billion tons per year by 2055. Princeton refers to this carbon savings as the “stabilization triangle.” The exercise they have developed is to take a number of existing technological changes that could be put into place starting today that would make a difference. The goal is to find 7 “wedges” each worth 1 BtC/yr in 50 years, that add up to the entire triangle.

What is a wedge? An activity that reduces emissions to the atmosphere that starts at zero today and increases linearly until it accounts for 1 BtC/yr of reduced carbon emissions in 2055 1 BtC/yr Total = 25B tonnes of carbon While these technologies are available today, they will need to be scaled up from a baseline of zero today to 1 BtC/yr in 50 years. This is a right triangle, so the area is ½ W x H = 25 billion tonnes of C. All 7 wedges added together would keep a total of ~175 billion tons of carbon from entering the atmosphere. 50 years

What are the options? Nuclear Fission CO2 Capture and Storage 14 BtC/y Fuel Switching Energy Efficiency and Conservation Stabilization Forests & Soils Triangle Options include nuclear power that could offset the use of coal-fired power plants; switching from coal to natural gas; energy efficiency, including increasing car fuel efficiency or building heat efficiency; using renewable energy like wind or solar power; changing land use practices, like switching to no-till agriculture, or reversing deforestation; and carbon sequestration, which entails capturing CO2 from power plants and storing it safely in geologic formations. YOUR TASK: You have definitions of each of the wedges, and what each would require. You must now look at each of the definitions and pick the 7 that you think most likely economically, politically, and socially. You have 20 minutes to pick your favorite 7 wedges, and then will be asked to explain your choices. NOTE: There is no “right” answer. The point of the exercise is that these are achievable goals, but each will require significant effort by society. Also, it is unlikely that any one technology can save the day alone – it will take a suite of technologies. 7 BtC/y 2005 2055 Renewable Electricity and Fuels

Your Task You have definitions of each of the wedges, and what each would require. You must now look at each of the definitions and pick the 7 that you think most likely economically, politically, and socially. You have 20 minutes to pick your favorite 7 wedges, and then will be asked to explain your choices. NOTE: There is no “right” answer. The point of the exercise is that these are achievable goals, but each will require significant effort by society. Also, it is unlikely that any one technology can save the day alone – it will take a suite of technologies.