>>Continued from page 1

Pacala and Socolow introduce the idea of a carbon “wedge,” any change that reduces carbon emissions by a 20 million-ton increment each year for 50 years in comparison to business-as-usual practices. At the end of 50 years, each wedge accounts for one billion metric tons of carbon not added to Earth’s atmosphere (Figure 1, right). They propose first stabilizing global carbon emissions with available technologies and then reducing emissions by employing more advanced technologies that will take longer to develop. Since this study was published four years ago, the calculations have been updated to account for recent emission increases and the lack of action between 2004 and 2007.3 According to the update, capping world CO2 emissions now requires eight wedges, compared with the 2004 estimate of seven wedges. Moreover, many climate scientists now believe that it is necessary to reduce CO2 emissions to avert the most severe effects of climate change. Stabilizing emissions is a start, but it won’t be enough to keep temperatures from rising past the 2oC threshold. But delay is harmful. If the world waits another five years to tackle emission reductions, simply capping CO2 emissions at 2004 levels could require 10 wedges.

The Choice is Ours: Selecting the Wedges

Pacala and Socolow identified 15 carbon wedges, each of which would prevent one billion metric tons of carbon emissions by 2054. They identified a technology as a wedge only if it was commercially available now or could be ramped up in scale very soon. Accomplishing any one of these will require major efforts. Table 1 shows the wedges arranged by types of energy production and consumption.

Table 1. Wedges (Pascala and Socolo, 2004,2007)

Type of Energy Consumption	Type of Energy Production
	Energy Efficiency and Resource Conservation	Renewable Energy	Nonrenewable energy
Transportation	1) 2X* vehicle mpg $ 2) Halve VMT $	7) 30X ethanol $$  8) H2 (wind) $$$	11) Synfuels (CCS) $$ 12) H2 (CCS) $$$
Electricity	3) 25% better $ 4) CFPP efficiency $	9) 40X wind $$ 10) 700X solar $$$	13) Natural gas $ 14) CCS at CFPP $$ 15) 3X nuclear $$
Biostorage	5) Protect forests $ 6) Agricultural practices $
*X = expansion of current conditions by factor indicated, mpg = miles per gallon, VMT = vehicle miles traveled, CFPP = coal-fired power plant, H2 = hydrogen, CCS = carbon capture and storage Relative costs of options are reproduced from <www.princeton.edu/wedges> and are intended as general approximations: $ = low cost, $$ = moderate cost, $$$ = high cost

Energy Efficiency

Energy use, especially in North America, is so inefficient that there are huge opportunities to reduce CO2 emissions through efficiency improvements, at very modest cost.

Transportation. Two wedges are achievable from changes in transportation. Doubling the average vehicle fuel-use efficiency from a hypothetical business-as-usual world average 30 mpg at mid-century to a readily attainable 60 mpg is one wedge (Table 1, wedge 1). Cutting the average vehicle miles traveled in half by traveling less, providing better mass-transit alternatives, and changing commuting patterns, provides another wedge (wedge 2). Doubling fuel efficiency and cutting vehicle miles in half together yield 1.5 wedges (being careful to avoid double-counting). Complete elimination of the remaining carbon emissions from vehicles could be achieved using carbon-neutral types of ethanol production and plug-in hybrids. Additional carbon savings could be achieved from increased efficiency in rail transport and by shifting from air travel to more efficient modes of transit.

Electricity. Increasing efficiency in consumption and production of electricity offers opportunities for two wedges. One efficiency wedge (wedge 3) comes from reducing energy use in buildings (much of which involves electricity) by 25 percent. More than one wedge is possible if energy-use improvements exceed 25 percent. This option is low-cost, and very doable.

A second electricity efficiency wedge (wedge 4) is available if mid-century coal-fired power plants produce twice as much electricity by operating at 60-percent efficiency, compared to the present 32 percent. If electricity continues to be produced from coal-fired plants, which appears likely as long as coal is inexpensive, doubling the energy output per unit of coal consumed will allow generating capacity to grow without increasing CO2 emissions. China and India currently account for 45 percent of world coal use, and in each of the two countries coal is the overwhelming energy source for electricity production, so they will certainly continue to use coal.

Continued on page 3>>