DOI: 10.1126/science.1100103 , 968 (2004); 305Science et al.S. Pacala,for the Next 50 Years with Current TechnologiesStabilization Wedges: Solving the Climate Problem www.sciencemag.org (this information is current as of May 26, 2007 ):The following resources related to this article are available online at http://www.sciencemag.org/cgi/content/full/305/5686/968version of this article at: including high-resolution figures, can be found in the onlineUpdated information and services, http://www.sciencemag.org/cgi/content/full/305/5686/968/DC1 can be found at: Supporting Online Materialfound at: can berelated to this articleA list of selected additional articles on the Science Web sites http://www.sciencemag.org/cgi/content/full/305/5686/968#related-content 66 article(s) on the ISI Web of Science. cited byThis article has been http://www.sciencemag.org/cgi/content/full/305/5686/968#otherarticles 12 articles hosted by HighWire Press; see: cited byThis article has been http://www.sciencemag.org/cgi/collection/atmosAtmospheric Science : subject collectionsThis article appears in the following http://www.sciencemag.org/about/permissions.dtl in whole or in part can be found at: this articlepermission to reproduce of this article or about obtaining reprintsInformation about obtaining registered trademark of AAAS. c 2004 by the American Association for the Advancement of Science; all rights reserved. The title SCIENCE is a CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the on May 26, 2007 www.sciencemag.orgDownloaded fromREVIEWStabilization Wedges: Solving the Climate Problemfor the Next 50 Years with Current TechnologiesS. Pacala1* and R. Socolow2*Humanity already possesses the fundamental scientific, technical, and industrialknow-how to solve the carbon and climate problem for the next half-century. Aportfolio of technologies now exists to meet the world’s energy needs over the next50 years and limit atmospheric CO2to a trajectory that avoids a doubling of thepreindustrial concentration. Every element in this portfolio has passed beyond thelaboratory bench and demonstration project; many are already implemented some-where at full industrial scale. Although no element is a credible candidate for doingthe entire job (or even half the job) by itself, the portfolio as a whole is large enoughthat not every element has to be used.The debate in the current literature about stabi-lizing atmospheric CO2at less than a doublingof the preindustrial concentration has led toneedless confusion about current options formitigation. On one side, the IntergovernmentalPanel on Climate Change (IPCC) has claimedthat “technologies that exist in operation or pilotstage today” are sufficient to follow a less-than-doubling trajectory “over the next hundredyears or more” [(1), p. 8]. On the other side, arecent review in Science asserts that the IPCCclaim demonstrates “misperceptions of techno-logical readiness” and calls for “revolutionarychanges” in mitigation technology, such as fu-sion, space-based solar electricity, and artificialphotosynthesis (2). We agree that fundamentalresearch is vital to develop the revolutionarymitigation strategies needed in the second halfof this century and beyond. But it is importantnot to become beguiled by the possibility ofrevolutionary technology. Humanity can solvethe carbon and climate problem in the first halfof this century simply by scaling up what wealready know how to do.What Do We Mean by “Solving theCarbon and Climate Problem for theNext Half-Century”?Proposals to limit atmospheric CO2to a con-centration that would prevent most damagingclimate change have focused on a goal of500 ⫾ 50 parts per million (ppm), or less thandouble the preindustrial concentration of 280ppm (3–7). The current concentration is ⬃375ppm. The CO2emissions reductions necessaryto achieve any such target depend on the emis-sions judged likely to occur in the absence of afocus on carbon [called a business-as-usual(BAU) trajectory], the quantitative details of thestabilization target, and the future behavior ofnatural sinks for atmospheric CO2(i.e., theoceans and terrestrial biosphere). We focus ex-clusively on CO2, because it is the dominantanthropogenic greenhouse gas; industrial-scalemitigation options also exist for subordinategases, such as methane and N2O.Very roughly, stabilization at 500 ppmrequires that emissions be held near thepresent level of 7 billion tons of carbon peryear (GtC/year) for the next 50 years, eventhough they are currently on course to morethan double (Fig. 1A). The next 50 years isa sensible horizon from several perspec-tives. It is the length of a career, the life-time of a power plant, and an interval forwhich the technology is close enough toenvision. The calculations behind Fig. 1Aare explained in Section 1 of the supportingonline material (SOM) text. The BAU andstabilization emissions in Fig. 1A are nearthe center of the cloud of variation in thelarge published literature (8).The Stabilization TriangleWe idealize the 50-year emissions reductionsas a perfect triangle in Fig. 1B. Stabilizationis represented by a “flat” trajectory of fossilfuel emissions at 7 GtC/year, and BAU isrepresented by a straight-line “ramp” trajec-tory rising to 14 GtC/year in 2054. The “sta-bilization triangle,” located between the flattrajectory and BAU, removes exactly one-third of BAU emissions.To keep the focus on technologies that havethe potential to produce a material difference by2054, we divide the stabilization triangle intoseven equal “wedges.” A wedge represents anactivity that reduces emissions to the atmospherethat starts at zero today and increases linearlyuntil it accounts for 1 GtC/year of reduced car-bon emissions in 50 years. It thus represents acumulative total of 25 GtC of reduced emissionsover 50 years. In this paper, to “solve the carbonand climate problem over the next half-century”means to deploy the technologies and/or lifestylechanges necessary to fill all seven wedges of thestabilization triangle.Stabilization at any level requires that netemissions do not simply remain constant, buteventually drop to zero. For example, in onesimple model (9) that begins with the stabi-lization triangle but looks beyond 2054, 500-ppm stabilization is