Rethinking wedges

Abstract

Abstract Stabilizing CO 2 emissions at current levels for fifty years is not consistent with either an atmospheric CO 2 concentration below 500 ppm or global temperature increases below 2 °C. Accepting these targets, solving the climate problem requires that emissions peak and decline in the next few decades, and ultimately fall to near zero. Phasing out emissions over 50 years could be achieved by deploying on the order of 19 ‘wedges’, each of which ramps up linearly over a period of 50 years to ultimately avoid 1 GtC y −1 of CO 2 emissions. But this level of mitigation will require affordable carbon-free energy systems to be deployed at the scale of tens of terawatts. Any hope for such fundamental and disruptive transformation of the global energy system depends upon coordinated efforts to innovate, plan, and deploy new transportation and energy systems that can provide affordable energy at this scale without emitting CO 2 to the atmosphere. 1. Introduction In 2004, Pacala and Socolow published a study in Science arguing that ‘[h]umanity 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’ [1]. Specifically, they presented 15 options for ‘stabilization wedges’ that would grow linearly from zero to 1 Gt of carbon emissions avoided per year (GtC y −1 ; 1 Gt = 10 12 kg) over 50 years. The solution to the carbon and climate problem, they asserted, was ‘to deploy the technologies and/or lifestyle changes necessary to fill all seven wedges of the stabilization triangle’. They claimed this would offset the growth of emissions and put us on a trajectory to stabilize atmospheric CO 2 concentration at 500 ppm if emissions decreased sharply in the second half of the 21st century. The wedge concept has proven popular as an analytical tool for considering the potential of different technologies to reduce CO 2 emissions. In the years since the paper was published, it has been cited more than 400 times, and stabilization wedges have become a ubiquitous unit in assessing different strategies to mitigate climate change (e.g. [2–5]). But the real and lasting potency of the wedge concept was in dividing the daunting problem of climate change into substantial but tractable portions of mitigation: Pacala and Socolow gave us a way to believe that the energy-carbon-climate problem was manageable. An unfortunate consequence of their paper, however, was to make the solution seem easy (see, e.g. [6, 7]). And in the meantime, the problem has grown. Since 2004, annual emissions have increased and their growth rate has accelerated, so that more than seven wedges would now be necessary to stabilize emissions and—more importantly—stabilizing emissions at current levels for 50 years does not appear compatible with Pacala and Socolow’s target of an atmospheric CO 2 concentration below 500 ppm nor the international community’s goal of limiting the increase in global mean temperature to 2 °C more than the pre-industrial era. Here, we aim to revitalize the wedge concept by redefining what it means to ‘solve the carbon and climate problem for the next 50 years’. This redefinition makes clear both the scale and urgency of innovating and deploying carbon-emissions-free energy technologies. 2. Solving the climate problem Stabilizing global climate requires decreasing CO 2 emissions to near zero [8–11]. If emissions were to stop completely, global temperatures would quickly stabilize and decrease gradually over time [8, 12, 13]. But socioeconomic demands and dependence on fossil-fuel energy effectively commit us to many billions of tons of CO 2 emissions [14], and at the timescale of centuries, each CO 2 emission to the atmosphere contributes another increment to global warming: peak warming is proportional to cumulative CO 2 emissions [15, 16]. Cumulative emissions, in turn, integrate all past emissions as well as those occurring during three distinct phases of mitigation: (1) slowing growth of emissions, (2) stopping growth of emissions, and (3) reducing emissions. Although they noted that stabilizing the climate would require emissions to ‘eventually drop to zero’, Pacala and Socolow nonetheless defined ‘solv[ing] the carbon and climate problem over the next half-century’ as merely stopping the growth of emissions (phases 1 and 2). Further reductions (phase 3), they said, could wait 50 years if the level of emissions were held constant in the meantime. But growth of emissions has not stopped (phase 2) or even slowed (phase 1), it has accelerated [17, 18]. In 2010, annual CO 2 emissions crested 9 GtC. At this level, holding emissions constant for 50 years (phase 2) is unlikely to be sufficient to avoid the benchmark targets of 500 ppm or 2 °C. To support this assertion, we performed ensemble simulations using the UK Met Office coupled climate/carbon cycle model, HadCM3L (see supplementary material available at stacks.iop.org/ERL/8/011001/mmedia ), to project changes in atmospheric CO 2 and global mean temperature in response to emissions scenarios in which seven wedges (W7) and nine wedges (W9) were immediately subtracted from the A2 marker scenario of the Intergovernmental Panel on Climate Change (IPCC)’s Special Report on Emissions Scenarios (SRES) [19] beginning in 2010 (figure 1). In the first half of this century, the A2 scenario is near the center of the plume of variation of the SRES emissions scenarios [20]. Indeed, actual annual emissions have exceeded A2 projections for more than a decade [21, 22]. During this period, strong growth of global emissions has been driven by the rapid, carbon-intensive growth of emerging economies [23, 24], which has continued despite the global financial crisis of 2008–9 [18]. For these reasons we believe that, among the SRES scenarios, A2 represents a reasonable ‘business-as-usual’ scenario. However, if emissions were to suddenly decline and follow a lower emissions business-as-usual trajectory such as B2, fewer wedges would be necessary to stabilize emissions, and deployment of seven wedges would reduce annual emissions to 4.5 GtC in 2060. Thus, mitigation effort (wedges) required to stabilize emissions is dependent on the choice of baseline scenario, but a half-century of emissions at the current level will have the same effect on atmospheric CO 2 and the climate regardless of what scenario is chosen. Figure 1. Modeled effects of deploying wedges. (A) Future CO 2 emissions under SRES A2 marker scenario and the A2 scenario reduced by deployment of 7 wedges (W7). The response of (B) atmospheric CO 2 and (C) global mean surface temperature under W7. (D) Future CO 2 emissions under SRES A2 marker scenario and stabilized at 2010 levels (reduced by approximately 9 wedges relative to the A2 scenario) (W9). The response of (

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