| 英文摘要: | Methods of removing CO2 from the atmosphere add vital flexibility to efforts to tackle climate change. They must be brought into mainstream climate policy as soon as possible to open up the landscape for innovation and development, and to discover which approaches work at scale. To achieve the widely held policy target of limiting average temperature change to 2 °C, integrated assessment models (IAM) increasingly depend on massive-scale 'negative emissions' through biomass energy with carbon capture and storage (BECCS), deployed in the second half of this century1, 2, 3, 4, 5, 6. Yet this key technology is technically immature today, and it is far from clear whether such large-scale deployment several decades in the future would be either feasible or desirable1. Hence a recent Commentary by Fuss et al. has branded BECCS a potentially “dangerous distraction”1. But before anyone dismisses what doesn't yet exist, we argue that the best way to determine “how safe it is to bet on negative emissions in the second half of this century”1 is to instigate a policy framework for greenhouse-gas removal (GGR) and invest in research and development innovation now.
Scalable GGR approaches bring unique flexibility to the mitigation toolbox. BECCS, along with other methods for removing greenhouse gases from the atmosphere, offer two key dimensions of flexibility; they decouple abatement opportunities from emissions sources in both space and time7, 8, 9. Decoupling in space allows GGR to indirectly mitigate emissions from areas of the energy system that are most difficult or expensive to decarbonize. Such 'project-level' negative emissions can in principle bring many benefits as a complement to conventional mitigation efforts, depending on the direction and efficacy of climate policy and GGR deployment. Possibilities include (i) buying time for the development of clean technologies, the replacement of locked-in sources, and changes in societal attitudes; (ii) reducing the total costs of meeting climate targets by displacing the most challenging and expensive emissions sources; (iii) making more aggressive emissions cuts feasible by simply adding new mitigation options; or (iv) allowing continuing use of fossil fuels in certain key sectors such as aviation7, 9. Decoupling in time raises the idea that GGR theoretically could be deployed at massive scale to generate global 'net-negative' emissions later this century, allowing us to recover from emitting too much earlier this century and overshooting CO2 concentration targets1, 4, 9. The negative emissions capacity outlined in the IPCC's Fifth Assessment Report3 implies BECCS input of up to 10 gigatonnes of CO2 abatement per year with global net negative emissions from around 2070. It is this second dimension of flexibility — decoupling in time — that Fuss et al. rightly caution against as “betting” on negative emissions1. They argue that our ability to reach or even assess the feasibility of a late-century, massive-scale BECCS scenario is severely constrained by (at least) four main groups of uncertainties surrounding: (i) access to sufficient biomass supply and storage space for captured CO2; (ii) the uncertain response of the global carbon cycle; (iii) relative costs and viability of untested technology; and (iv) social and political factors. Similar uncertainties face other GGR technologies. For example, estimates of the cost of direct air capture range from below US$100 to more than US$1,000 per tonne of CO2 abated10, and approaches based on interactions with natural systems such as soils and ocean alkalinity raise concerns over potential environmental impacts that are not yet fully understood11, 12, 13.
In the face of such uncertainties it can seem premature to commit to long-term policy support. A natural, scientific response is to call for a substantial interdisciplinary research agenda to explore and try to constrain the uncertainties1, so that we can best assess the future potential of GGR and guide policy through the remaining uncertainties. But at such vast scales of global deployment, and over such long timescales of technological, political and societal development, many of the uncertainties are inherent, and can only ever be loosely constrained by modelling and research9. This is well illustrated by, for example, existing estimates of global sustainable biomass resource in 2050 to 2100, which range from around 30 × 1018 J (30 EJ) per year to over 600 EJ yr−1 depending on assumed trends in diet, crop yields, land use and population14. The call to try to constrain the unconstrainable instead may lead to 'analysis paralysis', losing valuable time and helping to self-fulfil the prophecy that GGR cannot be realized at scale. A central problem is the framing of GGR as a large-scale, late-century approach that would inevitably entail major environmental and social consequences7. This presents multiple issues for policy, and immediately polarizes rather than nuances the debate. It risks both over-emphasizing the need for precaution and regulation over the possible advantages, and portraying reliance on these highly uncertain scenarios as an economically optimal policy. By concentrating on events more than 50 years in the future, it takes the debate away from current scientific knowledge, global experience and policy planning horizons, giving the false impression that effective policy engagement with GGR in the near term is of little value or urgency. This distracts attention from the nearer-term value that BECCS and other GGR could offer in supplementing ongoing mitigation efforts at more modest scales, and the urgency of the early technical and policy groundwork necessary to enable future scale-up7. Thus, although research to try to constrain long-term uncertainties is undoubtedly important, these uncertainties should not be used to justify inaction on more pressing near-term technology development and policy support needs. Indeed, an alternative response to such uncertainty is to start learning by doing. BECCS and its component technologies are at a relatively early stage of technical development, as are many other GGR options (Fig. 1). Individually, bioenergy and CCS technology and industries are themselves at an early stage, and integrating them poses further challenges to technical viability and achieving attractive economics7, 15, 16.
A first step forward can come from noting that the practical and conceptual difficulties in accounting, and to some extent the uncertainties, are shared to varying degrees by several emissions reduction technologies that are currently the focus of policy efforts. Life-cycle assessment methodologies, developing guidelines for carbon accounting in forestry and land-use change, approaches for reducing risks of indirect emissions from bioenergy and accounting, monitoring and liability mechanisms for geological storage are all transferable to different GGR methods, and these mechanisms can form the basis for policy integration. These ongoing overhauls of emissions accounting across all sectors represent a good opportunity to incorporate GGR. Based on the principles of integration with mitigation policy and building flexibility, we therefore propose four principles for a high-level strategy that can be applied in order to begin to make progress towards successful GGR integration7: • Fund research, development and demonstration of GGR systems, focusing on constraining uncertainties, developing practical accounting methods and bridging any other gaps between technology maturity and policy needs. Given the value of GGR in tackling the most difficult emissions sources, diverting some funding from more advanced and speculative clean energy research may pay off. • Build up support for low-cost, early opportunities through existing or new bottom-up policy mechanisms. Examples might include subsidies for electricity generated from early BECCS opportunities such as biomass co-firing in coal CCS plants, or inclusion of soil carbon enhancement or biochar in agricultural policies. This will help to capture early opportunities as well as stimulating development and innovation. • Commit to full integration of GGR into emissions accounting, accreditation and overall policy strategy in the longer term, including any carbon pricing mechanisms. This process will undoubtedly be complex, but the commitment will stimulate investment, research and long-term planning for GGR. • Develop steps to lay the broader groundwork for future GGR and to keep the GGR option open, avoiding lock-out of valuable opportunities. The first three principles will go some way towards achieving this, but there may be further steps that can be taken that are specific to each technology and must be identified through close engagement with stakeholders. An example of this might be 'capture-ready' requirements for bioenergy plants to ensure that they can be retrofitted with CCS when this option becomes viable. The challenge of meeting climate targets is huge, and we will need to make use of every tool at our disposal. GGR methods that can extract CO2 from the atmosphere itself can add vital flexibility to the efforts and must be brought into mainstream climate policy as soon as possible to open up the landscape for innovation and development. Effectively integrating such diverse approaches into policy will be challenging and complex, and the principles proposed here only point to the first stages of the process. But they represent essential steps that must be taken if we are not to miss the opportunity that GGR provides.
- Fuss, S. et al. Nature Clim. Change 4, 850–853 (2014).
- The Emissions Gap Report 2013 (UNEP, 2013).
- IPCC Climate Change 2014: Mitigation of Climate Change (eds Edenhofer, O. et al.) (Cambridge Univ. Press, 2014).
- Azar, C., Johansson, D. J. A. & Mattsson, N. Environ. Res. Lett. 8, 034004 (2013).
- Kriegler, E., Edenhofer, O., Reuster, L., Luderer, G. & Klein, D. Climatic Change 118, 45–57 (2013).
- Van Vuuren, D. P. et al. Climatic Change 118, 15–27 (2013).
- Lomax, G., Workman, M., Lenton, T. & Shah, N. Energy Policy 78, 125–136 (2015).
- Keith, D. W. Science 325, 1654–1655 (2009).
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