The trouble with negative emissions
Abstract
In December 2015, member states of the United Nations Framework Convention on Climate Change (UNFCCC) adopted the Paris Agreement, which aims to hold the increase in the global average temperature to below 2°C and to pursue efforts to limit the temperature increase to 1.5°C. The Paris Agreement requires that anthropogenic greenhouse gas emission sources and sinks are balanced by the second half of this century. Because some nonzero sources are unavoidable, this leads to the abstract concept of “negative emissions,” the removal of carbon dioxide (CO2) from the atmosphere through technical means. The Integrated Assessment Models (IAMs) informing policy-makers assume the large-scale use of negative-emission technologies. If we rely on these and they are not deployed or are unsuccessful at removing CO2 from the atmosphere at the levels assumed, society will be locked into a high-temperature pathway.
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The figure shows the median of the 76 IPCC scenarios that limit the global temperature rise to 2°C with 66% likelihood (2). Realized negative emissions are estimated by converting the BECCS energy consumption [exajoules (EJ) per year], assuming an average biomass emission factor of 100 metric tons of CO2 per terajoule (TJ) and assuming that 90% of the CO2 is captured. The emission pledges (INDCs) in 2030 are estimated based on cumulative emissions from 2011 to 2030 (5).
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Published In
Science
Volume 354 | Issue 6309
14 October 2016
14 October 2016
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Copyright © 2016, American Association for the Advancement of Science.
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Published in print: 14 October 2016
Acknowledgments
G.P. is funded by the Research Council of Norway (Strategic Challenges in International Climate and Energy Policy, project number 209701).
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RE: Troubles without negative emissions
"The trouble with negative emissions" (1) raises important technological risk and intertemporal equity arguments against overreliance on negative emissions technologies (NETs) to achieve ambitious climate targets. Instead of discounting the value of NETs (1, 2), we argue that NETs are indispensable in an era of planetary risks associated with an increasingly disturbed climate system, and need to be sufficiently prepared in advance. Climate mitigation strategies that deplete the full capacity of NETs in order to undo an emission overshoot in the first half of the 21st century indeed represent a dangerous high-stakes gamble. The use of NETs was originally conceived as a risk management tool to buffer against not only climate system uncertainties (3, 4), but also those associated with global politics (5), risks which have not dissipated (6). Clearly, ambitious climate targets such as that of the Paris Agreement would become unattainable without a sufficiently large negative emission buffer should natural land and ocean carbon sinks weaken or switch to sources of emissions. Likewise, massive amounts of negative emissions will be needed if major countries continue to believe that an emission peak in2030 is still compatible with the Paris Agreement (7).
Large investments are needed in the natural, technological and human resources necessary to form a NET bulwark against unanticipated carbon cycle emergencies. An acceleration of ecosystem restoration programs recarbonizing global landscapes, soils and wetlands with resilient native species appears as a first no-regret investment. Such a restoration and conservation of ecosystems would not only be an essential climate risk hedging measure, but also a step toward meeting the Sustainable Development Goals (8). Concurrently, substantial R&D investments are needed to enhance the sustainability of carbon removal technologies, taking advantage of a large set of biological and chemical pathways. Finally, the size and dynamic properties of a minimum negative emission reserve is still to be worked out based on a quantified political and climate system risk calculus.
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