PERSPECTIVE Well below 2 °C: Mitigation strategies for avoiding dangerous to catastrophic climate changes PERSPECTIVE Yangyang Xua,1 and Veerabhadran Ramanathanb,1 Edited by Susan Solomon, Massachusetts Institute of Technology, Cambridge, MA, and approved August 11, 2017 (received for review November 9, 2016) The historic Paris Agreement calls for limiting global temperature rise to “well below 2 °C.” Because of uncertainties in emission scenarios, climate, and carbon cycle feedback, we interpret the Paris Agree- ment in terms of three climate risk categories and bring in considerations of low-probability (5%) high- impact (LPHI) warming in addition to the central (∼50% probability) value. The current risk category of dangerous warming is extended to more categories, which are defined by us here as follows: >1.5 °C as dangerous; >3 °C as catastrophic; and >5 °C as unknown, implying beyond catastrophic, including existential threats. With unchecked emissions, the central warming can reach the dangerous level within three decades, with the LPHI warming becoming catastrophic by 2050. We outline a three- lever strategy to limit the central warming below the dangerous level and the LPHI below the cata- strophic level, both in the near term (<2050) and in the long term (2100): the carbon neutral (CN) lever to achieve zero net emissions of CO2, the super pollutant (SP) lever to mitigate short-lived climate pollutants, and the carbon extraction and sequestration (CES) lever to thin the atmospheric CO2 blan- ket. Pulling on both CN and SP levers and bending the emissions curve by 2020 can keep the central warming below dangerous levels. To limit the LPHI warming below dangerous levels, the CES lever must be pulled as well to extract as much as 1 trillion tons of CO2 before 2100 to both limit the preindustrial to 2100 cumulative net CO2 emissions to 2.2 trillion tons and bend the warming curve to a cooling trend. climate change | short-live climate pollutants | carbon capture | mitigation | air pollution The Paris Agreement and its intended nationally published model parameters. The warming esti- determined contributions (INDCs) to reduce emissions matesinthisstudyaccountforthewell-knowngreen- (1) are unprecedented first steps for stabilizing global house gases (GHGs) and various aerosols (Box 1). average warming to well below 2 °C (WB2C). It is gen- ii) Identify the constraints imposed by WB2C and the erally acknowledged that the INDCs must be strength- criteria for meeting WB2C, and thus sharpen the ened significantly to bend the climate emissions curve definition of WB2C. sufficiently and soon enough to limit the warming to iii) Explore the mitigation pathways that are still avail- WB2C (1–3). The overall objectives of this perspective able to meet the WB2C goal. piece are threefold: This perspective article weaves in science per- i) Assess the low-probability (5%) high-impact (LPHI) spectives with societal perspectives since the two are warming outcomes in the absence of a climate inextricably linked. For example, the mitigation path- mitigation policy after accounting for major un- ways we choose are largely motivated by the magni- certainties in: (a) future emission trajectories; tude and rapidity of societal as well as ecosystem (b) physical climate feedback involving water vapor, impacts (4) (Box 2). We recognize that the metrics clouds, and snow/ice albedo; (c) carbon cycle feed- for fully comprehending the societal impacts need back involving biogeochemistry; and (d) aerosol to extend beyond global average warming (5), but radiative forcing. We ensure that the extreme out- global warming is still a valuable and accepted metric comes projected in this study are consistent with for strategizing mitigation options (6). aDepartment of Atmospheric Sciences, Texas A&M University, College Station, TX 77845; and bScripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093 Author contributions: V.R. designed research; Y.X. and V.R. performed research; Y.X. and V.R. analyzed data; and Y.X. and V.R. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. This is an open access article distributed under the PNAS license. 1To whom correspondence may be addressed. Email: [email protected] or [email protected]. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1618481114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1618481114 PNAS | September 26, 2017 | vol. 114 | no. 39 | 10315–10323 Downloaded by guest on September 28, 2021 Box 1. The Non-CO2 Climate Pollutants The first category is SLCPs, which include GHGs such as methane, tropospheric ozone, HFCs, and aerosols such as BC, and coe- mitted OC. The lifetimes of these pollutants range from days (BC and OC), to months (tropospheric ozone), to about a decade (methane and HFCs), which explains the term “short-lived” in SLCPs. The second category of non-CO2 climate pollutants includes LLGHGs such as N2O and halocarbons other than HFCs (e.g., CFCs, HCFCs). Our model is forced by IPCC historical forcing of all non- CO2 gases and aerosols. The third category of non-CO2 climate pollutants is cooling aerosols (other than BC and OC) such as sulfates, nitrates, and dust. It should be noted that those cooling aerosols, along with the BC and OC aerosols included under SLCPs, are the major source of air pollution, leading to about 7 million deaths annually (31). Box 1 Figure shows the individual contribution of CO2, SLCPs, and cooling aerosols (other than those included in SLCPs) to the transient warming during the 20th and 21st centuries. All of the warming trends are relative to preindustrial temperatures. By 2015, the warming due to CO2 is about 0.8 °C and that due to SLCPs is about 1.1 °C. The sum of the CO2 and SLCP warming is already close to the Paris Agreement limit of 2 °C by 2015. On the other hand, the aerosols have a cooling (“masking”) effect of about −0.9 °C. When we add the sum of the CO2, SLCPs, and aerosol effects to the warming due to non-CO2 LLGHGs, the estimated warming by 2015 is ∼1.1 °C (black curve in Box 1 Figure), which can be compared with the observed warming of about 1 °C (SI Appendix, Fig. S1). The main inference from Box 1 Figure is that CO2 and SLCPs have exerted comparable warming effects (0.8 °C and 1.1 °C) to the past, while the aerosol masking effect is also comparable in magnitude but of opposite sign, with a cooling of −0.9 °C. 5 Transient Warming as in Baseline-fast due to CO2 only 4 due to aerosol only due to SLCPs only 3 2 1 Warming since 1900 (K) 0 -1 123456 Cumulative Emission since 1750 (Tt CO2) Box 1 Figure. Simulated transient warming (°C) following the baseline-fast scenario, as a function of the cumulative emission of CO2 (x axis; black line). The decades at which each additional trillion tons of CO2 was emitted and the corresponding CO2 concentration are shown at the top. The red, blue, and green lines illustrate transient simulated warming due to CO2, cooling aerosols, and SLCPs only, respectively. Projected Warming in the Absence of Climate Policies model and the climate model simulations have been extensively A convenient place to start the discussion is the projected validated by comparison with observations of atmospheric CO2 warming in the absence of climate policies. Determining this (7) (SI Appendix, Figs. S1 and S2), global mean temperature (8), baseline warming sets the stage for exploring and justifying mit- ocean heat content (box 1 of ref. 7), and sea-level rise (figure 2 of igation pathways. Published future CO2 emission scenarios (1, 4), ref. 9), as well as by comparison with published projections from along with historical emissions, are fed into a carbon cycle model 3D global climate model simulations (9, 10). to estimate the CO2 concentration during the 20th and 21st The projected warming (relative to 1900) at 2050 and 2100 is centuries (SI Appendix, Figs. S1 and S2). The calculated CO2 shown in Fig. 1, under the two baseline emission scenarios in the concentration is used to estimate its climate forcing (SI Appendix, absence of climate policy during the 21st century. The temporal Figs. S4 and S6). For climate forcing due to other atmospheric evolution of CO2 emissions, CO2 concentrations, and global av- compositions, we adopted the Intergovernmental Panel on Cli- erage temperatures are shown in SI Appendix, Fig. S1. To gen- mate Change (IPCC)-derived historical values (Box 1) and future erate the probability distribution curves shown in Fig. 1 (as well as projections (SI Appendix, Fig. S6). The temperature response is in SI Appendix, Figs. S3, S9, and S10), temperature under each estimated with an energy balance climate model (SI Appendix, scenario is simulated with 1,500 stochastic runs to cover the full range section 1). In a series of published studies (7–10), the carbon cycle of climate sensitivity involving feedback related to water vapor, clouds, 10316 | www.pnas.org/cgi/doi/10.1073/pnas.1618481114 Xu and Ramanathan Downloaded by guest on September 28, 2021 AB1.6 0.8 Baseline-fast Baseline-fast 1.4 Baseline-fast (Carbon-cycle Feedback) 0.7 Baseline-fast (Carbon-cycle Feedback) Baseline-default Baseline-default -1 1.2 -1 0.6 Baseline-fast (Aerosol Uncertainty) C) C) o o 1 0.5 0.8 0.4 0.6 0.3 0.4 0.2 Probability density ( Probability density ( 0.2 0.1 0 0 1 1.5 2 2.5 3 3.5 4 4.5 12345678910 2050 Warming(oC) 2100 Warming(oC) Fig. 1. Probability density function of projected warming for 2050 (A) and 2100 (B) for the baseline-fast (thick red line) and baseline-default (thick black line) scenarios.