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Amending the Inadequacy of a Single Indicator for Climate Impact Analyses Francesco Cherubini*,† and Katsumasa Tanaka‡ †
Industrial Ecology Programme, Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU), Trondheim, Norway ‡ Center for Global Environmental Research, National Institute for Environmental Studies (NIES), Tsukuba, Japan relative to CO2. The IPCC concludes that it is not possible to identify a best indicator, and “the most appropriate metric will depend on which aspects of climate change are most important to a particular application”1. A critical question thus arises: what climate impact indicator(s) shall we use in climate impact analysis? Attributing climate impacts to commodities, technological systems, or economic sectors typically relies on decisionsupporting tools like life-cycle assessment (LCA) and carbon footprints. These methods, including many others used at corporate and public levels to design climate mitigation priorities, aggregate GHGs into CO2-equivalents using GWP100. This practice has two evident inconsistencies with climate science. First, the default use of GWP100 is an arbitrary choice, maybe rooted in an “inadvertent consensus” from a partial interpretation of the first IPCC Assessment Report3. Second, it neglects possible contributions from NTCFs. In 2013, the UNEP/SETAC Life-Cycle Initiative launched an international process to provide guidance on the use of IPCC climate indicators and thereby revisit existing impact assessment methods in LCA and carbon footprints4. At the final Pellston Workshop in 2016, the plenary session approved a The fifth Assessment Report (AR5) from the Intergovernmenrevised approach based on two separate impact categories tal Panel on Climate Change (IPCC) synthesizes the assessing shorter-term and long-term impacts (http://www. contributions to climate change from greenhouse gases lifecycleinitiative.org/wp-content/uploads/2016/10/LCIA(GHGs) like CO2 and CH4, and a host of near-term climate publication-preview.pdf). The indicator for the long-term forcers (NTCFs) such as black carbon (BC) and sulfates impact category is GTP100, which unlike GWP, directly (SOx)1. GHGs have lifetimes ranging from less than a decade to measures temperature changes 100 years after an emission. In thousands of years, and NTCFs can be as short as a few days. contrast, GWP100 is more suitable for targeting shorter-term Climate system responses to emissions are thus fundamentally impacts and the rate at which the climate is changing. By explicitly accounting for all the forcing of an emission until the different. Long-lived gases like CO2 cause a warming for the TH, GWP100 captures a series of complex climate effects from centuries to come, whereas short-lived species like CH4 induce NTCFs and following cascading changes in CH4 and ozone, temperature changes that can be strong but temporary, and which contribute to the rate of warming. As GWP100 is mostly affect the rate of climate change2. Consequently, numerically close to GTP405, it can be interpreted as a proxy mitigation of short-lived gases alone will alleviate the rate of for temperature impacts after about four decades, a time scale warming but will not diminish the risks of exceeding future markedly shorter than that of GTP100. GWP20 is an additional temperature thresholds like the 2 °C stabilization target, which option to GWP100 for assessing the possible sensitivity to very are essentially dependent on cumulative emissions of CO2. AR5 short-lived components like BC, whose effects on the climate provides various indicators to capture these heterogeneities, like can be underrepresented by the longer integration under the Global Warming Potential (GWP) for a time horizon (TH) GWP100. of 20 or 100 years and the global temperature change potential By directly implementing the indicators from the IPCC, (GTP) for TH of 20, 50, and 100 years. These indicators work these climate impact categories would have the same units, as weighting factors converting impacts from different CO2-equivalents, despite them focusing on different climate emissions into common units, like CO2-equivalents, and vary effects. From a practical point of view, it is preferable to for the type of climate impact considered. GWP compares emissions based on their integrated radiative forcing until the TH using CO2 as reference, while GTP measures the Received: October 21, 2016 instantaneous impact on global temperature at the TH, again © XXXX American Chemical Society
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DOI: 10.1021/acs.est.6b05343 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology
(GTP100), which are almost 2 kg CO2-equivalents for rural India (approximately 11% higher than the other systems). Adapting existing approaches to developments in climate science and ensuring consistency between an indicator and the intended mitigation objective is crucial. There is an evident need for a distinction between short-term vs long-term climate impacts, requiring indicators that are scientifically sound and transparent. The practice of measuring climate impacts by only focusing on GWP100 and GHGs is clearly inadequate and can potentially lead to suboptimal mitigation strategies. For instance, the maritime shipping sector has high emissions of NTCFs that can either dampen (SOx) or exacerbate (BC) the warming from CO2 in the short term, thereby affecting the climate benefits of upgraded fuels or alternative shipping routes. The use of a single indicator can never yield exhaustive insights. The importance of short-lived species for the rate of climate change and the nearly irreversible dangers of delaying emission reductions of CO 2 for future temperature levels are complementary aspects that are worth to be clearly communicated to our society. We believe that the use of multiple indicators can better assist decision makers, and ultimately favor the deployment of effective mitigation policies supported by consistent indicators reflecting short (decades) and long (century) time scales of impacts.
differentiate the units to prevent confusion among analysts and decision-makers. A simple solution is to express short-term impacts in CH4 equivalents, that is, using CH4 as reference gas instead of CO2 for GWP20 and GWP100. CH4 is the most important short-lived component with respect to current radiative forcing1, and can be intuitively associated with shorter-term impacts, as opposed to long-term temperature changes measured in CO2-equivalents with GTP100. CH4equivalent indicators generally decrease with increasing TH if the pollutant has a lifetime shorter than CH4, whereas they increase if longer (Table 1). The use of CH4 (instead of CO2) as reference only affects the absolute outcomes and does not alter the relative contributions of emissions. Table 1. Indicators for selected GHGs and NTCFs in CH4equivalents (GWP20 and GWP100) and CO2-equivalents (GTP100). Values in CH4-equivalents come from dividing the value of GWP20 or GWP100 in CO2-equivalents by the GWP20 or GWP100 of CH4. Carbon-climate feedbacks are considered for all forcing agents (see IPCC table 8.SM.161). NTCFs are subject to larger uncertainty than GHGs and are provided with ranges (when available from the IPCC). GWP100 and GTP100 for NTCFs are adjusted to account for updated values for CO2
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→ time scale of temperature impacts →
forcing agent CO2 CH4 HCFC122a HFC134a CFC-11 N2O SF6 CF4 BC OC SOx NOx CO VOC
shorter-term
long-term
GWP20 (CH4-equiv)
GWP100 (CH4-equiv)
GTP100 (CO2-equiv)
n.a. 12.4 years 3.4 years
0.011 1 11
0.028 1 9
1 13 101
13.4 years
44
46
530
lifetime
45 years 121 years 3200 years 50 000 years weeks weeks weeks weeks months months
AUTHOR INFORMATION
Corresponding Author
very shortterm
82 3 204
157 8 725
3 490 297 33 631
58
216
9 560
37 ± 34 −1.84 ± 1.2 −1.62 −1.24 ± 0.4 0.09 ± 0.02 0.21 ± 0.08
28 ± 25 −1.4 ± 0.9 −1.3 −1 ± 0.3 0.07 ± 0.02 0.2 ± 0.1
119 (5 to 313) −6.7 ± 1.9 −5 −2.0 ± 1.9 −0.24 ± 0.10 0.83 (±0.46)
*Phone: 004773598942; e-mail:
[email protected]. ORCID
Francesco Cherubini: 0000-0002-7147-4292 Katsumasa Tanaka: 0000-0001-9601-6442 Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Myhre, G.; Shindell, D.; F.-M., Bréon; Collins, W.; Fuglestvedt, J.; Huang, J.; Koch, D.; Lamarque, J.-F.; Lee, D.; Mendoza, B.; Nakajima, T.; Robock, A.; Stephens, G.; Takemura, T.; Zhang, H., Anthropogenic and Natural Radiative Forcing. In Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change; Stocker, T. F.; Qin, D.; Plattner, G.-K.; Tignor, M.; Allen, S.K.; Boschung, J.; Nauels, A.; Xia, Y.; Bex, V.; Midgley, P. M., Eds. Cambridge University Press: Cambridge, United Kingdom and New York, NY, USA, 2013. (2) Smith, S. M.; Lowe, J. A.; Bowerman, N. H. A.; Gohar, L. K.; Huntingford, C.; Allen, M. R. Equivalence of greenhouse-gas emissions for peak temperature limits. Nat. Clim. Change 2012, 2 (7), 535−538. (3) Shine, K. The global warming potentialthe need for an interdisciplinary retrial. Clim. Change 2009, 96 (4), 467−472. (4) Frischknecht, R.; Fantke, P.; Tschümperlin, L.; Niero, M.; Antón, A.; Bare, J.; Boulay, A.-M.; Cherubini, F.; Hauschild, M. Z.; Henderson, A.; Levasseur, A.; McKone, T. E.; Michelsen, O.; i Canals, L. M.; Pfister, S.; Ridoutt, B.; Rosenbaum, R. K.; Verones, F.; Vigon, B.; Jolliet, O. Global guidance on environmental life cycle impact assessment indicators: progress and case study. Int. J. Life Cycle Assess. 2016, 21 (3), 429−442. (5) Allen, M. R.; Fuglestvedt, J. S.; Shine, K. P.; Reisinger, A.; Pierrehumbert, R. T.; Forster, P. M. New use of global warming potentials to compare cumulative and short-lived climate pollutants. Nat. Clim. Change 2016, 6 (8), 773−776.
We apply these indicators to the UNEP/SETAC case study, a life-cycle comparative analysis of producing 1 kg of cooked rice in urban China, Switzerland (using rice imported from the US), or rural India4. Impacts on the rate of climate change using GWP100 are 63 ± 10, 69 ± 5, and 99 ± 30 g CH4-eq , for urban China, Switzerland (U.S. rice), and rural India, respectively. With GWP20, they are 40 ± 13, 50 ± 7, and 82 ± 40 g CH4-eq. In all the cases, CH4 and BC are the major contributors to the shorter-term impact, followed by CO2. The highest short-term impact for the rural India case is mostly due to low rice yields and high emissions of NTCFs from the inefficient open fireplace used for cooking. On the other hand, CO2 largely dominates contributions to long-term impacts B
DOI: 10.1021/acs.est.6b05343 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
