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Home , Climate debt, Greenhouse gas emissions, Sea level rise

COMMENTARY

Lasting from past emissions COMMENTARY Tony E. Wonga,1

The emission of greenhouse gases into ’satmo- 100% sphereisaby-productofmodernmarvelssuchasthe Extremely likely by 2073−2138 production of vast amounts of energy, heating and 80% cooling inhospitable environments to be amenable to human existence, and traveling great distances 60% Likely by 2064−2105 faster than our saddle-sore ancestors ever dreamed . However, these luxuries come at a price: 40% changes in the form of severe , ex- Probability treme and temperatures, increased fre- 20% quency of flooding in coastal , global warming, RCP2.6 and -level rise (1, 2). Rising pose a severe risk RCP8.5 0% to coastal areas across the globe, with billions of 2020 2040 2060 2080 2100 2120 2140 US dollars in assets at risk and about 10% of the ’ when 50-cm sea-level rise world s population living within 10 m of threshold is exceeded (3–5). The price of our emissions is not felt immedi- ately throughout the entire , however, Fig. 1. Cumulative probability of exceeding 50 cm of sea-level rise by year (relative to the global mean sea because processes such as melt and the level from 1986 to 2005). The yellow box denotes the expansion of warming water act over the range of after which exceedance is likely [≥66% course of centuries. Thus, even if all greenhouse probability (12)], where the left boundary follows a gas emissions immediately ceased, our past emis- business-as-usual emissions scenario (RCP8.5, red line) sions have already “locked in” some amount of con- and the right boundary follows a low-emissions scenario (RCP2.6, blue line). The orange box denotes the range of tinued global warming and sea-level rise. In PNAS, years after which exceedance is extremely likely [≥95% Nauels et al. (6) examine how greenhouse gas emis- probability (12)]. sions since 1750, and anticipated future emissions, contribute to global sea-level rise. The 2015 Paris Climate Agreement represents a They find that emissions from 1750 to 1991 have landmark cooperative achievement by its participating committed us to about 60 cm of additional sea-level nations to recognize and address the threat posed rise by the year 2300, relative to the mean from 1986 by (7). The sets a to 2005, with another 24 cm stemming from green- goal to limit global warming to 2 °C above preindus- gases emitted between 1991 and 2016. So, trial* temperatures, with a more ambitious target of through our emissions thus far in modern human 1.5 °C. With the aim of achieving these global warming history, we have committed to a total sea-level rise of ’ about 84 cm over the next couple of centuries. This limits, each nation s planned reductions in greenhouse isthepricewepayfortheluxuryofabout200y gas emissions for the years 2016 through 2030 are of relatively unchecked . codified as the Nationally Determined Contributions To be clear, past emissions have already been (NDCs) (8). contributing to climate change. The sea-level com- Nauels et al. (6) isolate the impacts of past and mitments calculated by Nauels et al. (6) are the sea- anticipated future emissions under the Paris Agree- level rise that has yet to occur—the balance of ment by assuming that each country follows its NDCs our debt. from 2016 to 2030 then “shuts off” greenhouse gas

aSchool of Mathematical Sciences, Rochester Institute of Technology, Rochester, NY 14623 Author contributions: T.E.W. analyzed data and wrote the paper. The author declares no competing interest. Published under the PNAS license. See companion article on page 23487. 1Email: [email protected]. First published November 7, 2019. *“Preindustrial” refers to the state of the climate before the invention and widespread adoption of efficient steam engine technology in the late 1700s and is used as a baseline to measure the extent to which the emission of greenhouse gases by human activity is affecting Earth’s climate.

www.pnas.org/cgi/doi/10.1073/pnas.1917051116 PNAS | November 19, 2019 | vol. 116 | no. 47 | 23373–23375 Downloaded by guest on September 29, 2021 emissions. They find that emissions following the Paris Agreement Nauels et al. (6) offer a clear avenue to safeguard against some would successfully limit warming to within 1.5 °C but lead to underprotection regrets. about 20 cm of additional sea-level rise, for a total of just over Anthoff et al. (17) estimate the damages from sea-level rise 1 m of sea-level rise commitment by the year 2300. In the nearer-term thresholds by the year 2100 of 50 cm, 1 m, and 2 m, assuming future, the authors find that previous and planned greenhouse gas that nations all protect their coastlines following an economically emissions as per the NDCs commit us to about 43 cm of sea-level efficient strategy. For example, both and the United rise by 2100. This raises the question, What are the impacts of Kingdom incur damages upward of $2 billion as a result of 50 cm 43 cm of sea-level rise around the world? of sea-level rise (17), but the of the First, it is important to recognize that these estimates are an is about 5 times that of Thailand (18). With aggressively optimistic lower bound, as they do not account for 1mofsea-levelriseby2100[wellwithintherangeofcurrent nations exiting the Paris Agreement, or failing to meet their estimates (19)], , , , and Micro- emissions targets, as recent work has found to be the case (9). The nesia all suffer damages exceeding 1% of each nation’sgross committed sea-level rise calculated by Nauels et al. (6) is only the domestic product (17). part attributable to past emissions, so any future emissions be- yond 2030 will increase their estimates. In PNAS, Nauels et al. examine how greenhouse To account for a range of possible future emissions, we can use gas emissions since 1750, and anticipated future a recent set of temperature and sea-level projections (10) and emissions, contribute to global sea-level rise. examine the time frame on which a 50-cm threshold of sea-level rise will probably be exceeded. Approximate bounds on the These damages should also be read as an “optimistic” lower exceedance timing may be obtained by using 2 scenarios to rep- bound. Countries that fail to meet their NDC emissions goals or resent future emissions: Representative Concentration Pathway opt out altogether rack up additional , to be repaid (RCP) 8.5, which corresponds roughly to a “business-as-usual” disproportionately by smaller, low-lying coastal nations. The greenhouse gas emissions scenario, and the low-emissions sce- NDCs allow the authors to isolate the portions of sea-level com- nario RCP2.6, which is a more gradual real-world analog to the mitment that are attributable to each nation’s pledged emissions. authors’ scenario of zero emissions after 2030 (11). I calculate the They find that emissions from the period 2016 to 2030 by the top probability of exceeding the 50-cm threshold as the proportion of 5 emitters alone (, the , the , projections in a given year that have exceeded 50 cm of sea-level , and ) are responsible for committing the world to rise since the period from 1986 to 2005. about 12 cm of sea-level rise, out of a total commitment from that Sea-level rise is likely [≥66% probability (12)] to exceed period of 20 cm. The asymmetry between the countries emitting 50 cm by the year 2064 in the high-emissions scenario and by the most greenhouse gases and those who suffer the conse- 2105 in the low-emissions scenario (Fig. 1). First, this result cor- quences is striking. roborates the sea-level commitment from Nauels et al. (6) of A key lesson we can take from Nauels et al. (6) is that we have 43 cm by 2100. Second, this relates their result to the range of already put into the greenhouse gases sufficient to potential future emissions. The edges of the yellow box in Fig. 1 exceed the 50-cm and 1-m sea-level rise thresholds; it is only a represent the likely time horizon for 50 cm of sea-level rise, matter of when. With billions of dollars and millions of lives at assuming either strong emissions reductions (RCP2.6) or little/ stake, guarding against future damages is the economically and no emissions reductions (RCP8.5). These estimates shed light on ethically prudent decision. These results highlight the need to how the new study’s calculated sea-level commitment drives adapt coastal areas to guard against future sea-level rise, among serious risks to coastal areas because the true future emissions other climate risks. Indeed, if we neglect to adapt coastal areas to protect against future sea-level rise, damages may be amplified pathway will probably not be as rosy as zero greenhouse gas by more than 3-fold (17). emissions post-2030. The findings of Nauels et al. (6) make it clear that aggres- Thus, the sea-level commitments reported by Nauels et al. sive and immediate mitigation of greenhouse gas emissions is (6) can serve as a lower bound on future sea-level rise and an critical for avoiding severe climate damages. Even if emissions upper bound on the time horizon on which thresholds of sea- drop to zero after 2030, the specter of past emissions looms large level rise will be exceeded. This type of imprecise probabilistic as global temperatures will continue to warm for a few years and information is still quite useful from the perspective of a coastal sea levels continue to rise for more than a century, leading to decision maker. It is no secret that climate projections are billions of dollars in damages. To wait any longer to drastically deeply uncertain, which in turn leads to deep uncertainty in reduce emissions decreases the reliability of achieving the Paris assessments of coastal risk (13). This can lead to decision Agreement limits on global warming. Within about a decade, the paralysis while decision and policy makers contemplate the va- desired limits are no longer feasible (20). At present, the most riety of information presented to them (14). Regret can come in important decisions we can make are to 1) reduce near-term 2 forms: underprotection, leading to needlessly high losses, greenhouse gas emissions, even beyond what is pledged in the and overprotection, such as overspending on adaptation mea- Paris Agreement, to mitigate damage from future emissions and sures. An economically efficient protection strategy balances 2) provide the necessary support and to prepare over- versus underprotection. Often, this is chosen to minimize coastal areas for—at the very least—the committed sea-level rise total expected damages (15), but other measures of risk can from past emissions. Failure to do so endangers millions of vul- be optimized too (16). Knowledge of a lower bound on antici- nerable people and wastes billions of dollars due to excessive pated risks provides decision makers with a minimum level of damages from underprotection. We put those greenhouse gases protection needed to guard against those risks. In this way, into the atmosphere, and it is time to pay the piper.

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