Lasting coastal hazards from past greenhouse gas emissions

The emission of greenhouse gases into Earth’s atmosphere is a by-product of modern marvels such as the production of vast amounts of energy, heating and cooling inhospitable environments to be amenable to human existence, and traveling great distances faster than our saddle-sore ancestors ever dreamed possible. However, these luxuries come at a price: climate changes in the form of severe droughts, extreme precipitation and temperatures, increased frequency of flooding in coastal cities, global warming, and sea-level rise (1, 2). Rising seas pose a severe risk to coastal areas across the globe, with billions of US dollars in assets at risk and about 10% of the world’s population living within 10 m of sea level (3–5). The price of our emissions is not felt immediately throughout the entire climate system, however, because processes such as ice sheet melt and the expansion of warming ocean water act over the course of centuries. Thus, even if all greenhouse gas emissions immediately ceased, our past emissions have already “locked in” some amount of continued global warming and sea-level rise. In PNAS, Nauels et al. (6) examine how greenhouse gas emissions since 1750, and anticipated future emissions, contribute to global sea-level rise.

They find that emissions from 1750 to 1991 have committed us to about 60 cm of additional sea-level rise by the year 2300, relative to the mean from 1986 to 2005, with another 24 cm stemming from greenhouse gases emitted between 1991 and 2016. So, through our emissions thus far in modern human history, we have committed to a total sea-level rise of about 84 cm over the next couple of centuries. This is the price we pay for the luxury of about 200 y of relatively unchecked greenhouse gas emissions. To be clear, past emissions have already been contributing to climate change. The sea-level commitments calculated by Nauels et al. (6) are the sea-level rise that has yet to occur—the balance of our debt.

The 2015 Paris Climate Agreement represents a landmark cooperative achievement by its participating nations to recognize and address the threat posed by climate change (7). The Paris Agreement sets a goal to limit global warming to 2 °C above preindustrial* temperatures, with a more ambitious target of 1.5 °C. With the aim of achieving these global warming limits, each nation’s planned reductions in greenhouse gas emissions for the years 2016 through 2030 are codified as the Nationally Determined Contributions (NDCs) (8).

Nauels et al. (6) isolate the impacts of past and anticipated future emissions under the Paris Agreement by assuming that each country follows its NDCs from 2016 to 2030 then “shuts off” greenhouse gas emissions. They find that emissions following the Paris Agreement would successfully limit warming to within 1.5 °C but lead to about 20 cm of additional sea-level rise, for a total of just over 1 m of sea-level rise commitment by the year 2300. In the nearer-term future, the authors find that previous and planned greenhouse gas emissions as per the NDCs commit us to about 43 cm of sea-level rise by 2100. This raises the question, What are the impacts of 43 cm of sea-level rise around the world?

First, it is important to recognize that these estimates are an aggressively optimistic lower bound, as they do not account for nations exiting the Paris Agreement, or failing to meet their emissions targets, as recent work has found to be the case (9). The committed sea-level rise calculated by Nauels et al. (6) is only the part attributable to past emissions, so any future emissions beyond 2030 will increase their estimates.

To account for a range of possible future emissions, we can use a recent set of temperature and sea-level projections (10) and examine the time frame on which a 50-cm threshold of sea-level rise will probably be exceeded. Approximate bounds on the exceedance timing may be obtained by using 2 scenarios to represent future emissions: Representative Concentration Pathway (RCP) 8.5, which corresponds roughly to a “business-as-usual” greenhouse gas emissions scenario, and the low-emissions scenario RCP2.6, which is a more gradual real-world analog to the authors’ scenario of zero emissions after 2030 (11). I calculate the probability of exceeding the 50-cm threshold as the proportion of projections in a given year that have exceeded 50 cm of sea-level rise since the period from 1986 to 2005.

Sea-level rise is likely [≥66% probability (12)] to exceed 50 cm by the year 2064 in the high-emissions scenario and by 2105 in the low-emissions scenario (Fig. 1). First, this result corroborates the sea-level commitment from Nauels et al. (6) of 43 cm by 2100. Second, this relates their result to the range of potential future emissions. The edges of the yellow box in Fig. 1 represent the likely time horizon for 50 cm of sea-level rise, assuming either strong emissions reductions (RCP2.6) or little/no emissions reductions (RCP8.5). These estimates shed light on how the new study’s calculated sea-level commitment drives serious risks to coastal areas because the true future emissions pathway will probably not be as rosy as zero greenhouse gas emissions post-2030.

Fig. 1.

Cumulative probability of exceeding 50 cm of sea-level rise by year (relative to the global mean sea level from 1986 to 2005). The yellow box denotes the range of years after which exceedance is likely [≥66% probability (12)], where the left boundary follows a business-as-usual emissions scenario (RCP8.5, red line) and the right boundary follows a low-emissions scenario (RCP2.6, blue line). The orange box denotes the range of years after which exceedance is extremely likely [≥95% probability (12)].

Thus, the sea-level commitments reported by Nauels et al. (6) can serve as a lower bound on future sea-level rise and an upper bound on the time horizon on which thresholds of sea-level rise will be exceeded. This type of imprecise probabilistic information is still quite useful from the perspective of a coastal decision maker. It is no secret that climate projections are deeply uncertain, which in turn leads to deep uncertainty in assessments of coastal flood risk (13). This can lead to decision paralysis while decision and policy makers contemplate the variety of information presented to them (14). Regret can come in 2 forms: underprotection, leading to needlessly high losses, and overprotection, such as overspending on adaptation measures. An economically efficient protection strategy balances over- versus underprotection. Often, this is chosen to minimize total expected damages (15), but other measures of risk can be optimized too (16). Knowledge of a lower bound on anticipated risks provides decision makers with a minimum level of protection needed to guard against those risks. In this way, Nauels et al. (6) offer a clear avenue to safeguard against some underprotection regrets.

Anthoff et al. (17) estimate the damages from sea-level rise thresholds by the year 2100 of 50 cm, 1 m, and 2 m, assuming that nations all protect their coastlines following an economically efficient strategy. For example, both Thailand and the United Kingdom incur damages upward of $2 billion as a result of 50 cm of sea-level rise (17), but the gross domestic product of the United Kingdom is about 5 times that of Thailand (18). With 1 m of sea-level rise by 2100 [well within the range of current estimates (19)], Mozambique, the Bahamas, Palau, and Micronesia all suffer damages exceeding 1% of each nation’s gross domestic product (17).

In PNAS, Nauels et al. examine how greenhouse gas emissions since 1750, and anticipated future emissions, contribute to global sea-level rise.

These damages should also be read as an “optimistic” lower bound. Countries that fail to meet their NDC emissions goals or opt out altogether rack up additional climate debt, to be repaid disproportionately by smaller, low-lying coastal nations. The NDCs allow the authors to isolate the portions of sea-level commitment that are attributable to each nation’s pledged emissions. They find that emissions from the period 2016 to 2030 by the top 5 emitters alone (China, the United States, the European Union, India, and Russia) are responsible for committing the world to about 12 cm of sea-level rise, out of a total commitment from that period of 20 cm. The asymmetry between the countries emitting the most greenhouse gases and those who suffer the consequences is striking.

A key lesson we can take from Nauels et al. (6) is that we have already put into the atmosphere greenhouse gases sufficient to exceed the 50-cm and 1-m sea-level rise thresholds; it is only a matter of when. With billions of dollars and millions of lives at stake, guarding against future damages is the economically and ethically prudent decision. These results highlight the need to adapt coastal areas to guard against future sea-level rise, among other climate risks. Indeed, if we neglect to adapt coastal areas to protect against future sea-level rise, damages may be amplified by more than 3-fold (17).

The findings of Nauels et al. (6) make it clear that aggressive and immediate mitigation of greenhouse gas emissions is critical for avoiding severe climate damages. Even if emissions drop to zero after 2030, the specter of past emissions looms large as global temperatures will continue to warm for a few years and sea levels continue to rise for more than a century, leading to billions of dollars in damages. To wait any longer to drastically reduce emissions decreases the reliability of achieving the Paris Agreement limits on global warming. Within about a decade, the desired limits are no longer feasible (20). At present, the most important decisions we can make are to 1) reduce near-term greenhouse gas emissions, even beyond what is pledged in the Paris Agreement, to mitigate damage from future emissions and 2) provide the necessary support and infrastructure to prepare coastal areas for—at the very least—the committed sea-level rise from past emissions. Failure to do so endangers millions of vulnerable people and wastes billions of dollars due to excessive damages from underprotection. We put those greenhouse gases into the atmosphere, and it is time to pay the piper.