Overview of approach

On the basis of 2015 Automatic Identification System (AIS) data on shipping traffic (over 65,000 IMO registered vessels and over 7.6 billion motion records), we project 2020 geospatial shipping emissions inventories with and without implementation of the proposed standards. Using global chemical transport models with very high spatial and temporal resolution (10 × 10 km × 3 h), we evaluate the public health and climate forcing consequences of low-sulphur marine fuel policy implementation. Atmospheric transport and transformations of ship-emitted pollutants serve as inputs to modelling health effects, and to estimating direct and indirect radiative forcing potential. All computations incorporate existing regulations that limit sulphur emissions from ships in designated Sulphur Emissions Control Areas (SECAs). We report results of advanced shipping emissions inventories for 2015 and 2020 scenarios, high-resolution spatial and temporal chemistry-transport port model runs, health outcomes using linear and log-linear concentration–response functions, and radiative transfer evaluations of direct and indirect aerosol forcing changes.

Global shipping emissions in 2015 with projections to 2020

Global ship emissions were calculated using the Ship Traffic Emissions Assessment Model (STEAM) model19,20,21 and 2015 AIS ship traffic (see Methods). Emissions were projected to 2020 using vessel-type-specific annual growth rates (MEPC 70/5/322, 23 Table 166). Annual totals for emissions from global shipping for 2015 and 2020 are presented in Table 1, comparable with the 2012 results described in the Third IMO GHG Study12. Projected business-as-usual (BAU) emission results of the global fleet in 2020 are consistent with those of the Third IMO GHG Study, mainly reflecting growth assumptions; as shown in Table 1, emissions reductions associated with implementation of a low-sulphur fuel standard in 2020 (2020 Action) affect both SO 2 and PM (via reduction of sulphate emissions).

Table 1 Summary of emissions (and fuel consumption) of this work using STEAM for 2020 compared with Third IMO GHG Study (Smith et al.12) and all non-shipping emissions Full size table

Implementation of the 2020 standards results in ~75% reduction in shipping SOx emissions globally, along similar spatial distribution of emissions without the new standards (see Supplementary Figure 1). Supplementary Figure 1 also illustrates the presence of an IMO-approved sulphur emissions control area (SECA) designation for the United States (US) and Canada24; a Chinese domestic emissions control area (DECA) designation for the Bohai Sea, Yangtze River delta, and Pearl River delta in China25; IMO-approved SECA designation in parts of Europe; and additional European Union control requirements under the European Directive26. In the European and US/Canadian SECAs, ships are limited to burning 0.1% S fuel within 200 nautical miles of the coast, whereas the limit under the European Directive and in Chinese areas will be 0.5% S from the beginning of 2020 and 2019, respectively. These SECAs will be in force independent of the IMO decision to implement global fuel sulphur standards.

Impact of low-sulphur standards on particulate matter

Geospatial emissions inventories provide input into the Finnish Meteorological Institute’s (FMI) System for Integrated modeLling of Atmospheric coMposition (SILAM)27,28,29. SILAM was run at geospatial resolution of 10 x 10 km and a temporal resolution of 3 h (see Methods section). We used SILAM to predict pollutant concentrations under the following two scenarios: (1) business-as-usual (BAU), which assumes no implementation of a global, low-sulphur fuel standard; and (2) 2020 Action, which assumes on time implementation of the IMO’s 0.5% S low-sulphur fuel standard. Figure 1 shows the spatial distribution of atmospheric PM 2.5 near-surface concentrations under the BAU scenario; Fig. 2 presents the difference between the BAU scenario and the 2020 Action scenario. Related concentration maps provided in Supplementary Figure 1 shows consistency with previous distributions at a global level30.

Fig. 1 Mean annual PM 2.5 concentrations from all sources. Model results showing mean annual PM 2.5 concentrations in micrograms per cubic meter from all sources and with business-as-usual ship emissions in 2020 Full size image

Fig. 2 Reduction in annual PM 2.5 concentrations due to low-sulphur fuel standards. Model results showing the reduction in annual PM 2.5 concentrations in micrograms per cubic meter due to the implementation of the International Maritime Organization’s global fuel sulphur standard in 2020 Full size image

Considering all PM sources, typical concentrations of PM 2.5 amount to a few μg m−3, with much higher levels in arid areas and regions with strong fire activity (Fig. 1). Over the ocean, the major component of PM 2.5 is sea salt (over 50–75%), whereas over land terrestrial emissions are the major contributor. Ship contributions are visible mainly in the open ocean and over the busiest sea-lanes. Stricter limits on sulphur content in ship fuel decrease sulphate concentrations (2–4 μg SO 4 m−3 annual mean), leading to significant reduction of PM 2.5 in the vicinity of busy ship lanes (Fig. 2). However, total aerosol load reduction is limited since sulphates constitute on average less than 15% of total PM in the air (compare concentrations in Figs. 1 and 2).

Chemical links between sulphate and nitrate species resulted in some ammonia becoming available for forming ammonium nitrate, partly offsetting the sulphate aerosol reductions. However, this effect is small and the offset only exceeds 1% around the east coast of China, with a maximum of 15% over the Yellow Sea.

Health impacts of global shipping

We apply concentration–response (C–R) functions to 2020 population projections to estimate health impacts due to ship emissions in the BAU case and the 2020 Action case (see Methods section). We calculate adult mortality from lung cancer and cardiovascular disease and childhood asthma morbidity, and report results using a linear C–R function as discussed in Lepeule et al.31, and Zheng et al.31, 32. The vast majority of PM 2.5 exposure concentrations in our study area represent conditions similar to those in the Six Cities Study, indicating that functions derived from that study can also apply to our study.

Total premature mortality due to shipping in the 2020 BAU case is 403,300 per year (range of 212,300–595,400 based on the 95% confidence interval for relative risk); BAU mortality distribution is shown in Fig. 3. Total avoided premature mortality in 2020 with implementation of the low-sulphur fuel standards is expected to be 266,300 per year (range of 138,500–395,700), a reduction of ~ 34%); avoided mortality distribution is shown in Fig. 4. Childhood asthma morbidity due to shipping declines by 54%, from 14 million children affected in the BAU case, to 6.4 million children in the 2020 Action case (see Supplementary Note 1, Supplementary Figure 2. Detailed results are in Table 2, and Supplementary Table 1 and Supplementary Table 2 report regional results.

Fig. 3 Mortality due to ship emissions under a business-as-usual case. Map of combined mortality (from cardiovascular disease and lung cancer) due to PM 2.5 emissions from ships under a business-as-usual case for 2020. The in-grid-cell minimum and maximum mortality estimates are 0 and 2550, respectively Full size image

Fig. 4 Avoided mortality due to fuel sulphur standards. Map of avoided mortality (cardiovascular disease and lung cancer) from reduced ship PM 2.5 emissions due to implementation of the International Maritime Organization’s low-sulphur fuel standards in 2020. Annual avoided mortality minimum and maximum are 0 and 800, respectively Full size image

Table 2 Estimated annual health impacts of global shipping in 2020 in the BAU case and the 2020 Action case with IMO low-sulphur fuel standard, where parentheses indicate 95% confidence intervals based on relative risk calculations Full size table

More than 97% of the adult mortality benefits from ship emissions reductions will be in Asia (80%), Africa (12%), and Latin America and the Caribbean (5%). More than 98% of the childhood morbidity benefits from ship emissions reductions also occur in Asia (54%), Africa (33%), and Latin America and the Caribbean (12%). The different distributions are primarily due to the different distributions of adult and youth populations among nations. Europe, North America, and Oceania combined will receive <3% and <2% of the mortality and morbidity benefits of the global sulphur standard, respectively. This is primarily due to existing legislation (with or without the global standards) in North America, the Baltic Sea, the North Sea, the English Channel, and EU sea areas in general (see Supplementary Note 2 for regional tables).

Our assessment of the global health burden due to air pollution from ships are much higher than prior assessments primarily due to improved geospatial resolution of global models, updated inputs, and the use of linear C–R functions (see Methods section). First, by applying higher spatial and temporal resolution using the 2020 STEAM inventories and SILAM chemistry transport model, we more precisely assess proximal exposure concentrations, better quantifying peak exposure to at-risk communities (see Supplementary Note 2, Supplementary Figure 3). Second, the updates provided by Lepeule, et al.31, increase the health risks attributable to PM compared to previous assessments; e.g., the attributable fraction for lung cancer due to PM 2.5 exposure increases by a factor of 2.5. Third, we adopt the so-called linear relative risk function by Lepeule, et al.31, which differs from the log-linear functions of Pope, et al.33 and Ostro34 used in our previous health burden studies. The linear formulation produces characteristically higher health burden estimates for the range of ship pollution concentrations modelled (see Methods section and Supplementary Note 1). At higher concentrations, the combination of updated β coefficients and C–R functional form can increase the in-cell health burden estimate by orders of magnitude. Supplementary Table 3 presents results using log-linear C–R function. Supplementary Tables 4 and 5 present regional mortality and asthma results using log-linear C–R function, respectively. Lung cancer mortality using the linear function is more than three times the log-linear C–R results in Supplementary Table 3, and more than seven times the lung cancer estimates using coarser resolution inputs in Winebrake et al.10. (Note that Winebrake et al.10 computed premature cardiopulmonary mortality and cannot be compared directly with cardiovascular mortality as discussed in Methods section).

Our linear function BAU 2020 estimates in the East Asia region are an order of magnitude higher than the health burden estimated for 2013 by Liu et al.7 (238,100 vs. 24,000 air pollution deaths from shipping). This is clearly a result of the different choices of C–R functions, where Liu et al. use an assortment of relative risk functions within their exposure–response model based on Burnett et al.35. For our 2020 Action case, global controls for sulphur standards would reduce the shipping health burden in the East Asia domain of Liu et al. by 25% (with 34% reduction of all Asia as shown in Supplementary Table 1). Calculations using the log-linear C–R function in Supplementary Note 3 are 45% higher than the health burden estimated by Liu et al., and their estimates fall within the log-linear 95% confidence interval.

Ship pollution matters in the context of total health impacts from ambient (outdoor) air pollution, with or without the new standards. Despite demonstrated health benefits associated with a low-sulphur fuel standard, ship traffic using cleaner fuel will produce air pollution impacting mortality and morbidity in proximal coastal communities. The World Health Organization (WHO) estimates ~3 million deaths in 2012 attributed to ambient air pollution36, with 1.48 million attributed to lung cancer and cardiovascular disease; the World Bank estimates 5.5 million deaths in 2013 attributed to both household and ambient air pollution (2.9 million due to ambient PM 2.5 )37. Other scientific peer-reviewed journal papers report similar health burdens due to ambient PM, with estimates ranging from 2.2 to 3.3 million deaths annually38,39,40,41,42.

In terms of childhood asthma, the results are also quite significant. Under our BAU case and linear risk function, shipping emissions lead to ~14 million childhood asthma cases annually. Further, even with the low-sulphur fuel standards proposed by IMO, ships are still responsible for ~6.4 M childhood asthma cases annually. The 2014 Global Asthma Study estimated “as many as 334 million people in the world have asthma,” and statistics indicate 26% of the world population is 14 years or younger43. Without adjusting for higher prevalence for asthma among young and old persons, direct application of population statistics suggest 86 million children could suffer from asthma, and our linear-function results suggest that shipping is currently responsible for ~16% of these cases (~7.5% if the new IMO standards are implemented).

Most synthesis reports reference sources using log-linear C–R functions and matched β-coefficients35,36,37,38,39,40,41,42, and some meta-analyses do not explicitly report underlying C–R functions informing their health assessments. We can make bounding comparisons of the contribution of ship pollution to global cardiovascular and lung cancer mortality and asthma morbidity assuming that our log-linear specifications compare better with other studies (see Supplementary Note 3). Under the BAU scenario, shipping accounts for about 3.6% of WHO and World Bank mortality estimates from ambient air pollution, and nearly 7% of lung and cardiovascular disease mortality; shipping pollution under the BAU scenario contributes to ~3.6% of childhood asthma morbidity. Under the 2020 Action scenario, shipping would contribute about 2% of WHO and World Bank air pollution total mortality from ambient air pollution, about 4% of the lung cancer and cardiovascular disease mortality, and about 2.5% of childhood asthma. Cleaner ship fuels reduce exposure to ambient PM 2.5 air pollution, reducing total ambient air pollution deaths from lung cancer and cardiovascular disease by ~2.6% and reducing total childhood asthma by ~3%. Along major shipping lanes and ports near densely populated coastal regions, these health impacts are much greater.

Radiative forcing due to low-sulphur fuel

Global low-sulphur fuel standards also impose climate forcing consequences. Sulphates from ship emissions reduce radiative flux at the earth’s surface (i.e., contribute a cooling effect), previously estimated in the range of −47 to −8 mW m−2 (direct radiative effect) and −600 to −38 mW m−2 (indirect radiative effects)15, 30, 44,45,46.

Our analysis covered direct and the first-indirect effects (i.e., impact on the number concentration and size of cloud droplets and, consequently, cloud albedo via the Twomey effect). We estimated these effects for a variety of assumptions regarding the aerosol properties and meteorological conditions. We found that the global average of the direct radiative forcing of ship-emitted sulphates and nitrates strongly depends on the assumption of the single-scattering albedo (SSA) of the particles, consistent with Lohman et al.47 and Lesins et al.47, 48. The cooling under the BAU scenario is about −5.2 mW m−2 and −1.5 mW m−2 for sulphate and nitrate scattering aerosols, respectively (Table 3). Comparing BAU and 2020 Action scenarios, reduction of sulphates will lead to over 4 mW m−2 of reduced cooling, whereas the nitrate concentrations will increase due to the compensation effect, thereby adding extra cooling of almost 0.2 mW m−2. The net reduced direct cooling will be 3.9 mW m−2 for scattering particles.

Table 3 Summary of change in radiative forcing due to aerosol effects in BAU and 2020 Action scenarios Full size table

Estimation of the indirect effects remains very uncertain and depends on the assumption on the distribution of the ship-induced sulphate aerosols. We quantified the first-indirect effect using empirical relationships between the sulphate and sea salt concentrations derived from several cloud chemistry observation campaigns49,50,51 (see Methods section). This allowed estimating the sensitivity of the forcing in terms of the vertical in-cloud distribution of the sulphate aerosols. In order to compute an envelope of the possible variations, we considered two extreme cases: (1) when the sulphates are well mixed with the cloud water, i.e., the in-cloud convection mixes them perfectly; and, (2) when the in-cloud convection does not affect the sulphate vertical profiles. In the latter case, the sulphates and cloud-water vertical profiles are largely detached, and the upper levels of the clouds are practically not polluted by sulphates. For the well-mixed case, the cooling by ship-induced sulphates amounts to −86 mW m−2 for our BAU scenario and −19 mW m−2 for 2020 Action scenario. This means that the reduced cooling due to these first-indirect effects is 67 mW m−2.

Sensitivity simulations for direct and indirect effects highlighted several parameters affecting the final estimates. The direct radiative effect is strongly sensitive to aerosol optical features. The ship-induced sulphates and nitrates in nature are not pure scattering aerosols due to emissions of black carbon as part of PM. Black carbon serves as condensation centres attracting SO 2 and creating internally mixed particles with black cores coated by sulphates. Sensitivity computations suggested that at SSA of ~0.975 the direct cooling effect of ship plumes is zero, whereas darker aerosols are the warming agents (see Supplementary Figure 5, for results with SSA = 0.999, 0.99, and 0.95). The tipping point at SSA = 0.975 leads to all-zero direct forcing practically everywhere48. Therefore, the change of the direct radiative forcing due to low-sulphur standards will strongly depend on the technologies used for reaching the limitations, specifically how control technologies affect black carbon emissions52, 53.

Sensitivity of the indirect effect to the vertical profile of in-cloud sulphates seems to be most important to our estimates. In the detached sulphate profiles, the dominant effect of upper cloud layers, which are practically not polluted by sulphates, determines the overall cloud albedo regardless of the pollution at the cloud bottom, resulting in the lower estimate. Therefore, if most of sulphates are located near the cloud bottom, the reduced cooling is much less than for the well-mixed case: 17 mW m−2 instead of 69 mW m−2 (see Supplementary Note 4, Supplementary Figure 6). With high-resolution simulations, however, the well-mixing assumption is closer to observations, especially in tropical conditions, where horizontal size of the convective clouds is indeed of the order of 10 km. Therefore, the best estimate from this study is 69 mW m−2.

The above results are generally consistent with other literature15, 46, 54, 55. Thus, Eyring et al.54 roughly estimated direct cooling effect of sulphates of −11 to −26 mW m−2 in 2030 depending on traffic scenarios54, which is well in line with our BAU case plus the increase of ship traffic by 2030. Fuglesvedt et al.46 estimated indirect negative forcing from ship sulphate aerosols as −66 mW m−2 compare to preindustrial level (uncertainty range (−114, −38) mW m−2), which also compares well to our range of the first-indirect effect (−86, −22) mW m−2 46. Capaldo et al.56 suggested −110 mW m−2, but the evaluation against SO 2 mixing ratio observations showed almost an order of magnitude of over-estimation. A study of Dalsoren et al.57 considered a similar set of ship emission scenarios and estimated the total loss of sulphate-induced cooling at −70 mW m−2 with 10–15% variability depending on scenario of ship emission in 2030, which is in close agreement with our estimates. However, Dalsoren et al. suggested that direct and indirect effects are similar in amplitude, which differs from our and most other studies, which showed ~10-fold dominance of indirect effect(s).

Our BAU 2020 direct radiative forcing estimates are also consistent with the estimates by Liu et al.7 in East Asia7. This work estimates the BAU direct forcing in East Asia averages −23 mW m−2 (standard deviation of −14 mW m−2), with a region peak and minimum forcing of −55 mW m−2 and −1.5 mW m−2, respectively. We identify two local maxima for sulphate and nitrate radiative forcing (Fig. 3), one in the East Asia domain aligned with Liu et al., and a second region with greater maxima in the South China Sea. Liu et al. evaluated AIS-derived shipping only within their East Asia study domain, which included only a portion of the shipping traffic in the South China Sea. Our global AIS-derived shipping domain extends the major shipping lanes from mainland China to Singapore, and therefore better represents forcing peaks associated with the full extent of regional shipping routes.

A few studies showed much larger effects than the ones above. Tronstad Lund et al.15 based on regression between total sulphur emission and radiative forcing suggested the indirect cooling effect as high as −450 mW m−2. However, a relative reduction was estimated as 37–40% in direct and indirect aerosol forcing from fuel changes originally presented by Lauer et al.58, about half of the results reported in this work58. In turn, Lauer et al.58 estimated the direct effect at the same −11 mW m−2 level as came from our computations but got the indirect effect varying from −190 to −600 mW m−2 (about 75% of these attributed to sulphates) depending on ship emission inventory58. A similar study of Righi et al.59 showed the range from −280 to −400 mW m−2 but used higher ship SO x emission of 14 Mton per year. Balkansky et al. (2010) reported shipping direct effect of −20 mW m−2, and also pointed out the compensation by direct warming effect of road-traffic carbonaceous aerosols.44 For global estimates, including CO 2 , the net reduced cooling of this study amounts to nearly 35% of the total radiative forcing from transport46 and over 3% of anthropogenic radiative forcing13.

Considering the reasons for the differences from some of the previous work, one can notice a generally lower-sulphur lifetime and global mean burden in the current simulations in comparison with other studies. For example, we estimate a 0.6 Mton sulphur burden (2015), vs. 1.5 Mton sulphur burden (mean 1999–2004) assumed by Lauer, et al. Accounting for the lower emission of SO x in 2015 (111 vs. 150 Mton per year in Dentener et al.60 used by Lauer et al.), one ends up with the mean atmospheric lifetime of sulphur of about 2.8 days in the current study and 3.6 days in Lauer, et al. Species-specific lifetimes in our runs were 1.5 days for SO 2 (primary sink is oxidation to SO 4 ) and 2.8 days for sulphates, i.e. dry and wet deposition of sulphates are the primary mechanisms controlling the total-sulphur burden in SILAM. These lifetimes are within the range of values reported by other models61, with the SILAM estimate being more on the lower end. Evaluation of SILAM is outlined in Supplementary Note 5.

With our results comparing BAU and 2020 Action scenarios, Table 3 presents the estimated change in the radiative forcing from cleaner ship fuels on the following: (a) the shipping sector, (b) the transportation sector, and (c)all anthropogenic activity.

The cleaner fuels will also shift the composition of the forcing aerosols. In the BAU scenario, over two-thirds of the cooling is due to sulphates, whereas the contributions of sulphates and nitrates are practically the same under the 2020 Action scenario. Similar to patterns of near-surface PM 2.5 concentrations, highly heterogeneous ship traffic density and meteorological conditions determine the radiative forcing patterns (Fig. 5). In particular, the most-significant regional direct radiative effect is over the Persian Gulf, where very dense emissions coincide with large fraction of cloud-free days. Shipping in Southeast Asia is stronger and leads to higher ship-induced PM 2.5 concentrations covering wider area (Fig. 1). However, the resulting direct radiative forcing is lower than in the Persian Gulf region due to a larger number of cloudy days. For the indirect forcing, the pattern is likely to be the opposite: the effect will be more pronounced in cloudy regions of South-East Asia.