Shipping Emissions: From Cooling to Warming of Climate—and

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International shipping has been a fast growing sector of the global economy and its share of total anthropogenic emissions is significant, having effects on climate, air quality, and human health. The nature of the contribution to climate change is complex: In addition to warming by CO2 emissions, ship emissions of sulfur dioxide (SO2) cause cooling through effects on atmospheric particles and clouds, while nitrogen oxides (NOx) increase the levels of the greenhouse gas (GHG) ozone (O3) and reduce the GHG methane (CH4), causing warming and cooling, respectively. The result is a net global mean radiative forcing (RF) from the shipping sector that is strongly negative (1), leading to a global cooling effect today (Box 1). However, new regulations of SO2 and NOx, while reducing air pollution and its harmful effects on health and water/soil acidification (2), will reduce this sector’s cooling effects (3). Given these reductions, shipping will, relative to present-day impacts, impart a “double warming” effect: one from CO2, and one from the reduction of SO2. Therefore, after some decades the net climate effect of shipping will shift from cooling to warming.

Shipping Emissions: From Cooling to Warming of Climatesand Reducing Impacts on Health JAN FUGLESTVEDT* TERJE BERNTSEN Center for International Climate and Environmental Research Oslo (CICERO) VERONIKA EYRING Deutsches Zentrum fu ¨ r Luft- und Raumfahrt (DLR), Institut fu ¨ r Physik der Atmospha¨re, Oberpfaffenhofen, Germany IVAR ISAKSEN CICERO DAVID S. LEE Manchester Metropolitan University, Manchester, U.K.

Box 1

How should shipping emissions’ ability to cool or warm the climate be addressed?

Radiative forcing refers to the change in the Earth-atmosphere energy balance since the pre-industrial period. If the atmosphere is subject to a positive radiative forcing from, for example, the addition of a greenhouse gas such as CO2, the atmosphere attempts to re-establish a radiative equilibrium, resulting in a warming of the atmosphere. A negative RF on the other hand, causes climate cooling.

PHOTOS.COM

ROBERT SAUSEN DLR, Oberpfaffenhofen, Germany

Presently, more than 80% of world trade is transported by ships (4, 5). As melting of the Arctic ice continues, shipping may find itself even more detrimental to climate and environment, as new lanes may open up in a region perhaps even more sensitive than others to new and increasing emissions. The transnational nature of this sector makes it difficult to allocate emissions and responsibilities to states; emissions from international shipping are not regulated by the Kyoto Protocol. However, the role of this sector in new climate agreements is on the agenda for the UN Climate Change Conference (COP 15) in Copenhagen in December 2009. Thus, there are several reasons why research and policymaking need to pay attention to the development of the shipping sector. Regulations of SO2 emissions have been discussed for over a decade within the International Maritime Organization (IMO) because of shipping’s contribution to regional acidification and air pollution. The Marine Environment Protection Committee (MEPC) of the IMO recently adopted NOx regulations and progressive reduction in SO2 emissions from ships, with the global cap (a fuel S content limit) initially reduced from the current 4.5 to 3.5% by 2012, to be followed by progressive reduction to 0.5% by 2020, subject to a feasibility review by 2018 (6). In addition, the S limits in emission control areas will be reduced from the current level of 1.5 to 1% in 2010, and further to 0.1% in 2015. 10.1021/es901944r

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FIGURE 1. Global mean temperature responses to 1 y pulse emissions of CO2 and SO2 from shipping. This will lead to significant global health benefits (2): the potential contribution to annual premature mortalities due to cardiopulmonary disease and lung cancer attributed to particles caused by emissions from ships was estimated to be ∼60 000 (20 000-104 000) in 2001 (7). Compared to previous estimates, these shipping-related deaths account for 3-8% of the total number of worldwide deaths related to atmospheric particles (1). By 2012 control scenarios would reduce premature deaths by ∼43 500 if the S content is limited to 0.1% within 200 nautical miles (nmi; 370 km) of coastal areas and be reduced by ∼41 200 if the S content is reduced globally to 0.5% (2). Although emissions from shipping have been included in analyses of total anthropogenic climate effects, specific investigation of the climate impacts of shipping is a relatively recent endeavor (8-10). Our knowledge of the magnitude of shipping emissions (11-15) and their impacts has improved and the emerging picture is rather complex (1, 16-22), as the next section highlights.

Shipping Emissions and Forcing of Climate Current CO2 emissions of the ocean-going fleet (including domestic shipping and fishing, but excluding military vessels) are ∼1000 Tg CO2/y for 2006, representing 3.3% of total anthropogenic CO2 emissions (1, 15). The activity level (in terms of ton-miles [Editor’s Note: this non-SI unit is used in the cited report]) increased by almost 40% from 2000 to 2007 (23) accompanied by a strong increase in emissions. Since the mid-19th century, shipping’s emissions of CO2 have resulted in a RF of ∼37 mW/m2 by 2005 (1). However, a potentially large negative global mean RF from S emissions works in the opposite direction from that of CO2. S is emitted mainly as gaseous SO2 and is oxidized in the atmosphere to sulfate (SO42-) which will form particles. These particles have a direct impact on climate by scattering solar radiation and thus reducing the amount of shortwave radiation heating of the surface (10, 18, 19), amounting to a RF of -31 mW/m2 in 2005 (1). Additionally, SO2 indirectly affects climate by forming cloud condensation nuclei. Such activity increases droplet number densities and changes the reflectance and lifetimes of clouds (1, 18), causing a RF of -740 to -47 mW/m2. This contribution is significant because ships emit in regions with a clean environment and frequent low clouds. The potential impact of particulate matter due to shipping emissions is larger on the radiation budget given the relative albedo change over a dark ocean, as opposed to similar emissions over more reflective land surfaces. 9058

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Our understanding and ability to quantify the indirect effects of SO2 are limited mainly due to insufficient knowledge of the processes involved, but also from uncertainties in estimates of particle size distributions and in quantification and location of emissions. Thus, there are large uncertainties in the estimates of the temperature effects of SO2 emission. It is important to reduce these uncertainties to understand the anthropogenic perturbation of the climate system and to improve the usefulness of scenarios for future planning. Emission of NOx led to a warming (26 mW/m2) from O3 formation and a cooling (-33 mW/m2) from enhanced CH4 loss (1, 17, 19). The overall global mean RF for the sector on the order of -400 mW/m2 in 2005 (1) is associated with large uncertainties mainly from the indirect aerosol effect. All studies, however, agree that the central estimate of presentday net RF due to shipping is negative.

Is Shipping Good or Bad for Climate? Modeling the impacts of emissions on atmospheric chemistry and climate is a complex task, including different temporal and spatial scales in addition to the concept of climate change beyond global mean temperature. Specifically here, SO2 and CO2 effects operate on very different time scales (see Figure 1 for a schematic illustration). SO42- particles have an atmospheric residence time of a few days, and the climate response from SO42- is on the order of decades, mainly due to the thermal inertia of the oceans. Conversely, the effect of CO2 will last several centuries and a residual will last for much longer due to the slow removal of excess CO2. Thus, the warming by CO2 will dominate on a longer time scale. The short time scale of the SO2 effects compared to that of CO2 means that any potential proposal to maintain SO2 emissions to lower the rate of temperature change implies that future generations would be committed to continue elevated SO2 emissions. If for some reason (e.g., health issues, ocean acidification) the SO2 emissions must be reduced, future generations are beset with a warming effect that may be difficult to curb. Two hypothetical emission scenarios have been formulated to illustrate these temperature responses using a simple climate model with input from studies of shipping emissions and complex atmospheric processes including aerosolcloud interactions (19, 22). In one case the emissions are kept constant at year 2000 levels, in the second case the SO2 is reduced by 90% from year 2000 levels and thereafter held constant. Figure 2 illustrates the effect in terms of changes in global mean temperature (i.e., neglecting impacts of

FIGURE 2. Global mean temperature changes due to emissions of CO2 and SO2 and NOx-induced changes in O3, CH4, and O3PM (the latter being the primary mode ozone controlled by CH4), and the total temperature change (∆T TOT). Plotted are (a) the response to a scenario with all emissions kept constant at year 2000 levels and (b) the responses to a scenario with SO2 emissions reduced by 90% with all other emissions at year 2000 levels. historic emissions). For the constant emissions case (Figure 2a), the cooling effects of SO2 and NOx reduce the net effect to a maximum negative effect after 30 y. The long-term warming due to CO2 will lead to a switch from net cooling to net warming after ∼350 y. With the reduced SO2 emissions (Figure 2b) the net temperature effect switches to warming much earliersafter ∼70 y. The timing of the change from cooling to warming depends on the thermal inertia of the ocean and the magnitudes of the various RF components. Thus the switchover time is sensitive to the uncertainties in the indirect effect of SO42- on cloudsshere we have assumed -97 mW/m2 for the sum of the direct and indirect effects (from ref 19). (Using the uncertainty ranges from ref 1 for the S-related RF, we find that in the reference case the switchover occurs after 150 y for the minimum direct and indirect RF, while for the maximum values the switchover occurs on a scale >1000 y. In the S-reduction case, the corresponding numbers are ∼60 y and ∼200 y). During the first few years, the rapid increase in O3 results in a slight net warming (Figure 2b). The net long-term increase is mainly driven by increasing warming from CO2. If for some reason all emissions from the shipping sector were removed in the future (e.g., due to new technologies), then all nonCO2 effects would disappear within a few decades, while the warming from CO2 would still remain for several centuries (15). Thus, using emissions of SO2 to reduce global warming

may put us in a position where, unless CO2 is significantly reduced, we commit ourselves to long-term warming. In addition to changes in temperature, climate change manifests itself in many other ways, such as changes in precipitation and circulation patterns. Considering temperature alone, our current scientific understanding indicates that even if the strong negative RF from S is confined to certain regions, the pattern of temperature change seems to be determined by a combination of the forcing pattern and that of the feedback processes in the climate system (24-26). Thus, shipping will reduce warming not only in the regions with negative RF, but to some extent almost everywhere. The largest cooling is likely to be in the regions with strong negative RF, but cooling may also take place in regions with strong regional feedbacks (25). The extent to which heterogeneous RF patterns can lead to circulation changes that again can influence the temperature change pattern is an under-researched topic. Yet even with a cancellation between cooling caused by heterogeneous RF due to aerosols (or cloud effects) and warming from long-lived GHGs in terms of global mean values, a cancellation of the regional responses is questionable. It is important to realize that while global mean values are very useful for understanding the overall impact and evaluation of the various effects and their time scales, important information about the regional variation in the VOL. 43, NO. 24, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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responses is hidden. Furthermore, in the case of a perturbation that causes regional negative forcing, the use of global mean values may have some implicit effects. By summing up the local negative RF and positive RF with a different spatial pattern leading to a global mean cooling it is implicitly assumed that the negative RF is “good” for the climate. The negative RF caused by SO2 from shipping (possibly as high as 40% of the total anthropogenic CO2 positive RF) does indeed reduce global warming, but it may also have detrimental effects on climate by changing atmospheric circulation and the hydrological cyclesaspects of climate change that are not visible when global mean temperature is used as the indicator (25, 27, 28). In this case, a focus that emphasizes disturbance of climate may be better. Such disturbances could initiate changes in climate that require adaptation due to the many aspects of climate change. How such a “disturbance” metric could be formulated, however, is not presently obvious (29).

Inadvertent Geoengineering One might consider SO2 emissions as a form of inadvertent geoengineering due to the cooling effects. Indeed, the proposed geoengineering scheme of deliberately seeding lowlevel clouds over the oceans to enhance their albedo (30) would lead to a forcing mechanism similar to continued SO2 emissions from shipping. In a recent study, the climate effect of cloud seeding was simulated with the Hadley Centre’s comprehensive climate model (25): the strongest temperature responses were found in the regions where the RF is applied, but also in the Arctic where the strong snow/ice-albedo feedback operates. However, geoengineering that causes heterogeneous (i.e., spatially disparate) RF may have unforeseen effects such as changes in the hydrologic cycle and wind patterns (25, 27). In the Hadley Centre’s simulation, some of the current warming was masked, but both positive and negative changes in precipitation were modeled (25). Positive changes could be beneficial, but enhancing stratocumulus clouds in the South Atlantic and reduced precipitation in the Amazon may have serious impacts on rainforests and their resultant carbon-sink capacity. This demonstrates that such geoengineering may have unintended consequences and also result in irreversible processes. From these studies, one could argue that allowing continued SO2 emissions from shipping may have some similar impacts. Thus, available studies indicate that there is not a simple offset that gives a basis for a policy recommendation of using SO2 as a “climate-cooler”. These questions should be explored further by modeling studies of the climatic responses to the negative and heterogeneous RF from shipping if further consideration of such strategies is desired. A recent report (31) that assesses geoengineering techniques states that modification of surface or cloud albedo could have negative effects on regional weather patterns and ocean currents. It is also concluded that a great deal more research would be needed before these techniques could be seriously considered. Hence the available evidence suggests that “climate cooling” by continued shipping emissions of SO2 would not be advisible.

Beyond Cooling Effects In addition to the cooling effects addressed above, SO2 and NOx have several other indirect effects such as acidification and fertilization of the ocean, which affect the exchange of CO2 between the atmosphere and the oceans (32). Also, NOx produces O3 which may decrease the uptake of CO2 in the biosphere (33). In terms of ocean acidification, nitric acid (HNO3), formed from NOx, and sulfuric acid (H2SO4) from shipping emissions contribute a few percent compared to CO2 at a global scale (32), although the effects may be 9060

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significant in proximity to coastlines where the most sensitive biota are found. Biogeochemical changes caused by SO2 acidification and the effect of O3 decreasing carbon uptake will both serve to increase global warming. Conversely, fertilization by NOx has the opposite effect by stimulating growth of CO2-consuming plant life. These effects are not currently accounted for in RF calculations of shipping effects. With the strong reductions of SO2 and NOx emissions in Europe from land-based sources, shipping emissions have become an increasingly important source of S deposition and O3 formation in certain coastal areas (17, 34). As discussed in ref 1, 50% of the S deposition in the North Sea and Baltic region is from shipping and accounts for 10-25% of the total deposition along the western coasts of the UK and Scandinavia. Deposition of nitrates causes eutrophication which can harm ecosystems through asymmetric growth in N-poor regions (e.g., algae in rivers and lakes; lichens and mosses on hillsides) and in some regions encouraging invasive alien species (1). Almost 70% of ship emissions occur within 400 km of coastlines leading to reduced air quality in coastal areas and harbors. In addition, pollutants originating from ships may be transported several hundred kilometers. In this way, shipping may contribute to air quality problems over land and may counteract national land-based control measures, nonetheless increasing coastal pollution (1, 17, 35). In light of the discussion above, shipping initiates environmental effects that occur on very different temporal and spatial scales, with fundamentally different impacts on health, environment, and climate. Consequently, comparison of these effects is a very difficult task: monetizing climate impact costs is a difficult and contentious area, with no commonly accepted existent methods. Assessments of tradeoffs between impacts on human health and climate are an ongoing activity which is frequently discussed at meetings of MEPC. According to a recent study (20), the net contribution from shipping to anthropogenic global warming in 2000 was -7%. However, due to SO2 reductions, and depending on the scenario, the contribution will swing to a warming effect of a few percent in 2100. Health impacts are also related to climate change, and SO2 reductions will, in principle, increase the health problems related to global warming. The health effects per degree of global temperature increase have, however, not been well quantified to date.

Shipping In an International Climate Regime International shipping is not regulated by the Kyoto Protocol due to the methodological difficulties in allocating emissions to countries (36). Article 2.2 of the Protocol stipulates that reductions in emissions should be pursued by IMO, since shipping is a global industry (37). However, no agreement has yet been reached on how CO2 emission from shipping can be addressed, and incorporation into policy regimes is continuously under discussion. Currently, the European Union is investigating the feasibility of incorporating international shipping into its emissions trading scheme, and the role of this sector in new climate agreements is on the agenda for the United Nations Climate Change Conference (COP 15) in Copenhagen in December 2009. The UN Framework Convention on Climate Change requires that “policies and measures should... be comprehensive, cover all relevant sources, sinks, and reservoirs of greenhouse gases..., and comprise all economic sectors” (38). This was made operational by the Kyoto Protocol, which sets limits on emissions of CO2, CH4, N2O, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and SF6. Thus, the Kyoto Protocol is a multigas agreement, which is a step toward the UNFCCC’s aim of being comprehensive. This basket of components may be expanded in future climate agreements.

For example, inclusion of black carbon (soot) in international agreements and strategies has been discussed (39, 40). NOx, as a pollutant, is already regulated by separate protocols but with its dual climate effect and dependence on location of emissions (41) this gas is difficult to handle in a climate agreement (29). Inclusion of cooling agents such as SO2 has, as far as we know, not been suggested. It would not be based on solid and sound science if nations or sectors get credits for SO2 emissions; i.e., that these emissions in a regulatory framework will count as compensating the emissions that cause warming. The S reductions in the shipping sector are parallel to the SO2 reductions implemented in Europe and the U.S. in response to acid precipitation. One important difference, however, is that these new SO2 reductions take place over the oceans. Thus, there are reasons to expect that the effects of SO2 reductions from shipping (per unit emission) may be stronger than the land-based reductions due to the relative albedo effect noted above. The opposite effects of shipping emissionssa short-term cooling and long-term warmingsput this sector in a special position, both scientifically and in the context of policy making. The planned reductions of SO2 and NOx will have beneficial impacts on health, acidification, and eutrophication. But as a part of this “greening” of shipping, the sector will have a “double warming” effect which, together with international ambitions to limit level and rate of global warming, calls for an increased and broad focus on this sector, both from the science community and from policymakers. Jan S. Fuglestvedt is a research director of CICERO. Terje Berntsen is a senior scientist at CICERO and a professor in the Department of Geosciences, University of Oslo. Veronika Eyring is a senior scientist at the Institut fu ¨ r Physik der Atmospha¨re of the Deutsches Zentrum fu ¨ r Luft- and Raumfahrt (DLR) and a visiting professor at the Manchester Metropolitan University (MMU), UK. Ivar Isaksen is a senior scientist at CICERO and a professor in the Department of Geosciences, University of Oslo. David S. Lee is a professor of atmospheric science at MMU and Director of the Centre for Aviation, Transport and the Environment. Robert Sausen is head of the department for Atmospheric Dynamics at the Institut fu ¨ r Physik der Atmospha¨re of the Deutsches Zentrum fu ¨r Luft- and Raumfahrt (DLR) and a professor of meteorology at the Ludwig-Maximilians-Universita¨t Mu ¨ nchen. Please address correspondence regarding this article to [email protected].

Acknowledgments This commentary is based on work in the EU FP6 Integrated Project QUANTIFY (www.ip-quantify.eu), the EU Specific Support Activity ATTICA (www.ssa-attica.eu), the HelmholtzUniversity Young Investigators Group SeaKLIM, which is funded by the Helmholtz Association of German Research Centres, and the Deutsches Zentrum fu ¨r Luft- und Raumfahrt e.V. (DLR), and projects funded by the Norwegian Research Council. We thank Johannes Hendricks for useful discussions.

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