Article pubs.acs.org/est
Global-Mean Temperature Change from Shipping toward 2050: Improved Representation of the Indirect Aerosol Effect in Simple Climate Models Marianne Tronstad Lund,*,† Veronika Eyring,‡ Jan Fuglestvedt,† Johannes Hendricks,‡ Axel Lauer,§ David Lee,∥ and Mattia Righi‡ †
CICERO−Center for International Climate and Environmental ResearchOslo, P.O. Box 1129, Blindern, 0318 Oslo, Norway Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany § International Pacific Research Center, University of Hawaii at Manoa, Honolulu, Hawaii, United States ∥ Dalton Research Institute, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, United Kingdom ‡
S Supporting Information *
ABSTRACT: We utilize a range of emission scenarios for shipping to determine the induced global-mean radiative forcing and temperature change. Ship emission scenarios consistent with the new regulations on nitrogen oxides (NOx) and sulfur dioxide (SO2) from the International Maritime Organization and two of the Representative Concentration Pathways are used as input to a simple climate model (SCM). Based on a complex aerosol-climate model we develop and test new parametrizations of the indirect aerosol effect (IAE) in the SCM that account for nonlinearities in radiative forcing of ship-induced IAE. We find that shipping causes a net global cooling impact throughout the period 1900−2050 across all parametrizations and scenarios. However, calculated total net global-mean temperature change in 2050 ranges from −0.03[−0.07,−0.002]°C to −0.3[−0.6,−0.2]°C in the A1B scenario. This wide range across parametrizations emphasizes the importance of properly representing the IAE in SCMs and to reflect the uncertainties from complex global models. Furthermore, our calculations show that the future ship-induced temperature response is likely a continued cooling if SO2 and NOx emissions continue to increase due to a strong increase in activity, despite current emission regulations. However, such cooling does not negate the need for continued efforts to reduce CO2 emissions, since residual warming from CO2 is long-lived.
1. INTRODUCTION
Owing to the complexity of allocating emissions to individual nations, CO2 from international shipping was not included in the emission targets under the Kyoto Protocol.4 Emissions of nitrogen oxides and sulfur dioxide are already regulated by the IMO, and regulations for reducing CO2 are currently being debated.5 Furthermore, the revised EU Emission Trading Scheme (ETS) Directive requires the European Commission to consider the inclusion of shipping in the ETS from 2013 if the IMO process is not complete, and no agreement has been reached by the UNFCCC.6 In addition to CO2, other compounds such as ozone (O3) precursors (nitrogen oxides (NOx=NO+NO2), carbon monoxide (CO), and volatile organic compounds (VOCs)) and
Shipping plays an important role in transporting raw materials and goods around the world: 80% of global merchandise is carried by international shipping.1 Most ships are powered by combustion-efficient diesel engines, but because of the large number of ships (>100,000 in 2007, Lloyd’s Register − Fairplay 20072) and the associated large fuel consumption, shipping is a significant source of CO2 and other pollutants. The total emission of CO2 from shipping has been the subject of some controversy in recent literature with estimates ranging from 560 to 1360 Tg(CO2) yr−1 in 2000.3 However, a recent study commissioned by the International Maritime Organization (IMO)2 reconciled the various approaches and estimated total emissions of 1046 Tg(CO2) yr−1 from shipping in 2007. This corresponds to 3.3% of the total anthropogenic CO2 emissions in that year. International shipping accounts for 83% or 870 Tg(CO2) yr−1 of the total ship emissions in 2007. © 2012 American Chemical Society
Received: Revised: Accepted: Published: 8868
March 27, 2012 July 24, 2012 July 25, 2012 July 25, 2012 dx.doi.org/10.1021/es301166e | Environ. Sci. Technol. 2012, 46, 8868−8877
Environmental Science & Technology
Article
Figure 1. Historical emissions of shipping CO2, NOx, and SO2, and future scenarios to 2050 from IMO (blue: A1B (solid), A2 (dash-dot), B1 (dashed), and B2 (dotted)) and RCP (red: 4.5 (solid) and 8.5 (dashed)). Units are Pg(C) yr−1, Tg(N) yr−1, and Tg(S) yr−1, respectively.
sufficiently large number of ensemble members for a transient response, which is prohibitively expensive from a computational point of view. In such cases, simplified climate models can be used as a first order approximation to calculate temperature responses to imposed RFs.18 Such an approach is well established and is used by e.g. the International Panel on Climate Change (IPCC) for efficiently estimating the climate response to a wide range of emission scenarios.19,20 In this work, we utilize a range of future emission scenarios for shipping, including species that result in both warming and cooling, in order to determine the response of the climate system to a range of policy measures regulating the emissions from the shipping sector. We calculate global-mean RF and temperature changes using a simple climate model (SCM). The future emission scenarios take into account new regulations from IMO on the sulfur content of fuels and on NOx emissions. Previous global model studies (e.g., Lauer et al.17) suggest that the indirect aerosol effect (IAE) could have a very large contribution to the total climate effect of shipping and that it is mainly related to the sulfur emissions. However, the magnitude of the shipping IAE is still highly uncertain; estimates range from −600 to −38 mW m−2 in 2000.3 The dependence of the IAE on the amount of SO2 emitted needs to be represented thoroughly in the SCM, and the related uncertainties need to be investigated. In this study, we use recent estimates of RF from the shipping sector and a new, improved parametrization of the IAE of sulfate accounting for nonlinearities, which are related to e.g. strong regional variability and dependence on the geographical distribution of emission and contribution to IAE from various aerosol species.21 The calculated global-mean ship-induced RF and total net surface temperature change are first presented using the standard linear parametrization and reference RF value for the IAE in the SCM (Section 4.1). Next we show results where the standard linear parametrization of IAE is replaced with linear and logarithmic fits to data from global aerosol-climate model simulations (Section 4.2). These simulations cover various totals and geographical distributions of emissions from shipping as well as different assumptions on the initial aerosol size distribution. Finally, estimates of uncertainty in total RF and climate sensitivity are included (Section 5).
aerosols such as black carbon (BC) and organic carbon (OC) are emitted by ships. Shipping is an important source of sulfate (SO4) through oxidation of emitted sulfur dioxide (SO2) because the fuel sulfur content is usually high, averaging around 2.7% by mass.7,8 Other than these emissions from combustion associated with main engines and boilers, there are also emissions of ozone-depleting substances (CFCs/HCFCs/ HFCs) from refrigerant and air conditioning systems.2 The O3 precursors, SO2, and aerosol emissions have effects on atmospheric composition and climate and can affect human health9 and cause regional pollution and acidification.10 We focus here on the present-day and anticipated future climate impacts of ship emissions in terms of their contribution to global-mean radiative forcing (RF) and changes in surface temperature. The mechanisms involved are complex, and the emissions can result in both warming (positive RF) and cooling (negative RF) effects:3 (1) CO2 gives a positive RF; (2) NOx results in production of tropospheric O3 (positive RF) and a reduction of ambient CH4 (negative RF) [Reductions in CH4 leads to a longer-term reduction in O3 through changes in the atmosphere’s oxidation capacity (negative RF).]; (3) direct aerosol effect of OC and SO4 particles (negative RF); (4) direct aerosol effect of soot particles and reduced surface albedo when deposited on snow/ice covered surfaces (positive RF); (5) formation or change in low-level cloud properties, so-called indirect aerosol effect (negative RF). A value of −328 [−668, 26] mW m−2 for the overall RF (including the highly uncertain indirect aerosol effect) of shipping in 2000 from preindustrial has been estimated, implying that shipping has a net cooling impact today.3 Current regulations will lead to a reduced sulfur content in marine fuels11 and consequently to reductions in SO 2 emissions, which will result in a rapid decrease of the cooling effect due to the short atmospheric lifetime of SO4. CO2, on the other hand, has a long response time and accumulates in the atmosphere, leading to a long-term warming which at some point in the future will tend to overwhelm the cooling response from the shipping sector to a warming response.12−14 The RF effects summarized above have been assessed with a variety of models of varying complexity. Changes in atmospheric composition are assessed using complex chemical transport models (CTMs) or chemistry-climate models (CCMs);15,16 changes in the atmospheric aerosol loading and cloudiness are calculated with global aerosol-climate models.17 From the changes in radiatively active gases, aerosols, and clouds, the RF can be calculated with radiative transfer models, either offline or embedded within the model. Calculating the temperature response to relatively small perturbations represents a challenge in complex models, because an equilibrium response requires many years of integration or a
2. EMISSIONS As input to the SCM we use emissions from shipping from the Second IMO Greenhouse Gas study,2 where present-day (2007) emissions, historical emissions back to 1850, and future scenarios to 2050 consistent with the IPCC SRES storylines22 are estimated. We also use shipping emissions from the Representative Concentration Pathway (RCP) 4.5 and 8.523−26 8869
dx.doi.org/10.1021/es301166e | Environ. Sci. Technol. 2012, 46, 8868−8877
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AMVER ICOADS AMVER ICOADS ICOADS, coastal S reduction 2012 coastal 0.1 ICOADS, coastal S reduction 2012 global 0.5 ICOADS, global S reduction 2012 no action AMVER 2012 coastal 0.5 AMVER, coastal S reduction 2012 coastal 0.1 AMVER, coastal S reduction 2012 global 0.5 AMVER, global S reduction new IMO 2007 simulation (POM = 0.36 Tg/ ICOADS with yr): with SECAs SECAs new IMO 2007 simulation (POM = 0.36 Tg/ ICOADS without yr): without SECAs SECAs BIOCLEAN REF Paxian et al. 2010 BIOCLEAN REF Paxian et al. 2010 BIOCLEAN REF Paxian et al. 2010 BIOCLEAN MGO Paxian et al. 2010 BIOCLEAN MGO Paxian et al. 2010 BIOCLEAN MGO Paxian et al. 2010 BIOCLEAN PALM Paxian et al. 2010 BIOCLEAN PALM Paxian et al. 2010 BIOCLEAN PALM Paxian et al. 2010 BIOCLEAN SOYA Paxian et al. 2010 BIOCLEAN SOYA Paxian et al. 2010 BIOCLEAN SOYA Paxian e t al. 2010
inventory A inventory C inventory C 2012 no action 2012 coastal 0.5
