A recent study commissioned by the International Maritime Organization (IMO) estimated total emissions of 1046 Tg CO₂ yr⁻¹ from shipping in 2007, corresponding to 3.3% of the total anthropogenic CO₂ emissions in that year. Of that, international shipping accounts for 83% (870 TgCO₂ yr⁻¹ of the total ship emissions in 2007.

In addition to CO₂, other compounds such as ozone (O₃) precursors (nitrogen oxides (NO_x=NO+NO₂), carbon monoxide (CO), and volatile organic compounds (VOCs)) and 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. 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. The O₃ precursors, SO₂, and aerosol emissions have effects on atmospheric composition and climate and can affect human health and cause regional pollution and acidification.

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: (1) CO₂ gives a positive RF; (2) NO_x 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).

—Lud et al.

Prior work on the overall RF (including the highly uncertain indirect aerosol effect) of shipping in 2000 from preindustrial suggested that shipping has a net cooling impact today. The authors used a range of emission scenarios for shipping, consistent with the new regulations on nitrogen oxides (NO_x) and sulfur dioxide (SO₂) from the International Maritime Organization to determine the induced global-mean radiative forcing and temperature change.

Using on a complex aerosol-climate model, they developed and tested new parametrizations of the indirect aerosol effect (IAE), resulting in their finding of a net global cooling impact throughout the period 1900–2050 across all parametrizations and scenarios.

However, they noted, the wide range across parametrizations emphasizes the importance of properly representing the IAE in SCMs and to reflect the uncertainties from complex global models.

Because of the expected reductions in SO₂ and NO_x emissions reflected in the scenarios, the cooling impact of shipping is initially reduced. However, in the IMO scenarios the increase in activity outweighs emission reductions from more stringent regulations and the cooling effect strengthens again despite increases in CO₂. In the RCPs [Representative Concentration Pathways], reductions in SO₂ and NO_x continue up to 2050 and beyond and result in a continued weakening of the cooling. If current regulations are insufficient to continuously reduce emissions of SO₂ and NO_x, the net impact of shipping is likely to be a continued cooling until 2050. This will also exacerbate the significant additional negative impacts of shipping, such as acidification and health-related problems.

However, if short-lived cooling species are reduced, the long-term warming due to CO₂ dominates, and the net temperature change becomes positive. As has been emphasized in several previous studies, a continued cooling response does not negate the necessity for reductions in CO₂ emissions, which are crucial to limiting the global climate impact of the sector since the warming effect of CO2 is long-lived.

—Lund et al.

Resources

• Marianne Tronstad Lund, Veronika Eyring, Jan Fuglestvedt, Johannes Hendricks, Axel Lauer, David Lee, and Mattia Righi (2012) Global-Mean Temperature Change from Shipping toward 2050: Improved Representation of the Indirect Aerosol Effect in Simple Climate Models. Environmental Science & Technology 46 (16), 8868-8877 doi: 10.1021/es301166e