Impacts of shipping on the Climate

International shipping is a cornerstone of the global economic system since it accounts for about 90% of the world trade [International Chamber of Shipping ]. However, this growing transport sector consumes large proportions of fossil fuel (Figure 1) and releases as a result significant amounts of trace gases and particulate matter into the atmosphere. For instance, about 3% of the current global carbon dioxide (CO2) concentration has been attributed to the global shipping activity [Buhaug et al., 2009]. Furthermore, shipping emissions of nitrogen oxides (NOx) might well be about 10 times larger than those from aviation [Buhaug et al., 2009 ; Eyring et al., 2007]. Nevertheless, the Kyoto Protocol does not cover CO2 emissions from shipping and there is still no international agreement to curb emissions of non-CO2 species [UNFCCC, 1997]. Instead, the IMO has been given the task to reduce emissions of greenhouse gases and their precursors. Although the IMO has made substantial progress on this issue [MEPC 59 ; MEPC 60 ; MEPC 61 ; MEPC 62] but further work would be needed to avoid dangerous climate change.

A few scientific studies have looked at the impacts of shipping emissions on radiative forcing and climate [Endresen et al., 2003 ; Lee et al., 2006 ; Eyring et al., 2007 ; Fuglestvedt et al., 2008 ; Hoor et al., 2009 ; Eyring et al., 2009 ; Myhre et al., 2011 ; Hodnebrog et al., 2011]. Eyring et al. [2009] compiled radiative forcing estimates reported in the literature and calculated that global shipping could be responsible for -0.4 W.m-2 in 2005 when aerosols effects are included. However, significant uncertainties remain owing to the different methodologies, models, emissions and other input data employed in these studies (Figure 2).

CATE is committed to estimating the impacts of shipping on climate by using various methodologies and modelling approaches. A list of the projects that have been undertaken or are currently carried out by members of the group is given below and a more detailed description of them follows after:

1. Impacts of shipping emissions of CO2 and NOx on radiative forcing using a climate response model (CRM)
2. Impacts of shipping NOx emissions on radiative forcing using a Global Chemistry-Transport Model (GCTM) and a Radiative Transfer Model (RTM)

In addition, a report summarising the state of the science regarding the impacts of shipping emissions on climate has been produced by Lee et al. [2009].

1. Impacts of shipping emissions of CO2 and NOx on radiative forcing using a CRM

Lee et al. [2006]  calculated the radiative forcing of CO2 and NOx resulting from shipping emissions and estimated it to be respectively 43 mW.m-2 and 10 mW.m-2 respectively. Actually, NOx is not a significant greenhouse gas but has the potential to perturb the concentrations of O3 and CH4 in the atmosphere. The NOx radiative forcing may therefore be further decomposed into O3 (21 mW.m-2) and CH4 (-11 mW.m-2). An important finding from this study was the comparison of shipping RF with aviation RF which was estimated to be 1.8 to 2.7 times larger.

2. Impacts of shipping NOx emissions on radiative forcing using a GCTM and a RTM

As mentioned in the previous section, shipping NOx emissions alter O3 concentrations which in turn increase OH concentrations and result in a decrease of CH4 lifetime (depletion of CH4). With the help of the MOZART-4 GCTM and the Edwards-Slingo RTM, Hilaire et al., 2009 showed for the first time the spatial variability of the O3 and CH4 RF and proved that these forcings do not cancel out. This finding was later used in the second IMO study on greenhouse gases (Figure 3).

A more detailed description of this work as well as the content of Jerome’s thesis will be available this autumn.