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2.2.6 Future Emissions and the Total Contribution to Climate

As much as it is difficult to estimate current or even past emissions, a fortiori it must be much more difficult to estimate future emissions. Many scenarios for future GHG emissions from transportation, for example those presented for maritime transportation in Buhaug et al. (2009), are based on assumptions on global development in the IPCC Special Report on Emissions Scenario (SRES) storylines. Since projections of climate change depend heavily upon future human activity, climate models are run against scenarios. There are 40 different scenarios, each making different assumptions for future greenhouse gas pollution, land-use and other driving forces and divided into six categories: A1FI, A1B, A1T, A2, B1, and B2. Assumptions about future technological development as well as the future economic development are thus made for each scenario and most include an increase in the consumption of fossil fuels. These scenarios have been superseded by the so-called 'Representative Concentration Pathways' (RCPs), which are four greenhouse gas concentration (not emissions) trajectories adopted by the IPCC for its fifth Assessment Report (AR5) in 2014. They describe four possible climate futures, all of which are considered possible depending on how much greenhouse gases are emitted in the years to come. The four RCPs, RCP2.6, RCP4.5, RCP6, and RCP8.5, are named after a possible range of radiative forcing values in the year 2100 relative to pre-industrial values (+2.6, +4.5,

+6.0, and +8.5 W/m2, respectively). 'Radiative forcing' is the additional heat/

energy which is retained in the Earth's ecosystem through the addition of a gas to the atmosphere.

The transportation sector emits non-CO2 pollutants that are also climate forcers. These include CH4, NOx, SO2, CO, F-gases, black carbon, and non-absorbing aerosols (Ubbels et al., 2002). Although most current climate policies focus on GHGs, which have a relatively well-known behavior and radiative forcing of climate, there is strong evidence that the other emissions and mechanisms also play an important role for the transportation sector.

Quantifying these effects is a complex scientific undertaking because of the broad mix of substances and physical/chemical processes involved. Without getting into details, there are emissions that contribute to Global Warming and others that have a cooling effect. Several studies have investigated this interaction by specifically isolating the climate forcing from transportation (Fuglestvedt,

Fig. 2.5 Integrated global mean net RF per sector due to 2,000 transportation emissions, normalized to the values for road transportation for various time horizons (20, 100, and 500 years). Uncertainty ranges are given as one SD. Source: Fuglestvedt et al. (2008)

Berntsen, Myhre, Rypdal, & Skeie, 2008). For example, Fuglestvedt et al. (2008) showed that the transportation sector contributes significantly to man-made radiative forcing and that current emissions from transportation are responsible for 16 % of the integrated net forcing from all current anthropogenic emissions over the next 100 years. Sulfate emissions from shipping and rail result in a negative impact (see also Fig. 2.5).

International shipping has been a fast growing sector of the global economy and its share on total anthropogenic emissions has increased lately but the nature of the contribution to climate change is complex. In contrast to global warming induced by CO2 emissions, ship emissions of SO2 cause cooling through effects on atmospheric particles and clouds, while NOx increase the levels of the greenhouse gas ozone (O3) and reduce CH4, causing warming and cooling, respectively and the result is a net global mean radiative forcing from the shipping sector that is currently strongly negative (Eyring et al., 2009). However, due to new regulations on SO2 and NOx, their emissions will decrease and after 50 years the net global mean effect of current emissions will be close to zero (Buhaug et al., 2009; Eyring et al., 2009; Fuglestvedt et al., 2008). Eyring et al. (2010), a paper co-authored by some of the same authors involved in estimation of GHG inventories and also in the second IMO study state that in 2005 the total radiative force from shipping effect measured in W/m2 was -0.408, which means that currently shipping causes indeed cooling.

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