In all regions of the world, the transportation of people and goods are significant sources of emissions of air pollutants, GHGs, ODS (from automobile air-conditioning units) and PBTs (including lead [Pb] and other metals). Road transport, including petrol- (gasoline-) and diesel-fuelled passenger cars and heavy-duty trucks, account for a dominant fraction of NOx emissions, and a significant fraction of CO2, CO, NMVOC and BC emissions (see Figure 5.3; Hoesly et al. 2018). Road traffic also contributes to emissions of primary PM from tyre and brake wear and entrained road dust (not included in Figure 5.3). Because cars and trucks operate and emit pollutants near where people live and work, they have a larger impact on air pollution exposures and associated health impacts than is proportional to their fraction of total emissions.
Total road transport activity is higher in North America and Europe than in other regions and is therefore responsible for greater CO2 emissions, but those emissions have held steady for the last decade, with improvements in fuel efficiency keeping pace with increasing transport demand (Hoesly et al. 2018). The emissions of other transportation-related pollutants in North America and Europe have declined due to the introduction of vehicle emissions and fuel standards (see Section 12.2).
In developing countries, road transport emissions continue to rise as vehicle use is increasing faster than technological improvements, despite the introduction of emissions and fuel standards, which lag behind those in North America and Europe. Implementation of cleaner technologies is slowed by the trade in used vehicles from richer countries (UNECE and UNEP 2017). However, continued progress towards decreasing the sulphur content of fuel will enable the use of advanced emission control systems in all countries.
As emission standards are more widely applied to road vehicles, the relative fraction of emissions from non-road vehicles, such as heavy-duty construction equipment, is becoming increasingly important. Often running on diesel fuel, and with long lifetimes, such vehicles can be good candidates for retrofit control technologies or alternative fuels.
Maritime shipping is used to transport 80 per cent of global trade measured by volume (International Transport Forum 2017) and grew by more than 300 per cent between 1990 and 2015 when measured by ton-miles (United Nations Conference on Trade and Development [UNCTAD] 1997; UNCTAD 2017). Typically burning the heaviest petroleum products, ships are a significant source of SO2 and CO2 emissions globally and a source of SO2, NOx and BC emissions in coastal regions and port cities. Emission Control Areas have been established under international law (e.g. covering the North and Baltic seas and North American coastal waters) and national laws (e.g. covering Chinese ports and inland waters). The International Maritime Organization has announced new emission and fuel standards that are expected to dramatically decrease shipping emissions starting in 2020.
Aviation is a small but growing contributor to global emissions, accounting for less than about 2 per cent of global anthropogenic CO2 emissions from fuel combustion (IEA 2017). Between 2000 and 2016, global air passenger travel increased 235 per cent (measured in passenger-km) and airfreight increased 174 per cent (measured in tons-km) (International Civil Aviation Organization [ICAO] 2016a). Aircraft emit pollutants directly into the upper atmosphere where their impact on ozone formation and climate forcing is larger than if emitted near the surface. The contribution of aviation CO2 to radiative forcing is well quantified, but planes also emit water vapour, other gases and aerosols at high altitudes that trigger cloud formation and modify natural clouds and alter ozone and methane concentrations in the upper troposphere and lower stratosphere. The effects of these changes on climate forcing are not well quantified (Brasseur et al. 2016; Fahey et al. 2016). In 2016, the International Civil Aviation Organization (ICAO) adopted the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA), with a goal of capping the net CO2 emissions from international aviation at 2020 levels (ICAO 2016b).
