Editing IPCC:AR6/SR15/Chapter-4 (section)

==== 4.3.3.5 Shipping, freight and aviation ====

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International transport hubs, including airports and ports, and the associated mobility of people are major economic contributors to most large cities even while under the governance of national authorities and international legislation. Shipping, freight and aviation systems have grown rapidly, and little progress has been made since AR5 on replacing fossil fuels, though some trials are continuing (Zhang, 2016; Bouman et al., 2017; EEA, 2017) <sup>[[#fn:r367|367]]</sup> . Aviation emissions do not yet feature in IAMs (Bows-Larkin, 2015) <sup>[[#fn:r368|368]]</sup> , but could be reduced by between a third and two-thirds through energy efficiency measures and operational changes (Dahlmann et al., 2016) <sup>[[#fn:r369|369]]</sup> . On shorter intercity trips, aviation could be replaced by high-speed electric trains drawing on renewable energy (Åkerman, 2011) <sup>[[#fn:r370|370]]</sup> . Some progress has been made on the use of electricity in planes and shipping (Grewe et al., 2017) <sup>[[#fn:r371|371]]</sup> though no commercial applications have arisen. Studies indicate that biofuels are the most viable means of decarbonizing intercontinental travel, given their technical characteristics, energy content and affordability (Wise et al., 2017) <sup>[[#fn:r372|372]]</sup> . The lifecycle emissions of bio-based jet fuels and marine fuels can be considerable (Cox et al., 2014; IEA, 2017g) <sup>[[#fn:r373|373]]</sup> depending on their location (Elshout et al., 2014) <sup>[[#fn:r374|374]]</sup> , but can be reduced by feedstock and conversion technology choices (de Jong et al., 2017) <sup>[[#fn:r375|375]]</sup> .

In recent years the potential for transport to use synfuels, such as ethanol, methanol, methane, ammonia and hydrogen, created from renewable electricity and CO <sub>2</sub> , has gained momentum but has not yet demonstrated benefits on a scale consistent with 1.5°C pathways (Ezeji, 2017; Fasihi et al., 2017) <sup>[[#fn:r376|376]]</sup> . Decarbonizing the fuel used by the world’s 60,000 large ocean vessels faces governance barriers and the need for a global policy (Bows and Smith, 2012; IRENA, 2015; Rehmatulla and Smith, 2015) <sup>[[#fn:r377|377]]</sup> . Low-emission marine fuels could simultaneously address sulphur and black carbon issues in ports and around waterways and accelerate the electrification of all large ports (Bouman et al., 2017; IEA, 2017g) <sup>[[#fn:r378|378]]</sup> .

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