What role do emissions from international shipping and aviation play in the global climate?

And what do those sectors need to do to help keep warming below 1.5 degrees Celsius?

In advance of the United Nations Secretary General’s climate summit in September, many countries are vowing to ramp up their Paris agreement commitments to help limit the increase in global temperatures to 1.5 degrees Celsius. However, the growing emissions from two economic sectors – international shipping and international aviation – remain largely outside most of those commitments and could cause significant warming.

In a new study, researchers have found that, absent any climate action, the rising carbon dioxide emissions of international shipping and aviation could consume nearly one-third (15 to 30 percent) of our remaining “allowable warming” – the amount of additional warming that can occur before the world’s average temperature surpasses 1.5 to 2 degrees Celsius above preindustrial levels – by the end of the century.

The international shipping and aviation sectors need to implement policy solutions with integrity and extend them over time to reduce their future warming and align with the 1.5 °C global temperature threshold.

What’s the worst case scenario?

By 2018, our planet had already warmed 1 degree Celsius above preindustrial levels. This means there is an “allowable warming” of only 0.5 to 1.0 degree Celsius more if the Paris agreement thresholds of 1.5 or 2.0 degrees Celsius are to be met, respectively. Unabated CO2 emissions from international shipping and aviation could add 0.15 degrees of warming to that limit by the end of the century – which is 15 to 30 percent of the remaining “allowable warming” under these thresholds.

What can we do?

Already-proposed targets and policies within international shipping and aviation hold significant potential to reduce projected warming from the sectors, should they be implemented with integrity. For example, robust implementation of the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA), which caps the net CO2 emissions of most international flights at 2020 levels, could significantly reduce international aviation’s warming impact. The mitigation potential of the program is even greater if CORSIA’s targets are extended beyond 2035, and should the sector pursue further decarbonization by mid- or end-of-century the associated future warming from the CO2 emissions of international aviation may be reduced by roughly 90 percent.

Robust implementation of the targets the International Maritime Organization announced in 2018, i.e., cutting international shipping CO2 emissions by at least 50 percent by 2050 compared to 2008 with full decarbonization by the end of the century, could reduce the warming impact associated with the CO2 emissions of international shipping by more than 85 percent. However, the international shipping sector has considerably more work to do as the policies to achieve these targets do not yet exist.

What about other climate pollutants?

Ships and airplanes don’t just emit carbon dioxide. They also emit black carbon, carbon monoxide, sulfur dioxide, organic carbon, and nitrogen oxides. These pollutants affect the climate and some of them pose direct adverse impacts on human health and the health of ecosystems. Ships also release methane, a powerful greenhouse gas. When all pollutants are included in the analysis, the net warming attributed to each sector decreases, dominated by the indirect cooling effects of nitrogen oxides.

Targeting only these pollutants for cleanup could “unmask” the sectors’ greater warming effects. Measures such as fuel efficiency improvements or the expansion of high-quality sustainable alternative fuel use can mitigate both CO2 and non-CO2 climate impacts, but care must be undertaken not to allow some pollutants to increase in an effort to decrease others.

The study is currently published online in Atmospheric Chemistry and Physics Discussion, a discussion forum that encourages the peer review process to take place in a public setting. While the International Civil Aviation Organization’s 2019 Assembly considers next steps and the development of long term emissions goals, the International Maritime Organization is meeting for negotiations around its climate regulations May 7-17. Given the rapid progression of climate policy development in the transportation sector, the public availability of the analysis during its review process serves a unique benefit to the nexus of climate science and policy.