With the Paris climate talks coming to a close, participating nations are hashing out the details of how to hold each other to their carbon reduction goals and finance the whole transition to a cleaner world. Non-state actors are present, too; 400 cities signed a Compact of Mayors to set and track climate goals. And financial institutions have made big commitments to shift investment away from fossil fuels and better disclose climate-related business risks.

But there are two particular industries that must factor into any plan to cut carbon and yet aren’t directly represented in the current COP21 talks: international shipping and aviation.

They’re both big. International shipping produces 2.4 percent of global greenhouse gas emissions, equivalent to all of Germany. Meanwhile total aviation yields about 2 percent of global GHGs, and international flights account for 65 percent of that figure. These emissions won’t be covered by reductions being discussed at COP21, because they don’t happen within the boundaries of any specific countries. They’re also projected to rise dramatically by 2050.

Two major obstacles stand in the way of resolving emissions from international shipping and aviation. The first is procedural: those industries are not bound by the Paris climate deal. The second is practical: the world currently lacks a promising technology to replace carbon-based propulsion systems, as well as a promising alternative to carbon-based fuel.

The limits of COP21

The acronym-laden gathering of 196 nations in Paris is administered by the United Nations Framework Convention on Climate Change. That’s the organization created in 1994 to rein in greenhouse gases before they caused dangerous climate interference. The UN agencies charged with overseeing the environmental impacts of international transport are the International Civil Aviation Organization (ICAO) and the International Maritime Organization (IMO).