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 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).

The parties at COP21 could include language to direct those organizations to cut emissions from international shipping. But, as Politico reports, that requires a delicate balancing act: Many of the maritime nations most threatened by rising sea levels also rely on shipping and air travel for their economic health. As of Thursday evening, the draft text for the Paris treaty did not contain the words “shipping” or “aviation.”

Even if world leaders could determine carbon cuts for these industries, significant advances in technology and deployment would need to happen to make them possible.

Electric cars are growing cheaper and more accessible by the day, but electric propulsion doesn’t seem likely for international transit. When you’re out at sea or up in the air, you can’t stop to plug in and recharge your batteries. Additionally, air travel is extremely sensitive to the weight of an aircraft, and the batteries needed to power a long flight will weigh too much for the foreseeable future.

The industries can cut some emissions by looking at fuel efficiency, but not a lot. Airlines already did the more attainable upgrades in that regard during the fuel price spike of 2008, says the environmental consultant Suzanne Hunt, president of Hunt Green LLC. Fuel prices impose the largest cost airlines face (up to two-fifths of total costs), so they have a strong incentive to trim their demand wherever possible.

The shipping industry doesn’t face the same pressures, because in most cases the company hiring a ship to transport cargo pays for the fuel required, says Galen Hon, who manages the shipping efficiency operation at the Carbon War Room, a D.C.-based nonprofit that advocates market-based ways to reduce carbon emissions. That means the ship owner doesn’t feel financial pressure to improve efficiency, unless customers start factoring in environmental qualities when they select a transporter (Carbon War Room developed a tool to do just that).

In both shipping and aviation, then, the most significant carbon reductions will come from adopting different fuels or propulsion technologies.

In his ambitious roadmap for decarbonizing society by 2050, the Stanford engineering professor Mark Jacobson calls for all new aircraft to fly on liquid hydrogen by 2040. That’s what rockets burn, and if you want a rundown of the fantastically difficult requirements of a volatile fuel that must be kept at -423°F, head to NASA. The Soviet Union successfully flew an experimental hydrogen-propelled airliner called the Tu 155, so it can be done; it just can’t be done economically.

Part of the expense and time involved in creating a new generation of jet fuel is the safety requirements, Hunt says. That’s why she thinks the most likely non-fossil fuel for planes will be biofuels, which can be grown sustainably and poured into existing aircraft fuel tanks.

“There’s absolutely no margin for error, so the safety precautions for flying are really rigorous—it can take them many, many years to bring new technology into the airspace in terms of the aircraft,” she says. “One of the reasons people are so excited about aviation biofuel is that you don’t need to change the airplane or the engine.”

These biofuels typically derive from oils or fats (from plants or animals), or sugars. These are processed into hydrocarbons that function much like oil, but with the potential for much less carbon emission than conventional fuel (the accounting for this is very complicated, and some biofuels are more greenhouse gassy than others). More importantly, three types of biofuel have already been approved for blending with jet fuel for aviation and have entered into commercial use.

But just because they’re allowed doesn’t mean there will be enough of them. The biofuel industry, still quite young, needs to scale up to compete with traditional fuels, and so far the investment just isn’t there. Once it is, biofuel manufacturing will consume more raw ingredients, which calls for more farmland to grow them. If that takes away land from food production it creates tricky justice questions; if manufacturers use leftover materials from other industries, it might be hard to meet the global demand. The key is doing it at scale and at a price that airlines can actually pay.

“It’s really, really hard to make large quantities of sustainable biofuels for aviation,” Hunt says. “The industry is very much in its infancy and very capital intensive and it’s competing against the most powerful industry on the planet, oil.”

Electric propulsion has been gaining steam for smaller ships, but not for large cargo haulers. That’s because currently, electric generation on large ships tends to be a few percentage points less efficient than conventional propulsion, and those few points add up to a lot of money on a long voyage, says Hon of the Carbon War Room.

In the short term, then, carbon savings will come from efficiency upgrades to the hulls and rotors of ships. Some companies are turning back to wind power: The German company SkySails, for instance, produces large kite-like sails that attach to ships and give them a boost. A spokesperson for the company says they have installed the sails on five ships and that, in good wind conditions, they can save 30 tons of CO2 emissions per day. (Since winds don’t always blow steadily, the average comes down to two or three tons per day.) These can work well for bulk carriers—ships with large tanks for transporting grains or ore—but not on container ships, which carry almost all non-bulk cargo.

Again, the big carbon cuts will come from switching fuel sources. Ships burn what’s called bunker fuel, which is the heaviest, sludgiest oil left over when refineries distill gasoline, butane, and other useful hydrocarbons. Since it’s less refined, it’s also cheaper. And since ships burn it out at sea, the effects are less visible to the landlubbers who might be writing environmental policy.

The New York Times recently profiled the first container ships to run on liquefied natural gas, which burns cleaner than bunker fuel, especially in terms of particulate matter and acid-rain-producing sulfur dioxide. But this approach doesn’t look like it will provide much of a net gain in greenhouse-gas reductions, due to the technical requirements of running the ship and the possibility of methane leaks. It’s also expensive, because like liquid hydrogen, the liquefied natural gas must be kept at extremely low temperatures.

The most likely clean fuel for shipping might be biofuels or fuel cells, Hon says. Biofuels offer the same benefits and drawbacks as they do for airplanes—they don’t require technological changes to the craft, but they’re hard to source at scale. Fuel cells connected to electric engines would be a clean way of producing energy—they use chemical processes, not combustion, to provide the needed electrons.

“None of these technologies are viable right now,” he says. “Efficiency is totally possible with net-negative costs, that’s what we’re trying to figure out how to make happen sooner.”

Since international transportation is, well, international, any policies to clean it up need to be global in scope. If the European Union passes a strong law for lowering emissions from international flights, airlines could just divert air travel to other places with more lenient approaches to carbon. And a patchwork of different policies in different nations will make global travel exceedingly complicated.

A worldwide carbon tax could go a long way to driving cleaner performance from ships and aircraft and increasing market pressure for alternative fuels. Michael Gill, the director for aviation environment at the International Air Transport Association, says the aviation industry supports the ICAO developing a global, market-based regulation to cut carbon from flying, but they’re wary of a carbon tax: That might constrict the growth of the industry. Instead, he’d like to see a carbon offset scheme that includes incentives for switching to cleaner fuels.

If nations do want to act on their own, they could do a lot worse than ending subsidies for fossil fuels. A working paper by researchers at the International Monetary Fund estimated global post-tax subsidies for energy—mostly coal, natural gas, and oil, with a tiny sliver going to electricity—at an astounding $5.3 trillion for 2015, or 6.5 percent of global GDP. Stopping direct budgetary support for fossil fuels would be a logical place to start.

“Getting rid of subsidies for mature industries would be something you’d think would be palatable for liberals and conservatives alike,” Hunt notes.

It’s still possible, if unlikely, that some language about international transportation will make it into the final Paris treaty. Even if it doesn’t, there can still be progress.A successful deal at COP21 will generate political momentum for the ICAO’s next meeting in September 2016, says Gill. And the IMO will work on reducing emissions at a meeting in April, says Hon.

Environmentalists may well say that timeline isn’t fast enough. But that sounds like more of a testament to just how quickly the COP process has accelerated in the last few years. As recently as 2009, the question was: “Will the world ever forge a meaningful climate treaty?” Now we’ve advanced to asking: “Once we get the deal, how soon can international transit follow suit?”

This article appears courtesy of CityLab.