The COP26 climate summit has entered its thornier, more difficult stages — when scores of countries must come to a tangible agreement on steps to tackle climate change. Today’s theme is “transportation.”

The International Energy Association (IEA) reports that transport emissions are still increasing and account for roughly a quarter of all carbon dioxide released from fuel combustion. In the United States, the Environmental Protection Agency says that the sector accounts for 29 percent of the country’s greenhouse gas emissions — more than any other category.

“It’s absolutely critical to get it reined in,” said Lewis Fulton, an international transport and energy expert at the University of California at Davis. But, he added, “it’s a very challenging sector.”

What are the biggest climate problems we face under this theme?

The transportation sector can be broadly broken down into three subsectors. “You have ground transportation,” said Bob Noland, director of the Voorhees Transportation Center at Rutgers University. “Then you’ve got maritime transport and aviation.”

Christian Brand, a researcher at Oxford University’s Transport Studies Unit, pointed to a recent International Transport Forum report that projected transport activity to more than double by 2050. “They’re going up and up and up,” he said of emissions. “Passenger cars are by far the biggest.”

To electrify light-duty vehicles — such as cars and vans — Fulton said “the level of change in our lifestyles is kind of unprecedented.” And that’s among the easiest areas to tackle. Reducing emissions from long-haul trucking, aviation and maritime shipping will prove even trickier.

“We do not have technological solutions for all parts of the transport system yet,” Brand said.

Planes are especially troublesome, as their problems are twofold: not only do they produce greenhouse gases, including water-vapor, but they release them high in the atmosphere, where their climate impact is magnified. “The hardest is probably aviation,” Noland said of decarbonization sticking points. But, Fulton added, none of it will be particularly easy.

“All these fruits are pretty high up in the tree,” he said. “We’re going to need a ladder.”

What are potential solutions?

The most-discussed solution in the transportation space is electrification. According to the IEA, there are now more than 10 million electric vehicles on the road globally. Over 3 million of those are in China, while the Norwegian Road Federation reported that nearly 80 percent of new vehicles sold in the country this September were electric.

But electrification is not the answer for all types of transport. “The bigger the vehicle, the harder it is,” said Fulton, noting that hydrogen is one option being considered for freight trucks. And even after new passenger vehicles transition to electric, Brand said, it will take another 15 to 20 years to transition older fossil-fuel technology out.

“There’s a bit of an inertia in the system,” he said.

Brand sees curbing transport demand as an important first step in emissions reductions. “You have to think about reducing the need for travel,” he said. Scotland, where COP26 is being held, has already set a target of reducing the number of car kilometers driven by 20 percent by 2030.

Achieving goals like that means making it easier for people to operate without a car. “More needs to be done to reduce dependence on cars through infrastructure upgrades focusing on public transit,” said Sarav Arunachalam, an air pollution scientist at the University of North Carolina.

Led by Mayor Anne Hidalgo, Paris has been striving to become “la ville du quart d’heure” — the quarter-hour city, where a person’s basic needs are all within 15 minutes of their home. In Oregon, Portland’s climate action plan calls for 20-minute neighborhoods where “90% of Portland residents can easily walk or bicycle to meet all basic daily, non-work needs” by 2030.

Brand says speed reduction could also be a “quick win.” Seas At Risk, an association of environmental organizations from across Europe, reported that reducing shipping speeds by 20 percent would reap emissions savings of 24 percent. The German Environment Agency found that imposing even a 130 kilometer per hour (81 mph) speed limit on the country’s notoriously freewheeling freeways would save 1.9 million tons of carbon dioxide annually “instantly and at no significant additional cost.”

Fulton, though, says that demand side changes will only get the world so far. “If we need a 95 percent reduction, it really has to be the vehicles and the fuels that are revolutionized,” he said.

What solutions are already underway in the U. S.?

A self-described “car guy,” President Biden has made transportation, infrastructure and climate change cornerstones of his political agenda. An August executive order on electric vehicles contained a wish-list of investments, from a national charging network to transitioning the manufacturing supply chain.

Ultimately, Biden wants half of all vehicles sold in the U.S. to be plug-in by 2030. Currently, though, electric vehicles make up only about 2 percent of U.S. vehicle sales and Fulton called meeting Biden’s goal “pretty challenging.”

Congress, of course, also plays a role in making such a leap. But the recently passed infrastructure bill contains, among its other climate provisions, $7.5 billion for new electric vehicle charging stations. The reconciliation budget bill also being debated in Congress could include expanded electric vehicle credits and investment in supply chain transitions.

The Biden administration has also proposed plans to ratchet up fuel efficiency standards. On the aviation front, Biden wants to reduce emissions by 20 percent by 2030. But, Arunachalam said, “we have a long way to go.”

What developments are expected out of COP26?

Under the Paris agreement reached at the 2015 COP21 summit, countries don’t have to include international aviation or maritime emissions in their national contributions. The agreement makes no direct mention of cars either, leaving those emissions to countries to address in their individual action plans.

More recently, though, the United Nations Secretary General has called for phasing out the sale of internal combustion engines globally by 2040 — sooner in leading manufacturing countries. Some companies and governments are already moving in that direction.

Canada, for instance, “has set a mandatory target for all new light-duty cars and passenger trucks to be zero-emission by 2035.” Fulton says these types of moves put pressure on others to do the same.

“It shines a spotlight on who’s being aggressive and who isn’t. Countries that aren’t committing have to explain why,” he said. “I feel like there’s some momentum building.”