Air travel is surging. That’s a huge problem for the climate.

US airlines have an abysmal carbon footprint.

Greenhouse gas emissions in the United States appear to be on the rise again after years of decline. The Rhodium Group recently released preliminary estimates showing carbon dioxide emissions overall surged 3.4 percent in 2018, with the transportation sector leading the way as the largest source of emissions for the third year in a row.

Interestingly, the bump in transportation emissions didn’t come from cars. Car travel increased compared to 2017, but gasoline consumption decreased. That’s in part because overall fuel economy in passenger cars is improving as engines become more efficient and electric cars become more popular.

Instead, emissions from trucking and air travel helped contribute to the overall increase: Demand for both diesel and jet fuel increased about 3 percent in 2018.

Emissions relating to air travel are on the rise in the United States. Rhodium Group
Emissions relating to air travel are on the rise in the United States.

On the one hand, this shows just how hard it is to bring down greenhouse gas emissions when the US economy is growing — growth was 3 percent in 2018. With that came more manufacturing, more power use, more travel, and, yes, more greenhouse gases.

But it’s also a clear sign of just how difficult it is to decarbonize the airline industry, for which surprisingly few low-carbon technologies or fuels have been developed so far. That said, there are steps airlines can take to modestly reduce their impact on the environment. And on this front, a recent report from the German nonprofit atmosfair shows that US-based airlines have fared poorly compared to air carriers in other countries, failing to take climate change as seriously as some of their competitors abroad.

Demand for air travel is surging just when our window to limit catastrophic global warming is closing

In 2018, the total number of air passengers increased in the US, with some periods of the year experiencing all-time high air travel volumes. Around the world, airlines carried 4.3 billion passengers in 2018, an increase of 38 million compared to the year before. Aviation accounts for about 2 percent of global greenhouse gas emissions, and that share is poised to grow.

The International Civil Aviation Organization anticipates that by 2020, global international aviation emissions are projected will be 70 percent greater than in 2005. By the middle of the century, they are slated to increase by upward of 700 percent. Every round-trip trans-Atlantic flight emits enough carbon dioxide to melt 30 square feet of Arctic sea ice.

But the planet needs to cut its emissions from today by more than half to get on a path to limiting global warming this century to 1.5 degrees Celsius. The world may only have until 2030 to reach that milestone.

However, it’s a bit tricky for conscientious fliers to figure out just how much they’re contributing to climate change.

“Car drivers are used to easy and absolute climate efficiency indicators: grams [of] CO2 per kilometer or gallons per mile,” according to a December report from atmosfair. “This is not the case for aircraft: Every plane has to take off [and] climb out to a minimum altitude, regardless of how far it goes after that.”

Since planes take a huge amount of energy just to get off the ground, shorter flights actually have a larger CO2 footprint per passenger per mile than longer ones, so their overall carbon intensity can be higher.

Fuel consumption in liters per passenger for an Airbus A340 relative to distance traveled. atmosfair
Fuel consumption in liters per passenger for an Airbus A340 relative to distance traveled.

One also has to factor in the age of the aircraft, the type of aircraft, and the distance traveled. And aircraft emit more than carbon dioxide: They spew particles, nitrogen oxides, and sulfates. These compounds also trap heat and have an outsized effect on warming when they’re emitted at cruising altitude.

Aircraft emit a variety of compounds that can influence the climate. atmosfair
Aircraft emit a variety of compounds that can influence the climate.

Accounting for all these factors, atmosfair indexed 125 of the world’s airlines. The rankings are also subdivided in to short-, medium-, and long-haul flights. The methodology accounts for meteorological conditions on routes, passenger load, cargo load, aircraft type, engines, and efficiency ground operations. Airlines are then awarded efficiency points and are then ranked.

US air carriers have a lot of room for improvement in cutting their carbon emissions

The overall highest-ranked airline, according to atmosfair, was United Kingdom-based TUI Airways because of their efficient aircraft and high occupancy rates.

Airlines ranked by emissions efficiency in 2018. atmosfair
Airlines ranked by emissions efficiency in 2018.

The highest-rated US-based airline was Alaska Airlines, coming in at 22. The highest-ranked US legacy carrier is United Airlines, ranking 50th. All US air carriers slipped in the rankings compared to the year before, except for American Airlines, which ranked 58, rising from 66 in 2017. For country home to some of the world’s largest aircraft manufacturers, this is a dismal showing.

American Airlines’ fleet includes a combination of newer aircraft like the Boeing 737-800 and older, less efficient aircraft like the MD-80. The company has average to below-average occupancy on their shorter flights. “American Airlines still earns points compared to the previous year due to high occupancy on long-haul flights in combination with more efficient aircrafts,” according to the report. The company is aiming to retire its entire MD-80 fleet this year.

A big challenge for cutting emissions from aviation is that cleaner technology is still in its infancy. Fuel is the single-largest expense for airlines, so they have an incentive to use less fuel per passenger. That’s part of the appeal of new fuel-efficient aircraft like the Boeing 787 and the Airbus A350. But airlines make money filling seats, so they want to encourage more air travel.

Right now, we have almost no alternative that can match jet fuel’s energy density — no battery is going to get an airliner across the Atlantic. The engines themselves are pretty much at their upper limit for fuel economy and performance, so there’s little room for improvement there.

There is talk of electric aircraft and hydrogen-powered engines, but those flights are decades away. Airlines are now experimenting with biofuels that can be carbon-neutral. The big hurdle right now is price, but if oil prices rise and if production costs come down, biofuels could become a viable way to decarbonize air travel. Manufacturers like Airbus and Boeing are studying new wing designs that save fuel and hybrid systems, like electric motors for taxiing aircraft on the ground. The US Department of Energy and the military meanwhile are researching biofuels.

Regulations to limit carbon dioxide emissions from air travel would be a major step to spur more research on this front. The European Union has implemented some carbon rules for aircraft, but in the United States, the Trump administration is moving as far as possible from regulating greenhouse gases. That means the problem is likely to get worse before it gets better.