There's a parade that never seems to end on the edge of San Diego Bay.
One by one, crowded passenger jets roll onto the runway at San Diego International Airport, where they pause a moment, then soar off to places near and far.
It's a powerful symbol of human mobility. But the jets also represent a threat to the environment; they release greenhouse gases that significantly contribute to climate change.
And there's more to come. Commercial airline traffic is booming worldwide, including in San Diego, which handled 24.6 million passengers in 2018, a figure that jumped by more than 8 million in a decade.
The global surge has scientists and activists asking: "Can we overcome daunting technological challenges and develop and deploy environmentally friendly electric-powered jets for short-haul trips between cities?
The question is getting a deep look from David Victor, a UC San Diego professor who has joined a Europe-based study on the promise of electric aircraft.
"The world's diplomats have spent 30 years talking about the climate problem. That hasn't achieved much," said Victor, who specializes in international relations. "Global emissions of warming gases keep rising.
"Electric planes could be part of the solution. It's possible that we'll see people using them to take flights between cities like San Diego and San Francisco."
Victor is working with the Norway Research Association, which will spend $1.65 million to examine everything from battery technology to the public's perception of electric aircraft.
The association wants to help link Norway's cities with electric planes, and to help introduce the technology internationally.
Norway has 5.4 million people, making it one of the smaller countries in Europe. But it is deeply involved in the environmental movement. And it sees electric planes as a viable alternative to ones powered by fossil fuels. The Norwegian government has pledged to have all short-haul passenger flights performed by electric planes by 2040.
To press the issue, many Norwegians are joining other Europeans in "flight shaming" — or making people feel so guilty about using conventional aircraft that they turn to other forms of transportation, notably trains.
The environmental movement is largely driven by two key numbers — 2.5 and 3.5.
Scientists say that airline travel is the source of about 2.5 percent of global carbon dioxide emissions, a modest but significant factor in climate change. And aviation analysts predict that passenger traffic will rise by 3.5 percent a year through 2037. It's possible that passenger traffic will double, to 8.2 billion, from 2018 to 2037, according to the International Air Transport Association.
Boeing and Airbus, the world's two largest aircraft manufacturers, have been making their jets more fuel efficient. But analysts say the improvements won't fully offset the impact of rising emissions from soaring traffic.
That's shifting the spotlight to electric and hybrid-electric planes which, from a technological standpoint, are in their infancy.
Engineers and inventors have been trying to create such aircraft since the late 18th century. But it wasn't until 1973 that a person flew a full-size battery-powered plane for the first time. And current research is still largely focused on small planes that would carry small numbers of people.
There are many technological problems. The biggest one involves energy. Engineers need to greatly increase the power of batteries and keep them small in size. A breakthrough doesn't appear to be imminent. Even if it was, the aviation industry would also have to develop recharging hubs at airports and train pilots to fly aircraft that handle differently than conventional jets.
The industry would also need to convince the public that electric aircraft are safe.
Andreas Schafer, an energy and transport researcher at University College London, sees parallels to the Manhattan Project, the American-led effort that produced the first nuclear weapons during World War II. In today's dollars, the project cost about $23 billion.
"This number would roughly correspond to what one might expect when developing a completely novel, advanced-design, short-range, narrow-body, all-electric aircraft, excluding the battery-related development costs," said Schafer, who has collaborated with Victor for years.
The cost of separately developing new batteries also could approach $23 billion.
The overall price tag for introducing the aircraft "could be somewhere between one to two Manhattan Projects," Schafer said.
U.S.-based airlines — which would have to underwrite a lot of that research — have a more immediate worry.
Robin Hayes, the chief executive of JetBlue, recently said that American airlines are likely to face the same kind of flight shaming activism that has occurred in Europe.
"This issue presents a clear and present danger, if we don't get on top of it," Hayes told industry analysts.
Americans could end up feeling deeply conflicted about the matter.
"As people get informed about the dire impact of air travel, they start feeling ambivalence," said Erlinde Cornelis, a marketing professor at San Diego State University.
"On the one hand, you might feel positive about flying because it allows you to travel quickly, reaching faraway destinations, and it is often cheaper than taking a cross-country train. It's just convenient and tempting, plus, it's normalized behavior.
"On the other side, you feel bad about flying because you're contributing to the climate crisis while you know the world should be drastically reducing its fossil fuel dependency. Plus, for selfish reasons you might hate the cramped seats, the waiting at the airports."
Many people also wouldn't be able to switch to trains to meet their travel needs. The passenger rail network in the U.S. isn't as well developed as it is in many areas of Europe. For example, Amtrak doesn't provide direct passenger service between San Diego and San Francisco, two of the nation's largest cities.
It also appears that the California High-Speed bullet train project that's supposed to link Los Angeles and San Francisco may never be fully built.
Similar uncertainties exist about the future of electric planes.
"There is no question that this will be unbelievably complex and costly," Victor said.
"The only reason to do it is if we think that opening niches in the U.S. (and in other countries) will be the starting point to a broader application — eventually to medium-haul flights and beyond.
"One does this only if we need to make deep cuts in emissions. If we do shallow decarbonization there are other options that are cheaper — more natural gas, more renewables.
"But shallow decarbonization doesn't fix the climate. Only deep decarbonization fixes the climate."
The Detroit News and New York Times contributed to this story.
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