(Michael Parkin for The Washington Post)

Should I personally give up air travel, and should companies be pushed to reduce air travel by employees? If we cut our air travel by half, what impact would that have on the climate?

— Diane, California

How does long-distance travel affect climate change? (I.e., if I take Amtrak from D.C. to NYC instead of flying, how is that better given that the plane will still get there without me on it?)

— Kya, Washington

I want to take a trip to Florida to help out some ailing elderly relatives. For lowest emissions, should my wife and I fly round trip from the Washington, D.C., area or make the trip in a Tesla Model 3 at 133 mpg?

— Glenn, Maryland

Here is the stark and sad truth: There is almost no way to explore the planet without harming it.

Since 2017, the transportation sector has been the U.S. economy’s biggest source of greenhouse gases. Each year, people in the United States generate the equivalent of 1.9 billion metric tons of carbon dioxide to move ourselves and our stuff around — more than the cumulative emissions of every country in Africa put together. We’re driving, flying and cruising more miles than ever before — and we’re still relying almost entirely on fossil fuels to do so.

Canceling one round-trip ticket on a trans-Atlantic flight saves 1.6 tons of carbon dioxide — as much as the average citizen of India emits all year, according to a 2017 study published in the journal Environmental Research Letters. If Americans could slash our commercial aviation travel in half, we would avoid the equivalent of about 65 million metric tons of CO2; that’s as much annual savings as we’d get from replacing 2.5 billion conventional lightbulbs with energy-efficient LEDs.

This fact is sobering, but it should also be empowering. If you are lucky enough (and wealthy enough) to be among the tiny fraction of the world’s population who fly regularly, then a potent tool for protecting the planet is already within your reach. Go grab it.

Of course, there is some travel we cannot give up: essential work trips, visits to family and beloved friends. In those cases, a traveler must make the best of the options available to them. And that involves doing some math.

The standard unit for measuring carbon footprints is “tons of CO2 equivalent.” This describes the total global warming impact of all the greenhouse gases emitted by a person, business or country in terms of how much carbon dioxide it would take to achieve the same amount of warming over 100 years. Though a little unwieldy, this term lets scientists account for the fact that some gases — such as methane — are especially potent when it comes to trapping heat in Earth’s atmosphere.

But energy efficiency isn’t just about the total greenhouse gases emitted — it’s about how many people a vehicle can carry, and how far it can take those people. To evaluate modes of transport, scientists calculate the amount of CO2 equivalent generated to transport one passenger over a given distance.

These numbers vary depending on the length of the trip and the number of travelers. Since takeoff is the most fuel-intensive part of a plane trip, longer flights emit fewer greenhouse gases per mile than shorter ones, and nonstop flights are greener than ones that require connections. For this reason, a single traveler is better off flying directly from New York to Los Angeles than driving. But if you put more passengers in your car, or are going somewhere relatively close (say, from New York to Washington), it’s preferable to hit the road. Driving a hybrid or electric vehicle will make your trip greener than a flight of the same distance. Better still would be to take a train; according to a 2013 report from the National Academies of Sciences, Engineering and Medicine, the average emissions from a trip on a European high-speed train are half to a fifth of those from an equivalent flight.

What does that mean for Glenn’s journey from Washington to Florida? According to a calculator operated by Atmosfair, a German nonprofit that offers carbon offsets for travel-related emissions, a round-trip economy class flight from Dulles International Airport to Miami generates about 0.75 tons of CO2 equivalent per passenger. (That number more than doubles if you fly first class — as if waiting to board behind all the Platinum members wasn’t enough to turn you into a Jacobin.)

Gasoline produces about 20 pounds of carbon dioxide per gallon burned, and the average American car gets about 25 miles per gallon. This means the 2,100-mile round trip drive from Washington to Miami produces 1,680 pounds of CO2 — about 0.375 tons per single passenger. If Glenn drives his Tesla, he can get that number down to 0.07 tons.

Kya’s northward flight from Washington to New York would generate 0.21 tons of CO2 equivalent, according to Atmosfair. If she switches to Amtrak, she will produce just a quarter of those emissions.

Sure, the plane might still take off without her. The choices of one person can’t alter much in a world of 7 billion people, and too many people don’t have any choice at all.

Susan Shaheen, co-director of the Transportation Sustainability Research Center at the University of California at Berkeley, noted that many American communities lack sidewalks and reliable public transit systems. A shortage of affordable housing pushes people to live far from city centers, leading to lengthy commutes. The United States spends a much smaller fraction of its GDP on passenger rail infrastructure than most other developed nations; our rail network is among the least extensive in the world, and it has one of the worst safety records.

The dearth of environmentally sound transportation options is a global problem. This month, Greta Thunberg, the Swedish teenager whose weekly protest in front of her country’s parliament helped spark the youth climate movement, traveled to the United Nations’ climate conference in Spain via a zero-emissions sailboat. The 16-year-old doesn’t fly, so a multiweek boat voyage was the only way she could get across the Atlantic.

“I’m not traveling like this because I want everyone to do so,” she told reporters. “I’m doing this to sort of send a message that it is impossible to live sustainably today, and that needs to change.”

In travel, as in all other things, the people with the greatest capacity to combat warming are those with the money and power to introduce systemic change. Governments must regulate carbon-intensive modes of transportation and provide affordable, accessible alternatives to make a real difference. Corporations must invest in technologies to make travel more energy-efficient.

But individuals possess a form of power that governments and corporations don’t: People can transform culture. They can reduce the demand for unsustainable products, and they can inspire those around them to take action. Change is the cumulative effect of many individuals opting for a different, better path.

Kimberly Nicholas, the Lund University sustainability scientist who published the 2017 estimate of the climate costs of flying, said she has cut her air travel to just one flight a year — across the Atlantic to visit family. She bikes to work and vacations only in places she can reach by train. Whenever possible, she opts not to travel at all, giving work presentations via Skype and swapping in-person meetings for teleconferences.

To others, these might seem like sacrifices. But Nicholas said she’s come to appreciate the unexpected benefits of her choices — the tranquility of a train trip through snowy countryside; the comfort of having more time at home.

“We see the status quo as representing what’s good, and we overlook the problems,” she said. “We need to imagine the things that are better about a more sustainable system.”