The holidays are here, and you’ve got to decide whether to pack up the car or buy that plane ticket to go visit grandma.

Which is greener, flying or driving? It depends. And it involves a little math.

Either way, it can be a fraught choice. Each metric ton of heat-trapping carbon dioxide — one traveler’s share of the emissions on a one-way flight from San Francisco to New York — shrinks summer sea ice by about 32 square feet, according to recent research. Cross-country driving isn’t much better.

So a trip to see that fancy tree at Rockefeller Center sacrifices a slab of the ice the size of a king mattress. A round trip visit to Bangalore, with a layover in Hong Kong, could melt a small apartment-sized piece of the Arctic.

But for Pacific coast travel, there’s a lesser of evils.

To explain, we compared the carbon that’s emitted by both forms of travel to two cities — Los Angeles and Seattle — using a calculator created by Atmosfair, a German nonprofit that offers carbon offsets for travel-related emissions.

Driving all alone in an average car? For both trips, that’s worse than flying.

Solo driving to and from Los Angeles in a 25-mpg car produces a bit more carbon dioxide than an equivalent flight — about 0.3 tons vs. 0.27 tons.

Similarly, driving alone to Seattle, round trip, also produces more carbon than flying — 0.64 tons vs. 0.51 tons.

But that conclusion flips if your vehicle is more fuel-savvy or if you invite others along on your drive to share the total emissions. Then flying is much worse.

A new Prius, getting 53 miles per gallon on the highway, generates 0.14 tons of carbon on a trip to LA and back. That’s only half as much as flying.

If you’ve got someone else in your Prius, merely one-quarter as much carbon is emitted as two round-trip plane flights.

It gets even better with the new Tesla Model 3 with an advertised 132-mpg, the most energy-efficient vehicle on the road, according to Green Car Reports. It produces less than 0.06 tons of carbon on that trip to LA. That’s one-quarter as much as flying. For two passengers, that Tesla trip emits less than one-eighth as much carbon.

If Tesla’s seven-seater Model X SUV is packed full with friends, that could make a polar bear’s day.

What kind of vehicle melts more ice than a flight? A Ford 150 truck, for example. Averaging 16 mpg, it generates a whopping 0.46 tons of carbon — nearly double the 0.27 tons of carbon emitted in a flight to LA. A Hummer, Ferrari or Lamborghini are far worse.

Even if someone joins you in your pickup truck, you’re barely breaking even.

To be sure, it costs more carbon to go farther, whether driving or flying. A car trip from San Francisco to Seattle, at 810 miles, emits twice the carbon as a 378-mile trip to LA.

But longer flights are more efficient than shorter flights. That’s because taxiing, takeoff and landing are very fuel-intensive. And every plane has to climb to a minimum altitude, regardless of how far it goes after that.

So a Seattle flight is greener, per mile, than an LA flight. (That assumes no stops in, say, Portland. Nonstop flights emit less carbon than flights with connections.)

New planes are better than older ones, due to improved engineering. The most recent Atmosfair Airline Index shows that aircraft models such as Boeing 787-9, Airbus A350-900 or the A320neo achieve the greatest fuel efficiency.

The carbon footprint created by an elite Platinum traveler is far bigger than that of a poor plebe crammed in an economy seat. That’s because business class and first class seats take up more room, so fewer people are being moved by the same amount of fuel, according to a study by the World Bank.

In terms of overall impact, frequent fliers — those 12 percent of Americans who make more than six round trips by air a year — are worse offenders than the ordinary traveler taking a hard-earned holiday, according to an October study by the nonprofit research group International Council on Clean Transportation. Those fliers are responsible for about two-thirds of all aviation emissions.

“At the end of the day, the impact of an annual holiday flight gets swamped by emissions from American frequent flyers, who on average fly 14 times per year,” says Dan Rutherford, aviation director at the International Council on Clean Transportation. “So I wouldn’t feel too guilty about flying to visit family for Christmas. Just remember to ask Santa for cleaner planes and fuels next year.”

There are ways to lessen impacts from driving also. Regular tune-ups and well-inflated tires boost fuel efficiency. So does conservative driving: While each vehicle reaches its optimal fuel economy at a different speed, gas mileage usually decreases rapidly at speeds above 50 mph. According to the U.S. Department of Energy, each 5 mph you drive over 50 mph is like paying an additional $0.18 per gallon for gas.

All those gifts piled on the roof of the car? They don’t help. A large, blunt roof-top cargo box can reduce fuel economy by as much as 25 percent on interstate highways, according to the DOE. Rear-mounted cargo reduces fuel economy by much less.

Carbon offsets can help ease a traveler’s guilt. The money you pay for an offset — available online for $10 per metric ton — goes toward the construction of wind farms, tree planting, cleaner cook stoves and other ways to reduce greenhouse gas emissions.

Or we can simply stay home.

The planet is happiest when we turn down the thermostat and, in a kerchief or cap, with visions of sugar-plums dancing in our head, settle down for a long winter’s nap.

Do the Math

What’s the math behind our calculations of “CO2 equivalent,” the standard unit for measuring carbon footprint? (Carbon traps heat in the Earth’s atmosphere, warming the planet.)

We assumed that the carbon footprint for car and airline fuel is the same. Jet fuel produces an average of 21.1 pounds of CO2 per gallon, and aviation gas 18.4, while fuel for cars is 19.6, according to the U.S. Energy Information Administration.

Driving: Gasoline produces about 20 pounds of carbon dioxide per gallon burned, and the average car gets about 25 miles per gallon. We calculated pounds per mile traveled, then converted pounds to tons.

Flying: Atmosfair, a German nonprofit that offers carbon offsets for travel-related emissions, calculates a flight’s CO2 footprint between 22,600 cities for more than 200 airlines. (It also ranks the ‘greenest’ airline for your trip.) We used their calculation of kilograms of CO2 emitted by a flight, then converted kilograms to tons. Go to:


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