I’ve read trains are much more efficient than flying on a per-passenger-mile basis. Could we reduce carbon emissions by taking a boat across the Atlantic like the old days? —Greg

You mean a rowboat? A sailboat? Sure, go nuts. Past that, though, you’re not helping any. Which form of passenger travel is least lousy for the environment is an endlessly complicated question

How bad is boat travel? A 2017 study by the German environmental organization NABU estimated that a typical European cruise ship’s per-day particulate emissions are equivalent to a million cars combined. That’s just from smokestacks. Coming out the other end is about a billion tons of raw sewage annually into the world’s oceans, plus a hundred million gallons of leaked petroleum products.

Unless you’re hoofing it, though, most forms of travel take some earthly toll. A 2013 paper in Environmental Science and Technology figured that the “climate impact from a long-distance trip can easily vary by a factor of 10 per passenger depending on mode choice, vehicle efficiency, and occupancy.”

And there are endless ways to run the numbers. That paper looks at some factors less obvious than mere CO2 emissions: for instance, contrails left by planes in the thin upper air, which can intensify the greenhouse effect. If you really wanted to go deep, you could run what researchers call a life-cycle assessment, which accounts for not just the travel itself but everything that enables it: construction of automotive plants, track-laying requirements for intercity rail, taxi emissions en route to a suburban airport, etc. (Spoiler: viewed this way, infrastructure-heavy train travel loses a lot of points.)

For simplicity’s sake, let’s focus on the basic variables: emissions, passenger load, and distance. And assume we’re talking about a big trip—500 miles or more.

Pound for pound, according to the Union of Concerned Scientists, the very best bet is coach bus, followed closely by train travel. (City buses, we’ll note, are a different beast, their impact wildly dependent on how full they are; buses running at off-peak hours may generate eight times more emissions per passenger than during peak.) That’s probably unsurprising, but say there’s no train or bus option for where you’re going. Is driving next in line?

Not necessarily. We hear a lot about the eco-affronts of air travel, but those claims might be a bit oversold: there are conditions under which it makes more sense to travel by plane than car, and not just because seawater is hell on your Chevy’s undercarriage. Their emissions might be stupendous, but airplanes gain back lots of ground in load capacity and distance covered. Because most of a flight’s energy expenditure comes during takeoff and landing, the longer the plane spends at cruise altitude, the more environmentally friendly it becomes. If you’re flying from Chicago to Milwaukee, you’re basically lighting jet fuel just to watch it burn; New York to LA, though, is another story. And air travel is becoming ever more efficient for the same reasons everybody hates to fly: the airlines really pack those bodies in, and the fuller the flight the better the per-passenger efficiency.

Where driving sits in the rankings is largely a factor of who else is coming along. As a researcher put it, “Traveling alone in a large car can be as bad for the climate as flying, but driving with three in a small car could have an equally low impact as a train ride.” At greater distances, UCS figured two travelers is the threshold between whether it’s better, carbonwise, to fly or drive. If you’re a family of four on a 1,000-mile trip, driving even a big old SUV works out better than flying, or taking the train; if you’re a party of just one or two and your car’s not electric, you may as well go by air. And if you do, fly coach: a 2017 World Bank study estimated the per-passenger carbon emissions associated with first-class travel, because of cabin space used, can be nine times greater than economy.

Not to end on too dour a note here, but have you considered a staycation, Greg? A brand-new study out of Australia reckons that as of 2013, tourism’s carbon footprint accounted for 8 percent of world greenhouse emissions and was projected to keep growing, as demand for recreational travel stays ahead of emissions-reducing tech. Planes, trains, automobiles—it’s hard to think we’re not just rearranging deck chairs on the Titanic.