If you have to travel and you want to make as little impact as possible on the climate, your best bet is to go by train, bus or even ship, found a new study. Airplanes are the next best option. Cars are worst of all.
The study was the first to make a side-by-side, kilometer-by-kilometer, emission-by-emission comparison of the many ways that common transportation options impact climate. The results could help governments target environmental strategies and help travelers decide how best to get from Point A to Point B.
"This research is, to my knowledge, the first one which puts such well-justified and detailed numbers to this," said Jens Borken-Kleefeld, a physicist and climate scientist at the International Institute for Applied Systems Analysis in Laxenburg, Austria.
"In a way, it reiterates previous results to use public transportation as a general rule," he added. "Bus, coach and rail come up very, very well on climate impacts. If you want to buy a car, go for one that is the most fuel-efficient and has the most advanced emission-control standards."
The new paper, published in Environmental Science & Technology, offers just a few results from a 5-year research project that involved data collected by 45 research teams around the world.
Country by country, Borken-Kleefeld and colleagues began by tallying the myriad ways that people traveled in 2000. From there, they took inventory of all of the emissions spit out by each mode of transport, which included ships, trains, busses, airplanes, motorcycles and cars.
Many previous studies have measured carbon dioxide levels that come out of exhaust pipes. Instead, the new study included a full range of pollutants, including sulfur dioxide, nitrogen oxide and water vapor. The researchers also considered how long each gas lingers in the atmosphere and looked at how those gasses interact to influence climate change.
Using chemistry models that took global meteorological data into account, the researchers calculated projected temperature changes produced by each transport mode, looking into the future 5, 20, and 50 years.
The results were complicated and nuanced, but a few major findings emerged that allowed the researchers to rank transport modes by their contribution to climate warming. For moving freight, ships were best -- impacting climate up to 30 times less than trucking and actually often producing a cooling effect because of their sulfur emissions.
Rail scored nearly as well, but airplanes scored far worse, affecting climate up to 42 times more than trucking did.
For transporting passengers, effects on climate depended on the time scale. In the first year after a trip, results showed that, kilometer-per-kilometer, airplanes bumped temperature levels higher compared to road travel.
But in the long term, cars contributed more to warming than planes did on a kilometer-per-passenger basis. That's because cars are relatively inefficient and often carry just one or two people. Cars also spit out lots of carbon dioxide, which lasts, and continues to trap heat, for a really long time.
Overall, results showed, passenger airplanes and cars affected the climate three times more than buses and trains did.
"The really stand-out result is that after about 20 years, the warming impact from a single passenger in a car would be more than a person traveling by plane," said Nadine Unger, a climate scientist at Yale University in New Haven, Conn. "That's a very new perspective."
The results could influence decisions about environmental policies, including how best to invest in new technologies and mitigation strategies. The findings could also inform emission-trading schemes. And they might cause people to reconsider the way they travel.
For a trip from New York to Boston, for example, the new study suggests that flying might be a better choice for the sake of Earth's climate.
"From the point of view of the general public, there's been a level of anxiety that people feel recently about their carbon footprint when they go to airports," Unger said. "We should be feeling that way when we turn on our car ignitions."