What Is Greener? Flying vs. Driving

The Environmental Protection Agency's Inventory of U.S. Greenhouse Gas Emissions and Sinks, a 2019 report spanning almost three decades, revealed that transportation—flying, driving, rail, commercial shipping, etc.—is responsible for a larger share of domestic greenhouse gas (GHG) emissions than any other economic sector. It also experienced the greatest increase in emissions between 1990 and 2018, "due in large part to increased demand for travel," the report says.

Transportation alone generates nearly three times the GHG emissions of agriculture, and four times the amount produced by household and commercial properties in the United States. Both cars and planes have been blamed for accelerating climate change—but which is the worst offender? Experts say planes wreak planetary havoc due to their altitude, but are they more fuel-efficient, considering the vast number of passengers able to fit on a Boeing 737?

Learn more about the environmental effects of flying versus driving and which is the greenest way to travel for your next vacation.

Car Pollution

The World Health Organization estimates that 4.2 million people worldwide die from ambient air pollution every year. In the U.S. alone, more than 40% of the population live in areas plagued by poor air quality, and cars are one of the top polluters. 

Passenger vehicles produce several types of pollution: carbon monoxide, when carbon from fuel doesn't burn completely; hydrocarbons, a toxic combination of hydrogen and carbon emitted from car exhausts; nitrogen oxides, formed when nitrogen and oxygen react; and soot known as particulate matter, or PM. 

Cities are getting considerably smoggier because people are statistically driving more. According to the EPA's Greenhouse Gas Equivalent Calculator, driving a car 11,556 miles per year is equivalent, in GHG emissions, to powering a house for nine months, burning through 188 propane grill tanks, or charging a cell phone nearly 600,000 times. But the U.S. Department of Transportation's Federal Highway Administration says people are actually driving even more than the EPA's estimate. In fact, they're driving more miles per year—about 13,476—than ever before in American history. 

The 2021 EPA report revealed that passenger cars and light-duty trucks (including SUVs, pickup trucks, and minivans) together produce 57.7% of all transportation-related GHG emissions, more than eight times the emissions generated by commercial aircraft. On a positive note, automotive technology is only getting greener: new passenger vehicles, heavy-duty trucks, and buses are reportedly around 99% cleaner than 1970 models.

Cleaning Up Car Standards

The shift to cleaner cars is partially a result of emissions standards laid out by the EPA over the past 50 years. Whereas the toxic metal lead was once blended with fuel to boost octane levels, leaded gasoline is now prohibited—and has been for 25 years.

Today, about 2% of new passenger vehicles sold in the U.S. run on electric rather than fuel. The Environmental Defense Fund is now pushing for all new passenger vehicles sold by 2035 to be zero-emitting. According to its 2021 Clean Cars, Clean Air, Consumer Savings report, the organization's proposed pollution safeguards would reduce annual climate pollution by 600 million metric tons—the equivalent of 130,000,000 internal combustion engine vehicles (ICEVs) on the road for one year—and would prevent as many as 5,000 premature deaths annually by 2040.

Problems With Electric Cars

It's important to note that while electric vehicles (EVs) create almost no running emissions, the manufacturing process for most counteracts the benefits of fuel absence. EVs contain lithium-ion batteries, traction motors, and electronic controllers whose production creates up to 60% more carbon dioxide emissions than the production of ICEVs, according to a 2017 study comparing the life cycle of conventional and electric vehicles in China.

In order to determine whether an EV or ICEV is greener, one must weigh the GHG emissions over the lifetime of the vehicle. Experts maintain that EVs offer a greener future, because manufacturing is becoming more widespread (as opposed to being mostly limited to China) and because battery recycling will in time become more efficient, leading to a reduced need for new material extraction. However, the EVs of today are not a perfect solution.

Airplane Pollution

Although passenger cars currently account for the largest portion of transportation-related GHG emissions, air travel is one of the fastest-growing polluters. As of 2018, aircraft were responsible for 9% of U.S. transportation sector GHG emissions and 2.4% of total carbon dioxide emissions globally. The United Nations International Civil Aviation Organization (ICAO) predicts global emissions from aircraft will triple by 2050, and estimates from another study by the International Council on Clean Transportation overshoot the UN's predictions by 150%.

The carbon dioxide emitted from one round-trip flight from New York to London totals about 1,414 pounds per passenger, according to ICAO's Carbon Emissions Calculator—that's more than the average citizen of Kenya (and more than 30 other countries) emits over a full year. What's worse, CO2 is only half the problem.

Like cars, planes emit carbon dioxide and other greenhouse gases when they burn fuel. But unlike cars, planes also leave behind those wispy ice trails—called contrail clouds—that are even more polluting than the CO2 they produce, according to a study on global radiative forcing.

The word "contrails" is a compound of "condensation" and "trails," they occur when exhaust gases mix with low-temperature, high-humidity air. Contrails are damaging not just because they block sunlight, but also because they trap heat coming up from the ground, ultimately creating a warming effect below. This kind of anthropogenic warming is called radiative forcing.

Sustainable Aviation Fuel

Today, alternative fuels that are similar in chemistry to traditional fossil jet fuel, but made from waste and excess raw materials instead, are becoming more commonplace. According to SkyNRG, a sustainable aviation fuel global market leader, this cleaner concoction can be blended with traditional jet fuel and requires "no special infrastructure or equipment changes."

San Francisco International Airport has already started delivering sustainable aviation fuel through a pipeline; American, JetBlue, and Alaska Airlines are some of the industry players that have committed to using it. SkyNRG says this new fuel could reduce CO2 emissions by at least 80%.

Altitude Changes

New research suggests that because contrail clouds form only in very low temperatures, reducing the altitude of flights even slightly could dramatically minimize contrail climate forcing. 

One Imperial College of London study found that just 2% of flights in Japan's airspace were responsible for 80% of that space's radiative forcing. The same study estimated that if even 1.7% of flights reduced their altitude by 2,000 feet—which is a normal amount of variation from the flight path anyway—the climate impact of contrails could be cut by 59%.

Which Is Greener?

Because cars and aircraft impact the environment in different ways, there are several factors to consider when weighing which mode of transportation is greener. Firstly, emissions must be broken down into per-mile, per-person estimates, which can be done using the EPA's Greenhouse Gas Equivalent Calculator for vehicles and the ICAO's Carbon Emissions Calculator for planes. Whereas the average passenger vehicle fits five to eight, a passenger jet can fit up to 220.

Keep in mind that the ICAO calculator only measures carbon dioxide emissions and not the impact of contrail radiative forcing. It's this non-CO2 greenhouse gas that usually tips the scale in favor of driving. For instance, according to 2019 data from the U.K. Department for Business, Energy and Industrial Strategy (BEIS), a domestic flight will emit about 22% less carbon dioxide per person, per mile than a diesel car with one passenger. But, when you also factor in contrail clouds, a domestic flight generates 49% more emissions overall. 

You must also consider the length of the trip. A 2014 University of San Francisco study revealed that emissions from aviation's landing and takeoff cycle alone can represent up to 70% of an airport's total emissions inventory. Because cruising altitude is gentler on fuel, long-haul flights are actually more efficient than short trips, and direct flights are much better for the environment than connecting flights.

There is no hard and fast rule when deciphering the "greenness" of flying vs. driving. While flying could be better for long-distance travel, short road trips shared between several people may result in lower per capita emissions. 

To further reduce your carbon footprint while traveling, the Center for Climate and Energy Solutions suggests driving an electric vehicle rather than one that runs on gasoline, cruising at a steady pace instead of regular breaking and accelerating to avoid fuel waste, taking public transportation whenever possible, packing light, and always choosing direct flights.