Do you wonder about the impact of driving a car or taking a flight? Do you want to know how to do the least harm when you travel? Since research indicates that transportation contributes to climate change, it may help to think critically about our travel habits and modes of transportation.

Carbon dioxide (CO2) is a significant contributor to climate change. CO2 is released into the atmosphere when we burn fossil fuels such as car gasoline and plane jet fuel. Some forms of transportation contribute to climate change more than others. For example, the least planet-friendly ways to travel are by plane, van, and car. The best are a self-powered bicycle, electric car with solar panels, regular electric car, and train.

Climate change causes forest fires, floods, droughts, heatwaves, health impacts, food shortages, sea-level rise, altered marine ecosystems, coral bleaching, acidic oceans, damage to infrastructure, economic disruption, hurricanes, and the destruction of communities. But people may not be spurred to take action because they don’t feel like they can identify the cause of climate change or assume there’s nothing they can do to prevent it. Fortunately, everyone can help.

Planes are harmful to the planet. A plane weighs about 130,000 pounds. It flies about 35,000 feet above ground. To keep it suspended in the air for hours takes a massive amount of jet fuel. That fuel releases carbon dioxide (CO2) and other greenhouse gases into the air (an especially potent hit given their elevation) and oceans. Those gases block heat from escaping the atmosphere, which causes the planet’s temperatures to increase.

How much carbon dioxide and greenhouse gases are you responsible for when you fly? A flight from New York to California generates about one metric ton per person. Fly from the U.S. to Asia and you’re responsible for about five metric tons. How bad is that? Five metric tons is the average amount of carbon dioxide each person produces per year. Thus, taking one international flight or a few domestic flights in a year can double your annual carbon footprint. The United States comprises only 4% of the world’s population and yet we’re responsible for about 50% of the world’s CO2 emissions from air travel. About 85% of the world’s population has never flown on an airplane.

Cars are also problematic. According to the EPA, the typical car releases about 4.6 metric tons of CO2 per year. Every time we burn a gallon of gasoline, it creates about 9,000 grams of CO2. Therefore, for every 1,000 gallons of gasoline we burn, we’re adding about one metric ton of CO2 into the air. The CO2 warms the planet and leads to fires, floods, hurricanes, and other disasters.

Parsing climate math is no small feat though. A deep dive into carbon calculators reveals figuring out how best to travel depends on several factors, e.g., how many people travel in a car, the tremendous amount of fuel often used on a runway, factoring in the water vapors caused by planes that release a host of other greenhouse gases that trap heat, the CO2 output caused by maintaining bridges, roads, tunnels, and other infrastructure needed to pave the way for cars that isn’t needed in the air, and a host of other factors. The negative impact of planes extends beyond their CO2 emissions, and the adverse effects of cars include more than the gasoline they burn. As you can see, determining if it’s better to travel by plane or car isn’t easy.

Government regulations to limit carbon dioxide emissions from air and car travel increased innovation. Unfortunately, the Trump administration deregulated the industry to allow plane and car manufacturers to maximize profits at the expense of the planet. But while cleaner technology for planes, and the development of electric planes are still in their infancy, fuel is the highest cost for airlines so they have an incentive to increase the efficiency of their planes regardless. Also, some car companies are continuing to improve fuel economy standards despite Trump’s rollback. The U.S. Clean Air Act also mandates companies use lighter and more fuel-efficient planes and cleaner jet fuels to reduce carbon pollution, although enforcement has lagged.

The silver lining is that everyone can reduce CO2 emissions. If you choose to fly, consider reducing your carbon footprint by purchasing carbon offsets or flying nonstop (taking off and ascending uses the most fuel). Buy an electric car, walk, bike, or take a train or carpool whenever possible. Taking a train emits about 90% less CO2 emissions than comparable travel by car or plane. Advocate for more bike-sharing stations and infrastructure investment to build more railroads. Interested in discovering more ways to help? Calculate your carbon footprint.