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In 2020, carbon dioxide was the biggest source of greenhouse gas emissions in the US, about 80%. The other 20% were from other greenhouse gases such as methane and nitrous oxide.

And in the US, the largest source of carbon dioxide emissions was from transportation. The transportation sector includes the movement of people or goods through vehicles such as passenger cars, trucks, and motorcycles, or through aircrafts, boats, railways, pipelines, or public mass transit systems. Burning fossil fuels such as gasoline or diesel accounted for about a third of all US carbon emissions and about a quarter of US greenhouse gas emissions.

For the last 30 years, carbon dioxide emissions from the transportation sector rose at a steady clip, despite newer technologies reducing the average emissions for vehicles. Then the COVID-19 pandemic hit.

Carbon emissions from transportation decreased 15% between 2019 and 2020.

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The pandemic impacted the world economy, energy markets and energy-related CO2 emissions. Efforts to reduce the spread of the virus caused a decrease in travel and an increase in working from home. As a result, US emissions related to transportation dropped.

From 1990 to 2019, US energy-related carbon emissions increased by 2.1%. During that time, transportation carbon emissions rose by 21%.

But in the pandemic’s first year, US energy-related carbon emissions decreased by 11%. The transportation emissions fell even more by 15%.

Motor gas was the source of more than half of all transportation carbon dioxide emissions.

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The transportation sector gets most of its energy from burning petroleum-based products. The use of fossil fuels in transportation is the primary reason the sector is the biggest source of CO2 emissions in the US. More than half of all transportation emissions come from burning gasoline for cars and other passenger vehicles. Diesel fuel for trucks and jet fuel for aircraft are the other major CO2 emissions sources.

How do different vehicles contribute to emissions?

Passenger cars and freight trucks were the largest contributors to transportation sector greenhouse gas emissions, producing 63.7% in 2020. Rails, ships, and boats produced 4% of emissions that year. The remainder came from light-duty trucks, aircraft and pipelines.

From 1990 to 2019, passenger car emissions rose by 19.3%. Population growth, economic growth, urban sprawl, and periods of low gas prices all led to Americans driving more miles over the last three decades. The number of vehicle miles traveled by passenger cars and light-duty trucks increased 47.5% from 1990 to 2019.

A typical passenger vehicle emits about 4.6 metric tons of carbon dioxide per year or about 404 grams of carbon dioxide per mile. This number can vary based on a vehicle’s fuel, fuel economy, and the number of miles driven per year, according to the EPA. Overall, the equation is simple; more miles driven equals more vehicle emissions.

The pandemic halted the trends of more driving and rising emissions. Between 2019 and 2020, passenger vehicle miles traveled decreased 12.2%, according to the EPA. Passenger vehicle emissions dropped by 20.8%.

Commercial aircraft emissions decreased by 32% between 2019 and 2020. But aircraft emissions were decreasing before the pandemic. Starting around 2007, improvements in aircraft and engine technologies caused a reduction in fuel burn and emissions. These advances, along with other factors, helped cause a 35% decrease in emissions since 2007. The steep decrease in commercial aircraft emissions from 2019 to 2020 was primarily due to COVID-19 impacts on scheduled passenger air travel.

Passenger cars are consistently the highest greenhouse gas emitters of all vehicles.

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The USAFacts State of the Earth report contains more analysis about carbon emissions and other environmental data.

Inventory of US greenhouse gas emissions and sinks
US energy-related carbon dioxide emissions