Uber and Lyft vehicles make up a relatively small share of the cars on Los Angeles’s streets. But a recent report from the California Air Resources Board suggests they produce a disproportionate amount of greenhouse gas emissions.

Authors of the report analyzed data from millions of trips made with ride-hailing apps in 2018, concluding that ride-hailing vehicles across the state produced emissions at a rate of 301 grams of CO2 per passenger mile traveled—nearly 50 percent higher than other automobiles.

That’s mainly because Uber and Lyft drivers spend more than one-third of their total driving time by themselves. Passengers are in the car only about 61 percent of the time, according to the report. Miles drivers travel without a fare amount to additional emissions, since they add on to the journeys actually taken by passengers.

Don Anair, research and deputy director of the clean vehicles program at the Union of Concerned Scientists, calls the numbers highlighted in the report “troubling.” He also says they may undersell the true climate impacts of ride-hailing vehicles, since not all of these trips would have otherwise been taken by car.

A 2018 report by transportation consultant Bruce Schaller found that 60 percent of trips made with Uber and Lyft in large U.S. cities, including Los Angeles, would have been made by walking, biking, or taking transit, had ride-hailing not been an option.

Ride-hailing advocates say that, on the flip side, services like Uber and Lyft could encourage people to ride transit more by giving some users easier access to train stations and bus stops. But the authors of the air resources board report find these benefits to be “negligible.”

Schaller writes in an email that the board’s findings highlight the fact that ride-hailing services are “making achievement of emissions reduction goals that much harder” in California.

State leaders aim to reduce greenhouse gas emissions to 40 percent below 1990 levels over the next decade. Transportation is California’s largest source of emissions, and reducing the raw number of miles driven in the state will be a key part of meeting that threshold.

Statewide, Uber and Lyft account for only a small portion of overall driving; trips taken with ride-hailing services amounted to a little over 1 percent of those taken in all passenger vehicles. In Los Angeles, they account for closer to 3 percent of trips, according to a 2019 study by transportation analyst Fehr and Peers.

But air resources board staffers note in the report that the industry is growing quickly. Bloomberg New Energy Finance projects that by 2040 nearly one in five auto trips will be made using ride-hailing services.

A state bill passed two years ago mandates that ride-hailing companies allow California to track their contribution to overall vehicle emissions. The recent air resources board report sets a baseline, so that state leaders can ensure reductions occur in the future.

Anair says that local officials and state regulators should push companies like Uber and Lyft to make good on promises to make sure more trips are made with electric vehicles—and to support drivers in purchasing less pollutive cars. He says ride-hailing companies could also encourage more customers to use carpool options, which can lessen the distance drivers travel by themselves.

“This report really underlines how important it is to enforce emissions standards for these companies—so it’s something they have to do, not just something they’d like to do,” Anair says.

A spokesperson for Lyft wrote in a statement that the company “is striving to make every ride 100 percent electric over time. We have worked with Colorado lawmakers on policies to make electric vehicles there more affordable, and hope to similarly collaborate with California in order to speed the transition to an all-electric fleet.”

Adam Gromis, public policy manager on sustainability and environmental impact with Uber, says the company “is serious about addressing environmental impact on our platform.” In a statement, he writes that Uber is working on “innovations” that will decrease the amount of time drivers spend picking up fares, boost the number of riders per trip, and give passengers other alternatives to driving—like bikes and scooters.