Governors' Biofuels Coalition

There are no signs that trend has improved since then, said Anup Bandivadekar, who heads the passenger vehicle program at the International Council on Clean Transportation, a nonprofit think tank affiliated with the United Nations initiative. “It’s making progress in fuel economy increasingly difficult,” he said.

The global S.U.V. boom is a roadblock in the march toward cleaner cars that has been aided by advances in fuel-saving technology and hybrid or electric vehicles. Compared to smaller cars, S.U.V.s are less efficient, generally by about 30 percent.

S.U.V.s are also less likely to go electric soon. There are technological hurdles to powering a larger car with batteries, and the perception among many automakers remains that drivers of S.U.V.s value power and performance, and don’t want to be constrained by the range anxiety of battery-powered cars.

For the moment, Tesla’s Model X is the only major fully electric S.U.V. on the market. The company has sold about 40,000 since they went on sale in 2015.

The S.U.V.-building bonanza contrasts with promises made by automakers of big investments in electric vehicles and other low-emitting vehicles. At the same time, they are pouring resources into far more polluting S.U.V.s.

General Motors, which unveiled its Chevy Bolt electric car in 2016, has sold about 25,000 of them in the United States, and the model hasn’t received updates that might spur sales this year. This month, though, the automaker announced it was spending $265 million to build its new Cadillac XT4 crossover S.U.V. at its plant in Kansas City, Kan.

General Motors continues to invest in clean car technology, including all-electric cars, a spokeswoman, Laura Toole, said. “It’s encouraging to see overall industry fuel economy improving — especially given the significant market shift to crossovers and S.U.V.s,” she said in an email.

Volkswagen, which made no S.U.V.s to speak of a decade ago, plans to sell almost 20 new S.U.V. models worldwide by 2020, and expects those models to make up 40 percent of its global sales. Currently, Volkswagen sells just four S.U.V. models.

A Volkswagen spokeswoman, Jeannine Ginivan, said the company’s S.U.V.s were filled with technology that bolsters the car’s efficiency. Still, its most popular Tiguan model, which weighs almost 4,000 pounds, achieves just 27 miles per gallon on the highway, compared to 36 miles for Volkswagen’s Passat, its midsize sedan, which weighs closer to 3,000 pounds.

The S.U.V.’s global dominance today has roots in the United States of the 1970s, when automakers started confronting stricter auto-safety and environmental rules. Authorized by a Clean Air Act amendment to set federal tailpipe rules for the first time, the Environmental Protection Agency required automakers to more than double their average fuel efficiency over the following decade. But vans, pickup trucks and other off-road vehicles escaped with fewer restrictions than traditional passenger cars.

Detroit’s riposte: Turn the truck into America’s new family car.

The Jeep Cherokee became an early runaway hit — a “versatile family fun machine that has the ruggedness, reliability and go-anywhere heritage Jeep vehicles are known for,” declares the brochure for its 1974 model — leading the way for others to turn working vehicles into family rides. Cherokees and Ford Explorers soon began replacing Tauruses and other smaller cars on driveways across America. In North America, S.U.V.s and pickup trucks outsell all other car categories combined.

The vehicles are now particularly popular in China, where the big cars have offered a sense of status, stability on bumpy roads, and room for larger families emerging with the phasing out of the one-child policy. McKinsey, the global consulting firm, predicts that by 2022, one in every two cars sold in China will be an S.U.V. That could make it tougher for China to tame its urban air-pollution crisis.

Even western Europe — where in 2004, Ken Livingstone, who was mayor of London at the time, declared S.U.V. drivers “complete idiots” and threatened to charge them 25 pounds to enter the city — has fallen for the S.U.V. Sales in that region have more than doubled over the past five years, a clip four times as fast as the overall market, according to JATO data.

Automakers have a strong financial incentive to build and try to sell more S.U.V.s, which tend to be higher-end offerings with luxury trimmings that command premiums over the basic vehicles they are based on. A $60,000 truck, for example, can generate tens of thousands of dollars in operating profit.

On the other hand, most automakers still lose money on each electric vehicle they sell. The credit rating firm Moody’s recently warned that electric vehicles will likely generate low returns for automakers through the early 2020s.

Automakers point out that the bulk of the global growth has been in S.U.V. crossovers, which are based on car underbodies (not trucks) and achieve better fuel economy than larger S.U.V.s. Current E.P.A. rules remain ambiguous on whether crossovers are cars or trucks, allowing manufacturers to still classify many models as trucks, evading tougher fuel economy rules.

Crossovers have also been harmful to overall emissions goals because market research shows that many first-time buyers of crossovers had previously driven cleaner, smaller sedans.

More recent regulations have not helped. Fuel economy standards around the world tend to become more lenient as car weight and size increase. That means S.U.V.s typically do not have to meet as strict a fuel-efficiency standard as smaller cars do.

In the United States, automakers have now come full circle. They point to the shift toward larger cars as a reason to push back tougher fuel regulations that they agreed to, under the Obama administration, in return for a federal bailout of billions of dollars at General Motors and Chrysler.

One question that worries experts: Will the world embrace the S.U.V.’s even-more-polluting cousin, the all-American pickup truck?

There are signs this is already happening. Last year, Ford sold more than one million F-series pickup trucks — a fifth of them outside the United States — putting it within striking distance of unseating Toyota’s Corolla as the world’s best selling vehicle, according to tallies from JATO and Toyota.

A Ford spokeswoman, Dawn McKenzie, said that trucks had “moved from being seen as only relevant for work” to “being much more versatile,” and that recent models had made strides in fuel economy. The most fuel-efficient F-series model gets about 25 miles per gallon on the highway, though a new model this year could get closer to 30 miles a gallon.

The trucks, which cater to drivers looking for power and hauling might, are unlikely to go electric soon. One of the few hybrid choices right now is the Workhorse, a hybrid pickup made by an Ohio company that specializes in fleet vehicles. Workhorse says it has received about 5,000 orders so far for the truck, which goes about 80 miles on a full battery charge.

“It seems to be an American male thing to think: ‘I may want to haul things,’” said Lewis Fulton, who co-directs the sustainable transportation program at the University of California, Davis, Institute of Transportation Studies. “If that caught on in other countries, it really wouldn’t help.”