Exhaust emissions from tailpipe [photo: Simone Ramella, 2005, used under Creative Commons 2.0]
That huge intake of breath you may have heard from across the Atlantic Ocean was the sound of German carmakers gasping at a court decision handed down on Tuesday.
After a week's delay in issuing its ruling, a German court decided that individual cities in the country may ban vehicles with diesel engines to reduce air pollution.
The decision could affect the value of up to 15 million vehicles now on the road's in Europe's largest car market.
The Federal Administrative Court in Leipzig ruled that city authorities in Düsseldorf and Stuttgart may legally ban older diesel vehicles with minimal emission controls from city centers.
It ruled only on whether such bans were legal, and the cities themselves must now put rules into place for any such limits.
In its decision, the court recommended that any such restrictions should be introduced only gradually and that exemptions be permitted for a variety of reasons.
2004 Volkswagen Touareg TDI
But acknowledging the impact on the values of the older diesel vehicles, one judge wrote that "certain losses will have to be accepted," according to a summary in The Guardian on Tuesday.
Moving to reassure drivers, German environment minister Barbara Hendricks noted that no bans were imminent and it was her goal to ensure they would not be needed.
Similarly, Chancellor Angela Merkel weighed in to stress that the bans were localized and did not affect the majority of German car owners.
The older diesels likely to be banned emit far more nitrogen oxides than those under the latest emission limits, known as Euro 6b, which went into effect on January 1, 2017.
Those limits brought EU-market limits on nitrogen oxides down to roughly the same level they have been in the U.S. since January 1, 2008.
Roughly six dozen German cities had levels of airborne nitrogen oxides above the EU threshold last year; the chemicals are a key contributor to photochemical smog formation.
1970s Los Angeles smog depicted in the Honda short film
Paris has planned for two years to ban the oldest diesels from its city center, using stickers of different colors in vehicle windshields to indicate the level of emissions rules they were certified under.
Various European cities have proposed different types of bans on high-emitting vehicles in general, especially those with diesel engines.
The German decision could affect not only the values of used diesel cars, however, but may also be viewed as another nail in the coffin for the prospects of new cars fitted with diesel engines.
CHECK OUT: How Paris will ban the dirtiest diesels: color-coded stickers (Jan 2017)
Porsche said earlier in February it had ended production of its last diesel model, and Fiat Chrysler is expected to announce it will no longer fit diesel engines to passenger cars from 2022, though it will continue their use in light trucks and commercial vehicles.
The cost of modern urea-based nitrogen-injection systems to cut nitrogen-oxide emissions renders smaller diesel engines unaffordable against small, direct-injected and turbocharged gasoline engines of similar output.
Meanwhile, while the Volkswagen diesel emission scandal is now largely out of the news in North America, a criminal investigation into the actions of numerous high-ranking former and current VW Group executives remains underway in Germany.
diesel and AdBlue fillers in Audi Q7 TDI
Any ban on diesels would be especially symbolic in Stuttgart, which houses the global headquarters of giant automaker Daimler.
That company's Mercedes-Benz unit introduced the first passenger car with a diesel engine in 1936, and developed the urea-injection system to treat emissions of nitrogen oxides more than 10 years ago.
The court ruling also represents something of a rebuke to Merkel's government, which has usually supported the German auto industry and lobbied against restrictions on its vehicles—including city emission bans like the ones made legal by the court.