Earth is getting hotter, . One new reason hotter temperatures are bad news: diesel engines spew far more toxic particles when the temperature rises about 30 degrees Celsius (86 Fahrenheit), by The Real Urban Emissions (TRUE) initiative, and many times higher than the average emissions permitted under EU regulations.

Using fancy lasers, the researchers were able to measure “the real-world [nitrogen oxide], particulate and CO2 emissions from over 180,000 vehicles - including L-category vehicles and buses” during the summer of 2018 in cooperation with Paris Mayor Anne Hidalgo.

The findings are unnerving. Even the most compliant diesel models, known as Euro 5 and Euro 6 diesels, still emitted 20 to 30 percent more nitrogen oxide at temperatures above 30 degrees Celsius. And motorcycles emitted 11 times more carbon dioxide and six times more nitrogen oxide than the average gasoline passenger car on a per-kilogram of fuel consumed basis (of course, motorcycles consume a lot less fuel).

“The results show that the excess emissions from diesel passenger cars in particular - which previous TRUE results show produce EU limits – rise in higher temperatures,” TRUE wrote, “amplifying the severe health and environmental impacts of excess emissions.”

The 30 degrees marker is particularly noteworthy as more European cities like Paris regularly top it, which used to be quite rare. But according to TRUE, Paris exceeded 30 degrees on 20 days this past summer.

“As the effects of climate change intensify,” TRUE warns, “the number of days with comparably high or higher temperatures in Europe will rise, especially in urban heat islands such as Paris.”