Can smog be beneficial?

A new study says yes, sort of.

Tiny pieces of air pollution — also called fine particulates — contribute to human cases of asthma, lung disease, even premature death and therefore are a target for removal by state and federal air quality agencies looking to protect public health.

But researchers from the Jet Propulsion Laboratory in La Canada Flintridge and Caltech in Pasadena have discovered these same microscopic dirt balls fouling the air act as barriers that ricochet sun beams back into the atmosphere, blocking the rise in surface temperatures and possibly slowing global warming.

Known as aerosols, these microscopic pieces of diesel exhaust, dust, soot and factory emissions form what some dubbed “a warming hole,” meaning they create a cool spot over an increasingly warming Earth.

“I would never say air pollution is beneficial because obviously it is bad for human health. And I’m not going to say it has a positive effect, but it appears in some cases it slows down warming,” said Mika Tosca, lead scientist on the study released last week and published in the journal Remote Sensing in July.

Yes, there’s a catch.

The warming holes disappear once the concentration of pollution particles is reduced. Then, after that, surface warming accelerates, Tosca said. It’s like putting a movie on pause and then fast forward.

Tosca and her team began the study by noticing that temperatures in the Southeast United States — Mississippi, Alabama, Georgia and parts of Louisiana — had remained steady as compared to the rest of the world. In the 20th century, this part of the country saw no temperature rises, while the average temperature in the continental U.S. rose by almost 1 degree Fahrenheit.

Temperatures actually decreased in the Southeast at times but began to rise in the 1990s. From 2000 to 2015, the temperatures rose sharply, by 1.5 degrees Fahrenheit, higher and faster than the rest of the country or the globe.

Using satellites launched by NASA in 1999 and one put into space in 2006 by NASA and the French space agency, CNES that measure atmospheric particles, her team found aerosols greatly decreased in that time frame. They noted the drop coincided with stricter air pollution controls on fine particulates imposed by the U.S. Environmental Protection Agency on coal plants and industrial companies beginning in 2006.

In short, the air got cleaner. But blue skies no longer blocked sunlight, so the surface temperatures shot up.

Is this what is happening in Southern California?

Tosca did not study California. But she did note the tremendous progress toward cleaner air in the L.A. Air Basin in the last 35 years and the rise in summer temperatures. (California recorded the hottest year on record in 2014 and nearly broke a record in 2015).

“It is possible some of the warming is part of that,” she said. “When you do clean the air, you suddenly see all this warming that should’ve been realized 20 or 30 years ago.”

Air pollution hides the warming only to unleash it hotter and more rapidly once the air gets cleaned up, Tosca said.

What happens to the sun’s rays in the warming hole? While most are reflected, some may get absorbed in the oceans or in the clouds, she said. That may explain why the warming accelerates in the absence of particulate pollution.

“The aerosols are acting as an extra cloud,” she said.

Clouds could be another variable keeping away warmer temperatures. That is the subject of future studies, she said.

The research could give support to using geoengineering techniques to slow global warming, which is the warming of the earth due to heat that is trapped by excessive amounts of carbon dioxide, methane and other greenhouse gases from vehicles, utilities, landfills and cows.

One method involves shooting aerosol particles high into the atmosphere to block sunlight. Tosca does not endorse this approach, saying it could have a negative ripple effect on global weather patterns.

Instead, she hopes places with high levels of particle pollution, such as China and India, will adjust their climate change goals or at least consider her research when planning for greenhouse gas reductions in the future.

“Cleaning up the air there could give us more global warming than our models have shown,” she said. “It may change some of those targets.”