Canada’s tree-mendous discovery: Forests limit harmful ozone gas

It was a Tuesday in early February 2015 when Sylvie Gravel walked into her colleague Paul Makar’s office carrying printouts that would mystify both of them.

The two scientists at Environment and Climate Change Canada were staring at computer model forecasts of ozone gas. Up in the Earth’s atmosphere, the ozone layer helps block harmful ultraviolet radiation. But closer to the surface, the reactive gas has been linked to health problems and smog, so the federal government department monitors it in air quality forecasts.

The problem was that the forecasts were way off. The computer model had predicted significantly more ozone across Eastern Canada and the United States than what was actually observed in the air. Something wasn’t being factored in—but what?

“I did not understand the behaviour, and I said, ‘Do you see any reason why we would see something like that?’” Gravel recalled asking Makar. She left the graphs with him to mull over.

Makar spent several days pondering what could be so special about that part of the continent that it would affect ozone production in such a profound way. He went home that weekend, he said, with the question fading into the back of his mind.

“I basically forgot about it until five a.m. Sunday morning,” said Makar, “when I woke up with this thought going through my head: ‘Holy cow, that’s where the forests are.’”