Once again, vast expanses of Canadian wilderness are on fire. …They’ve been pouring smoke—once again—into northern cities in the United States. That haze is loaded with a more obscure form of carbon, compared to its famous cousin CO2: black carbon. By May 16, the fires’ monthly carbon emissions surpassed 15 megatons, soaring above previous years. Black carbon consists of tiny particles generated from the incomplete combustion of fuels—whether that’s Canadian trees and soils, cooking fuels like wood and charcoal, or coal. “The problem is they don’t burn efficiently,” says Yusuf Jameel, who researches black carbon at the climate solutions nonprofit Project Drawdown. “So they emit a lot of particles and poisonous gases.” …If black carbon wafts from such wildfires in the Arctic, it darkens ice and snow, dramatically accelerating melt. “It’s a huge health issue. It’s a big climate issue,” says Jameel.