Plants protect themselves from heat waves at our peril (thanks to our pollution)
When it’s hot outside, the human body tries to cope by sweating, resting, and if you’re in the sun directly, creating freckles or moles to try to block damage from ultraviolet light. As much as plants need sunlight for photosynthesis, too much heat can be a problem for them too, but rather than emitting some stinky sweat like us, they end up emitting a group of chemicals called isoprenes (C5H8). This helps plants avoid damage from the sun, but some of those isoprenes can also combine with air pollutants, forming new compounds like ozone (O3). As you’ll see on air quality forecasts, this all combines to dangerous conditions in congested areas, even beyond the heat itself.
Worse than the sum of their parts
Plants evolved heat-beating chemistry long ago, and obviously the world has weathered many heat waves. While we’re just starting to appreciate the role plants can play in ozone creation, the most recent addition to this system is actually pollution from human activity, like burning fossil fuels to power cars and other power plants. One group of gasses, nitrous oxides, can readily react with isoprene, making these pollutants into a health hazard. The combination is great at making ozone at ground level, right where it can be inhaled by humans to do damage to our lungs.
In simulations of various temperature conditions, these effects don’t just scale up as things get hotter. A 77º Fahrenheit day, is likely to push plants to increase their isoprene output a bit, resulting in anywhere from a 6 to 20 percent increase in ozone levels in a city. However, as the plants get further from their comfort zone, their rate of isoprene output increases. A 86º Fahrenheit day can end up with 60 percent higher ozone, posing a significantly higher health risk.
Changing the equation
It’s important to stress that while plants are part of this dynamic, they’re not really the problem. In places without human emissions to provide reagents like nitrous oxide, the extra isoprenes in the air shouldn’t be a big problem. The fact that plants also help capture carbon dioxide, which contributes to overall climate change, also points to their net value in air quality management. Fewer plants mean more loose carbon dioxide, and then more heat being trapped in the atmosphere. Some heat, and heat waves, is of course inevitable, leaving us with… oh right— the cars and other exhaust emissions! If we can reduce those emissions, a day hot enough to stress our trees doesn’t have to become a dangerous day for us.
Source: When it’s hot, plants become a surprisingly large source of air pollution by Ashley Yeager, Science News