The author of today's post, Lisa Moore, Ph.D., is a scientist in the Climate and Air Program.
If you're in or near a big city, you've probably heard your local news give ozone alerts. Those warnings mean that smog levels are high enough to affect your lungs. Even moderate pollution causes respiratory problems for kids with asthma. Really high levels of ozone make it dangerous for even the healthiest adult to be outdoors.
Now scientists are warning that smog could make global warming worse because of its effects on plants.
It turns out that ozone not only hurts your lungs, it also decreases plants' ability to take up carbon dioxide (CO2). That's a problem – plants remove heat-trapping CO2 from the atmosphere – but how much of a problem? Is it enough to affect global climate?
Scientists have studied the effects of elevated CO2 and ozone on plants for years. Researchers expose leaves, plants, or whole ecosystems to combinations of elevated CO2 and ozone and measure the plants' responses over time. But these are small-scale experiments.
A few weeks ago, researchers led by Dr. Stephen Sitch published a paper in Nature titled "Indirect radiative forcing of climate change through ozone effects on the land-carbon sink [PDF]" (paid subscription required) that attempts to quantify the problem on a global scale. Using data from the small-scale experiments, they modified a global-scale carbon cycle model to include the effects of CO2 and ozone on plant CO2 uptake. (See my previous post for more on how climate models work.) Then they ran the model using a business-as-usual scenario of CO2 emissions, with and without projected increases in ozone concentration.
The results were striking. In all cases, plants continued to take up extra carbon, but when the effects of ozone were included, they took up significantly smaller amounts. In other words, as smog increased, the rate at which CO2 accumulated in the atmosphere also increased. The upshot is that ground-level ozone does seems to add to global warming.
The same chemical reactions that produce smog also increase ozone in the upper troposphere, where it acts as a greenhouse gas. The authors estimate that the indirect warming effects of ozone through its effects on plants could be as large as the direct warming impact of ozone in the upper troposphere. (Ozone is also found in the stratosphere. See Bill's post on "Global Warming and the Ozone Hole".)
So the bad news is that smog could exacerbate global warming. The good news is that existing pollution controls can lead to rapid air quality improvements, meaning less smog and less global warming.