Ozone air pollution harms people and plants. Worldwide, exposure to ozone causes about 700,000 excess deaths each year, as well as impaired breathing, asthma attacks, and reduced crop yields. Changes in vehicle and industrial emissions in the United States required by the Clean Air Act have improved surface ozone air quality significantly in many urban and rural regions. In a recent letter to Nature, I show that these trends of falling ozone help explain some changes in the way forests use water that have been observed, but previously not fully explained. Trees take carbon dioxide from the atmosphere in order to photosynthesize sugars and biomass, but lose water through transpiration in the process. The ratio of carbon dioxide gained to water lost is water-use efficiency. Long-term measurements in many forests in North America and Europe have revealed that forests have increased their water-use efficiency over the last 20 years. Rising carbon dioxide levels in the atmosphere (due to fossil fuel use and deforestation) explain part of the water-use trend, but not all of it. Using empirical relationships between ozone and water use in controlled experiments on many tree species, I show that the ozone trends explain a significant part of the water-use efficiency trends. These ozone-water interactions should be added to future climate and biosphere models to better understand and predict how air pollution affects climate and the carbon and water cycles.
At rural sites throughout the Northeastern and Midwestern United States (left) surface ozone (O3) pollution has been falling for the last 15 years. Trees and crops injuries from ozone are related to their cumulative exposure to ozone over a season, which is measured as AOT40 (right). AOT40 has fallen 5% per year in the Midwestern US and 7% per year in the Northeastern US. These ozone trends explain about one-sixth of the trend in water-use efficiency that has been observed at nearby forest sites, which is more than can be explained by other factors.