Indirect radiative forcing of climate change through ozone effects on the land-carbon sink

Top Cited Papers
Publisher Website
Google Scholar
Abstract
Plants and soil are currently slowing down global warming by storing about a quarter of human carbon dioxide emissions, but this mitigating effect could be undermined by increases in near-surface ozone. Unlike high-altitude ozone, which blocks harmful ultraviolet rays, low-level ozone damages plants, reducing their capacity to take up carbon dioxide and accelerating global warming. Many climate models include a warming factor for low-altitude ozone as a weak greenhouse gas, but not its effect on vegetation. A new study suggests that projected increases of ozone concentration from industrial sources will markedly reduce plant productivity. This indirect effect could contribute more to global warming than the direct effect of ozone as a greenhouse gas. The impact of projected changes in ozone levels on the land-carbon sink are estimated with the help of a global carbon cycle model, which accounts for interactions between ozone and carbon dioxide through stomatal closure. A significant suppression of the global land carbon sink as increases in ozone concentrations affect plant productivity is found. The resulting indirect radiative forcing by ozone effects on plants could contribute more to global warming than the direct radiative forcing. The evolution of the Earth’s climate over the twenty-first century depends on the rate at which anthropogenic carbon dioxide emissions are removed from the atmosphere by the ocean and land carbon cycles1. Coupled climate–carbon cycle models suggest that global warming will act to limit the land-carbon sink2, but these first generation models neglected the impacts of changing atmospheric chemistry. Emissions associated with fossil fuel and biomass burning have acted to approximately double the global mean tropospheric ozone concentration3, and further increases are expected over the twenty-first century4. Tropospheric ozone is known to damage plants, reducing plant primary productivity and crop yields5, yet increasing atmospheric carbon dioxide concentrations are thought to stimulate plant primary productivity6. Increased carbon dioxide and ozone levels can both lead to stomatal closure, which reduces the uptake of either gas, and in turn limits the damaging effect of ozone and the carbon dioxide fertilization of photosynthesis6. Here we estimate the impact of projected changes in ozone levels on the land-carbon sink, using a global land carbon cycle model modified to include the effect of ozone deposition on photosynthesis and to account for interactions between ozone and carbon dioxide through stomatal closure7. For a range of sensitivity parameters based on manipulative field experiments, we find a significant suppression of the global land-carbon sink as increases in ozone concentrations affect plant productivity. In consequence, more carbon dioxide accumulates in the atmosphere. We suggest that the resulting indirect radiative forcing by ozone effects on plants could contribute more to global warming than the direct radiative forcing due to tropospheric ozone increases.