Tropospheric ozone is known to damage plants, reducing plant primary productivity and crop yields, yet increasing atmospheric carbon dioxide concentrations are thought to stimulate plant primary productivity. Increased carbon dioxide and ozone levels can both lead to stomatal closure, reducing the uptake of either gas, and in turn limiting the damaging effect of ozone and the carbon dioxide fertilization of photosynthesis.

The researchers estimated 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 closure.

They found 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. They 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.

Climate models have largely ignored atmospheric chemistry but in this research we have identified a cause of potentially increased warming with elevated levels of surface ozone likely to suppress plant growth.

—Dr Stephen Sitch, Met Office Hadley Centre

We estimate that ozone effects on plants could double the importance of ozone increases in the lower atmosphere as a driver of climate change, so policies to limit increases in near-surface ozone must be seen as an even higher priority.

—Professor Peter Cox, University of Exeter

Resources:

• “Indirect radiative forcing of climate change through ozone effects on the land-carbon sink”; S. Sitch, P. M. Cox, W. J. Collins & C. Huntingford; Nature advance online publication 25 July 2007 | doi:10.1038/nature06059