Sensitivity of global greenhouse gas budgets to tropospheric ozone pollution mediated by the biosphere

Tropospheric ozone (O 3 ), a harmful secondary air pollutant, can affect the climate via direct radiative forcing and by modifying the radiative forcing of aerosols through its role as an atmospheric oxidant. Moreover, O 3 exerts a strong oxidative pressure on the biosphere and indirectly influences the climate by altering the materials and energy exchange between terrestrial ecosystems and the atmosphere. However, the magnitude by which O 3 affects the global budgets of greenhouse gases (GHGs: CO 2 , CH 4 , and N 2 O) through altering the land–atmosphere exchange is largely unknown. Here we assess the sensitivity of these budgets to tropospheric O 3 pollution based on a meta-analysis of experimental studies on the effects of elevated O 3 on GHG exchange between terrestrial ecosystems and the atmosphere. We show that across ecosystems, elevated O 3 suppresses N 2 O emissions and both CH 4 emissions and uptake, and has little impact on stimulation of soil CO 2 emissions except at relatively high concentrations. Therefore, the soil system would be transformed from a sink into a source of GHGs with O 3 levels increasing. The global atmospheric budget of GHGs is sensitive to O 3 pollution largely because of the carbon dioxide accumulation resulting from suppressed vegetation carbon uptake; the negative contributions from suppressed CH 4 and N 2 O emissions can offset only ∼10% of CO 2 emissions from the soil–vegetation system. Based on empirical data, this work, though with uncertainties, provides the first assessment of sensitivity of global budgets of GHGs to O 3 pollution, representing a necessary step towards fully understanding and evaluating O 3 –climate feedbacks mediated by the biosphere.