Sensitivity of global greenhouse gas budgets to tropospheric ozone pollution mediated by the biosphere
Tropospheric ozone (O3), 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, O3 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 O3 affects the global budgets of greenhouse gases (GHGs: CO2, CH4, and N2O) through altering the land-atmosphere exchange is largely unknown. Here we assess the sensitivity of these budgets to tropospheric O3 pollution based on a meta-analysis of experimental studies on the effects of elevated O3 on GHG exchange between terrestrial ecosystems and the atmosphere. We show that across ecosystems, elevated O3 suppresses N2O emissions and both CH4 emissions and uptake, and has little impact on stimulation of soil CO2 emissions except at relatively high concentrations. Therefore, the soil system would be transformed from a sink into a source of GHGs with O3 levels increasing. The global atmospheric budget of GHGs is sensitive to O3 pollution largely because of the carbon dioxide accumulation resulting from suppressed vegetation carbon uptake; the negative contributions from suppressed CH4 and N2O emissions can offset only ̃10% of CO2 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 O3 pollution, representing a necessary step towards fully understanding and evaluating O3-climate feedbacks mediated by the biosphere.