220.127.116.11 Effects of Climate Change
Climate change can affect tropospheric ozone by modifying emissions of precursors, chemistry, transport and removal (European Commission, 2003). These and other effects are discussed below. They could represent positive or negative feedbacks to climate change.
18.104.22.168.1 Effects on emissions
Climate change affects the sources of ozone precursors through physical response (lightning), biological response (soils, vegetation, biomass burning) and human response (energy generation, land use, agriculture). It is generally expected that lightning will increase in a warmer climate (Price and Rind, 1994a; Brasseur et al., 2005; Hauglustaine et al., 2005), although a GCM study by Stevenson et al. (2005) (errata) for the 2030 climate finds no global increase but instead a shift from the tropics to mid-latitudes. Perturbations to lightning could have a large effect on ozone in the upper troposphere (Toumi et al., 1996; Thompson et al., 2000; Martin et al., 2002; Wong et al., 2004). Mickley et al. (2001) find that observed long-term trends in ozone over the past century might be explainable by an increase in lightning.
Biomass burning in the tropics and at high latitudes is likely to increase with climate change, both as a result of increased lightning and as a result of increasing temperatures and dryness (Price and Rind, 1994b; Stocks et al., 1998; A. Williams et al., 2001; Brown et al., 2004). Biomass burning is known to make a large contribution to the budget of ozone in the tropical troposphere (Thompson et al., 1996), and there is evidence that boreal forest fires can enhance ozone throughout the extratropical NH (Jaffe et al., 2004). With climate warming, it is likely that boreal fires will increase due to a shorter duration of the seasonal snowpack and decreased soil moisture (Kasischke et al., 1995).
Biogenic VOC emissions may be highly sensitive to climate change. The most important global ozone precursors are CH4 and isoprene. The effect of climate change on CH4 is discussed in Section 7.4.1. The effect on NMVOCs was examined by Constable et al. (1999), Sanderson et al. (2003b), and Lathière et al. (2005). Although biogenic NMVOC emissions increase with increasing temperature, all three studies concur that climate-driven changes in vegetation types unfavourable to isoprene emissions (notably the recession of tropical forests) would partly compensate for the effect of warming in terms of ozone generation.
22.214.171.124.2 Effects on chemistry
Changes in temperature, humidity and UV radiation intensity brought about by climate change could affect ozone significantly. Simulations with GCMs by Stevenson et al. (2000) and Grewe et al. (2001) for the 21st century indicate a decrease in the lifetime of tropospheric ozone as increasing water vapour enhances the dominant ozone sink from the oxygen radical in the 1D excited state (O(1D)) plus water (H2O) reaction. Stevenson et al. (2006) find similar results in an intercomparison of nine models for 2030 compared with 2000 climate. However, regional ozone pollution may increase in the future climate as a result of higher temperatures (see Section 7.6, Box 7.4).
126.96.36.199.3 Effects on transport
Changes in atmospheric circulation could have a major effect on tropospheric ozone. Studies using GCMs concur that STE should increase in the future climate because of the stronger Brewer-Dobson stratospheric circulation (Sudo et al., 2002a; Collins et al., 2003; Zeng and Pyle, 2003; Hauglustaine et al., 2005; Stevenson et al., 2006) (errata). Changes in vertical transport within the troposphere are also important, in view of the rapid increase in both ozone production efficiency and ozone lifetime with altitude. Convection is expected to intensify as climate warms (Rind et al., 2001), although this might not be the case in the tropics (Stevenson et al., 2005). The implications are complex, as recently discussed by Pickering et al. (2001), Lawrence et al. (2003), Olivié et al. (2004), Doherty et al. (2005) and Li et al. (2005). On the one hand, convection brings down ozone-rich air from the upper troposphere to the lower troposphere where it is rapidly destroyed, and replaces it with low-ozone air. On the other hand, injection of NOx to the upper troposphere greatly increases its ozone production efficiency.