Drivers of Dry Day Sensitivity to Increased CO₂

Abstract

The latest generation of coupled ocean-atmosphere global climate models project a global increase in annual mean precipitation of 1%-3% for every 1°C of warming . This increase is determined by a robust response to global mean surface air temperature (2%-3% per 1°C) that is partly offset by fast adjustments to atmospheric radiative heating by greenhouse gases and aerosols (Allan et al., 2020;Fläschner et al., 2016). More intense but less frequent precipitation events have been observed across many regions (Donat et al., 2019;Giorgi et al., 2011), with projections of an increased incidence of extreme precipitation events coupled with longer dry spells (Sillmann et al., 2013;Thackeray et al., 2018). Yet the projections of regional precipitation remain highly uncertain, and their total variance is still dominated by model uncertainty rather than emission scenarios or internal climate variability Lehner et al., 2020).Beyond the fast adjustment due to its radiative effect, increased atmospheric carbon dioxide (CO 2 ) may also trigger a fast vegetation adjustment that is now accounted for in most Earth System Models. In response to high ambient CO 2 concentrations, many plants reduce their stomatal conductance and transpiration in order to minimize water loss, thereby increasing their plant water-use efficiency. This decrease in transpiration, however, may be partly offset by a CO 2 fertilization effect that favors photosynthesis and vegetation density. The net physiological CO 2 effect can affect the amount and variability of soil moisture and evapotranspiration, which in turn can influence the frequency, intensity, and duration of precipitation events. It may thus contribute to model uncertainty, especially in forested areas (