The Role of Plant CO₂ Physiological Forcing in Shaping Future Daily-Scale Precipitation

Abstract

Continued anthropogenic CO2 emissions are expected to drive widespread changes in precipitation characteristics. Nonetheless, projections of precipitation change vary considerably at the regional scale between climate models. Here, it is shown that the response of plant physiology to elevated CO2, or CO2 physiological forcing drives widespread hydrologic changes distinct from those associated with CO2 radiative forcing and has a role in shaping regional-scale differences in projected daily-scale precipitation changes. In a suite of simulations with the Community Climate System Model, version 4 (CCSM4), reduced stomatal conductance from projected physiological forcing drives large decreases in transpiration and changes the distribution of daily-scale precipitation within and adjacent to regions of dense vegetation and climatologically high transpiration. When atmospheric conditions are marginally favorable for precipitation, reduced transpiration dries the boundary layer and increases the likelihood of dry day occurrence. In CCSM4, the annual number of dry days increases by upward of 15 days yr−1 over tropical land and the continental midlatitudes. Decreases in transpiration from physiological forcing also increase the number of heavy precipitation events by up to 8 days yr−1 in many tropical forest regions. Despite reductions in the land surface contribution to atmospheric moisture, diminished surface latent heat fluxes warm the forest boundary layer and increase moisture convergence from nearby oceans, enhancing instability. The results suggest that consideration of the radiative impacts of CO2 alone cannot account for projected regional-scale differences in daily precipitation changes, and that CO2 physiological forcing may contribute to differences in projected precipitation characteristics among climate models.

Why does Amazon precipitation decrease when tropical forests respond to increasing CO2?

“…A significant fraction of this dipole of precipitation change is attributed to the physiological response of vegetation to atmospheric CO 2 [Swann et al, 2016;Bonfils et al, 2017;Skinner et al, 2017;Kooperman et al, 2018;Richardson et al, 2018]. Stomatal resistance increases under elevated CO 2 , leading to a decrease in evapotranspiration and less water loss during photosynthesis [Field et al, 1995].…”

“…The common approach in these experiments is to confine CO 2 increases in an ESM to the land surface alone, which isolates this vegetation-driven feedback from the radiative effects of CO 2 [e.g., Sellers et al, 1996;Betts et al, 2004;Pu and Dickinson, 2014]. A subset of eight modeling centers produced these experiments under CMIP5, permitting intermodel comparisons on how the physiological effect will influence temperature, precipitation, and drought [Swann et al, 2016;Lu et al, 2016;Lemordant et al, 2018;Skinner et al, 2017Skinner et al, , 2018. Only some of these studies, however, have explored the structure of changes within the Amazon specifically [e.g., Abe et al, 2015;Skinner et al, 2017;Richardson et al, 2018;Kooperman et al, 2018].…”

“…A subset of eight modeling centers produced these experiments under CMIP5, permitting intermodel comparisons on how the physiological effect will influence temperature, precipitation, and drought [Swann et al, 2016;Lu et al, 2016;Lemordant et al, 2018;Skinner et al, 2017Skinner et al, , 2018. Only some of these studies, however, have explored the structure of changes within the Amazon specifically [e.g., Abe et al, 2015;Skinner et al, 2017;Richardson et al, 2018;Kooperman et al, 2018]. These papers focus on multidecadal to century-long timescales and attribute the wet Andes, dry Amazon dipole to an increase in the column-integrated export of moisture or moist static energy (MSE) from the Amazon lowlands toward the Andean cordillera [Skinner et al, 2017;Kooperman et al, 2018].…”

“…Only some of these studies, however, have explored the structure of changes within the Amazon specifically [e.g., Abe et al, 2015;Skinner et al, 2017;Richardson et al, 2018;Kooperman et al, 2018]. These papers focus on multidecadal to century-long timescales and attribute the wet Andes, dry Amazon dipole to an increase in the column-integrated export of moisture or moist static energy (MSE) from the Amazon lowlands toward the Andean cordillera [Skinner et al, 2017;Kooperman et al, 2018]. To date, no study has assessed the rapid evolution of the forest's response to CO 2 as it develops in time.…”

Plants and Drought in a Changing Climate

“…On a global scale, Earth system models tend to grow more leaf area under future climate conditions but with significant variation across models [56]. However, modeled leaf area increases are not sufficient to compensate for decreased stomatal conductance under large increases in CO 2 [12,57], although increases in leaf area could be of sufficient magnitude regionally [58]. Further, significant uncertainty remains in the expected response of leaf growth to increasing CO 2 , and, if underestimated, could suggest that plant water savings are unlikely to be realized.…”