Droughts in a Changing Climate

Plants absorb CO$_2$ through valves on their leaves called stomata, and simultaneously lose water to the atmosphere by means of transpiration through the same pathway. Higher atmospheric CO$_2$ concentrations allow plants to reduce water losses per unit of carbon gain, in part by reducing stomatal conductance when the gradient of CO$_2$ between the atmosphere and the leaf interior increases. The fact that CO$_2$ concentrations influence plant stomata is nothing new to plant physiologists, but the response at the global scale is under-appreciated by climate scientists and plant physiologists alike.

In work led by Swann, published in PNAS in 2016, we show that the physiological response of plants to rising CO$_2$ (e.g. allowing stomata to close) overwhelms the large increases in the demand for water from the atmosphere due to increasing temperatures as well as increases in water flux from CO$_2$ induced growth of plants. Including the biological control over water fluxes is therefore critical for assessing climate impacts of global warming—without doing so we would predict much drier conditions on land and more frequent and severe drought, a mistake that is widespread through the climate impacts literature. Our findings further demonstrate that ecosystems and climate must be considered together as a coupled system, particularly when assessing the response of the climate system to change. This work has led to related collaborations on tropical precipitation patterns driven by plant responses to increased CO$_2$ ( Kooperman et al. 2018) and an assessment of the ability of different “wetness” metrics to capture plant influences ( Leomordant et al. 2018).