Importance of carbon dioxide physiological forcing to future climate change

Abstract

An increase in atmospheric carbon dioxide (CO 2 ) concentration influences climate both directly through its radiative effect (i.e., trapping longwave radiation) and indirectly through its physiological effect (i.e., reducing transpiration of land plants). Here we compare the climate response to radiative and physiological effects of increased CO 2 using the National Center for Atmospheric Research (NCAR) coupled Community Land and Community Atmosphere Model. In response to a doubling of CO 2 , the radiative effect of CO 2 causes mean surface air temperature over land to increase by 2.86 AE 0.02 K (AE1 standard error), whereas the physiological effects of CO 2 on land plants alone causes air temperature over land to increase by 0.42 AE 0.02 K. Combined, these two effects cause a land surface warming of 3.33 AE 0.03 K. The radiative effect of doubling CO 2 increases global runoff by 5.2 AE 0.6%, primarily by increasing precipitation over the continents. The physiological effect increases runoff by 8.4 AE 0.6%, primarily by diminishing evapotranspiration from the continents. Combined, these two effects cause a 14.9 AE 0.7% increase in runoff. Relative humidity remains roughly constant in response to CO 2 -radiative forcing, whereas relative humidity over land decreases in response to CO 2 -physiological forcing as a result of reduced plant transpiration. Our study points to an emerging consensus that the physiological effects of increasing atmospheric CO 2 on land plants will increase global warming beyond that caused by the radiative effects of CO 2 .global warming | runoff | evapotranspiration | hydrological cycle | plant stomata I ncreased atmospheric CO 2 content affects global climate not only through its greenhouse radiative effect, but also through its effect on plant physiology. Plant stomata open less widely under elevated CO 2 concentrations, leading to reduced plant transpiration (1-3). A decrease in canopy transpiration tends to reduce evapotranspiration (the sum of canopy evaporation, canopy transpiration, and soil evaporation), triggering changes in atmospheric water vapor and clouds, and affecting surface radiative fluxes, thus producing changes to temperature and the water cycle. This driver of climate change, referred to as "CO 2 -physiological forcing," has been detected in both field experiments (4, 5) and climate modeling studies (3,(6)(7)(8)(9)(10)(11).In this study, we examine the climate effect of CO 2 -physiological forcing using a coupled global atmosphere-land surface model (12,13). While previous studies have looked at the response of temperature and runoff to CO 2 -physiological forcing, the focus of this study is to examine the nature of climate response to CO 2 -physiological forcing in terms of both magnitude and pattern, and contrast it with the effect of CO 2 -radiative forcing. Most previous modeling studies on the climate effect of CO 2 -physiological forcing (6-11) were performed within the modeling framework of the Met office Hadley Center models using the "MOSES" scheme (7) as...

Climate forcing due to optimization of maximal leaf conductance in subtropical vegetation under rising CO 2

“…Our simulations with the stomatal optimization model predict that a doubling of present CO 2 will decrease the annual transpiration flux from subtropical vegetation in Florida by ≈60 W·m −2 . This decrease is considerable because the current annual evapotranspiration flux in Florida is ≈120 W·m −2 and transpiration constitutes ≈50% of this total (12,30). Feedbacks at regional and continental scale could potentially compensate for reduced canopy transpiration and shift the fractional contribution away from transpiration (31).…”

“…This climatic effect is termed the physiological forcing of CO 2 , which acts beside and independent of its radiative forcing. Despite advances to quantify this physiological forcing with global climate models (11,12), these models rely on semiempirical relations to simulate g s from environmental variables (13,14). Alternative models have been proposed on the mechanism that stomatal adaptations optimize carbon gain under the constraint of a cost of water loss (15,16).…”

Global CO 2 rise leads to reduced maximum stomatal conductance in Florida vegetation

“…Consequently, the adaptation within the phenotypic plasticity is likely to constrain epidermis structural adaptation in the near future when phenotypic response limits are reached (35,37). Current increase in CO 2 and the coinciding reduction in plant transpiration already results in increased continental run-off (46), and climate models predict surface temperature increases arising from reduced evaporative cooling (6,7). The mechanisms of optimization of carbon gain to water loss described here could be used to better estimate this physiological forcing for the past and future CO 2 but should be considered within the framework of species-specific phenotypic plasticity (37).…”

Stomatal Function across Temporal and Spatial Scales: Deep-Time Trends, Land-Atmosphere Coupling and Global Models

“…Stomatal closure in response to elevated CO 2 is a waterconserving mechanism that reduces transpiration but, as a consequence, increases canopy temperature. This physiological forcing adds to the radiative forcing effect of increasing atmospheric CO 2 concentration on climate (Sellers et al, 1996a;Betts et al, 1997;Cao et al, 2010). The potential magnitude of physiological climate forcing varies substantially from tropical to temperate to boreal landscapes (Bonan, 2008).…”