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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...

Forestry – including afforestation (the planting of trees on land where they have not recently existed), reforestation, avoided deforestation, and forest management – can lead to increased sequestration of atmospheric carbon dioxide and has therefore been proposed as a strategy to mitigate climate change. However, forestry also influences land‐surface properties, including albedo (the fraction of incident sunlight reflected back to space), surface roughness, and evapotranspiration, all of which affect the amount and forms of energy transfer to the atmosphere. In some circumstances, these biophysical feedbacks can result in local climate warming, thereby counteracting the effects of carbon sequestration on global mean temperature and reducing or eliminating the net value of climate‐change mitigation projects. Here, we review published and emerging research that suggests ways in which forestry projects can counteract the consequences associated with biophysical interactions, and highlight knowledge gaps in managing forests for climate protection. We also outline several ways in which biophysical effects can be incorporated into frameworks that use the maintenance of forests as a climate protection strategy.

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