Earlier Leaf-Out Warms Air in the North
The timing of leaf growth in the spring is sensitive to climate and has shifted earlier in recent decades. Experiments with a climate model show that earlier spring greenup can affect atmospheric circulation and the radiative balance, and thereby warm surface air.
We found that the extra warming due to increased water vapor and clouds in the atmosphere, which results from earlier leaf-out, is sufficiently large that it can amplify climate change.
Plant leaves interact with the climate system in a variety of ways: they absorb sunlight, emit water vapor through transpiration, and increase the surface roughness of the atmosphere. The climate effects of plant phenology changes are typically assumed to be controlled by these effects on the surface energy budget. In this paper, we show that extra warming results when plant leaves grow earlier in the spring, resulting in increased transpired water building up in the atmosphere, thereby changing the atmospheric water vapor profile and moisture transport. Because water vapor is a powerful greenhouse gas, it can further warm the climate. This effect is in addition to the surface energy balance effects that might be expected. Because the dynamics of plant phenology are uncertain and poorly represented in climate models, this finding emphasizes the importance of accounting for earlier leaf-out and the resulting water vapor and atmospheric circulation changes.