Local temperatures are affected not only by global climatic factors, but also by radiative (albedo) and non-radiative (evapotranspiration and convection) mechanisms related to local vegetation cover. Through evapotranspiration, solar energy is converted to latent heat and released from the planet’s surface, while convection refers to the turbulent mixing of air that dissipates sensible heat. The authors state that while albedo (reflectivity of land surface, which is often lower on forested land) is increasingly accounted for alongside greenhouse gases in climate models, the non-radiative mechanisms are not. However, the evapotranspiration and convection facilitated by vegetation have an important cooling effect and should therefore be included in models to avoid the risk of “promoting land sector policies that may be counter to the aims of mitigation or adaptation” [Bright 2017: 296].
The authors demonstrate that “non-radiative mechanisms dominate the local response in most regions for eight of nine common LCMC perturbations” [Bright 2017: 296]. Land cover and land management changes (LCMC) considered in the study include converting cropland or grassland to deciduous or evergreen forests. The authors found that gains in forest cover increased annual cooling in all but the northernmost latitudes, where the lower albedo of forests compared to grasslands had a warming effect that was stronger than the cooling effect of non-radiative mechanisms. In many regions, including much of Europe, the US, and the tropics, non-radiative cooling dominated albedo effects. “Over annual timescales, forest cover gains result in net cooling for many of the densely populated regions of the planet” [Bright 2017: 298].
Bright et al. conclude that “benchmarking the locally driven LCMC effect to that driven by global forcers (such as CO2) can provide an additional perspective by which to support the valuation of vegetated ecosystems and the local climate regulation services that they provide” [Bright 2017: 301].
Bright, Ryan M., et al., 2017, Local temperature response to land cover and management change driven by non-radiative processes, Nature Climate Change 7, http://doi.org/10.1038/NCLIMATE3250.