Forests, atmospheric water and an uncertain future: the new biology of the global water cycle

Abstract

Theory and evidence indicate that trees and other vegetation influence the atmospheric water-cycle in various ways. These influences are more important, more complex, and more poorly characterised than is widely realised. While there is little doubt that changes in tree cover will impact the water-cycle, the wider consequences remain difficult to predict as the underlying relationships and processes remain poorly characterised. Nonetheless, as forests are vulnerable to human activities, these linked aspects of the water-cycle are also at risk and the potential consequences of large scale forest loss are severe. Here, for non-specialist readers, I review our knowledge of the links between vegetation-cover and climate with a focus on forests and rain (precipitation). I highlight advances, uncertainties and research opportunities. There are significant shortcomings in our understanding of the atmospheric hydrological cycle and of its representation in climate models. A better understanding of the role of vegetation and tree-cover will reduce some of these shortcomings. I outline and illustrate various research themes where these advances may be found. These themes include the biology of evaporation, aerosols and atmospheric motion, as well as the processes that determine monsoons and diurnal precipitation cycles. A novel theory-the 'biotic pump'-suggests that evaporation and condensation can exert a major influence over atmospheric dynamics. This theory explains how high rainfall can be maintained within those continental land-masses that are sufficiently forested. Feedbacks within many of these processes can result in non-linear behaviours and the potential for dramatic changes as a result of forest loss (or gain): for example, switching from a wet to a dry local climate (or visa-versa). Much remains unknown and multiple research disciplines are needed to address this: forest scientists and other biologists have a major role to play. New ideas, methods and data offer opportunities to improve understanding. Expect surprises.

Comments on “Is Condensation-Induced Atmospheric Dynamics a New Theory of the Origin of the Winds?”

“…More importantly, removing major sources of water vapor, e.g. through large-scale deforestation, will influence atmospheric circulation, modify ocean-to-land moisture transport and impact the terrestrial water cycle (Makarieva and Gorshkov, 2007;Makarieva et al, 2013a;Nobre, 2014;Sheil, 2018). Independent observation-based studies testify in favor of a significant impact of vegetation cover on ocean-to-land circulation and moisture import (e.g., Levermann et al, 2009;Chikoore and Jury, 2010;Andrich and Imberger, 2013;Poveda et al, 2014;Herzschuh et al, 2014;Levermann et al, 2016;Boers et al, 2017).…”

Unravelling the functions of biogenic volatiles in boreal and temperate forest ecosystems

“…Better understanding of these loops can improve forecasts of climate and vegetation resilience. Variability in terrestrial vegetation growth and phenology can modulate fluxes of water and energy to the atmosphere (Sheil 2018), thus affecting the climatic conditions that regulate vegetation dynamics. The BVOCs from natural vegetation may significantly affect cloud formation (Joutsensaari et al 2015;Zhao et al 2017) and precipitation in different vegetation regions.…”

The need for green and atmospheric water governance

“…Flowchart for average global annual green and blue water flows and storage on the terrestrial surface. Source : Flow estimations are based on Rockström and Gordon (), Rockström, Lannerstad, and Falkenmark (), Rost et al (), Hoekstra and Mekonnen (), Wang‐Erlandsson et al (), and Sheil ()…”

“…When following a water particle throughout a basin, a continuous partitioning process between blue and green water can F I G U R E 3 Flowchart for average global annual green and blue water flows and storage on the terrestrial surface. Source: Flow estimations are based on Rockström and Gordon (2001), Rockström, Lannerstad, and Falkenmark (2007), Rost et al (2008), Hoekstra and Mekonnen (2012), Wang-Erlandsson et al (2014), and Sheil (2018) occur before the particle is returned to the atmosphere or ocean, which is referred to as "the water cycle continuity" (Falkenmark, 2001). Human activities can influence this cycle and the partitioning process by irrigation practices (converting blue water to green water), and land management (Falkenmark, 2008;Jewitt, 2006), which can have direct impacts on aquifer replenishment, ecosystem service generation, or downstream blue water availability.…”