The role of ecosystem transpiration in creating alternate moisture regimes by influencing atmospheric moisture convergence

Abstract

The terrestrial water cycle links the soil and atmosphere moisture reservoirs through four fluxes: precipitation, evaporation, runoff, and atmospheric moisture convergence (net import of water vapor to balance runoff). Each of these processes is essential for sustaining human and ecosystem well-being. Predicting how the water cycle responds to changes in vegetation cover remains a challenge. Recently, changes in plant transpiration across the Amazon basin were shown to be associated disproportionately with changes in rainfall, suggesting that even small declines in transpiration (e.g., from deforestation) would lead to much larger declines in rainfall. Here, constraining these findings by the law of mass conservation, we show that in a sufficiently wet atmosphere, forest transpiration can control atmospheric moisture convergence such that increased transpiration enhances atmospheric moisture import and results in water yield. Conversely, in a sufficiently dry atmosphere increased transpiration reduces atmospheric moisture convergence and water yield. This previously unrecognized dichotomy can explain the otherwise mixed observations of how water yield responds to re-greening, as we illustrate with examples from China's Loess Plateau. Our analysis indicates that any additional precipitation recycling due to additional vegetation | 2537 MAKARIEVA et al.

Re-appraisal of the global climatic role of natural forests for improved climate projections and policies

“…Such an endeavor requires a plausible alternative concept, and we propose that a global climate model built around the stabilizing impact of natural ecosystems can become such an alternative. This will require an interdisciplinary effort and an account of global transpirational cooling, the role of natural ecosystems in the long-distance moisture transport (Makarieva and Gorshkov, 2007;van der Ent et al, 2010;Ellison et al, 2012;Poveda et al, 2014;Molina et al, 2019;Makarieva et al, 2023) and water cycle stabilization (Zemp et al, 2017;Baudena et al, 2021;O'Connor et al, 2021) and the distinct impact of ecosystems at different stages of ecological succession on the surface temperature and fire regime (e.g., Aleinikov, 2019;Baker and Spracklen, 2019;Lindenmayer et al, 2022) and the cloud cover (Cerasoli et al, 2021;Duveiller et al, 2021). Living systems function on the basis of solar energy that under terrestrial conditions can be converted to useful work with a near 100% efficiency.…”

Even cooler insights: On the power of forests to (water the Earth and) cool the planet

“…Increasing cloud production, as illustrated by a quick comparison with Figures 2 and 3 above, leads to increased downward LW radiation toward the land surface and a decline in outgoing LW radiation. Moreover, since increasing concentrations of CO 2 /GHGs and clouds (water vapor) both act as GHGs, their persistent accumulation in the atmosphere appears to weaken any causal pathway (assuming one exists, see, e.g., Colman & Soden, 2021; Jeevanjee et al., 2021, 2022; Makarieva et al., 2023; Stevens & Bony, 2013) by which a share of the latent heat flux might move beyond the lower atmosphere and out into space. Thus, while Zeng et al., for example, suggest that increased ET is one of the principal drivers of cooling global temperatures (Zeng et al., 2017), the evidence presented here suggests something different: increasing latent heat production, cloud cover, and atmospheric CO 2 /GHG concentrations hinders the potential space‐bound release of outgoing LW radiation, further exacerbating the climate problem.…”

Targeted rainfall enhancement as an objective of forestation

“…Yet, it has also been proposed that a larger latent heat flux would locally enhance rainfall if atmospheric moisture content is high enough. Close to saturation, a small increase in atmospheric moisture would trigger rainfall, which would induce moisture convergence and thus rainfall again (Makarieva et al., 2023; not shown in the simple Figure 1). This would add to another biological pathway of local rainfall initiation via emissions of biogenic volatile organic compounds that stimulate cloud formation (Peñuelas & Staudt, 2010).…”