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

Abstract

Along with the accumulation of atmospheric greenhouse gases, particularly carbon dioxide, the loss of primary forests and other natural ecosystems is a major disruption of the Earth's system and is causing global concern. Quantifying planetary warming from carbon emissions, global climate models highlight natural forests' high carbon storage potential supporting conservation policies. However, some model outcomes effectively deprioritize conservation of boreal and temperate forests by suggesting that increased albedo upon deforestation could cool the planet. A potential conflict of global cooling vs. regional forest conservation could harm environmental policies. Here we present theoretical and observational evidence to demonstrate that, compared to the carbon-related warming, modeling skills for assessing climatic impacts of deforestation is low. We argue that estimates for deforestation-induced global cooling result from the models' limited capacity to account for the global effect of cooling from evapotranspiration of intact forests. Specifically, transpiration of trees can change the greenhouse effect via small modifications of the vertical temperature profile. However, due to their convective parameterization (which postulates a certain critical temperature profile), global climate models do not properly capture this effect. This may lead to an underestimation of warming from the loss of forest evapotranspiration in both high and low latitudes. As a result, conclusions about deforestation-induced global cooling are not robust and could result in action that immediately worsened global warming. To avoid deepening the environmental crisis, these conclusions should not inform policies of vegetation cover management, especially as studies from multiple fields are accumulating that better quantify the stabilizing impact of natural ecosystems evolved to maintain environmental homeostasis. Given the critical state and our limited understanding of both climate and ecosystems, an optimal policy with immediate benefits would be a global moratorium on the exploitation of all natural forests.

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.…”