The Hidden Effects of Deforestation on Our Planet.

By Colm Gorey of Frontiers.



The Unseen Effects of Deforestation. Biophysical Effects on Climate.

Research published in Forests and Global Change offers the most comprehensive and detailed evidence to date that forests are more important to the climate – both globally and locally – than we think due to the way in which they physically transform the atmosphere. The first-ever research conducted by a team from the US pinpointed the local, regional and global non-CO2 benefits of specific forest zones worldwide to find that the entire world gains the most benefits from the band of tropical rainforests spanning Latin America, central Africa and Southeast Asia.

It finds that, together, forests keep the planet at least half of a degree Celsius cooler when we account for the understudied biophysical effects – from chemical compounds to turbulence and the reflection of light.
These effects in the tropics alone deliver planetary cooling of one-third of a degree Celsius; when combined with the CO2, the cooling effect is more than 1 degree Celsius.



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Please read the detailed research works presented below.


ORIGINAL RESEARCH article

Front. For. Glob. Change, 24 March 2022
Sec. Forest Disturbance.

This article is part of the Research Topic titled,

Global Patterns and Drivers of Forest Loss and Degradation Within Protected Areas.


Research done by, Deborah Lawrence, Michael Coe, Wayne Walker, Louis Verchot and Karen Vandecar.


Department of Environmental Sciences, University of Virginia, Charlottesville, VA, United States

The Woodwell Climate Research Center, Falmouth, MA, United States

The Alliance of Bioversity International and the International Center for Tropical Agriculture, Cali, Colombia.



Climate policy has thus far focused solely on carbon stocks and sequestration to evaluate the potential of forests to mitigate global warming.

These factors are used to assess the impacts of different drivers of deforestation and forest degradation as well as alternative forest management. However, when forest cover, structure and composition change, shifts in biophysical processes (the water and energy balances) may enhance or diminish the climate effects of carbon released from forest aboveground biomass. The net climate impact of carbon effects and biophysical effects determines outcomes for forest and agricultural species as well as the humans who depend on them.

Evaluating the net impact is complicated by the disparate spatio-temporal scales at which they operate. Here we review the biophysical mechanisms by which forests influence climate and synthesize recent work on the biophysical climate forcing of forests across latitudes. We then combine published data on the biophysical effects of deforestation on climate by latitude with a new analysis of the climate impact of the CO2 in forest aboveground biomass by latitude to quantitatively assess how these processes combine to shape local and global climate. We find that tropical deforestation leads to strong net global warming as a result of both CO2 and biophysical effects.

From the tropics to a point between 30°N and 40°N, biophysical cooling by standing forests is both local and global, adding to the global cooling effect of CO2 sequestered by forests. In the mid-latitudes up to 50°N, deforestation leads to modest net global warming as warming from released forest carbon outweighs a small opposing biophysical cooling. Beyond 50°N large scale deforestation leads to a net global cooling due to the dominance of biophysical processes (particularly increased albedo) over warming from CO2 released. Locally at all latitudes, forest biophysical impacts far outweigh CO2 effects, promoting local climate stability by reducing extreme temperatures in all seasons and times of day. The importance of forests for both global climate change mitigation and local adaptation by human and non-human species is not adequately captured by current carbon-centric metrics, particularly in the context of future climate warming.



Introduction.

Failure to stabilize climate is in itself a large threat to biodiversity already at risk from deforestation.

Protection, expansion, and improved management of the world’s forests represent some of the most promising natural solutions to the problem of keeping global warming below 1.5–2 degrees (Griscom et al., 2017; Roe et al., 2019). Forests sequester large quantities of carbon; of the 450–650 Pg of carbon stored in vegetation (IPCC, 2013), over 360 Pg is in forest vegetation (Pan et al., 2013). Adding the carbon in soils, forests contain over 800 PgC, almost as much as is currently stored in the atmosphere (Pan et al., 2013). In addition, forests are responsible for much of the carbon removal by terrestrial ecosystems which together remove 29% of annual CO2 emissions (∼11.5 PgC; Friedlingstein et al., 2019).

Globally, forest loss not only releases a large amount of carbon to the atmosphere, but it also significantly diminishes a major pathway for carbon removal long into the future (Houghton and Nassikas, 2018). Tropical forests, which hold the greatest amount of aboveground biomass and have one of the fastest carbon sequestration rates per unit land area (Harris et al., 2021), face the greatest deforestation pressure (FAO, 2020). Given the long half-life and homogenous nature of atmospheric CO2, current forest management decisions will have an enduring impact on global climate through effects on CO2 alone. However, forests also impact climate directly through controls on three main biophysical mechanisms: albedo, evapotranspiration (ET) and canopy roughness.

The direct biophysical effects of forests moderate local climate conditions. As a result of relatively low albedo, forests absorb a larger fraction of incoming sunlight than brighter surfaces such as bare soil, agricultural fields, or snow. Changes in albedo can impact the radiation balance at the top of the atmosphere and thus global temperature. The local climate, however, is not only impacted by albedo changes but also by how forests partition incoming solar radiation between latent and sensible heat. Deep roots and high leaf area make forests very efficient at moving water from the land surface to the atmosphere via ET, producing latent heat. Thus, beneath the forest canopy, the sensible heat flux and associated surface temperature are relatively low, especially during the growing season when ET is high (Davin and de Noblet-Ducoudré, 2010; Mildrexler et al., 2011; Alkama and Cescatti, 2016).

This cooling is enhanced by the relatively high roughness of the canopy, which strengthens vertical mixing and draws heat and water vapor away from the surface. Higher in the atmosphere, as water vapor condenses, the latent heat is converted to sensible heat. As a result, warming that began with sunlight striking the canopy is felt higher in the atmosphere rather than in the air near the land surface. These non-radiative processes stabilize local climate by reducing both the diurnal temperature range and seasonal temperature extremes (Lee et al., 2011; Zhang et al., 2014; Alkama and Cescatti, 2016; Findell et al., 2017; Forzieri et al., 2017; Hirsch et al., 2018; Lejeune et al., 2018). Their impact on global climate, however, is less clear.

Despite high spatial variability, forest biophysical impacts do follow predictable latitudinal patterns. In the tropics, higher incoming solar radiation and moisture availability provide more energy to drive ET and convection, which in combination with roughness overcome the warming effect of low albedo, and result in year round cooling by forests.

At higher latitudes, where incoming solar radiation is highly seasonal, the impacts of ET and surface roughness are diminished (Anderson et al., 2011; Li et al., 2015) and albedo is the dominant biophysical determinant of the climate response. In boreal forests, relatively low albedo and low ET cause strong winter and spring warming. In the summer, higher incoming radiation and somewhat higher ET result in mild cooling by boreal forests (Alkama and Cescatti, 2016). In the mid-latitudes, forest cover results in mild biophysical evaporative cooling in the summer months and mild albedo warming in the winter months (Davin and de Noblet-Ducoudré, 2010; Li et al., 2015; Schultz et al., 2017).

The latitude of zero net biophysical effect, the point at which the annual effect of the forest shifts from local cooling to local warming, ranges from 30 to 56°N in the literature (Figure 1). These generalized latitudinal trends can be modified by aridity, elevation, species composition, and other characteristics, which vary across a range of spatial scales (Anderson-Teixeira et al., 2012; Williams et al., 2021).



FIGURE 1. Latitude of net zero biophysical effect of forests on local temperature varies from 30 to 56°N. Above the line, forest cover causes local warming; below the line, forest cover causes local cooling. The thickness of the line indicates the number of studies that show forest cooling up to that threshold. Data sources as indicated.



Various mechanisms can amplify or dampen a forest’s direct effects on the energy and water balance, with climate impacts in the immediate vicinity, in remote locations, or both (Bonan, 2008). Indirect biophysical effects are particularly important in the boreal region where snow-forest albedo interactions are prevalent. Low albedo forests typically mask high albedo snow, resulting in local radiative warming (Jiao et al., 2017).

At the larger scale this forest-induced warming is transferred to the oceans and further amplified by interactions with sea ice (Brovkin et al., 2004; Bala et al., 2007; Davin and de Noblet-Ducoudré, 2010; Laguë and Swann, 2016). In fact, indirect biophysical feedbacks appear to dominate the global temperature response to deforestation in the boreal region (Devaraju et al., 2018). Future climate warming may alter the strength of such feedbacks, depending on the rate at which forests expand northward and the extent and persistence of spring snow cover in a warmer world.

In the tropics, where ET and roughness are the dominant biophysical drivers, forests cool the lower atmosphere, but also provide the water vapor to support cloud formation (Teuling et al., 2017). Clouds whiten the atmosphere over forests and thus increase albedo, at least partially offsetting the inherently low albedo of the forest below (Heald and Spracklen, 2015; Fisher et al., 2017).

However, the water vapor in clouds also absorbs and re-radiates heat, counteracting some of the cloud albedo-induced cooling (Swann et al., 2012). In the Amazon basin, evidence suggests that deep clouds may occur more frequently over forested areas as a result of greater humidity and consequently greater convective available potential energy (Wang et al., 2009). The impact of tropical deforestation on cloud formation is modified by biomass burning aerosols (Liu et al., 2020) and the net impact on global climate is unclear. Quantifying these indirect biophysical feedback effects is an ongoing challenge for the modeling community particularly in the context of constraining future climate scenarios.

Forest production of biogenic volatile organic compounds (BVOC), which affect both biogeochemical and biophysical processes, further complicate quantification of the net climate impact of forests. BVOC and their oxidation products regulate secondary organic aerosols (SOA), which are highly reflective and result in biophysical cooling. SOA also act as cloud condensation nuclei, enhancing droplet concentrations and thereby increasing cloud albedo, which leads to additional biophysical cooling (Topping et al., 2013). On the other hand, SOA can also cause latent heat release in deep convective cloud systems resulting in strong radiative warming of the atmosphere (Fan et al., 2012, 2013). Furthermore, through impacts on the oxidative capacity of the atmosphere, BVOC increase the lifetime of methane and lead to the formation of tropospheric ozone in the presence of nitrogen oxides (Arneth et al., 2011; McFiggans et al., 2019).

The persistence of ozone and methane (both greenhouse gases) results in a biogeochemical warming effect. The net effect of forest BVOC at both local and global scales remains uncertain. Current evidence, from modeling forest loss since 1850, suggests that BVOC result in a small net cooling, if indirect cloud effects are included (Scott et al., 2018). The strongest effect is in the tropics, where BVOC production is highest (Messina et al., 2016).
An improved understanding of the combined effects of forest carbon and biophysical controls on both local and global climate is necessary to guide policy decisions that support global climate mitigation, local adaptation and biodiversity conservation. The relative importance of forest carbon storage and biophysical effects on climate depend in large part on the spatial and temporal scale of interest.

Local surface or air temperature may not be sensitive to the incremental impact of atmospheric CO2 removed by forests growing in a particular landscape or watershed. In contrast, local temperature is sensitive to biophysical changes in albedo, ET and roughness. At regional and global scales, where the cumulative effects of forests on atmospheric CO2 become apparent in the temperature response, we can usefully compare these impacts. Estimates of the relative impact of biophysical and biogeochemical (e.g., carbon cycle) processes on global or zonal climate have been provided primarily by model simulations of large-scale deforestation or afforestation (Table 1). These studies generally show that CO2 effects on global temperature are many times greater than the biophysical effects of forest cover or forest loss. In models depicting global or zonal deforestation outside the tropics, however, global warming from CO2 release offsets only 10–90% of the global biophysical cooling. The global CO2 effects of total deforestation in the tropics greatly outweigh the global biophysical effects (Table 1).

With the exception of Davin and de Noblet-Ducoudré (2010), these studies have estimated the net contribution of biophysical processes, without isolating the individual biophysical components. Here, we provide a new analysis of CO2-induced warming from deforestation by 10° latitudinal increments (Supplementary Information 1). We then compare the CO2 effect with the only published determination of biophysical effects by latitude (Davin and de Noblet-Ducoudré, 2010) to clarify the potential net impact of forest loss in a particular region on local and global climate.


Source, Frontiers.org the original publisher.

Citation, Lawrence D, Coe M, Walker W, Verchot L and Vandecar K (2022) The Unseen Effects of Deforestation: Biophysical Effects on Climate. Front. For. Glob. Change 5:756115. doi: 10.3389/ffgc.2022.756115

Received, 10 August 2021. Accepted, 02 March 2022. Published on 24 March 2022.

Edited by, Christos Mammides, Frederick University, Cyprus

Reviewed by, Alvaro Montenegro, The Ohio State University, United States.

Rita Von Randow, São Paulo State Technological College, Brazil.