Journal About Deforestation

[Ariel Wascom]

Figure1. Latitude of net zero biophysical effect of forests on local temperature varies from 30 to 56N. 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.

Table 1. Forest effects on global temperature in modeling experiments from biogeochemical (CO2) versus biophysical impacts (albedo, evapotranspiration and roughness as well as changes in atmospheric and ocean circulation, snow and ice, and clouds).

Figure 3. Local temperature change in response to deforestation by season and time of day in the various climate zones as determined by comparing neighboring forested and open land (space for time approach) or measuring forest change over time. Warm/dry season response, averaged over the entire diurnal cycle, in red shading and cold/wet season response in blue shading. Daytime response, averaged over the entire annual cycle, in yellow shading and nighttime response in gray shading. See Supplementary Information 3 for data sources.

Figure 4. Effect of complete deforestation on local annual temperature by climate factor, averaged across the land surface within a 10 latitudinal band. Complete deforestation was implemented globally and analyzed by 10 latitudinal bands (Davin and de Noblet-Ducoudr, 2010). The CO2 effect was determined from total aboveground biomass in each 10 band after Walker et al. (2020) and scaled by CERA-derived sensitivity by latitude. Inset distinguishes the sum of all local biophysical effects from local CO2 effects.

Copyright 2022 Lawrence, Coe, Walker, Verchot and Vandecar. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

Global land-cover map (1-km resolution), produced by NASA, emphasizing with red rectangles the three regions in which all tropical forests (green color) are replaced with a mixture of shrubs and grassland in our deforestation experiments.

Past studies have indicated that deforestation of the Amazon basin would result in an important rainfall decrease in that region but that this process had no significant impact on the global temperature or precipitation and had only local implications. Here it is shown that deforestation of tropical regions significantly affects precipitation at mid- and high latitudes through hydrometeorological teleconnections. In particular, it is found that the deforestation of Amazonia and Central Africa severely reduces rainfall in the lower U.S. Midwest during the spring and summer seasons and in the upper U.S. Midwest during the winter and spring, respectively, when water is crucial for agricultural productivity in these regions. Deforestation of Southeast Asia affects China and the Balkan Peninsula most significantly. On the other hand, the elimination of any of these tropical forests considerably enhances summer rainfall in the southern tip of the Arabian Peninsula. The combined effect of deforestation of these three tropical regions causes a significant decrease in winter precipitation in California and seems to generate a cumulative enhancement of precipitation during the summer in the southern tip of the Arabian Peninsula.

In the version used for this study, the NASA GISS GCM II has 12 vertical layers and a horizontal grid size of 4 by 5. Heat and humidity are advected with a quadratic upstream scheme, and momentum is advected with a second-order scheme. The model has both shallow and deep convection, and a second-order closure planetary boundary layer scheme for moisture and heat transfer is applied at the surface. The model uses six soil layers and a hydrology scheme that accounts for soil moisture transfer and root extraction (Rosenzweig and Abramopoulos 1997), the latter of which depends on the vegetation specified within a grid element.

This land-cover change is expressed in the model by a change of albedo (from a value of 0.06 for tropical forest to 0.1 for the deforested land), vegetation height that affects surface roughness (from 25 to 5 m), leaf area index (from 6 to 1), cumulative root distribution, and stomatal conductance. The cumulative root distribution is defined by root depth and two empirical parameters, which are vegetation specific. For tropical forest, these three parameters are 0.8, 1.1, and 0.4 m, respectively. Corresponding values for the deforested land are 1.5, 0.8, and 0.4 m. Stomatal conductance is controlled by three empirical constants, which are also vegetation specific. It significantly affects the redistribution of energy received at the ground surface into sensible and latent heat. Werth and Avissar (2004b), Costa et al. (2004), and Avissar and Werth (2004) discuss the impact of this parameter on the Amazonian hydroclimatology simulated with the NASA GISS GCM II and other models.

In evaluating the significance of this bias between the model and the GPCP data, one needs to keep in mind that the model is forced by multiyear, monthly mean SSTs and fixed land-cover types dated from the 1960s. These conditions eliminate some of the interannual variability, which might have an impact on the monthly, annual, and global mean, through nonlinear interactions in the climate system. Furthermore, at the resolution of the NASA GISS GCM II, topography is only schematically represented by the model, and the numerical techniques used to approximate atmospheric flow near topography tend to create erroneous motion in the mountain lee.

However, it is also noticeable that the three regions where the deforestation experiments are being performed, namely Amazonia, Central Africa, and Southeast Asia, are generally well simulated by the model.

Figure 6 shows those locations worldwide where precipitation has either significantly decreased or increased for at least 3 months of the year as a result of deforestation of Amazonia (see next section for statistical analysis). The annual cycle and change of monthly mean rainfall at a few continental locations mostly affected in this scenario are also provided. The selection of these locations was subjectively made based on our perception of where the most impressive impacts occur, both in terms of absolute magnitude and time duration. For brevity, it is not possible (and justified) to present such an annual cycle for all locations (which are indicated by the color-coded points on the world map), and the purpose of the few examples presented here is just to illustrate various intricate ways by which the annual cycle of precipitation is perturbed by the deforestation in Amazonia.

While the major impact of deforestation on precipitation is found in and near the deforested regions themselves, a strong impact is propagated by teleconnections along the equatorial regions and, to a lesser yet still statistically significant extent, to midlatitudes and even high latitudes. One can notice that, as a result of the deforestation of Amazonia, the largest decrease of precipitation in continental regions outside of the Tropics is seen in North America, where this deforestation causes a decrease of rainfall in the Gulf of Mexico region, with a particularly severe impact in Texas (about 25%) and northern Mexico, during the spring and summer seasons. Asia, mostly in a region spreading from Turkistan in the west to the Gobi Desert in the east, is also affected by the deforestation of Amazonia. However, the extent of the impact in that region is less significant from a water resource perspective given the small, absolute amount of rainfall received there.

We explain these findings as follows: As a result of tropical deforestation, the sensible and latent heat released into the atmosphere is considerably altered (Shukla and Mintz 1982). The associated change of pressure distribution modifies the zones of atmospheric convergence and divergence, which shift the typical pattern of the Polar Jet Stream and the precipitation that it engenders as far away from the Tropics as mid- and high latitudes. Such a mechanism is not unique to deforestation, as it is also the probable reason for the impacts of El Nio on the global weather and climate (e.g., Trenberth et al. 1998).

3a8082e126