Usually, when scientists and policymakers consider the amount of carbon that a forest can extract from the atmosphere (‘sequester’), they are simply considering the amount of carbon stored in the biomass (simply the trees). However, a recent study sheds light on a parallel phenomenon that is equally important in storing carbon from the atmosphere.
Writing in Frontiers in Forests and Global Change, Lawrence et al. (2022) report on the impact of change of forest cover on abiotic processes such as water and energy balance. These biophysical processes have a large – but hitherto studied – impact on the carbon sequestration potential of the ecosystem. These biophysical effects are:
- Albedo, ie, the amount of incoming radiation reflected by the ground
- Evapotranspiration (ET) or the evaporation of water from land and the evaporation of water vapor from plant stomata
- Roughness of the canopy – essentially a metric to measure the irregularities of the surface of the canopy. A high roughness of the canopy stimulates vertical mixing and draws heat and water vapor from the surface.
The tropics inevitably receive more sunlight and moisture, which provides more energy to drive ET and cool the air near the surface. At higher latitudes, albedo is the most prominent biophysical driver, because vegetation is flat and sunlight is seasonal. Another physicochemical phenomenon is the release of volatile organic compounds (VOCs) by forests. Their reaction with atmospheric oxygen produces secondary organic aerosols that are not only highly reflective (this causes cooling), but also concentrate clouds (aka ‘cloud condensation nuclei’) that increase ‘cloud’ albedo.
The authors examined effects of forest cover change on carbon stocks elucidated by abiotic factors, and then separated them by latitudes. To this end, they compiled quantitative data on biophysical factors from published literature. The data consists of both ground-based exams and remote sensing. Here, both techniques usually use an area covered with forests and robbed of one as a proxy for deforestation and deforestation respectively. This was done on three scales – locally, regionally and globally – and for tropical rainforests in three continents: Latin America, Central Africa and Southeast Asia.
Surface temperatures in areas under forest cover are ‘significantly lower’ than in areas without cover, according to the study. In tropical forests, an average local surface cooling of 0.96 degree C was observed, while in moderate forests, the average cooling was 0.4 degree C. In boreal (Arctic) forests it was 1 degree C.
Results showed that biophysical cooling effects brought about by biophysical / abiotic drivers change by latitude in a fairly predictable manner. From the equator to 30-40 degrees N, biophysical effects increase CO₂ sequestration while cooling the global environment. In the middle latitudes up to 50 degrees N, deforestation leads to a ‘modest’ net global warming. Beyond 50 degrees N deforestation leads to increased cooling.
In case of deforestation, albedo induces changes, although this is compensated by the cooling effect of lost canopy roughness. In the tropics, once the warming of lost evapotranspiration is calculated, the net biophysical effect of tropical deforestation is global warming.
At higher latitudes (i.e. 20-30 degrees N) the albedo compensates for the combined effect of canopy roughness, evapotranspiration, VOCs, ‘resulting in almost zero net biophysical effect on global temperature.’ At even higher latitudes (30-40 degrees N) albedo is the most powerful biophysical driver and so deforestation at these latitudes leads to non-cooling.
These biophysical effects of forests add a matte impact to the local and regional climate, which explains why, after deforestation, warm days become more frequent, even at medium and high latitudes. “Historical deforestation accounts for nearly a third of today’s increase in the intensity of the hottest days of the year at a particular location,” the article says.
In a press release, Deborah Lawrence, lead author of the study, issued a preliminary warning: ‘A recent major UN climate report showed that we must now act urgently to prevent the worst-case scenarios for our planet … If we lose these forests , we will get there 10 years faster. If we protect these forests, they will protect us from extreme climate disasters, droughts and impacts on our food and agriculture. We now take advantage of the tropics that keep us cooler; they already make us feel these extremes, ‘she said.
According to the Food and Agriculture Organization (FAO), tropical forests, which have one of the fastest carbon sequestrations per unit land area, have the greatest deforestation pressure. Tropical forests store nearly a quarter of the earth’s carbon on the planet and can also cool the earth with as much as 1 degree C, and even more if we count biophysical effects. The study further adds that restoring forests in the 0-10 degree N region would provide 25 percent more global cooling than expected based on CO2 sequestration alone.
Considering both biophysical and biochemical effects of forests in tandem can give us a healthier picture of the potential of forests in compensating for warming and, thus, help governments in devising better conservation and climate strategies.
The author is a researcher at the Indian Institute of Science (IISc), Bengaluru, and a freelance science communicator. He tweets on @criticism