How do land degradation, desertification, and land rehabilitation affect the global climate?

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To answer this question well requires a multidisciplinary understanding of the issues, to gain some perspective let us consider the following sub-questions…

Is it true that land degradation and rehabilitation are potent considerations in the the world’s cimate system?

This is so. In the previous chapter we have discussed the enormous risks created by global warming. In this chapter we try giving an overview how land degradation altered and still alters global climate and how restoration efforts can be applied to counter the threat of global warming and to induce GHG negative sustainable growth and development across the globe.

Desertification and land degradation are global scale problems profoundly affecting drylands, but also more humid ecosystems. Land degradation is generally initiated by deforestation, removal of other perennial vegetation or overgrazing, followed by soil degradation and erosion leading to loss of biological productivity and biodiversity. Another category involves clearing and tilling for agricultural crop production and abandonment when the soils are depleted or degraded, resulting in long-term or irreversible ecosystem degradation.

Those processes release most of the organic carbon bound in biomass and soil into the atmosphere in the form of CO₂, a powerful greenhouse gas responsible for human induced global warming, and other greenhouse gases, from decomposing biomass and loss of soil organic matter (= soil degradation), mechanisms rather well understood and quantified.

A recent summary of those emissions from FAO are summarized in the following infographic:

Interestingly, numerous approaches to dramatically reduce those GHG emissions rapidly are not only well known, but actually profitable if more than a five year’s business horizon were to be considered!

Can vegetation completely change surface radiation interactions and therefore local and global climate?

The physical changes from vegetated surfaces to bare, exposed degraded soils also induce often dramatic climatic changes that can result in permanent transformation of formerly fertile woodlands or savannas to barren deserts or thorny scrub. Climate relevant aspects of dryland rehabilitation such as albedo changes, and competition between perennial and annual vegetation were claimed to have adverse effects on the environment and climate, which has been used to argue against using afforestations especially in drylands. However, a wide range of climate relevant interactions of vegetation with radiation have to be studied (see Solar Radiation) and additional climate relevant environmental and economic services of conserved ecosystems have to be considered.

Fig. 11: Schematic comparison of the interactions of solar radiation with a degraded surface (left), compared to a forested area. The balance of the different impacts and effects strongly depends on the amounts of moisture available, but also on the tree species planted, planting density and many other parameters.

The result of land rehabilitation is a complete change in the way vegetated or bare surfaces interact with solar radiation and climate as schematically summarized in Fig. 11. Plants do in fact absorb most sunlight, while bare soil reflects some light back into space so that bare soil might have a cooling effect at the first glance. However, a wide range of biochemical and physical interactions of green plants with green vegetation induce a complex and differential response resulting in most cases in vegetated areas actually having a cooling effect on global climate.

Fig. 12: Trees and other green vegetation affect temperature and climate in many ways.

Leaves absorb radiation and transfer heat and radiation to the surrounding air (Fig. 12 top left), creating hot air that starts rising and creating a low pressure system (bottom left). If the trees have sufficient water they will also transpire water to cool their leaves, resulting in warm humid air rising. When this warm humid air finally encounters cooler air layers, clouds and precipitation are induced (bottom left and right side), whereby dense forest induces far more clouds and precipitation than deforested areas (right side, Fig. 12). This cloud cover also blocks out further light from reaching earth’s surface and thus further can counter global warming

Such a cloud induction effect is also noticeable, though has not been quantified, over Yattir forest in the Northern Negev where convective clouds are forming regularly in early summer when soil moisture in the forest is still significant (Fig. 13).

Fig. 13: Convective cloud formation over Yattir forest, spring 2014. Though this is a rare event, the resulting clouds reflect light back into space, and shadebelow areas from the intensive sunrays.

Can trees and perennial vegetation improve local climate and GFG balance by storing water and reflecting radiation into space?

Further contributions of trees on climate are their capability to transport water deep into the soil, and return it to the surface using sophisticated root transport mechanisms resulting in modulation of the local climate (Fig. 14). Trees further modulate climate by storing moisture deep underground during the rainy season using their tap roots, while reusing the same moisture during the dry season for evapotranspiration and cooling, and maintaining essential moisture in top soil. This only recently documented mechanism is worth further investigation and application, providing a sophisticated natural climate control to dry environments.

Fig. 14: Trees modulate microclimate by sophisticated mechanisms of water translocation between tap roots and lateral roots.

Fig. 15: Reflection wavelengths from vegetated areas (solid line) or bare soils (dotted line). The longer wavelengths reflected from bare soil interact directly with the major absorption bands of CO₂, Methane, and water causing warming of the atmosphere, whilethe shortwave reflections from vegetation can mostly escape to space unhindered.

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