How can evapotranspiration be reduced

Land use

Coverage of the land surface according to satellite data from NASA (November 2000 to October 2001). Shade key: see picture below
Color key for the picture above

1 Introduction

Under Land use one understands any kind of use of soils and land areas (parts of the solid surface of the earth) by humans. That does not necessarily mean that there are no plants or plants that are different from the natural ones; natural ecosystems are almost always managed; For example, cutting trees or mowing meadows removes biomass. The concept of land use or land use change consequently includes not only the change in land cover, but also the way in which this land cover is dealt with, e.g. management practices, fertilization, fire prevention and the type of plowing. Since the effects of such effects are extremely difficult to assess, climatological studies usually only relate to the type of land cover and its influence on the climate.

The climate close to the ground is decisively shaped by the properties of the surface, because energy, water, trace gases and impulses (which describe the movement of the air) are exchanged there. It is therefore not surprising that changes in land cover or management of a region can have an impact on the local climate and material cycles. In general, such interventions have an immediate effect and remain limited to the corresponding area (local or regional). For example, when a forest area is replaced by a lighter arable area, more solar radiation is reflected. This also changes the precipitation, as arable plants store less water than trees. Globally, such effects have little effect. However, this is different with the release of greenhouse gases, in particular carbon dioxide, through deforestation, which are distributed all over the world and therefore have a global effect.

The influence of such land cover changes on the exchange of energy, water and momentum are summarized as so-called biogeophysical effects (see also the biosphere in the climate system). They are therefore differentiated from biogeochemical effects that affect the carbon cycle and thus the greenhouse effect.

2 Change in land use over time

Two films on the DKRZ server show the expansion of agricultural land for the periods 800-2100 and 1860-2100.

The transformation of the land surface is arguably the first significant influence that humans had on the climate, albeit locally limited. In particular, the intensive clearing of forests and the conversion into pasture or arable land can result in a local reflection power of -5 W / m due to an increased albedo2 lead, especially over the agricultural areas of North America and Eurasia. In addition, forests were cleared because of the great need for timber for houses and ships. This way of influencing the climate was much more decisive than the emission of greenhouse gases before the beginning of industrialization, but it did not have a global effect. Already in 1750 about 6-7% of the land surface was used for agriculture, especially in Europe, India and China. About 11 million km2 Forests have fallen victim to clearing over the past 300 years.[1] In the last 50 years there have been hardly any changes in the global agricultural area. In the middle and high latitudes, there has even been a slight increase in forest area.[2] However, deforestation is advancing faster than ever in the tropics, with drastic consequences for the local climate, among other things.

The historical land use changes made by humans had a number of physical consequences on the climate and led to a total of around a third of the total carbon dioxide emitted to date. However, these emissions occur over many centuries and could thus be compensated for by the ocean in particular. Since the carbon dioxide content of the air increased as a result of industrialization, the plants on land also take in additional CO2 on, especially in the tropics. It is still not clear whether the historical land use changes have warmed or cooled the earth. This is due to the numerous interactions that ultimately contain all the properties of the climate. The next section gives a more comprehensive overview of the “biogeophysical” effects of deforestation.

3 Potential climate impacts from deforestation

The biogeophysical interactions between vegetation and atmosphere consist in the exchange of energy, momentum and water. However, these exchange processes are not independent of one another, so that a change in vegetation cover generally has a chain of further consequences. The newly established climate is a result of these many relationships and the result can clearly depend on the region and the characteristics of the new vegetation. The three physical key variables that are changed during deforestation are the albedo of the earth's surface, its roughness and evapotranspiration.

3.1 Albedo

In general, it is assumed that deforestation generally leads to an increase in the albedo, firstly because forests represent very dark surfaces and secondly because they are geometrically complex, so that the light is reflected several times and a larger part is swallowed up. However, the albedo of the earth's surface depends on many properties of the soil and vegetation, in particular on the leaf area, leaf orientation, transparency and reflectivity of the leaves, soil structure and moisture; in the case of snow cover, the temperature and age of the snow, and even the position of the sun and the amount of light scattered. This shows that the weather itself has a decisive influence on the albedo and not just the other way around. For example, during a period of drought, many leaves wither, increasing the albedo so that less sunlight is absorbed.

An increase in the albedo, α, has a direct influence on the radiation balance, as the energy absorbed at the surface is reduced.

short wave ↓ + long wave ↓ - long wave ↑ = net radiation

The long-wave radiation falling on the ground (the second part of the balance) comes from the clouds and greenhouse gases, especially water vapor. The net radiation is an excess of energy that has to be transported into the atmosphere in the form of sensible and latent heat. A sensible (perceptible) heat flow means that heat is transported from the ground into the atmosphere via air vortices. A latent heat flow, on the other hand, is the transport of water vapor via these eddies, which only releases its heat later when it condenses back into liquid water.

Net radiation = sensible + latent heat flow

If the albedo increases, the temperature decreases because suddenly less solar radiation is absorbed. From the lower net radiation it clearly follows that the two heat flows must decrease together. How exactly the division between the two is, is complicated in individual cases and also a challenge for complicated climate models. Unfortunately, this division also has important consequences and further feedback. Here is just one example: If the sensible heat flow increases into the lower atmospheric layer (boundary layer), this increases and turbulent air eddies arise, which results in mostly drier air being mixed in from above. So it becomes drier and the evaporation, which is stimulated when it is dry, and with it the latent heat flow, become stronger again.

In addition, the occurrence of thunderstorms, which mainly bring precipitation in the tropics, depends heavily on the relationship between sensible and latent heat flow.

3.2 Water balance

In addition, the water balance also changes: A reduction in surface roughness and surface cover reduces the proportion of precipitation that is absorbed by the vegetation. The evaporation from the surfaces of the vegetation decreases, on the other hand it is facilitated on the ground itself. Evaporation is usually combined with the transpiration of plants as evapotranspiration. Perspiration is the evaporation of water from the stomata (stomata) of the plants, which was taken from the soil reservoir via the roots. Without the deep roots of the trees, a large part of the soil moisture can no longer evaporate.

The latent heat flow resulting from evapotranspiration causes a link between the water balance and the surface energy balance: If less water is available for evaporation, there is also less evaporative cooling and the temperature rises.

Due to the way in which the stomata in the leaves (stomata) work, transpiration can also be severely restricted in the case of forest: When there is a lack of water, the stomata contract to prevent the plant from drying out. This means that less water gets outside, which further supports the dry conditions. In addition, the CO2-Content has an influence on transpiration: With more CO2 In the air, the gas exchange through the stomata can take place more efficiently because the plant no longer needs as much air to absorb a certain amount of carbon dioxide. This means that it also loses less water. Each individual leaf therefore generates less evaporative cooling. However, a higher CO2-Concentration also lead to more and more lush plants growing, because they have more food - the total area of ​​all leaves on the earth can therefore increase. The more leaf area can thus compensate for the less transpiration, so that the temperature does not necessarily have to rise additionally. The feedbacks that affect the vegetation density and the surface are called structural feedbacks, those that affect the metabolism of a single leaf are called physiological feedbacks.

The disruption of the water cycle also results in changes in the other parts of the above energy balance: Due to the change in the water vapor content due to evapotranspiration and possible changes in the cloud cover, there are repercussions on the incident solar radiation and the long-wave counter-radiation, which decreases with a decrease in the water vapor content. At the same time, the terrestrial radiation depends heavily on the temperature and somewhat on the emissivity (radiation capacity) of the surface. The optical properties of the surface (albedo and emissivity) are in turn dependent on the soil moisture and snow cover, which can have further repercussions.

Because of the reduced evapotranspiration, there is initially less water available in the atmosphere, which has a lowering effect on precipitation. However, this only applies if more water than before is not carried into the corresponding region as a result of changes in circulation in the atmosphere. This merging of water over one point is called moisture convergence and corresponds to an imbalance between precipitation and evapotranspiration:

Moisture convergence = precipitation - evapotranspiration

This is a simple balance sheet for the atmosphere. But what happens to the excess water on the ground? Of course, this water will not stay there forever, but will flow to other places in streams and rivers. However, some of it is temporarily stored in the soil so that it is moist even on sunny days. (Remember, for example, digging in the sandpit. The sand on the ground is always wetter and therefore darker and more stable.)

However, fewer roots reduce the soil's storage capacity for water and can thus lead to stronger and faster runoff. The consequences of deforestation for the water cycle can be observed regularly in the form of flood disasters, some of which are caused by the fact that water runs off directly into the valleys during heavy rain instead of being held back by the tree roots.

3.3 surface roughness

The third influencing factor mentioned above, surface roughness, has an impact on how efficiently heat is exchanged between the ground and the atmosphere. The greater the roughness, the more energy can also be removed from the floor via the air vortex. With less roughness (e.g. a pasture instead of a forest), therefore, not as much energy can be dissipated as is originally available through the radiation balance. Therefore, the earth's surface becomes warmer until it emits enough long-wave radiation and the balance between energy gain and loss is balanced again. For surfaces that are not uniform, e.g. with few trees and grass in between, the assessment is not always that easy: Compared to a forest with closed treetops, the roughness is higher because air vortices can easily form between the free-standing trees transport a lot of heat. How the roughness changes when trees are cut down is therefore not easy to assess in individual cases.

Another influence of the roughness is the change in the wind close to the ground: Since this is not slowed down much with a smooth surface, it can become faster and thus bring about an even better energy transport (because faster wind also creates stronger air vortices). This effect counteracts the previous one: it has a cooling effect with low roughness, instead of warming. In addition, the wind direction can also be affected, since the Coriolis force reacts to the braking of the wind and the balance to the force of the air pressure difference can no longer be maintained. The local air circulation is affected, which in turn can have an impact on moisture transport, precipitation and ultimately evapotranspiration and the energy balance.

3.4 Global Effects

In addition to local climate changes, it is also possible that areas outside the area affected by deforestation may be affected. In addition to the direct transport of air masses, this happens through changes in regional or even global circulation. Such circulations are created by temperature differences, intensified by the water cycle and the release of latent heat and influenced by the influence of friction on the ground, so it is not surprising that changes in land use do not leave the circulation on earth unchanged.

Greenhouse gas emissions, known as biogeochemical effects, are of particular importance for the global effects of deforestation. Forest destruction causes sinks of carbon dioxide to be lost, which is emitted into the atmosphere, spreads evenly around the globe relatively quickly and warms the atmosphere, particularly in the middle and high latitudes of the northern hemisphere through snow / ice albedo feedback. Another consequence of the land use changes are methane emissions from the spread of wet rice cultivation in Asia. Methane is also an important, long-lived greenhouse gas that spreads globally in the atmosphere. There is currently broad agreement that the biogeochemical effects outweigh the biogeophysical effects globally.[3]

4 Evaluation and Legal Regulation

Physical influencing variables of forest cover and their most important feedbacks. A plus sign next to an arrow from A to B means that process B has been strengthened by a fictitious strengthening of A, a minus sign means a weakening (so the signs do not reflect the ultimate influence of the deforestation). Both paths, reduced evapotranspiration and roughness (red) and albedo increase (blue), are valid everywhere. In boreal forests, however, the blue, cooling path predominates, in tropical forests the red, warming path.

It turns out that the cutting down of forests in different parts of the world can change the climate in a completely different way because the interaction of the physical effects shifts (see the main article on deforestation in the tropics, middle and high latitudes). It also depends on what the spatial pattern of this deforestation looks like and what type of land use will be carried out there later (in the tropics, for example, cattle breeding is an important source of methane).

The cooling via the albedo effect (the blue branch in the diagram) is in boreal forest also globally and long-term averaged so strong that the warming effect of the carbon dioxide released during logging is exceeded. Planting trees in these regions does not reduce climate change, but intensifies and accelerates it! Against this background, it seems absurd that political agreements such as the Kyoto Protocol allow countries to allow boreal forests to be credited as a positive contribution to climate protection. A warning against jumping to conclusions is appropriate here: In addition to storing carbon, forests have a whole range of important properties, of which biodiversity, recreational value, the provision of food and wood and the regulation of the water balance are only a few the destruction of forests to mitigate climate change does not appear to be a particularly appropriate strategy.

In the Tropics the influence of the reduced evapotranspiration (red branch in the diagram) is locally much more decisive than the albedo change. Therefore, deforestation is warming the tropics. Outside the areas affected by deforestation, however, the lower water vapor content may lead to a cooling, which, however, is exceeded by the warming caused by released carbon dioxide. A reforestation of the tropics would therefore have a cooling effect on the global climate, the size of which is not well known.

Overall, however, it must be taken into account that the global mean temperature in connection with land use is not a suitable measure for assessing the dangers of climate change and its effects on people's lives. The areas that have been deforested so far are large overall, but not contiguous, but rather spread over the globe as a patchwork quilt. There may therefore have been significant climate changes at every single location, which, on a global average, almost cancel each other out. However, every person individually experiences the specific effects of climate change at any point in time at their place of residence. A long-term and global mean value of a cause will therefore not be able to describe the totality of the effects.

Due to the climate relevance of land use, land use changes and agriculture (and especially deforestation), attempts are currently being made to regulate these areas more closely in global and European climate protection law and to include them more in greenhouse gas reduction efforts. This has proven to be difficult up to now due to measurement and comprehensibility problems, enforcement problems, impending relocation effects and the question of the correct baseline (as a computational inventory from which reductions can be initiated and then checked).[4]

5 individual proofs

  1. ↑ Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. Lowe, G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz and R. Van Dorland, 2007: Changes in Atmospheric Constituents and in Radiative Forcing. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA
  2. ↑ FAO, 2006: Global Forest Resources Assessment 2005. Progress towards sustainable forest management. FAO, Rome
  3. ↑ Smith, MC, JS Singarayer PJ Valdes, JO Kaplan, and NP Branch (2016): The biogeophysical climatic impacts of anthropogenic land use change during the Holocene, Climate of the Past 12, 923-941, doi: 10.5194 / cp-12 -923-2016
  4. ↑ Ekardt, F. / Hennig, B., 2010: Land use, climate change and emissions trading. In: Journal for New Energy Law, Issue 6

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