What are the typical effects of deforestation

Deforestation (high latitudes)

Geographical location of the boreal forests

The boreal forests (also called taiga) essentially extend over Canada, Scandinavia and Siberia and cover about 12 million square kilometers of the earth. So far, only a small proportion of these forests has been cleared (with the exception of Eastern Europe), which is why the possible influence of severe deforestation is usually understood with computer simulations. For the coming decades, the forests are not expected to decline, but rather to expand at high latitudes, partly because of the abandonment of agricultural areas, partly also due to the expected strong warming as a result of climate change, so that the tree line will move north could move. The statements made in this article about the possible consequences of far-reaching deforestation are therefore not to be understood as a realistic scenario, but are intended to illuminate the role of forests in the climate system. Conversely, the question of afforestation is of political relevance, since carbon storage in forests is often discussed as a way of mitigating climate change, while other (physical) effects of the forest are ignored. The latter are particularly important in the forests at high latitudes.

1 The energy balance of boreal forests

There is snow for a large part of the year, which makes the albedo the most important factor influencing the vegetation. Since they protrude above the snow, the trees such as spruces, pines and aspens only have an albedo between 0.2 and 0.4 in winter, in contrast to completely snow-covered grass with 0.75 - 0.9 [1]. The much lighter snow is thus masked by the dark trees. In Eastern Siberia there are mainly larch forests that shed their needles in winter, as no other trees grow due to the extremely cold winters with absolute lows down to -70 ° C. However, the snow cover lasts for a long time during the year. The low position of the sun also contributes to the fact that even summer-green trees have a low albedo due to the trunks.

The difference between the surface albedo with and without forest is therefore nowhere else as great as in boreal forests. Although this difference exists throughout the winter, the influence on the net radiation and thus the temperature difference is by far the clearest in spring and early summer, since only then is the position of the sun sufficiently high and the duration of sunshine long. In autumn, however, due to the thermal inertia of the soil, there is only snow when the solar radiation is very low anyway. This means that if the boreal forest were deforested, the spring temperature would be several degrees lower than it is today[2][3].

Spruce forest in Alaska. A typical feature of the taiga is that the trees are significantly lower and not as close together as in the tropics.

Due to the lower temperature, the snow would melt again 1-2 months later in spring, which increases the temperature difference and maintains it in early summer[4]. Mind you, this only applies to hypothetical, large-scale deforestation that would affect a large part of the forest. As a counterexample, imagine a clearing. Here, the air temperature would be influenced by the surrounding forest, but the snow would be exposed to direct sunlight - in this case, the snow would melt earlier there. Just because the entire forest keeps the temperature high, the melt will be delayed, should it be removed. In this context, however, the ocean also plays an important role. The feedback becomes even stronger via the sea ice cover and the additional ice albedo feedback and its thermal inertia: Since it is colder in spring than on a forested continent, more ice forms on the ocean, which melts later and reflects more sunlight back.

Model calculations show that taking the ocean into account, the cooling is stronger, more extensive and more evenly distributed over the year.[5][2] Despite the significantly lower temperatures and the associated lower terrestrial radiation, the increase in long-wave net radiation remains quite limited, since the lower humidity and temperature of the atmosphere lower the counter radiation. The resulting reduced net radiation must therefore be compensated for by lowering the sensible and latent heat flows. In addition to the decrease in temperature and the lower roughness, the latent heat flow is also reduced by the greater snow cover, since the sublimation of snow is weaker than the evaporation from wet soil.

2 Water cycle in boreal forests

In summer and autumn, the transpiration of trees can be an important source of moisture. The melt water from the snow and ice of winter is now absorbed by the roots and then transpired. Without the forest, this cooling influence would disappear, which would cause the temperature to rise. However, the models show that the temperature is lower than in the forested fall, even in this time of year, but for contradicting reasons. Apart from the simple predominance of the still existing albedo effect, it is possible that evaporation on the ground will increase. This is not unrealistic insofar as the conditions in boreal forests are extremely dry (there is no more rainfall there than in desert regions!), So that trees significantly limit transpiration via their stomata.

The already low precipitation would decrease even further in the event of deforestation, as less water enters the atmosphere. This is especially true for the summer, since in winter the water is bound as snow and ice anyway, little of which is converted into water vapor (sublimed). On the other hand, cloud cover increases due to the cooling near the surface.

The increased cloud cover can mean that less sunlight reaches the ground, which further increases the cooling. However, the cloud cover is also the most important reason for doubts about the strength of the snow masking: Especially where a lot of snow falls, the cloud cover is ultimately high, so that the sunlight is reflected by it and less on the surface of the earth where the trees cover the snow can mask.

3 Changes in circulation in the ocean and atmosphere

Large-scale deforestation of the boreal forests would ultimately also result in changes in circulation, which could trigger climate changes even in distant places in the northern hemisphere:
The cooling of high latitudes changes the temperature difference between north and south, which shifts the jet stream and the trajectories of the low pressure areas to the south. Thus there could be more precipitation than before in southern Europe. In addition, the temperature difference between land and ocean would be weakened, the Eurasian cold spring would be stronger in winter and the heat depression would be weaker in summer. All of this could result in weaker and delayed monsoons in India, Southeast Asia, and Arabia.[3]

These processes change the patterns of cloud cover and snow cover, which in turn changes the energy balance there and triggers further changes in circulation. The ocean circulation would also be affected: lower temperatures at high latitudes strengthen the thermohaline circulation and thus lead to more heat transport to the north. The cooling over the North Atlantic and Europe would therefore be weakened somewhat.

4 Outlook

All of these are the results of idealized experiments. The course of the future climate also depends on other factors: In the future, due to the anthropogenic climate change that is taking place anyway, the snow cover could decrease, so that the albedo difference between forest and soil decreases. For the future of the boreal forest it is also important how quickly climate change takes place: If it is slow enough, the forests could spread to the north and lead to additional warming there and thus to better living conditions. But if it happens too quickly, the trees will suddenly find themselves in an area in which they can no longer grow and the forest would decline sharply (see also tipping points in the climate system). The latter possibility is supported by the knowledge that the boreal forest in particular grows very slowly and may still adapt to the climate change since the last ice age.

5 individual proofs

  1. ↑ Betts, A.K., Ball, J.H. (1997): Albedo over the boreal forest, Journal of Geophysical Research, 102 (D24), pages 28901-28910
  2. 2,02,1Bonan, G.B., D. Pollard and S. L. Thompson, 1992: Effects of boreal forest vegetation on global climate. Nature, 359, 716-718.
  3. 3,03,1Douville, H. and J.-F. Royer, 1997: Influence of the temperate and boreal forests on the Northern Hemisphere climate in the Météo-France climate model. Climate Dynamics, 13, 57-74.
  4. ↑ Bonan, G.B., F. S. Chapin III, S. L. Thompson, 1995: Boreal forest and tundra ecosystems as components of the climate system. Climatic Change, 29, 145-167.
  5. ↑ Ganopolski, A., V. Petoukhov, S. Rahmstorf, V. Brovkin, M. Claußen, A. Eliseev, C. Kubatzki, 2001: CLIMBER-2: a climate system model of intermediate complexity. Part II: model sensitivity. Climate Dynamics, 17, 735-751.

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