Introduction to climate dynamics and climate modelling - Interactions between climate and the terrestrial biosphere

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4.3.3 Interactions between climate and the terrestrial biosphere

The terrestrial biosphere plays an important role in the global carbon cycle (section 2.3.3). Changes in the geographical distribution of the different biomes, can modify carbon storage on land and so have an impact on the atmospheric concentration of CO₂. This can potentially lead to feedbacks that are commonly referred to as biogeochemical feedbacks as they involve the interactions between climate, biological activity and the biogeochemical cycle of an important chemical element on Earth (the carbon).

**Figure 4.15:** The difference in albedo between snow-covered forest and grass. The forest appears darker as it absorbs more of the incoming solar radiation. Photo courtesy of Ali Gillet.

In addition, changes in vegetation have a clear impact on the physical characteristics of the surface, in particular the albedo and water exchanges between the ground and the atmosphere (section 1.5). Feedbacks implying physical variables influenced by the terrestrial biosphere are referred to as biogeophysical feedbacks. One important biogeophysical feedback is the tundra-taiga feedback that can be observed at high latitudes. The albedo of a snow-covered forest is much lower than that of snow over grass (Table 1.3, Fig. 4.15). As a consequence, if, because of warming, trees start to grow in the tundra (transforming the region in a taiga) the surface albedo will tend to decrease, in particular in spring (see section 4.2.3). This will lead to more warming and thus a positive feedback.

**Figure 4.16:** Flow graph illustrating the positive tundra-taiga feedback . A positive (negative) sign on an arrow sign means that an increase in one variable induces an increase (decrease) in the one pointed by the arrow. The positive sign inside a circle indicates that the overall feedback is positive.

However, one of the most spectacular examples of biogeophysical feedbacks occurs at low latitudes, linked to the relatively fast desertification of the Saharan region, which it has been estimated has been going on for between about 6 and 4 kyr BP (see section 5.5.1) The standard explanation of this decrease is related to a positive atmosphere-vegetation feedback triggered by the comparatively slow changes in orbital forcing (see section 5.5.1). Because the intensity of the African monsoon decreased (due to the decrease in summer insolation), precipitation decreases in the Sahara during the Holocene. This induces a decrease in the vegetation cover and thus an increase in the surface albedo (Table 1.3). As a consequence, there was an additional cooling and reduction of precipitation that amplified the initial decrease in vegetation cover.