Climate and Climate Change Implications

Climate and vegetation are especially tightly coupled in the tropics. The land-cover type determines both the available energy at the land-surface (through its albedo), and also the partitioning of this available energy into sensible heat and evapotranspiration. The aerodynamic roughness of the land, which is strongly dependent on the height of the vegetation, also influences the global pattern of atmospheric circulation. Through these mechanisms vegetation can influence climate both locally (e.g. by affecting cloud cover and rainfall) and remotely (through atmospheric teleconnections).

The other half of the climate-vegetation feedback loop concerns the effects of climate on vegetation. Water availability is the primary requirement for vegetation, so land-cover in the tropics is strongly influenced by rainfall, through both soil moisture and decreased fire frequency in wet conditions.

Thus tropical climate strongly affects the land-cover, and the land-cover influences the tropical climate. In some parts of the tropics the climate-vegetation feedback may lead to strong non-linear effects and even multiple vegetation-climate states. The Sahellian drought of the 70s and 80s may have been perpetuated by positive vegetation feedbacks. Model simulations suggest that parts of the Amazon Basin can potentially support both stable savanna and forest states.

Some climate carbon cycle models also show strong feedbacks leading to predictions of forest/savanna transitions under global warming. The initial cause of forest loss in these simulations is a reduction of rainfall under enhanced CO2, with this initial rainfall reduction amplified through further rainfall reductions as forest is replaced by savanna (Fig 2). As rainfall decreases (right to left), rainforest contracts increasing the area of savanna (S). This in turn, exerts a positive feedback on the regional climate by further decreasing rainfall, and further decreasing the forest cover. An additional positive feedback is a substantial release of carbon to the atmosphere associated with biome transition accelerating global warming.

Future changes in the position of the major rainforest-savanna boundaries around the globe will have significant implications for the soil organic carbon (SOC) pool, as SOC inventories of forest soils can be up to double those of savanna soils. While at the global level the distribution of tropical forest and savannas are controlled by climate, particularly by rainfall amount and seasonality, within a regional savanna-forest ecotone, factors such as soil texture and disturbance history, particularly fire frequency, become important. Hence, prediction of the impacts of future climate-driven changes in forest/savanna distribution for the global SOC pool requires a more detailed and predictive understanding of the interactions between vegetation, climate, edaphic and disturbance effects than is currently available. Our knowledge of the carbon stocks of rainforests and savannas and their dynamics is quite sparse, especially for soils.

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