Abstract

The climate of the Earth is, to a considerable degree, controlled by (he terrestrial biosphere. Its surface properties (albedo and roughness) are important parameters for the surface exchanges of energy and momentum, Even more important are the tightly coupled exchange fluxes of water, carbon dioxide, and oxygen both by diffusion through the stomata of leaves during photosynthesis and through soils and organic matter by respiration. By means of process models of terrestrial biogeochemistry and coupling these to general circulation models in various ways, one can attempt to estimate the degree to which this terrestrial biospheric control is effective. Furthermore, the exchanges of these gases are reflected in spattotemporal variations of their atmospheric distribution. Modeling of these patterns and comparing them to observations constitutes a powerful t»K)l to evaluate the performance of terrestrial biogeochemical models.

A recent sensitivity model simulation with the European Centre Hamburg Atmosphere Model (ECUAM) general circulation model demonstrates that the presence or absence of terrestrial vegetation induces near surface land temperature changes of as much as 8 °C, doubled precipitation, and nearly threefold changes in evapotranspiration. The largest contributions to these changes are found to be caused by the enhanced surface water recycling in the presence of a "green world," as compared w ith a global desert. The simulation sets upper bounds on possible climate modifications induced by anthropogenic changes in land use. Terrestrial CO? exchanges play a crucial role in the global carbon cycle, not unly because of the never-ending quest for the "missing sink" but also for the quantification of climatic feedbacks, in particular in the context of global warming. Atmospheric observations clearly demonstrate that climatic fluctuations significantly reduce or enhance the growth rate of atmospheric CO?* at least on inierannual to decadal time scales. It is also clear that the terrestrial biosphere contributes significant!) to these \ariations. Parallel to H2O and G(>2<, oxygen is also exchanged w ith the terrestrial biosphere. Oxygen is not a greenhouse gas, but it provides an important diagnostic for terrestrial and oceanic biogeochemical processes. Furthermore, oxygen is composed of three different stable isotopes, which, through fractionation effects, provide additional information on the global biogeochemical cycles of H2O, OO2, and oxygen.

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4.1 Introduction

There is no doubt that the biosphere has the potential to considerably influence the climatic environment on the surface of the Earth, as witnessed, for example, by a walk on a hot summer day from an open field into a neighboring forest. During recent years il has become increasingly evident that this influence is not restricted only to local modifications of the environment but that there exist controls of the biosphere on the climate system that have the potential to significantly modify the climate on a global scale. There exist three different fundamental interaction pathways between the physical climate system and the biosphere, operating on different temporal and spatial scales. They include the following:

lr The biophysiological controls by which the vegetation on land actively controls évapotranspiration and thus the surface balances of energy and water;

2. V global, biogeochemical control through emissions and uptake of trace gases, such as carbon dioxide, methane, nitrous oxide, and others, which influence the radiation balance of the atmosphere;

3. A biogeographical feedback exerted by the presence of vegetation regimes w ith different physical properties (albedo, roughness, conductivity, etc) determining the surface exchanges of energy, momentum, and water.

The quantification of these biospheric controls on the Earth's climate and the determination of global-scale potential impacts induced by anthropogenic modifications require that these feedback processes be explicitly incorporated in comprehensive models of the Earth System, Traditionally, various scientific communities working on different temporal and spatial scales (Figure4.1 A) have explored this. Accordingly, the last interactions are being explored w ith soil-vegetation-atmosphere-transport (SVAT) models; terrestrial biogeochemical models (TBMs) arc used to describe the exchanges of the trace gases w ith the atmosphere, and biome models have been developed to investigate the biogeographical feedbacks {Prentice et al., 1992). Recently, first attempts have been made to consistently combine these three approaches in the form of dynamical global vegetation models (DGVMs), as shown schematically in Figure 4.IB.

The extent to which present-day DGVMs are able to describe biosphere-climate interactions under present, past, or future conditions is being described by Prentice (Chapter 11, this volume). Here I illustrate the state of the art in modeling biospheric exchanges of water, carbon dioxide, and oxygen.

4.2 Water Cycle

Evapotranspiration plays a crucial role in the surface energy balance over land. Biological controls on évapotranspiration arc foremost the stomata in plant leaves, which determine the conductivity; A second important factor is the amount of soil water that is available for évapotranspiration, an amount that is determined to a considerable degree by the rooting depth of plants (Shuklla and Mintz, 1982; Nepstad et al., 1994; Kleidon and Meimann, 1998).

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Intermediate: l(M<r vrs

Stow: 10-10* vrs time scale

Hlophvsiokjgical Feed bucks (local I

Biuceochemical Feedbacks (global)

H ideographical Feedbacks I local-regional-global)

IXchange of Fnergv Momentum Moisture

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