Ecosystems and the Global Energy Balance

Ecosystems affect the global energy balance directly by utilizing solar radiation by photosynthesis, and also indirectly by altering components of the surface energy balance such as surface albedo and evapotranspiration rates, and by strongly affecting biogeochemical fluxes and atmospheric composition.

Direct Biotic Effects

Photosynthesis utilizes about 8 W m~2 of solar radiation (Table 3) Since most of the carbohydrates are respired within relatively short time at the same location, most of the energy is released as heat by respiration. Hence, the energy fluxes associated with photosynthesis and

Indirect Biotic Effects and Feedbacks

Ecosystems interact with their physical and geochemical environment. The effect of ecosystems on the energy balance are categorized into two types of effects:

1. Biogeophysical effects modify components of the surface energy balance and the physical functioning of the climate system. These effects are strongest for terrestrial vegetation, which affects the energy balance over land in various ways: (a) the surface albedo of vegetated surfaces is generally darker than bare surfaces (Table 4), thereby enhancing absorption of solar radiation; (b) a heterogeneous canopy cover enhances the aerodynamic roughness of the surface, which enhances turbulent fluxes and frictional dissipation; (c) vegetation root systems enhance the ability to recycle soil moisture through transpiration, thereby affecting the latent heat flux; and (d) vegetated surfaces modulate the partitioning of sensible and latent heat through stomatal

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Figure 7 Dominant feedback processes that shape the response of the global energy balance and surface temperature to external change. The diagrams show the variables involved in four important feedback processes. The (+/-) signs at the arrows indicate positive/negative influences. (a) The thermal radiation feedback. An external change in forcing that would increase surface temperature would also increase the emission of long-wave radiation (a '+' influence). An increased emission would result in a lower surface temperature (a '-' influence). The enhanced emission of long-wave radiation therefore counteracts the initial change, resulting in a negative feedback loop. The same line of reasoning also applies for an external change that would reduce surface temperature. (b) The snow/ice albedo feedback. An external change that warms the surface reduces the presence of snow, lowers the surface albedo, thereby amplifying the warming (a positive feedback). (c) The water vapor feedback. An external change that warms the surface heats the lower atmosphere. Since warmer air can hold more moisture, this enhances surface evaporation and the amount of water vapor in the atmosphere. More water vapor results in a stronger atmospheric greenhouse effect, thereby amplifying the initial change (a positive feedback). (d) Two types of cloud feedbacks. Continuing from the water vapor feedback, more water vapor in the atmosphere can result in more clouds. Depending on the balance of increased cloud cover on shortwave reflection (path A) or increased greenhouse forcing (path B), cloud feedbacks can form both positive and negative feedback loops on surface temperature.

functioning. These effects have considerable effects on the surface energy and water balance and the overlying atmosphere (Figure 8) and result in two biogeophysical feedback processes (Figure 9).

2. Biogeochemical effects modify chemical cycling of elements and the atmospheric composition (Table 1). Some ofthese have important consequences for the global energy balance and atmospheric dynamics, such as

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Figure 8 Climatic differences of a 'desert world'. Annual mean differences in (a) near-surface air temperature, (b) precipitation, and (c) cloudiness between the simulated climate of a 'desert world' void of terrestrial vegetation and the simulated present-day climate. These climatic differences result from the effect of vegetation on surface albedo, aerodynamic surface roughness, and the depth of the rooting zone.

(a) carbon cycling affects the strength of the greenhouse effect, (b) oxygen concentrations affect stratospheric absorption of sunlight, and (c) the production of some compounds, such as dimethyl sulfide by marine algae, act as cloud condensation nuclei, thereby modifying cloud

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Figure 9 Vegetation feedbacks on the surface energy balance. The diagrams show the two major feedback loops by which vegetation directly affects the physical functioning of the surface energy balance. (a) The snow-masking feedback. An external change in forcing that would increase surface temperature in regions where temperature limits terrestrial productivity (such as the Arctic) increases the length of the growing season. A longer growing season would result in higher productivity, which extends the boreal forest cover in temperature-limited regions. Enhanced boreal forest cover masks the presence of snow at the surface, thereby lowering the surface albedo. This results in enhanced absorption of solar radiation, which amplifies the initial change, resulting in a positive feedback loop. (b) The water cycling feedback. An external change that results in enhanced precipitation in regions where water limits productivity (such asthesemiarid tropics) increases the length of the growing season, resulting in higher productivity. This extends vegetative cover, and thereby evapotranspiration, atmospheric moisture content, resulting in more precipitation. The initial change is hence amplified, resulting in a positive feedback.

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Figure 10 Emergence of temperature regulation in the conceptual 'Daisyworld' model. The 'Daisyworld' model is a conceptual model of a virtual world in which the planetary albedo is regulated by the population dynamics of black and white daisies. (a) The fractional cover of black and white daisies for different values of solar luminosity, expressed as the fraction of its present-day value. (b) The different proportions of daisies result in an overall planetary albedo that results in constant temperature conditions over a wide range of solar luminosity values.

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biospheric dynamics in Daisyworld is highly simplistic. Yet the challenge to find general organizing principles that can explain the interactions of the biosphere with the global energy balance remains an active research topic.

See also: Carbon Cycle; Climate Change 3: History and Current State; Coevolution of the Biosphere and Climate; Energy Flows in the Biosphere; Entropy and Entropy Flows in the Biosphere; Estuarine Ecohydrology; Evapotranspiration; Gaia Hypothesis; Radiation Balance and Solar Radiation Spectrum; Temperature Patterns; Water Cycle.

Figure 10 Emergence of temperature regulation in the conceptual 'Daisyworld' model. The 'Daisyworld' model is a conceptual model of a virtual world in which the planetary albedo is regulated by the population dynamics of black and white daisies. (a) The fractional cover of black and white daisies for different values of solar luminosity, expressed as the fraction of its present-day value. (b) The different proportions of daisies result in an overall planetary albedo that results in constant temperature conditions over a wide range of solar luminosity values.

Gaia Hypothesis

In the extreme form, strong negative biotic feedbacks on temperature can regulate the global energy balance into a state of homeostasis (i.e., no temperature sensitivity to external forcing). This has been suggested by the Gaia hypothesis of James Lovelock, stating that the atmosphere is regulated by and for the biosphere. This notion was originally motivated by the observation that the Earth's atmospheric composition is far from thermodynamic equilibrium, and is maintained in that state by the photo-synthesizing biota.

The conceptual 'Daisyworld' model was developed to demonstrate the possibility of global homeostasis. This model describes a world where the planetary albedo is determined by the fractions of black and white daisies, and of bare ground. Using equations of population dynamics and a temperature-dependent growth parame-trization, 'Daisyworld' demonstrates that homeostasis is a possible outcome of population dynamics coupled to the global energy balance (Figure 10).

However, the notions of Gaia and Daisyworld remain controversial. Surface temperatures in Earth's history have been far from constant, and the representation of

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