Friday, March 16, 2007

Earths thermostat the Biosphere

As we previously showed Vernadsky postulated the biosphere regulates the transformation of energy on the planet.

The Biosphere is devoted to calculations on the fraction of total solar energy used by photosynthesizing organisms to produce biomass.In the context of these calculations Vernadsky argues that it is an inherent characteristic of the biosphere that living matter is distributed on the Earth’s surface in a way that solar radiation is completely captured.In order to optimize the utilization of solar energy and to create a sufficient surface,green biomass appears in different forms in different biotopes.On land,plants have to develop three-dimensional structures in order to create a sufficiently thick film for optimal use of solar radiation.In oceans,primary production is dominated by phytoplankton because it can easily distribute over the depth of the photic zone.

Arguments against solar variance is on the basis of minimal changes to the overall energy transfer in the sun-earth coupling, however the changes are dominant in the spectra that are transformers of cellular growth i.e. the biosphere attenuation and amplification.

The total solar irradiance, or TSI, along with Earth's global average albedo, determines Earth's global average equilibrium temperature. Because of selective absorption and scattering processes in the Earth's atmosphere, different regions of the solar spectrum affect Earth's climate in distinct ways. Approximately 20 - 25 % of the TSI is absorbed by atmospheric water vapor, clouds, and ozone, by processes that are strongly wavelength dependent. Ultraviolet radiation at wavelengths below 300 nm is completely absorbed by the Earth's atmosphere and contributes the dominant energy source in the stratosphere and thermosphere, establishing the upper atmosphere's temperature, structure, composition, and dynamics. Even small variations in the Sun's radiation at these short wavelengths will lead to corresponding changes in atmospheric chemistry. Radiation at the longer visible and infrared wavelengths penetrates into the lower atmosphere, where the portion not reflected is partitioned between the troposphere and the Earth's surface, and becomes a dominant term in the global energy balance and an essential determinant of atmospheric stability and convection.

Assuming climate models include a realistic sensitivity to solar forcing, the record of solar variations implies a global surface temperature change on the order of only 0.2° C. However, global energy balance considerations may not provide the entire story. Some recent studies suggest that the cloudy lower atmosphere absorbs more visible and near infrared radiation than previously thought(25% rather than 20%), which impacts convection, clouds, and latent heating .Also, the solar ultraviolet, which varies far more than the TSI, influences stratospheric chemistry and dynamics, which in turn controls the small fraction of ultraviolet radiation that leaks through to the surface.

Vernadsky derives an expression for the “kinetic geochemical energy of living matter”. The kinetic geochemical energy of an organism is related to its mass and its speed of transmission. The latter depends on the size of the organism and the optimal number of generations per day and is normalized to the surface area of the Earth. Vernadsky frequently refers to the geochemical energy in The Biosphere especially to emphasize the enormous biogeochemical potential of microorganisms.

The interaction with and effect of solar radiation on Earth’s energy budget is strongly dependent on the electromagnetic spectrum. Planetary albedo is only relevant to short-wave radiation; hence, all far-red radiation impinging at the top of Earth’s atmosphere will either be absorbed by gases in the atmospheric column, or will be transmitted to the surface.

Some of the short-wavelength radiation impinging on the top of the atmosphere is scattered and reflected back to space,while the majority penetrates to the surface. Over 70% of Earth’s surface is covered by liquid water that absorbs about 95% of incident solar irradiance.In its upper 3 m the ocean contains the equivalent heat capacity of the entire atmosphere of the planet.As the average depth of the oceans is about 3800 m, this geophysical fluid acts as a huge heat storage system for the planet.That is,the oceans do not directly affect the radiation budget of the planet, but rather, affect the time constant by which the planet “experiences” changes in radiation.

As in the atmosphere, the direct physical interactions between solar radiation and the ocean are wavelength dependent. Water itself effectively absorbs all incident infrared solar radiation (e.g., Morel and Antoine, 1994),and this direct radiative transfer process provides roughly half of the heat to the ocean surface waters. An example of potential interactions between solar radiation and physical and biological processes is, where the following sequence is illustrated:

(1)forcing of the upper ocean physical condition through the input of solar radiation, including light, heat, and indirectly momentum at the ocean surface;

(2)upper ocean physical responses, including stratification and turbulent mixing that result in

(3) phytoplankton vertical and horizontal motions, which, in turn, lead to

(4) feedbacks on distributions of pigments and photosynthetic available radiation (PAR),

(5) modulation of the upper ocean heating via phytoplankton, and their associated
optical properties.

The balance between primary production and grazing determine the concentration of phytoplankton at any moment in time and both processes must be considered in biological–physical interactions.

The interactions among these processes occur on many time and space scales.Long-term changes (millennia to millions of years) in ocean circulation are driven by changes in radiative forcing resulting from orbital variations, albedo feedbacks, and continental configuration). Short-term changes (seconds to decades) are driven by atmospheric conditions (e.g., aerosols, cloud cover, albedo, and ozone concentration) and thermal contrasts between continents and the oceans and from the equator to the poles. Together, both long- and short-term variations in radiative transfer of broadband, as well as visible solar energy, determine the depth of the upper mixed layer, turbulent kinetic energy, and the vigor of large-scale oceanic circulation, which ultimately determines, on a global scale, the distribution and productivity of phytoplankton.

Light entering the ocean has only two possible fates; it can be absorbed or scattered. Within this context it is convenient to classify bulk optical properties of the ocean as either inherent or apparent. Inherent optical properties (IOPs) depend only on the medium and are independent of the ambient light field.

The color of the ocean is strongly influenced by suspended particulate materials, dissolved constituents, and bubbles (Kirk, 1992). The absorption and scattering cross sections of each of the components affect the spectral quality of light penetrating through the water column and the upwelling irradiance spectrum. To first order, the photosynthetic pigments of phytoplankton are the major constituents that modify open ocean color from the color blue. These pigments evolved to absorb and transform energy.

One of the clearest examples of biology affecting physical processes is the modulation of upper ocean heating rates by variability in phytoplankton and their associated pigment concentrations and related optical characteristics. Because phytoplankton absorb visible radiation in spectral regions that are relatively transparent for water itself, these photosynthetic organisms are potentially capable of altering the upper ocean heat budget. The extent to which this occurs depends on the concentration and vertical distribution of pigments within the water column, as well as the incident spectral irradiance. Intuitively, one can understand the effect by considering two bodies of water, lying side by side—swimming pools, for example. If one adds black ink to one pool while keeping the second clear, the darker pool will absorb virtually all of the incident solar radiation and become warmer faster. This effect is used to heat water in rooftop solar systems for homes. Similarly, the addition of phytoplankton to the upper ocean can have measurable effects on the rate of heating of the euphotic zone, with consequences for the depth of the upper mixed layer and vertical eddy diffusivity (e.g., Lewis et al., 1983, 1988).

Direct measurements of bio-optical as well as physical variables have been made in the warm-water pool of the western Pacific (Siegel et al., 1995; Ohlmann et al., 1998). This work is supportive of the previous assertions concerning the importance of the penetrative component of solar radiation and more generally biogeochemical processes. For example, it was determined that common values of the penetrative solar flux are about 23 W m−2 at 30 m (the climatological mean mixed layer depth), and thus a large fraction of the climatological mean net air–sea flux of about 40 W m−2. Synoptic scale forcing (e.g., wind bursts) were found to lead to tripling of phytoplankton pigment concentrations and a reduction in penetrative heat flux of 5.6 W m−2 at 30 m, or a biogeochemically mediated increase in the radiant heating rate of 0.138C/ month. In-depth analysis of the radiant heating and parameterizations of light attenuation for this experiment are given in Ohlmann et al. (1998)


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