Energy Budgets

An energy budget describes the ways in which energy is transformed from one state to another within some defined system, including an analysis of inputs, outputs, and changes in the quantities stored. Ecological energy budgets focus on the use and transformations of energy in the biosphere or its components.

Solar electromagnetic radiation is the major input of energy to Earth. This external source of energy helps to heat the planet, evaporate water, circulate the atmosphere and oceans, and sustain ecological processes. Ultimately, all of the solar energy absorbed by Earth is re-radiated back to space, as electromagnetic radiation of a longer wavelength than what was originally absorbed. Earth maintains a virtually perfect energetic balance between inputs and outputs of electromagnetic energy.

Earth's ecosystems depend on solar radiation as an external source of diffuse energy that can be utilized by photosynthetic autotrophs, such as green plants, to synthesize simple organic molecules such as sugars from inorganic molecules such as carbon dioxide and water. Plants use the fixed energy of these simple organic compounds, plus inorganic nutrients, to synthesize an enormous diversity of biochemicals through various metabolic reactions. Plants utilize these biochemicals and the energy they contain to accomplish their growth and reproduction. Moreover, plant biomass is directly or indirectly utilized as food by the enormous numbers of heterotrophic organisms that are incapable of fixing their own energy. These organisms include herbivores that eat plants, carnivores that eat animals, and detritivores that feed on dead biomass.

Worldwide, the use of solar energy for this ecological purpose is relatively small, accounting for much less than 1% of the amount received at Earth's surface. Although this is a quantitatively trivial part of Earth's energy budget, it is clearly very important qualitatively, because this is the absorbed and biologically fixed energy that subsidizes all ecological processes.