Energy Budgets

As noted previously, energy can be transformed among its various states. For example: (a) electromagnetic energy can be absorbed by a dark object and converted to thermal kinetic energy, resulting in an increased temperature of the absorbing body; (b) gravitational potential energy of water high on a plateau is transformed into the kinetic energy of moving water and heat at a waterfall, or it can be mobilized by humans to drive a turbine and generate electrical energy; and (c) solar electromagnetic radiation can be absorbed by the chlorophyll of green plants, and some of the absorbed energy can be converted into the chemical potential energy of sugars, and the rest converted into heat.

All transformations of energy must occur according to certain physical principles, known as the laws of thermodynamics. These are universal laws, which means that they are always true, regardless of circumstances. The first law states that energy can undergo transformations among its various states, but it is never created or destroyed, so the energy content of the universe remains constant. A consequence of this law for energy budgets is that there must always be a zero balance between the energy inputs to a system, the energy outputs, and any net storage within the system.

The second law of thermodynamics states that transformations of energy can only occur spontaneously under conditions in which there is an increase in the entropy of the universe. (Entropy is related to randomness of the distributions of matter and energy). For example, Earth is continuously irradiated by solar radiation, mostly of visible and near-infrared wavelengths. Some of this energy is absorbed, which heats the surface of Earth. The planet cools itself in various ways, but ultimately this is done by radiating its own electromagnetic radiation back to space, as longer-wave infrared radiation. The transformation of relatively short-wave solar radiation into the longer-wave radiation emitted by Earth represents a degradation of the quality of the energy, and an increase in the entropy of the universe.

A corollary, or secondary proposition of the second law of thermodynamics is that energy transformations can never be completely efficient, because some of the initial content of energy must be converted to heat so that entropy can be increased. Ultimately, this is the reason why no more than about 30% of the energy content of gasoline can be converted into the kinetic energy of a moving automobile, and why no more than about 40% of the energy of coal can be transformed into electricity in a modern generating station. Similarly, there are upper limits to the efficiency by which green plants can photo-synthetically convert visible radiation into biochemicals, even in ecosystems in which ecological constraints related to nutrients, water, and space are optimized.

Interestingly, plants absorb visible radiation emitted by the Sun, and use this relatively dispersed energy to fix simple inorganic molecules such as carbon dioxide, water, and other nutrients into very complex and energy-dense biochemicals. The biochemicals of plant biomass are then used by heterotrophic organisms to synthesize their own complex biochemicals. Locally, these various biological syntheses represent energy transformations that substantially decrease entropy, rather than increase it. This is because relatively dispersed solar energy and simple compounds are focused into the complex biochemicals of living organisms. Are biological transformations not obeying the second law of thermodynamics?

This seeming physical paradox of life can be successfully rationalized, using the following logic: The localized bio-concentrating of negative entropy can occur because there is a constant input of energy into the system, in the form of solar radiation. If this external source of energy was terminated, then all of the negative entropy of organisms and organic matter would rather quickly be spontaneously degraded, producing heat and simple inorganic molecules, and thereby increasing the entropy of the universe. This is why life and ecosystems cannot survive without continual inputs of solar energy. Therefore, the biosphere can be considered to represent a localized island, in space and time, of negative entropy, fueled by an external (solar) source of energy. There are physical analogues to these ecological circumstances—if external energy is put into the system, relatively dispersed molecules of gases can be concentrated into a container, as occurs when a person blows energetically to fill a balloon with air. Eventually, however, the balloon pops, the gases re-disperse, the original energy input is converted into heat, and the entropy of the universe is increased.