Nature’s economy The earth and its atmospheric envelope constitute a majestic system, maintained in stability by a web of physical and chemical interactions, all driven ultimately by energy from the sun. Radiation from the sun warms the earth making life possible; differential warming between the equator and the poles drives large scale convective circulation in the oceans and the atmosphere determining regional climates, seasons and rainfall patterns, storms and droughts. Energy from the sun evaporates water from the surface of the oceans which becomes condensed in clouds and precipitated back to earth as rain or snow to feed terrestrial plants and animals, enter streams and groundwater systems, and eventually return to the ocean. Light energy from the sun drives processes of photosynthesis upon which life on earth depends. Green plants on land combine the simple chemical molecules of carbon dioxide and water with energy from sunlight to produce carbohydrates (such as glucose) and oxygen:
6CO2 + 6H2O + energy →← C6H12O6 + 6O2
In a reversal of this process, the energy stored in carbohydrates when combined with oxygen from the air, is released as food in both plants and animals in processes collectively referred to as respiration. Plants and animals die and are decomposed by soil micro-organisms and ultimately oxidized to carbon dioxide, to water, and to other simple molecules. Nitrogen, the major component of the earth’s atmosphere, is ‘fixed’ by nitrogen-fixing bacteria in soil, and in the form of nitrate is available for absorption by plants and the formation of amino acids and proteins, which become available as food throughout the food chain. When plants and animals excrete wastes, nitrogen is returned to the atmosphere, and when they die decomposer organisms break down nitrogenous molecules into ammonia and nitrate which can again be fixed by nitrifying bacteria, or further broken down to elementary nitrogen and returned to the atmosphere. Similar processes occur in the upper layer of the seas, where photosynthesis is mediated by algae and phytoplankton, marine organisms fix nitrogen, and the
products of these processes are provided for the growth and development of marine animals and plants. Micro-organisms constitute the main biomass of the seas, and when they die they rapidly dissolve to soluble organic matter, although some living matter sinks to the ocean depths. These biological processes have cycle times that vary from hours or days to years. In contrast, geological processes take place on timescales counted from tens if not hundreds of thousands, or even millions, of years. Inorganic materials of the earth have been thrust up or down by large scale tectonic processes, and new rock masses formed by the laying down of marine sediments, by volcanic activity, and by high temperature and pressure at depth introducing metamorphic changes to existing rocks. Changes in sea level followed by the laying down of sediments on top of plant and animal remains have provided the total stock of the world’s coal, petroleum, and natural gas; metamorphic processes have concentrated metallic ores in veins and reefs; the weathering of rocks by water and climatic processes have produced soils suitable for the growth of plants. The continuity over time of the fossil record in sedimentary rocks has indicated that, overall and despite some fluctuations, the physical and chemical conditions of the earth’s surface and atmosphere have maintained these conditions since the establishment of the original oxygen-respiring species about 600 million years ago (Press and Siever 1974).