Life and Biogeochemical Cycles:
Life on earth is inextricably linked to climate through a variety of interacting cycles and feedback loops. In recent years there has been a growing awareness of the extent to which human activities, such as deforestation and fossil fuel burning, have directly or indirectly modified the biogeochemical and physical processes involved in determining the earth's climate. These changes in atmospheric processes can disturb a variety of the ecosystem services that humanity depends upon. In addition to helping to maintain relative climate stability and a self-cleansing, oxidizing environment, these services include protection from most of the sun's harmful ultraviolet rays, mediation of runoff and evapotranspiration (which affects the quantity and quality of fresh water supplies and helps control floods and droughts), and regulation of nutrient cycling among others.

Before further discussion of these services, it is important to review briefly how life and climate interact. The transport and transformation of substances in the environment, through life, air, sea, land, and ice, are known collectively as biogeochemical cycles. These global cycles include the circulation of certain elements, or nutrients, upon which life and the earth's climate depend. One way that climate influences life is by regulating the flow of substances through these biogeochemical cycles, in part through atmospheric circulation. Water vapor is one such substance. It is critical for the survival and health of human beings and ecological systems and is part of the climatic state. When water vapor condenses to form clouds, more of the sun's rays are reflected back into the atmosphere, usually cooling the climate. Conversely, water vapor is also an important greenhouse gas in the atmosphere, trapping heat in the infrared part of the spectrum in the lower atmosphere. The water or hydrologic cycle intersect with most of the other element cycles, including the cycles of carbon, nitrogen, sulfur, and phosphorus, as well as the sedimentary cycle. The processes involving each one of these elements may be strongly coupled with that of other elements, and ultimately, with important regional and global scale climatic or ecological processes.


Managing and finding solutions to many of the important environmental problems facing humanity begins with understanding and integrating biogeochemical cycles and the scales at which they operate. Examples of these links include world climate and the potential threat of global climate change; agricultural productivity and its strong reliance on climatic factors including temperature and precipitation, and on the availability of nutrients; the cleansing of toxics in soils and streams through precipitation and runoff; acid precipitation and the perturbation of ecosystem processes; the depletion of stratospheric ozone and its potential threat to human health and the food chain; and the often destructive interaction with natural cycles of other manmade compounds such as pesticides and synthetic hormones.

The Hydrologic and Sedimentary Cycles:
While the total amount of water found on earth may seem huge, the amount of precipitating freshwater available to people is a tiny fraction of this total. Earth's renewable supply of water is continually distilled and distributed through the hydrological cycle. It falls from the sky as precipitation, collects in lakes, rivers and oceans or seeps into the ground, and eventually evaporates or transpires, accumulating as water vapor in clouds, ready to begin the sun powered cycle again. Water is transferred to the air from the leaves of plants primarily from a process called transpiration. This, combined with evaporation from bodies of water and the soil, is known as evapotranspiration. Evaporation of ocean water is about six times as large globally as evapotranspiration on land, although in the centers of continents evapotranspiration may be the main local source of water vapor. Changes in the global climate may cause changes in the hydrologic cycle. Increases in temperature and evaporation are expected to cause increases in precipitation, which may further affect runoff and soil moisture, and eventually influence vegetation patterns and world agriculture.
The sedimentary cycle is tied to the hydrological cycle through precipitation. Water carries materials from the land to the oceans, where they can be deposited as sediments. On a shorter time-scale, the sedimentary cycle includes the processes of physical or chemical erosion, nutrient transport, and sediment formation for which water flows are mostly responsible. On a geologically longer time-scale, the processes of sedimentation, chemical transformation, uplift, sea floor spreading, and continental drift operate. Both the hydrological and sedimentary cycles are intertwined with the distribution of the amounts and flows of six important elements - hydrogen, carbon, oxygen, nitrogen, phosphorus, and sulfur. These elements, or macronutrients, combine in various ways to make up more than 95% of all living things. Appropriate quantities of them in proper balance and in the right places are required to sustain life. Although great stocks of all of these nutrients exist in the earth's crust in different (but not always accessible) forms, at any one time the natural supply of these vital elements is limited. Therefore, they must be recycled for life to regenerate continuously. We describe three of these cycles critical to important ecosystem services in the following sections.

The Nitrogen Cycle:
Nitrogen exist in a variety of forms in natural systems and its compounds are involved in numerous biological and abiotic processes. Nitrogen, in its gaseous form of N2, makes up almost 80 percent of the atmosphere. This constitutes the major storage pool in the complex cycle of nitrogen through ecosystems. Some of this gas is converted in the soils and waters to ammonia (NH3), ammonium (NH4+), or many other nitrogen compounds. The process is known as nitrogen fixation, and, in the absence of industrial fertilizers, is the primary source of nitrogen to all living things. Biological nitrogen fixation is mediated by special nitrogen-fixing bacteria and algae. On the land, these bacteria often live on nodules on the roots of legumes where they use energy from plants to do their work. In freshwater and, possibly, in marine systems, cyanobacteria fix nitrogen. Once nitrogen has been fixed in the soil or aquatic system, it can follow two different pathways. It can be oxidized for energy in a process called nitrification or assimilated by an organism into its biomass in a process called ammonia assimilation.

Plants incorporate the appropriate forms of fixed nitrogen into their tissues through their root systems. The plants then use it to manufacture amino acids and convert it into proteins. Nitrogen, fixed as proteins in the bodies of living organisms, eventually returns via the nitrogen cycle to its original form of nitrogen gas in the air. The process of denitrification starts when plants containing the fixed nitrogen are either eaten or die. Fixed nitrogen products in dead plants, animal bodies, and animal excreta encounter denitrifying bacteria that undo the work done by the nitrogen-fixing bacteria. Generally, N2 is the end-product of denitrification, but nitrous oxide (N2O) is also produced in much smaller quantities (up to ten percent).

The disruption of the nitrogen cycle by human activity plays an important role in a wide-range of environmental problems ranging from the production of tropospheric (lower atmosphere) smog to the perturbation of stratospheric ozone and the contamination of groundwater. Nitrous oxide, for example, is a greenhouse gas like carbon dioxide and water vapor that can trap heat near the earth's surface. It also destroys stratospheric ozone. Eventually nitrous oxide in the stratosphere is broken down by ultraviolet light into nitrogen dioxide (NO2) and nitric oxide (NO), which can catalytically reduce ozone. Nitrogen oxides are chemically transformed back to either N2 or to nitrate or nitrite compounds, which may later get used by plants after they are washed by the rain back to the earth's surface. Nitrate rain is acidic and can cause ecological problems as well as serve as a fertilizer to vegetation.

The Sulfur Cycle:
Another example of a major biogeochemical cycle of significance to climate and life is the sulfur cycle. Living things require certain safe, low levels of this nutrient. The sulfur cycle can be thought of as beginning with the gas sulfur dioxide (SO2) or the particles of sulfate (SO4=) compounds in the air. These compounds either fall out or are rained out of the atmosphere. Plants take up some forms of these compounds and incorporate them into their tissues. Then, as with nitrogen, these organic sulfur compounds are returned to the land or water after the plants die or are consumed by animals. Bacteria are important here as well since they can transform the organic sulfur to hydrogen sulfide gas (H2S). In the oceans, certain phytoplankton can produce a chemical that transforms to SO2 that resides in the atmosphere. These gases can re-enter the atmosphere, water, and soil, and continue the cycle.


In its reduced oxidation state, the nutrient sulfur plays an important part in the structure and function of proteins. In its fully oxidized state, sulfur exists as sulfate and is the major cause of acidity in both natural and polluted rainwater. This link to acidity makes sulfur important to geochemical, atmospheric, and biological processes such as the natural weathering of rocks, acid precipitation, and rates of denitrification. Sulfur is also one of the main elemental cycles most heavily perturbed by human activity. Estimates suggest that emissions of sulfur to the atmosphere from human activity are at least equal or probably larger in magnitude than those from natural processes. Like nitrogen, sulfur can exist in many forms: as gases or sulfuric acid particles. Sulfuric acid particles contribute to the polluting smog that engulfs some industrial centers and cities where many sulfur containing fuels are burned. Such particles floating in air (known as sulfate aerosols) can cause respiratory diseases or cool the climate by reflecting some extra sunlight to space.


The lifetime of most sulfur compounds in the air is relatively short (e.g. days). Superimposed on these fast cycles of sulfur are the extremely slow sedimentary-cycle processes or erosion, sedimentation, and uplift of rocks containing sulfur. In addition, sulfur compounds from volcanoes are intermittently injected into the atmosphere, and a continual stream of these compounds is produced from industrial activities. These compounds mix with water vapor and form sulfuric acid smog. In addition to contributing to acid rain, the sulfuric acid droplets of smog form a haze layer that reflects solar radiation and can cause a cooling of the earth's surface. While many questions remain concerning specifics, the sulfur cycle in general, and acid rain and smog issues in particular are becoming major physical, biological, and social problems.




The Carbon Cycle:

Carbon, the key element of all life on earth, has a complicated biogeochemical cycle of great importance to global climate change. The carbon cycle includes four main reservoirs of stored carbon: as CO2 in the atmosphere; as organic compounds in living or recently dead organisms; as dissolved carbon dioxide in the oceans and other bodies of water; and as calcium carbonate in limestone and in buried organic matter (e.g. natural gas, peat, coal, and petroleum). Ultimately, the cycling of carbon through each of these reservoirs is tightly tied to living organisms.


Plants continuously extract carbon from the atmosphere and use it to form carbohydrates and sugars to build up their tissues through the process of photosynthesis. Animals consume plants and use these organic compounds in their metabolism. When plants and animals die, CO2 is formed again as the organic compounds combine with oxygen during decay. Not all of the compounds are oxidized, however, and a small fraction is transported and redeposited as sediment and trapped where it can form deposits of coal and petroleum. Carbon dioxide from the atmosphere also dissolves in oceans and other bodies of water. Aquatic plants use it for photosynthesis and many aquatic animals use it to make shells of calcium carbonate (CaCO3). The shells of dead organisms (e.g. phytoplankton or coral reefs) accumulate on the sea floor and can form limestone that is part of the sedimentary cycle. The relevant time-scales for these different processes vary over many orders of magnitude, from millions of years for the rock cycle and plate tectonics to days and even seconds for processes like photosynthesis and air-sea exchange.


CO2 is a trace gas in the earth's atmosphere that has a substantial effect on earth's heat balance by absorbing infrared radiation. This gas, like water vapor (H2O), CH4, and N2O, has a strong greenhouse effect. Life can alter the global concentration of CO2 over very short time periods. During the growing season, CO2 decreases in the atmosphere of the temperate latitudes due to the increasing sunlight and temperatures which help plants to increase their rate of carbon uptake and growth. During the winter dormant period, more CO2 enters the atmosphere than is removed by plants, and the concentration rises because plant respiration and the decay of dying plants and animals occurs faster than photosynthesis. The land mass in the northern hemisphere is greater than in the southern hemisphere, thus the global concentration of CO2 tracks the seasonality of terrestrial vegetation in the northern hemisphere.