a - Ecosystem Ecology: Carbon Cycle Summary

Ecosystem Ecology and the Carbon Cycle

An ecosystem comprises a community of organisms and their physical environment, influencing each other. Ecosystem ecology explores the links between the physical and biological aspects of Earth, focusing on biogeochemical cycles.

The Carbon Cycle

The carbon cycle is a network of biological and physical processes that moves carbon among rocks, soil, oceans, air, and organisms. Photosynthesis and respiration drive carbon cycling. Photosynthetic organisms remove approximately 210,000,000,000 metric tons of carbon from the atmosphere annually, converting carbon dioxide into sugars. Cellular respiration uses oxygen to oxidize organic molecules, releasing carbon dioxide and water. Carbon also remains in biological systems through consumption.

Annual Patterns of Atmospheric Carbon Dioxide

Charles Keeling's monitoring in 1958 revealed seasonal oscillations in carbon dioxide concentrations, increasing in winter and spring and decreasing in early fall (Northern Hemisphere seasons). This is due to terrestrial geography and the Earth's axial tilt. During winter months plants are not photosynthesizing, allowing carbon dioxide to accumulate. Increased sunlight in late spring through early fall boosts photosynthesis, drawing down atmospheric carbon dioxide. There has been a steady increase in atmospheric carbon over the past 65 years.

Short-Term vs. Long-Term Carbon Cycle

The short-term carbon cycle is driven by respiration, photosynthesis, human activities, geologic inputs (volcanoes, mid-ocean ridges), and geologic removal (weathering of rocks). Keeling's work shows a 30% increase in atmospheric carbon dioxide in the past 65 years which correlate to the industrial revolution. Scientists use ice cores to analyze gas concentrations and compare them to the Keeling curve. Isotopic composition of atmospheric carbon dioxide compared to increased burning of fossil fuels reveals carbon isotopes, carbon 12, carbon 13, and carbon 14.

Carbon Isotopes

The ratio of carbon 13 to carbon 12 has decreased, indicating added carbon dioxide has less carbon 13. Fossil fuels have the correct ratio to account for isotopic changes in atmospheric carbon dioxide. Humans add about 4,000,000,000 metric tons of carbon to the atmosphere annually. Half of the carbon dioxide released by human activities is stored in the oceans.

Long-Term Carbon Cycle and Reservoirs

Long-term carbon dioxide concentrations correlate with temperature changes. Carbon dioxide is a greenhouse gas that can dramatically change temperature. Historical data correlate with the growth and decay of continental ice sheets. Carbon reservoirs include atmosphere, surface and deep ocean waters, living organisms, soils, and sediments. Sediments and sedimentary rock are the largest carbon reservoir. Fluxes are the rates at which carbon flows from one reservoir to another, influenced by photosynthesis, respiration, and human activity. Sensitivity of reservoirs depends on size relative to flux. Accumulation of carbon in sediments occurs through weathering, burial of organic matter, sedimentation, and burial of organic matter in ocean sediments. Release occurs through oxidation of coal, oil, and volcanism. Human use of fossil fuels greatly outweighs these geological carbon outputs.