BIOL/CSES 4684: Carbon Cycle

BIOGEOCHEMICAL CYCLES

ENVIRONMENTAL MICROBIOLOGY

BIOL/CSES/CEE/ENSC 4164

THE CARBON CYCLE

Introduction to the Carbon Cycle

Most of the carbon within organisms comes from the carbon dioxide (CO₂) in the air. The atmosphere is 0.03 mol % in CO₂. However, the greatest physical reservoir of carbon is not atmospheric carbon dioxide but instead is located in the Earth's crust and is not easily accessible to biological organisms. There are essentially four reactions involving the carbon cycle that must balance in nature. These are the uptake and evolution of carbon dioxide (CO₂) and the uptake and evolution of methane (CH₄).

The following figure shows the major sinks and reservoirs within the Global Carbon Cycle. Units are 10¹⁵ gm (or one PETAGRAM) of carbon per year.

ANNUAL RATES OF CARBON FLUX	GLOBAL WARMING
CO₂ UPTAKE	CO₂ EVOLUTION
DEGRADATION RATES	CH₄ UPTAKE AND EVOLUTION

Some Imbalances in the Carbon Cycle Caused by H. sapiens

1. Coal formation occurs very slowly, on a geological time scale. On the other hand, the burning of fossil fuels by man happens very fast, and the rate of utilization of these resources is increasing. As a result, the amount of carbon dioxide in the atmosphere is increasing.

2. Plants growth has been stimulated by the "green revolution" and by this additional carbon dioxide in the atmosphere. In addition, deforestation of land has accelerated in this century, producing yet more decaying plants. Plant decay releases methane and even more carbon dioxide into the atmosphere.

3. Agricultural ruminants, such as cattle, eat grasses and grains (grown on deforested lands that are no longer available to fix the increasing carbon dioxide in the air!). Together with symbiotic microorganisms, they digest the grasses and grains, forming more carbon dioxide and, in addition, methane.

As a result of these and other events, the levels of carbon dioxide and methane in the atmosphere have increased. For instance, there has been a 2% increase in the atmospheric methane in the last few years alone.

Why is this noteworthy? Much of the infrared radiation (IR) that comes to Earth from the sun does not reach the surface of the planet because it bounces away. Gases like carbon dioxide and methane trap IR and prevent it from radiating away from the Earth. This is known as the greenhouse effect. Without this phenomenon, the average temperature of the planet would be -18^oC. As a result of the increasing levels of CO₂ and CH₄ in the atmosphere, more IR is being "captured" and not bouncing away from the planet. The Earth is getting warmer! Increased levels of methane are especially dangerous because this gas absorbs five times more IR than carbon dioxide. Methanotrophs oxidize methane, but they are simply not efficient enough to use up methane in the concentrations found in the atmosphere. Current research is focused on manipulating these organisms to increase their affinity for methane and their rates of methane utilization.

This link includes some graphics (not very good quality) that demonstrate what are believed to be the people-associated causes of GLOBAL WARMING.

URL http://www.filebox.vt.edu/users/chagedor/biol_4684/Cycles/Ccycle.html
Last Updated 1/14/2008