Change and Cycles
"Where Land, Air, and Water Meet"
The Earth is often compared to a gigantic greenhouse. Energy in the form of sunlight passes through the planet's atmosphere. Some of the energy that strikes the land and water is reflected back into space. Most of the rest is absorbed by the land and water, converted to heat, and radiated back into the atmosphere. This radiated energy is mostly absorbed by carbon dioxide (CO2) and other atmospheric gases, which act much like the glass in a greenhouse, warming the atmosphere.
Since the middle 1800's, scientists have wondered about the importance of CO2 as a "greenhouse gas" in the regulation of Earth's climate. According to the "greenhouse effect" theory, increasing levels of CO2 in the atmosphere will trap more and more heat, raising the planet's overall temperature and affecting regional climates, sea levels, distribution of arable land, animal and human habitats, and more.
The widespread burning of fossil fuels-coal, oil, and natural gas-releases greenhouse gases, including CO2, into the atmosphere. Concern about the possible climatic effects of increasing atmospheric CO2 levels was voiced in 1957 by Roger Revelle, then director of the Scripps institute of Oceanography in La Jolla, California. He wrote, " ... human beings are now carrying out a large-scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future. Within a few centuries we are returning to the atmosphere and oceans the concentrated organic carbon stored in sedimentary rocks over hundreds of millions of years."
In the late 1950's scientific information on the levels of CO2 in the atmosphere was sketchy. So Revelle and others commenced an international effort to monitor CO2 concentrations in the atmosphere from stations in Alaska, Antarctica, and Hawaii, where measurements would not be affected much by nearby sources of greenhouse gases such as factories.
The Mauna Loa, Hawaii, station has been operating since November 1958. The record compiled there over more than 30 years reveals some interesting insights into the global carbon cycle. A chart of changing CO2 patterns is reproduced on the back of the poster. Notice two patterns: a sawtooth pattern of seasonal variations of CO2 of about 5 parts per million [ppm] superimposed over a long-term increase of about 1.3 ppm per year.
This long-term increase in CO2 levels raises new questions. What are the sources of this "new" CO2? How much comes from burning fossil fuels? How much may be caused by the clearing and burning of forests? What are the "sinks," or storage places, for the "excess" CO2, and how big are they? Is the increase in atmospheric CO2 changing the global climate?
Such new tools as computer simulation models and satellite images are used to better estimate the sources and sinks of the carbon cycle. CO2 dissolves readily in sea water, where it is absorbed by microscopic plants known collectively as phytoplankton. When phytoplankton die, some are incorporated into the ocean bottom sediments, trapping their carbon for millions of years. Sensors borne on satellites can measure electromagnetic energy reflected from the Earth, providing images of the extent and vigor of plant growth. These satellite images are being used to map phytoplankton activity at the ocean's surface and estimate the role of the oceans as a sink for atmospheric carbon.
On land, plants absorb and store CO2. A large part of the carbon that is fixed in the leaves of plants, as they grow, is released back into the atmosphere when the plants die and decay. During the growing season, plants take in CO2 and give off oxygen. In the dormant winter months, no CO2 is taken in. This causes a seasonal oscillation of CO2 levels in the atmosphere, with maximum concentrations in early spring and minimum ones in the fall. In the longer term, carbon is stored either as wood, as humus in soil, or in certain rocks. Satellite sensors can be used to better delineate the world's forests and grasslands as both sources and sinks for CO2.
All this information is being integrated into computer models to help scientists simulate how the oceans, the biosphere, and the atmosphere interact in the global carbon cycle. The idea is to learn more about effects of human activity on carbon levels and, thus, on climate.
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