into their body; eventually they die and decompose, returning some of that carbon to the ground and some back to the atmosphere.
This carbon cycle ties together the atmosphere, the biosphere, and the geosphere. The atmosphere contains only about two-fifths as much carbon as does the biosphere and about one-fiftieth as much as the oceans, but enough carbon passes through the atmosphere to make CARBON DIOXIDE effect the second most important greenhouse gas after water vapor. There is just enough CARBON DIOXIDE effects to keep people alive without frying them to a crisp. Venus and Mars, on the other hand, have kept their carbon under much stable conditions. Nearly all of the carbon on Venus is suspended in the atmosphereÃ¢â‚¬â€about 350,000 times more carbon per gallon of air than on Earth. This thick soup of CARBON DIOXIDE bakes the planet with intense heat. Mars, on the other hand, is too frigid for carbon to stay up in the air. Most of the red planet's CARBON DIOXIDE lies frozen as dry ice at the planet's poles leaving a very weak greenhouse effect that makes even the sunniest spots colder than Antarctica.
If the earth's unique carbon cycle keeps people alive, it also leaves them extraordinarily vulnerable. CARBON DIOXIDE, more than any other gas, is directly tied to the lives and actions of human beings. If the world's populations grow, if people continue to build factories, if they burn or cut down forests, if they drive rather than walk to work, they create CARBON DIOXIDE. Every decision a person makes inevitably affects how much CARBON DIOXIDE is spewed into the atmosphere and how the greenhouse effect evolves. Consequently, CARBON DIOXIDE has become the primary focus of plans to curb global warming in the future. The ability to regulate how and when this gas is produced will determine whether it sustains or endangers future civilizations.
CARBON DIOXIDE was first discovered in the 17th century, by Flemish alchemist Johann Baptista Van Helmont. For the next 200 years, scientists attempted to trace the sources of CARBON DIOXIDE and its concentration in various areas. Eventually, a rough outline of CARBON DIOXIDE'S origins emerged: Humans, animals, and plants breathed it out; plants breathed it in; burning wood, paper, oil, and coal released it into the air; and smokestacks belched it into the sky in great blasts.
Until the 20th century, CARBON DIOXIDE was mainly of interest to scientists. It existed only in trace quantities in the atmosphereÃ¢â‚¬â€about 300 parts per millionÃ‚Â and seemed harmless to human beings. It was not until Svante Arhenius, at the turn of the century, finally connected CARBON DIOXIDE to the greenhouse effect that CARBON DIOXIDE suddenly took on larger significance, and even then to only a few visionary minds. If CARBON DIOXIDE is a greenhouse gas, Arhenius reasoned in the April 1896 issue of London, Edinburgh, and Dublin Philosophical Magazine, then increasing the level of CARBON DIOXIDE effects in the air must bring about a "change in the transparency of the worldÃ¢â‚¬Â.