It's hard to find a reasonably simple, yet informative, diagram of the carbon cycle, but we searched far and wide to find this excellent example. The red lines are the ones to pay attention to. They show the "anthropogenic flux," which is a fancy way to say that humans are responsible for the movements of carbon in shown in red.

We can think of the carbon that's being absorbed (downward-pointing arrows) and released (upward-pointing arrows) as carbon dioxide molecules, since CO₂ is what's released by burning of fuels, and CO₂ is what's removed from the atmosphere by photosynthesis.

If you add all the sources of CO₂ (upward arrows) and subtract all the sinks (downward arrows; places where CO₂ is removed from the atmosphere), you'll find that there's a net increase in the atmosphere of about 3.1 gigatonnes of CO₂ per year. That's almost 7 trillion pounds a year, or a little over 3 pounds of CO₂ per day for every man, woman, and child on Earth. Carbon dioxide, of course, is a greenhouse gas -  find out more how CO₂ contributes to global warming.

Notice that humans are involved in two activities that affect the carbon cycle: changes in land-use and the burning of fossil fuels. One particularly worrisome land-use change is conversion of tropical rainforest to palm oil plantations.

We can make changes to reduce greenhouse gas emissions in both arenas. Reducing conversion of forests to farmland and reducing the conversion of farmland to cities helps reduce greenhouse gas production.

This is actually pretty important, since the destruction of forests and farmland not only destroys plants that remove CO₂ from the atmosphere, but the dead plants and disturbed soil actually release large amounts of CO₂ as decay happens. Unfortunately, the use of some kinds of biofuels, like biodiesel made from palm oil, requires the destruction of tropical rainforests, which are some of the best CO₂-removing habitats around.

We use carbon-containing molecules for energy. This includes both food that we eat and fuel that we burn. The amount of CO₂ that our bodies produce is tiny, about 2 pounds per day -- fairly discouraging for those of us who are trying to lose weight, since lost weight is turned into CO₂ and exhaled.

On the other hand, the fossil fuel we burn (or cause to be burned) each day may produce very large amounts of CO₂. For example, burning one gallon of gasoline - which weighs about 6.5 pounds - produces about 20 pounds of CO₂. So burning one gallon of gasoline produces 10 times as much CO₂ as our bodies produce in a day. Many of us find it necessary to burn many gallons of gasoline or other fossil fuels each day. In fact, the average American -- whether directly or indirectly -- produces about 160 pounds of CO₂ per day -- about 50 times the world average! [There's lots more information about CO₂ production at the Carbon Dioxide Information Analysis Center at Oak Ridge National Laboratory.]

Find out more on:
Global Warming
Carbon Dioxide chemistry