The falling water collects again in oceans, lakes, and streams, and
the cycle continues. The total amount of water on Earth remains constant;
when we talk about a “shortage of water,” we really mean
the water’s not available in the form that we’d like to
have it — water
that’s fit for drinking, water that can be useful for agriculture,
or water that we can put on our lawns, for example.
 |
| One view of the carbon cycle. This graphic, by the National Aeronautics and Space Administration's Earth Science Enterprise, shows just how complex the carbon cycle can be. Don't even try to understand it; many scientists don't, either. For more details about the cycle, and an explanation of the graphic, visit NASA's excellent report on the Earth Observatory Web site at http://earthobservatory.nasa.gov/Library/CarbonCycle/ |
Another cycle.
The hydrological cycle is one of the
most important systems on Earth. Without it, we could not live and
Earth
wouldn’t be Earth as we know it. But there’s another cycle
that's just as important — some might say far more so. It’s
called the carbon cycle.
|
Busy
consumers. . . What
do human beings have in common with green-leaved plants,
rabbits, and foxes? We’re all consumers — organisms
that eat other organisms below themselves in what we call
the “food
chain.” At
the base of this consumer pyramid are the herbivores, or
plant-eating animals (the rabbit, for example), who feed
on green plants. They are called the "primary consumers." Then
come the "secondary consumers," or meat-eating
carnivores, those who feed only on herbivores (the fox,
perhaps). The "tertiary consumer" is one that
feeds on other carnivores. An eagle, for example, might
eat the fox. The creature at the top of the scale is known
as the "top carnivore." This chain of consumerism
exists both on land and in the water environment. . . . and decomposers. Another essential member of this eat-and-be-eaten drama are the decomposers, also called the reducers. These include the bacteria, fungi, and some protozoa who feed on dead plant and animal matter and their feces. In the process, the decomposers create carbon dioxide and heat, and clean and enrich the soil. You may think of this as a yukky job, but someone's got to do it; without the decomposers, we'd be shoulder deep in stuff we'd rather not think about. |
The cycle works like this (remember that, as in the water cycle, there’s no real starting or ending point. Like a bicycle wheel, it just goes ’round and ’round):
Green plants use carbon from the air in the form of carbon dioxide
or dissolved in water in combination with water and light to create
organic chemicals necessary for growth and continued life. Algae in
bodies of water undergo the same conversion. The changeover process
is known as photosynthesis.
Other creatures, known as consumers (See the box, “Busy consumers”), eat the plants and the chemicals that are stored within those plants, and convert them to other life-giving uses. For example, you (a consumer) eat dinner and convert part of what you eat into energy to keep you healthy and growing.
You say you didn’t eat a plant for dinner, but had chicken instead? Remember that the chicken came from an animal that consumed a great deal of plant material.
Consumers get consumed, too.
The consumers play their
part in the carbon cycle. They excrete wastes (in the case of animals)
and die and decay (in the case of both plants and animals). When
they
excrete and decay, they stop being consumers and start being consumed
themselves — by bacteria and fungi and other tiny organisms
that are known as decomposers or reducers. In the process, the
carbon dioxide
is released back into the atmosphere, and the cycle continues.
Obviously, the carbon cycle is one of Earth’s most important processes. But until just a few years ago, scientists and government officials and people in general paid little attention to the movement of carbon through our lives. Like water and air, Earth’s carbon was something we took for granted. These resources have been here all along, we figured (if we thought about them at all), and undoubtedly they’ll be here forever.
Now that has all changed dramatically. We have a new concern for our environment, and we are learning now just how fragile that environment is.
Scientists no longer are able to ignore what’s happening to the
basic elements of our lives, and the people who make our laws and
policies
no longer are able to avoid dealing with the water, air, and carbon
that make life possible.What changed our thinking was the prospect
that
Earth’s climate is undergoing rapid and dramatic change — a
change that could cause big-time problems for Earth and its inhabitants
in the not-too-distant future.