Our home planet, the Earth, is the largest of the terrestrial planets with a diameter of 12,742 kilometers and a mass of 5.977 × 1024 kilograms. It has moderately-thick atmosphere that is 78% nitrogen (N2) and 21% oxygen (O2). It has the right surface temperature and atmospheric pressure for life and liquid water to exist. It is the only place that has either of these things. Some water is also in the form of water vapor and ice. It is also a very beautiful place.
Most of the Earth's water is
liquid and some is frozen. The rest that is water vapor
works with carbon dioxide in the atmosphere to create a small greenhouse effect,
raising temperature about 30° C. This natural greenhouse effect makes it warm
enough on the surface for liquid water to exist. Besides making life possible, the
liquid water
also helps to keep the amount of atmospheric carbon dioxide from getting too high.
Carbon dioxide dissolves in liquid water to form ``carbonic acid'' (soda water).
Some of the dissolved carbon dioxide will combine with minerals in the water and settle
to the ocean floor to form limestone. The amount of atmospheric carbon dioxide is
also kept in check by biological processes.
Aquatic plants extract carbon dioxide dissolved in the water to use in their photosynthesis process. Aquatic animals use the carbon dioxide and calcium in the water to make shells of calcium carbonate (CaCO3). When the animals die, their shells settle to the ocean floor where, after years of compacting and cementing, they form limestone, locking up the carbon dioxide. Some of the locked up carbon dioxide is released into the atmosphere via geologic heating processes such as volcanism.
Earth is unique in that its crust is broken up into chunks called "plates" and these "plates" jostle about because of the convection motion of the mantle below the crust. Among other things plate tectonics does in keeping our planet habitable, plate tectonics play a key role in regulating the amount of carbon dioxide in the Earth's atmosphere. Carbon dioxide in the atmosphere dissolves in rainwater. The slightly acidic rainwater erodes land rocks and the broken down minerals are carried to the oceans via the runoff. Calcium in the broken down minerals combines with the dissolved carbon dioxide in the oceans to create carbonates such as limestone at the ocean floor. Through plate tectonics, the limestone and other carbonate minerals are carried to subduction zones where they are melted. The melted rock releases carbon dioxide through volcanoes. Volcanoes are the major natural (non-human) way that carbon dioxide is released back into the atmosphere over tens of millions of years of time (in the short term they are just a hundredth of the human annual contribution).
The formation of the limestone happens most easily in shallow water. On Earth the presence of continents makes it possible for places of shallow water to exist. The continents are created from lower-density material than oceanic plates through the subduction process of plate tectonics. Liquid water plays a key role in helping the subduction process of plate tectonics to work by lowering the melting point of the oceanic crust and keeping the lithosphere pliable enough to bend and descend far enough into the mantle to melt. In turn the temperature regulation of plate tectonics enables liquid water to remain on the Earth's surface.
The temperature regulation happens because of a negative feedback process that cools the Earth if it gets too hot and warms the Earth if it gets too cool. The rate that carbon dioxide is removed from the atmosphere depends on the temperature such that the higher the temperature, the higher the rate that carbon dioxide is removed. If the Earth warms up, there will be more evaporation and rainfall, resulting in greater removal of atmospheric carbon dioxide. The reduced atmospheric carbon dioxide leads to a weakened greenhouse effect that counteracts the initial warming and cools the Earth back down. If the Earth cools off, the rainfall decreases, resulting in less removal of atmospheric carbon dioxide. The atmospheric carbon dioxide level will build back up because of the outgassing of volcanoes. A strengthened greenhouse effect counteracts the initial cooling and heats the Earth back up.
This whole process of the cycling of the carbon dioxide in the water, life, rocks, and air is called the carbon cycle or the carbon dioxide cycle in geology and oceanography. There is the equivalent of 70 atmospheres of CO2 locked up in the Earth's rocks. The contribution of carbon dioxide from the burning of fossil fuels by humans is a new input into this cycle with unknown consequences. Could this artificial input into the carbon cycle upset the natural balance and create a runaway greenhouse effect like that on Venus? Maybe. How much would it take to tip the balance? We don't know. We are in the midst of a great planetary experiment. We do know that the amount contributed by human activity has had an effect that is long-lasting and the increase has happened in a very short time scale compared to past natural fluctuations in the carbon dioxide levels (see THE source on the science of climate change: Intergovernmental Panel on Climate Change). Unfortunately, it would take many decades, even centuries, for the additional (human-generated) amount of carbon dioxide to be removed via the processes described above even if we stopped all burning of fossil fuels today. Also, the climate change effects take decades to build up so people have been unwilling to make the needed changes in energy generation systems today. Sir John Houghton's 2005 testimony to the United States Senate Committee on Energy & Natural Resources gives a concise summary of the science of global warming and the resulting impacts on humanity from a significantly warmer planet.
Like the debate over Intelligent Design, the controversy over global warming is being fought over in the popular media even though the consensus of the scientific community that is actually researching climate change is that humans are having a greater impact on global warming than natural sources. As a measure of the certainty of this, major insurance companies are now revising their business projections to take the IPCC's conclusions into account. Rather than belabor the point about the human-caused global warming by countering all of the objections of the skeptics on this page, please see my "Answers to Global Warming Skeptics" page for more on this.
A flowchart of the carbon cycle on the Earth is given on the Earth-Venus-Mars page. While the flowcharts for Venus and Mars show what happened long ago, the flowchart for the Earth shows the cycle as it currently operates.
The temperature of the atmosphere decreases with increasing altitude above the surface up to the ozone layer. The temperature below the ozone layer is below the freezing point of water. If water vapor gets up too high in the atmosphere, it condenses and rains back to the surface. This height is below the ozone layer, so the water vapor does not get high enough to be dissociated---there is a ``cold trap'' below the ozone layer.
Within the past couple of decades, the ozone layer has been partially destroyed by some of the chemicals used in modern devices. One class of ozone-destroying chemicals are called chlorofluorcarbons (CFC's) that are used in aerosol sprays, the cooling fluid in older refrigerators and air conditioners and in making styrofoam. Most of the industrialized nations have taken steps to reduce the production of CFC's, but the CFC's already in the atmosphere will take some time to disappear and the ozone destruction will continue for a while. You may have heard that some ozone is produced by engines here on the surface. Unfortunately, that ozone does not make it to the ozone layer high up. Also, ozone can damage our respiratory system.
Go to pictures from the surface of our home planet
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last updated: October 17, 2007