Earth is a planet that is friendly and hospitable to human habitation. Even though it includes the frozen ice caps of Greenland and the hot sands of the Sahara desert, for the most part our planet has climate conditions that make for pleasant living with only minor adjustments. As far as we know, earth is the only planet with a climate so favorable to human beings. On some of our closest neighbor planets, we find temperatures 900 degrees Fahrenheit higher or 200 degrees Fahrenheit colder than those found on earth. What makes earth so comfortable is its large complex system of gases, about 218 miles (350km) thick, called the atmosphere. This "blanket" interacts with the land, the oceans, and the sun to produce the earth's weather and climate.

The atmosphere consists of four layers: the troposphere, stratosphere, mesosphere, and thermosphere. Closest to the earth's surface, the troposphere contains more than 90 percent of all the gases in the atmosphere and is composed of about 78 percent nitrogen, 21 percent oxygen, trace gases, water droplets, dust, and other particles. It is in the troposphere that most weather occurs.

Weather-snow, rain, sunny skies, freezing cold-is a result of short-term conditions. Air rises in the troposphere heated by the sun, then falls earthward as it cools, and intermixes with evaporated water from the planet's bodies of water to form clouds and precipitation. The sun's uneven heating of the earth's surface along with earth's rotation creates rising and falling movements, as well as horizontal air movement (winds).

Climate is the average of the short-term weather conditions over time, usually in blocks of thirty years. Earth's climate naturally fluctuates between warm and cold phases over time-scales from centuries to thousands of years.

The constant temperature on earth (about sixty degree Fahrenheit) is a result of the "greenhouse effect." Without the greenhouse effect the earth's average temperature would be about 0 degrees Fahrenheit. The greenhouse effect is a result of greenhouse gases, which block the transmission of long-wave radiation. The earth is heated by the sun, and the earth heats the air. The sun's rays reach the earth's atmosphere as short-wave radiation. The earth's surface, heated by the sun's energy, then reradiates this heat back into space as long-wave, or infrared radiation-heat. The greenhouse gases absorb some of the energy carried by the radiation and redirect the majority of it back to earth, thus making for a lower atmosphere close to earth that is warmer than the cooler upper atmosphere. Greenhouse gases (trace gases) constitute less than 0.1 percent of the atmosphere, and include water vapor, carbon dioxide, methane, nitrous oxides, and chlorofluorocarbons (CFCs). Considered responsible for 55 percent of the greenhouse effect, atmospheric carbon dioxide (CO2) is the most significant greenhouse gas and is part of the natural carbon cycle. Carbon is one of the fundamental building blocks of life (We are carbon-based life forms!) and the carbon cycle exists on a long-term as well as a short-term scale. The biological/physical carbon cycle-carbon converted to biological form in plants, consumed by animals, and returned to the atmosphere in respiration-can travel this path over a very short period of days, weeks, or months. The geological carbon cycle takes place over hundreds of millions of years and involves the cycling of carbon through the layers of the earth. Carbon buried under the ocean floor might take tens of millions of years to return to the atmosphere, and the emission of CO2 from deep below the planet's surface happens in geological events such as volcanic eruptions.

By extracting fossil fuels (hydrocarbons) such as oil and coal, humans have tapped into the long-term geological carbon cycle. Since the Industrial Revolution, we have been burning vast quantities of these fossil fuels to power our developing technological and global civilization. When fossil fuels are burned, carbon is released, and as a result we have been increasing the atmospheric concentration of CO2.

Another way in which humans have increased atmospheric CO2 levels, is through forest clearing. Large amounts of carbon are stored in trees and other vegetation. When trees die and decompose, some of their stored carbon is released to the air as CO2. However, when humans clear large expanses of forest, using fire, this enhances atmospheric carbon levels in two ways. First, in combustion, the trees' stored carbon is released directly into the air as CO2. Second, clearing trees takes away a significant mechanism for removing CO2 from the atmosphere.

Burning fossil fuels and clearing forests have released stores of carbon dioxide into the atmosphere at a faster rate than would occur from natural cycling processes. Since the 1800s, CO2 concentrations worldwide have increased from approximately 280 ppm (0.028%) to around 365 ppm (0.0365%). The increase seems trivial, but it also means that some three gigatons (three billion metric tons) of CO2 are being added to the atmosphere every year.

At the same time over the past 130 years the earth's global average temperature has increased by approximately 0.5 degrees centigrade or about 1 degree Fahrenheit. Six of the last six years were the hottest years on record.