The atmosphere can be thought of as a thin layer of gases that surround the Earth. The two major elements—nitrogen and oxygen—make up 99 percent of the volume of the atmosphere. The remaining 1 percent is composed of what are referred to as trace gases. The trace gases include water vapor, methane, argon, carbon dioxide, and ozone. Although they only make up a small portion of the atmosphere, the trace gases are very important. Water vapor in the atmosphere is variable: Arid regions may have less than 1 percent, the Tropics may have 3 percent, and over the ocean there may be 4 percent.
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The Earth's atmosphere is composed of 78 percent nitrogen, 21 percent oxygen, and 1 percent trace gases; the atmosphere is divided into four layers based on temperature; the troposphere is the layer in which all the weather occurs. It averages seven miles (11 km) high.
The atmosphere is not uniform. It is divided vertically into four distinct layers: (1) the troposphere, (2) the stratosphere, (3) the mesosphere, and (4) the thermosphere. The four levels are divided by height and temperature, as shown in the illustration on page 11.
The lowest level, closest to the Earth's surface is the troposphere. It extends upward to an average height of seven miles (11 km). This level is critical to humans because all of the Earth's weather occurs in this layer. In this level, the temperature gets cooler with increasing height. At Earth's surface the temperature averages 59°F (15°C) and at the tropopause (the top of the troposphere) the temperature is only -71°F (-57°C). Moisture content also decreases with altitude in this layer. Above the troposphere there is not enough oxygen to sustain life and winds increase with height. One of the most pronounced wind systems—the jet stream—is located at the top of the troposphere, as shown in the illustration.
The climate of an area is the result of both natural and anthropogenic (influenced by humans) factors. The natural factors come from the
The human factor influences climate when it alters land and resource uses. For example, when people change a natural forested area to a city, it has a direct effect on climate.
When climate change occurs naturally, it is so slow (over thousands of years) that it is not readily detectable. Climate changes caused by people occur much faster and are noticeable within a few generations, or less. Often, a change in one part of the climate produces changes in other parts as well, because the Earth is a global system.
Since the Industrial Revolution began in the late 18th century—and especially since the introduction and use of fossil fuels involved in the rapid modernization of the 20th and 21st centuries—the global average temperature and atmospheric carbon dioxide (CO2) concentrations have increased notably. Because CO2 levels are higher now than they have been in the past 650,000 years and surface temperatures on Earth have risen significantly during the same time, scientists have concluded that humans are responsible. In support of this, scientists at both NOAA and NASA have run two types of computer models: one of climate systems with natural climate processes alone and another with natural climate processes combined with human activities. The models that include the human activities more accurately resemble the actual climate measurements of the 20th century, giving scientists further proof that human activity does play a significant role in global warming.
Since the 1970s, the Earth's climate has been monitored by instruments on satellites. In addition, measurements of the atmospheric CO2 have been obtained since 1957, when the world's first monitoring station was built on top of Mauna Loa—the highest mountain on Hawaii and the Earth's largest volcano. Scientists wanted to monitor the atmospheric CO2 levels to check if they were increasing. They were able to determine a definite increase—one that is still occurring today. Currently, the CO2-monitoring network has expanded to more than 100 stations globally in order to track concentrations of carbon dioxide, methane, and other greenhouse gases.
Scientists know natural climate variability and cycles will continue, and they expect CO2 levels to rise and global warming to increase because of human influences. How much climate change occurs will ultimately depend on the choices humans make concerning population, energy, technology, and global cooperation.
Dr. Rajendra Pachauri, chairman of the IPCC, said at an international conference attended by 114 governments in Mauritius in January 2005, that the world has "already reached the level of dangerous concentrations of carbon dioxide in the atmosphere." He recommended immediate and "very deep" cuts in the pollution levels if humanity is to "survive." He also said: "Climate change is for real. We have just a small window of opportunity and it is closing rather rapidly. There is not a moment to lose."
In a report in the Guardian in February 2006, Dave Stainforth, a climate modeler at Oxford University, said: "This is something of a hot topic but it comes down to what you think is a small chance—even if there's just a half percent chance of destruction of society, I would class that as a very big risk."
The world's first functioning CO2-monitoring station is on top of Mauna Loa—the highest mountain on Hawaii and the Earth's largest volcano. Here scientists are able to monitor atmospheric CO2 levels. This facility has been instrumental in providing well-documented evidence that CO2 levels are steadily rising—evidence supporting the existence of global warming. (NOAA)
Chris Rapley, head of the British Antarctic Survey, commented in 2006 that the huge West Antarctic ice sheet may be starting to disintegrate, an event that would raise sea levels around the world by 16 feet (5 m). He said, "The IPCC report characterized Antarctica as a slumbering giant in terms of climate change. I would say it is now an awakened giant. There is real concern."
According to the American Geophysical Union, "Natural influences cannot explain the rapid increase in the global near-surface temperatures observed during the second half of the 20th century."
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