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Background Information


What is global warming?

Global warming is a term that has been used to describe a period in recent history when the earth’s surface temperature warmed. There have been three periods of global warming since 1850; from 1860 to 1880, 1915 to 1945, 1975 to 1998 (see graph). The most recent warming was originally thought to be caused by the emissions by man of carbon dioxide, commonly called by its scientific name CO2. Each period of warming was separated by a period of global cooling. The warming/cooling cycles, and their associated change in precipitation patterns are called climate change. Climate change has been occurring for the past 11,000 years since the last ice age. Recent research has shown that this steep curve is probably a product of what is known as the heat island effect rather than global warming.
 

 
  Global mean surface temperature anomaly 1861-2004. Anomaly means the departure from a fixed starting point.  (Source:  Wikipedia)  
 
 

 

An illustration showing the exchanges of energy between outer space, the earth’s atmosphere, and the earth’s surface. The greenhouse effect results from the ability of greenhouse gases in the atmosphere to capture and recycle energy emitted by the earth’s surface. (Source: Wikipedia)

 

What are greenhouse gases?

Greenhouse gases are those gases that allow most solar radiation (mainly visible light) to penetrate to the earth’s surface, but absorb thermal infrared radiation emitted by the earth’s surface. As solar energy reaches the earth’s surface some of it is reflected back out into space, some is absorbed by the atmosphere, and the rest is absorbed by the earth and warms it. The warm earth then emits infrared radiation which we cannot see. Most of this infrared radiation is absorbed by the various greenhouse gases, much of which is emitted back down to the surface of the earth where it starts the cycle again. This cycle is called the greenhouse effect because it acts somewhat like a greenhouse in that it allows the sunlight to penetrate the greenhouse, but as the greenhouse warms, the glass ceiling and walls prevent most of the heat from escaping. The most powerful greenhouse gas is water vapor, not carbon dioxide.

 

What gases have a greenhouse affect?

Most gases in the atmosphere do not have greenhouse gas properties. Of the various gases that have greenhouse properties, water vapor, clouds, carbon dioxide, methane, nitrous oxides are the most important. Carbon dioxide is not the most powerful greenhouse gas. This fact is rarely mentioned. In fact, it is a distant second. Water vapor and clouds are the most powerful by far. The greenhouse potential of each gas is difficult to measure because the earth’s atmosphere is constantly in a state of flux due to constantly changing weather patterns and seasons. Furthermore, some greenhouse gases are more effective in trapping heat than others. For instance, methane is about 20 times more effective as a greenhouse gas than carbon dioxide. So even though there is far less methane in the atmosphere than carbon dioxide, methane is still a very important greenhouse gas. When all of these factors are considered, the potential effectiveness of a greenhouse gas can be estimated. Water vapor accounts for 31 to 70 percent of all the greenhouse gas potential in the atmosphere at any one time. When clouds are added, this increases to 65 to 85 percent. Carbon dioxide, on the other hand, only accounts for 9 to 27 percent. Since most carbon dioxide in the atmosphere is produced naturally from plant decay, exhaling from animal life, soil microbiotic  activity and other sources, only 3 percent of the maximum 27 percent is from burning fossil fuel and other human activity. Methane and other greenhouse gases make up the balance of greenhouse gases. Of all greenhouse gases, water vapor and clouds are by far the most powerful greenhouse gases.

 

 

Of the most important greenhouse gases, water vapor and clouds are by far the most important. Carbon dioxide comes in a distant second and man-caused carbon dioxide is hardly a blip on the scale.

 

The molecular structure of CO2

 

The atmospheric concentrations of CO2 has increased from about 315 ppm in the late 1950s to 380 ppm in 1998

 

 

 

What is carbon dioxide?

Carbon dioxide (CO2 = 1 carbon, 2 oxygen atoms) is a colorless, odorless, tasteless gas which is about 1.5 times as heavy as air. Under normal atmospheric conditions it is stable, inert (does not react with other substances or gases), and nontoxic. In solid form it is known as dry ice. The atmosphere contains approximately 0.033 percent carbon dioxide by volume. Nearly all animal life produces carbon dioxide during respiration which is exhaled (breathed out). Nearly all plant life absorbs CO2 as the building block for food production in a process called photosynthesis. When animal and plants decompose, carbon dioxide is released as a by-product. Carbon dioxide is also a byproduct of burning plant material and fossil fuels. Since it is also a greenhouse gas, many consider human-emitted carbon dioxide to be the major source of global warming in the twentieth century. The atmospheric concentrations of CO2 has increased from about 270 ppm (parts per million) in the early 20th century to 380 ppm in 1998. Most of this increase has come from the burning of fossil fuels, but could also come from volcanoes and a warming ocean. As the ocean warms, it cannot hold as much carbon dioxide and billions of tons of CO2 is released into the atmosphere.

 

The effectiveness of carbon dioxide as a greenhouse gas decreases with increasing CO2 concentrations

As carbon dioxide concentrations increase it becomes increasingly difficult for additional CO2 molecules to further increase the greenhouse effect. The first 20 ppm (parts per million) of atmospheric carbon dioxide has a huge effect. The second 20 ppm is much less. By the time atmospheric carbon dioxide reaches the modern day level of 380 ppm, additional increases have an extremely small affect. The carbon dioxide caused global warming theory is not based on a direct cause and effect relationship, but an indirect relationship based on the belief that carbon dioxide will increase high elevation cirrus cloud formation. Cirrus clouds trap heat in the upper troposphere, around 5 to 8 miles in altitude, causing it to warm faster than the troposphere at the earth’s surface. The troposphere is the layer of atmosphere we live in and extends to about 8 miles in altitude. As you will learn in Lesson 2, actual measurements are showing this warming is not occurring.

 

 

The effectiveness of carbon dioxide as a greenhouse gas dramatically diminishes with increasing carbon dioxide concen­tration. At modern day concentrations, CO2 will not directly cause much warming. Instead, the global warming theory says that  CO2 increases high elevation cirrus clouds which then increase the temperature.

(Source: Climate Outlook to 2030)

 
Image showing when a very strong El Nino strikes surface waters in the Central equatorial Pacific Ocean warmed           
When the trade winds shift and push warm water from the western pacific to the eastern Pacific, the jet streams change, changing the weather patterns and climate. When the trade winds change back the ocean cools and it is called La Nina. See a NASA animation.

El Nino

A change in the Pacific Ocean’s surface temperature by a few degrees Celsius when wind pushes warm water from the western Pacific to the eastern Pacific Ocean. These changes cause atmospheric conditions often resulting in rain, thunderstorms or drought around the world. When the wind no longer pushes the warm water east, cool water returns and is called La Nina. A corresponding, but less pronounced cycle occurs in the Atlantic Ocean. El Nino's are thought to be caused by solar activity working through the Pacific Decadal Oscillation.

   Student Links:
See a NASA visualization: http://www.globalwarmingclassroom.info/el_nino.htm
            http://meteora.ucsd.edu/~pierce/elnino/whatis.html
http://www.pmel.noaa.gov/tao/elnino/el-nino-story.html