Global warming and the greenhouse effect

First, these are not the same thing. The greenhouse effect is the natural, normal phenomenon by which the earth is as warm as it is. "Life as we know it" needs the greenhouse effect. Global warming is an exaggeration of the greenhouse effect, a warming far more than normal.

Normal warming, called the greenhouse effect, involves the absorption of outgoing infrared (heat) energy from the earth's surface by two gases in the atmosphere: water vapor and carbon dioxide.

FYI Many of the charts and graphs linked in this section come from a National Oceanic and Atmospheric Administration website.

The carbon cycle
Because carbon dioxide in the atmosphere is a key component of the greenhouse effect and is also strongly implicated in the possible global warming, a review of the ways carbon dioxide enters and exits the atmosphere as part of the carbon cycle is needed.

How carbon dioxide enters the atmosphere

• Respiration, the oxidation of carbon compounds with the production of carbon dioxide. This is photosynthesis run backwards. All animal life that use oxygen produces carbon dioxide via respiration. This includes humans, microorganisms (biodegradation), and vegetation. In reaction form: carbon compound + oxygen --> carbon dioxide and water vapor.

• Combustion of carbon compounds with the production of carbon dioxide. This is the same reaction as respiration.

How carbon dioxide exits the atmosphere

• Photosynthesis, the production of carbon compounds and oxygen by green plants using chlorophyll, carbon dioxide, and water. In reaction form: carbon dioxide + water --> carbon compounds + oxygen

• Dissolving in water. Carbon dioxide gas dissolves in rainfall (natural acidity of rainfall) and in surface waters: streams, lakes, and, especially because of their size, the oceans. The reaction is carbon dioxide + water --> carbonic acid. Now the carbonic acid (H₂CO₃) will break apart somewhat to yield H⁺ (hydrogen ion- think acidity) and HCO₃^-1(bicarbonate ion). The bicarbonate will also break apart somewhat to yield H⁺ and CO₃^-2 (carbonate ion). In the ocean (mainly) the carbonate ions are used by marine life to make shells, combining calcium and magnesium ions with the carbonate to make calcium carbonate and magnesium carbonate (solids). The reactions: CO₃^-2 + Ca⁺² --> CaCO₃ and CO₃^-2 + Mg⁺² --> MgCO₃. The shells from marine life eventually sink to the ocean bottom and are buried. This sequesters (out of the active cycle) carbon for a long time.

Where carbon is stored in the biosphere: the atmosphere, in the bodies of living creatures (animals and plants, with forests being much longer term storage reservoirs), and in the water (especially the ocean, the single largest carbon storage area).

Know: how carbon enters, exits the atmosphere

See the Woods Hole web pages on the carbon cycle and know the answers to these questions:

1. What percent of our bodies are carbon?

2. How much carbon was released to the atmosphere from changes in land use, worldwide, between 1850 and 2000? How much is a petagram?

3. The boxed equation at the bottom of the page shows annual additions and subtractions of carbon from the atmosphere. What is the source of the largest addition per year? What is the main subtraction (sink)? Don't worry about the missing sink discussion, although you may find it interesting.

The earth's radiation balance

Incoming radiation is from the sun. The primary types of electromagnetic energy in solar radiation are ultraviolet, visible, and infrared (heat), with the sun's peak wavelength in the visible part of the spectrum. In percentage terms, about nine percent of incoming sunlight is ultraviolet, and the rest in split between visible and infrared energy. The peak energy, remember is in the visible.

Outgoing radiation (which must equal incoming) is from the earth. The earth's outgoing energy is all in the form of infrared.

Why the sun's peak energy is in the visible and the earth's is in the infrared part of the electromagnetic spectrum is explained by Wien's Law which states that the peak wavelength of a radiating body is equal to a constant divided by the absolute temperature of the body. In equation form:

peak wavelength (nanometers) = 2,898,000 / K (2.8 million divided by the absolute temperature, in K)

Where K is the absolute temperature in kelvins.
K = Celsius + 273

The sun's radiating temperature is about 6000 K; therefore its peak wavelength is 2,898,000/6000 or about 480 nanometers. This is in the visible part of the spectrum, which runs from about 400 nanometers to about 750 nanometers (see Set 4). The earth's radiating temperature is about 300 K; therefore its peak wavelength is 2,898,000 divided by 300 or about 10,000 nanometers. This is in the infrared part of the spectrum.

Therefore the earth absorbs solar energy (combination of ultraviolet, visible, and infrared) and reradiates it as infrared energy (only).

Infrared absorption

Heat energy, on its way out of the earth's atmosphere, is absorbed by gases. The two most important of these absorbing gases are water vapor and carbon dioxide. If it were not for the IR absorption by water vapor and carbon dioxide, the earth's average temperature would be much lower, about zero degrees Celsius. This natural absorption of outgoing IR energy and the consequent warming of the planet is the greenhouse effect.

Gases in the atmosphere (other than water vapor) that absorb infrared energy are called greenhouse gases. (Water vapor is normally excluded from the list of greenhouse gases, but don't forget that it is a strong absorber of infrared radiation and contributes greatly to the normal warming of the earth.) Besides carbon dioxide, methane, nitrous oxide (N₂O, not NO₂), ozone, and chlorofluorocarbons are strong IR absorbers. Carbon dioxide is the most important greenhouse gas, contributing about 50% of the IR absorption; the others, together, account for the other 50%. See the following chart.

Greenhouse gases' potential contribution to global warming:

Chapter 6 of the Global Change Student Guide discusses global warming and the greenhouse gases in turn. Also see Chapter 2 of the Global Change Student Guide and United Nations Environment Programme website.

Know the answers to the questions below.

1. What is climate (radiative) forcing? or

2. What is the effect of an increase in radiative forcing?

3. What units are used for radiative forcing?

4. What does GWP stand for?

5. What is an atmospheric sink? What is the main sink for methane?

6. Which gas has contributed most to the total increase in radiative forcing? What percent of the total does this gas account for? One place for this answer is this section of the Student Guide.

Changes in greenhouse gas concentrations

Although water vapor concentrations fluctuate greatly in space and time, overall the amount of water vapor in the earth's atmosphere is not changing. The greenhouse gases are increasing in air concentration:

Carbon dioxide has been increasing during the last century, according to many estimates. Accurate long term measurements have been collected in Hawaii for the last 40 years. Methane, nitrous oxides, and CFCs have increased also. This table shows the rates of increase for the greenhouse gases. This table from the Carbon Dioxide Information Analysis Center has more extensive lists of gases, warming potentials, and atmospheric residence times. The data from Hawaii are discussed in the CDIAC pages. Know the answers to these questions from the CDIAC:

1. What is the name of the instrument used to measure carbon dioxide?

2. How often are the Mauna Loa measurements taken?

3. Click on the Digital Data icon at the top of the page. When did these measurements begin?

FYI All things global warming are found at http://globalchange.gov/

Human Actions

There is strong evidence of human actions being the cause of the increased carbon dioxide levels. Combustion of fossil fuels (petroleum, coal, natural gas) produces carbon dioxide and of course their use has increased dramatically during the last century. Also, worldwide, more vegetation (forest) is being removed than is being replanted. This deforestation results in less carbon being held as part of trees and more carbon dioxide transfer to the air. The deforestation is mainly the result of clearing additional land for crops or grazing, and the vegetation is burnt (combustion conversion to carbon dioxide) or is allowed to rot (biodegradation = respiration, conversion to carbon dioxide by the decomposers).

The greenhouse gases and their humans sources are listed in the table.

Prediction

If greenhouse gases absorb outgoing infrared energy and this causes the greenhouse effect, then additional greenhouse gases should absorb more infrared energy and increase the greenhouse effect beyond its normal bounds (global warming).

The computer models project a 1.5 - 4.5 degree Celsius increase in global temperature if the carbon dioxide levels in the atmosphere double (from about 350 ppm to 700 ppm). According to mid-range projections by the models, carbon dioxide levels will reach 700 ppm by the year 2100.

This chart shows projections of global average temperatures calculated by computer models.

Adverse effects of global warming

1. A change in weather patterns. The current rainfall levels seen (and expected for agriculture) in the American Midwest could shift northward into Canada.

2. Rising sea levels, due to the thermal expansion of the oceans and the melting of the polar icecaps and the Greenland Ice Sheet, flooding coastal areas and damaging estuaries.

Here is the USEPA list of possible adverse effects. See the sidebar listing various effects.

Uncertainties (aspects of global warming that should be acknowledged as not completely understood).

1. The actual temperature record. The earth's temperature increased about 0.5 degrees Celsius during the period 1880-1940, prompting commentators during the 1930s to coin the term "greenhouse effect". From 1940-1970, temperatures decreased about 0.2 degrees C, and there was talk of a possible small ice age. From 1970 to the present the trend has been up, about 0.3 degrees C. A question arises: why should the most pronounced increase occur 1880-1940, when the greenhouse gases were increasing only slowly and why did global temperatures decrease during 1940-1970, when greenhouse gas concentrations were increasing rapidly?

2. Temperature record bias. Temperatures that are used for tracking global trends have come from meteorological stations. These stations were usually located on the edge of an urban area; in the United States during the 20th century it has been common for the National Weather Service to be at the airport. So in many cases 50 years or more of temperature readings for an urban area have come from the airport weather station. Now, urban areas are warmer than the surrounding countryside because in the city a smaller area is covered by grass and trees, which have a cooling effect and a greater area is covered by buildings, asphalt, and concrete, all of which absorb and retain heat. Also, the urban area will have many heat sources itself, from fuel combustion mainly. This phenomenon is called the urban heat island ( FYI see, e.g., Project ATLANTA - Urban Heat Island Study and Heat Island Group Home Page). Well, cities have grown during the time that the weather station has been recording temperatures and the urban heat island has approached (or surrounded) the weather station. The weather station has recorded higher temperatures, but why? Because the city has grown toward it, not because it has detected a global warming. The statisticians studying global temperature trends acknowledge this upward bias in the temperature record; the question is what correction factor to apply, i.e., can the urban heat island explain all or only part of the observed higher air temperatures?

3. Computer simulation models, although powerful, are only as good as the relationships that are coded in them and the data they use as input. The atmosphere is incredibly complex and the models used to predict changes in the weather, much less the climate, are primitive compared with the phenomenon they are trying to simulate.

4. Feedback effects in the computer simulation models. This is an extension of the model discussion above. How the model handles changes in its variables as time proceeds can greatly affect its long-term prediction. For example, how does the model treat clouds? If the earth gets warmer, then more evaporation will create more clouds. More clouds in the atmosphere mean more heat-absorbing water vapor, which means more atmospheric warming, which means more evaporation, more clouds, etc. But clouds also increase the albedo of the earth. The albedo is the percent of the incoming solar radiation that is reflected back to space. More cloud cover, more albedo, more reflection. With more reflection, less solar radiation gets to the ground to be absorbed and reradiated as infrared. Less infrared, less warming. So do clouds increase or decrease warming? The model will simulate both effects; which of the two will predominate is a matter of the relationships in the computer code. Another potentially important factor is the type of clouds, which can either cool or warm the earth.

5. Long term trends. As with ozone layer depletion, the data set we are using to make predictions to the middle and end of the 21st century are limited. The earth's temperature fluctuates naturally, we know. On what time scale and by how much and when is uncertain. Are the changes in temperature that we are seeing the result of natural fluctuations? (yes, but we don't know how much). Are they the result of human activity? (probably, but by how much we don't know).

6. Other pollutant gases are cooling the atmosphere. Sulfates formed from sulfur dioxide emissions exert a cooling effect on the atmosphere by blocking incoming solar energy. For a period after Mount Pinatubo erupted, which sent millions of tons of sulfur compounds into the atmosphere, the earth's temperature declined (1991-1993).

Solutions

Most of the solutions put forward against global warming are focused on CO₂.

• Lower fossil fuel consumption, which will lower carbon dioxide emissions.

• Reforestation. Plant more trees, which will remove carbon dioxide from the atmosphere and keep the carbon in the vegetation for several decades (as opposed to an annual plant which dies and is decomposed within one year).

International agreements to work against global warming: Global Warming: Actions -- Global.

The most recent of the international agreements is the Kyoto Protocol (Read the summary for the basics) (full text , FYI). The UN Framework on Climate Change has a beginner's guide that is useful.
Know these basics of the Kyoto Protocol:

• binding greenhouse gas emissions targets,

• credit given for planting trees,

• developed countries have different requirements than developing countries,

• international emissions trading allowed. This is similar to the U.S. sulfur dioxide emissions trading under the acid rain control provisions of the Clean Air Act.

The Intergovernmental Panel on Climate Change produces periodic reports which have influenced the debate on global warming. Know what the IPCC is and its basic purpose.

For further reading

• The Carbon Dioxide Information Analysis Center at Oak Ridge, Tennessee has a comprehensive set of information. See their FAQs, for example.

• Globalchange.gov A comprehensive site.

• The USEPA global warming page: The EPA Global Warming Site

• The National Oceanic & Atmospheric Administration climate site: The Climate Diagnostics Center

• Is global warming for real? Science Has Spoken: Global Warming Is a Myth

• Other explanations The Iceman Cometh and Goeth

• Another skeptical article (includes the urban heat island discussion) National Policy Analysis #218: Buenos Aires Conference On Global Warming: Much Ado About Nothing - October 1998

• Global Climate Web Sites This Australian site has a wealth of information.

Last modification: 11/29/2004
Contact: Dr. Wyman
http://www.faculty.mcneese.edu/wyman/GlobalWarming.htm