Greenhouse Gases

Greenhouse Gases

Big Problems Require Bold Solutions - Global Warming, Greenhouse Gases, the Ocean, and Limiting Reactants

There is now general agreement in the scientific community that the average temperature of the earth is increasing and that this global warming is potentially a major problem. In 1995, the United Nations Intergovernmental Panel on Climate Changes (IPCC) published a report that described this global warming. Written by about 500 scientists and reviewed by about 500 others, it states that the average global air temperature has increased 0.3 to 0.6 C in the past century and is expected to increase an additional 1 to 3.5 C in the next century. This increase in temperature is expected to cause more frequent and intense heat waves; ecological disruptions that could lead certain types of forests to disappear and some species to become extinct; a decline in agricultural production that could result in hunger and famine; an expansion of deserts; and a rise in sea level.

Greenhouse Gases

The global system that regulates the earths temperature is very complex, but many scientists believe that the increase in temperature is caused by an increase of certain gases in the atmosphere that trap energy that would otherwise escape into space. These gases, called greenhouse gases, include carbon dioxide, methane, nitrous oxide, chlorofluorocarbons (CFCs), and the ozone in the lower atmosphere. To get an idea of how an increase in these gases in the atmosphere could lead to global warming, lets first look at the role that some greenhouse gases normally play in regulating the flow of energy into and out of the earths atmosphere.

There is a natural balance between the energy coming into our atmosphere from the sun and the energy escaping from the earth back into space. A simplified description of this energy balance depicts the energy that comes in as relatively short wavelength UV radiation and the energy that goes out as longer wavelength IR radiation. Much of the high-energy radiant energy from the sun is absorbed in the upper atmosphere, but some of the ultraviolet radiant energy passes through. The lower-energy end of the ultraviolet portion of the light spectrum reaches the earths surface and warms it. As the earth cools down again, it releases energy as infrared radiation (Figure 1)

The greenhouse gases absorb some of the IR energy released as the earth cools. When they re-emit it, some of the energy  is sent back toward the earth, rather than out into space. If the greenhouse gases were not there to trap some of this IR energy, the earth would be about 33 C (60 F) colder than it is. The gases are called greenhouse gases because their effect is like the glass panels in a greenhouse. Radiant energy passes through the panels to enter the greenhouse, but the panels slow down the passage of heat energy leaving the greenhouse. On a cold day, more energy comes into the greenhouse than goes out, so the air in the greenhouse is warmed. Even though the actual mechanism is very different, the net effect of the greenhouse gases in the atmosphere is similar: they too tend to trap energy that would otherwise escape, keeping the temperature of the system higher than it would be otherwise.

Figure 1   The Greenhouse Effect     Some of the ultraviolet light that comes from the sun passes through the earths atmosphere and warms the earth. As the earth cools, it emits infrared radiation. Some of this infrared radiation is absorbed by the greenhouse gases. When the gas particles re-emit the IR radiation, a portion of it is sent back toward the earth. Thus, the greenhouse gases trap some of the energy that would have otherwise escaped. This trapped energy leads to temperatures on the earth that are higher than they would be without the greenhouse gases.  

Increases in the levels of the greenhouse gases trap too much of the escaping energy. It is this situation that scientists believe is causing global warming. Table 1 compares todays levels of five greenhouse gases with their levels 100 years ago. The concentrations are described in parts per million (ppm) or parts per billion (ppb). Parts per million is the equivalent of milligrams of substance per kilogram total. (There are a million, or 106, milligrams per kilogram.) For example, 360 ppm CO2 in the atmosphere means that each kilogram of atmosphere contains 360 mg of CO2. Parts per billion (ppb) is the same as micrograms of substance per kilogram total. (There are a billion, or 109, micrograms per kilogram.) For example, 310 ppb N2O in the atmosphere means that there are 310 mg of N2O per kilogram of atmosphere. The table also lists some of the human, or anthropogenic, sources of these gases.

Table 1     Greenhouse Gases

Greenhouse Gas



Percent Increase

Anthropogenic Sources


280 ppm

360 ppm


burning fossil fuels; deforestation


0.95 ppm

1.7 ppm


agriculture; fuel leakage



0.7 ppb




250 ppb

310 ppb


agriculture; combustion


15 ppb

20-30 ppb


urban pollution

Because carbon dioxide plays a major role in global warming, many of the proposed solutions to our problem are aimed at decreasing the levels of carbon dioxide in the atmosphere. This might be done in three general ways. One approach would be to stop the emission of carbon dioxide into the atmosphere. Because the burning of fossil fuels accounts for about 80% of these emissions, one important focus is to develop alternatives to fossil fuel use. For example, if electricity generated by solar energy could take the place of electricity generated by coal-burning power plants, the release of carbon dioxide into the atmosphere would be greatly reduced.

A second approach is to continue generating carbon dioxide, but to trap it before it is released into the air. One suggestion is to liquefy the CO2 produced at fossil fuel power plants and pump the liquid deep into the ocean. It would remain in the ocean water and not return to the surface for a long time. In the interim, scientists would be developing alternatives to fossil fuels.

A third general approach to lowering the carbon dioxide levels in the atmosphere would be to remove the CO2 after it has been released into the air. Because CO2 mixes throughout the atmosphere, this is generally considered to be too big a task, but at least one suggestion for removing it has been considered seriously: alter the chemistry of the earths oceans such that they will absorb the excess CO2.

A Bold Solution

Huge amounts of carbon dioxide are constantly dissolving in the ocean from the air and at the same time leaving the ocean and returning to the air. The ocean takes about 100 billion tons of carbon dioxide out of the atmosphere per year, and ultimately returns 98 billion tons of it. The remaining 2 billion tons end up as organic deposits on the sea floor. If the rate of escape from the ocean to the air could be slowed, the net shift of CO2 from the air to the ocean would increase, and the levels of CO2 in the air would fall. The goal is to find some way to increase the use of CO2 in the ocean before it can escape.

Phytoplankton, the microorganisms that form the base of the food web in the ocean, take CO2 from the air and convert it into more complex organic compounds. When the phytoplankton die, they fall to the sea floor, and  the carbon they contain becomes  trapped in the sediment. If the rate of growth and reproduction of these organisms could be increased, more carbon dioxide would be used by them before it could escape from the ocean into the air. The late John Martin of the Moss Landing Marine Laboratories in California suggested a way of making this happen.

It has been known since the 1980s that large stretches of the worlds ocean surface receive plenty of sunlight and possess an abundance of the major nutrients and yet contain fairly low levels of phytoplankton. One possible explanation for this low level of growth was that the level of some trace nutrient in the water was low. This nutrient would be acting as a limiting reactant in chemical changes necessary for the growth and reproduction of organisms.

Dr. Martin hypothesized that the limiting factor was iron. Iron is necessary for a number of crucial functions of  phytoplankton, including the production of chlorophyll. He suggested that an increase in the iron concentration of the ocean would stimulate phytoplankton growth, and that more carbon dioxide would be drawn from the atmosphere to fuel that growth. In a 1988 seminar at Woods Hole Oceanographic Institution, Dr. Martin claimed that fertilizing the oceans with 300,000 tons of iron could remove 2 billion tons of CO2 from the atmosphere. He said, Give me a tankerload of iron, and Ill give you an ice age. Dr. Martin and others set out to provide the proof.

The first tests were done in the laboratory, and although the results supported the hypothesis, the conclusions were criticized because the conditions in the laboratory did not fully duplicate the conditions in the ocean. Thus, Dr. Martin made his next bold suggestion. He proposed a large-scale test in the ocean itself. Due to the difficulty of controlling conditions in the real world, ocean tests on the scale he suggested had never been done.

It is sad that Dr. Martin did not live long enough to see the results of the tests, but two of his ocean experiments have now been done. They were conducted in the Pacific Ocean about 300 miles south and 800 miles west of the Galapagos Islands. In October of 1993, about 450 kg of iron(II) sulfate was spread over about 64 square kilometers of ocean, and the chemistry and biology of the area were monitored for nine days. The rate of phytoplankton growth increased significantly, and the levels of CO2 in the water decreased. These results confirmed Dr. Martins main hypothesis, that iron was a limiting factor in the growth and reproduction of phytoplankton. Unfortunately, the CO2 concentration leveled off after only one day at about 10% of the expected drop. A second test in 1995 led to 20 times the normal abundance of phytoplankton and a significant drop in CO2 levels in the ocean, but as soon as the iron was gone, the levels returned to normal.

Because the decrease in CO2 in the water was less than expected and because the increase in phytoplankton growth requires a constant addition of iron, it seems unlikely now that fertilizing the oceans with iron is going to solve the problem of global warming. This is disappointing, but it should not surprise us that a problem this big cannot be reversed by a single solution. The fact that fertilizing the oceans with iron did stimulate the phytoplankton growth to a marked degree encourages others to seek similarly bold solutions without being discouraged by the magnitude of the task.

The innovative ideas and the experiments of scientists like John Martin have shown us that, although we may be harming our environment by releasing chemicals that destroy the ozone layer or cause global temperatures to rise,  we may also have the potential to make significant changes in our surroundings that can heal the damage we have done.

The Complexity of Science

The issues presented in this section provide good examples of the complexity of science. For example, the experimental fertilizing of the ocean with iron was based on a fairly straightforward hypothesis: if the ocean has too little iron for maximum growth of phytoplankton, then adding iron should solve the problem. However, the experiment led to only 10% of the expected decrease in the CO2 levels of the waters. It is not known for sure why this happened, but there are some theories. Perhaps when the iron(II) ions interacted with the organic substances, some insoluble iron compounds were formed. Thus, the explanation could be as simple as the iron precipitating from the waters more quickly than expected. Other possible explanations relate to the fact that a change in one component of an ecosystem leads to changes in other components. As the growth rate of the CO2-consuming phytoplankton increased, they may have stimulated the growth rate of the CO2-producing zooplankton. Another possibility is that some other trace nutrient, like zinc or manganese, becomes the limiting reactant when iron is in excess.

The concept of global warming itself is controversial because of the complexity of our global weather system and the difficulty of predicting changes in it. For example, as the concentrations of the greenhouse gases in the atmosphere increase, the temperature of the earth increases, and more water evaporates from the surface of the earth. The effect of this mechanism is difficult to predict. Water itself is a greenhouse gas, so an increase in the gaseous H2O concentrations in the atmosphere would trap more IR radiation and could lead to an even greater increase in temperature. On the other hand, the increased water vapor also leads to increased clouds. These, especially at low altitudes, can reflect the incoming UV radiation back into space and lead to cooling. Meanwhile, the greenhouse effect of the water molecules in high clouds seems to outweigh the reflective effect of the clouds. The IR energy sent back to Earth from the water molecules in the high clouds is greater than the energy from UV radiation that the high clouds reflect back into space. As you can see, the complexity of the system makes it very difficult to predict whether more water in the atmosphere will lead to warming or cooling (Figure 2).


Figure 2    Greenhouse Gases: Warming or Cooling?

Another example of the complexity of global warming stems from a relationship between this topic and stratospheric chemistry. If the IR radiation trapped by excess greenhouse gases in the lower atmosphere had been allowed to move into the stratosphere as the natural cycle would have allowed, the gases in the stratosphere would have absorbed some of it and been warmed. Thus, the increase in the greenhouse gases is thought to have caused a cooling of the stratosphere. This cooling may have increased the formation of ice crystals there, and provided a surface on which ozone molecules are destroyed at a greater rate than would otherwise occur. Less ozone allows more UV radiation to reach the earths surface. This can damage the plants that take CO2 from the air and lead to an increase in the CO2 levels in the atmosphere. The extra CO2 absorbs IR radiation that would have reached the stratosphere, and the cycle repeats (Figure 3).


Figure 3    Greenhouse Gases and Stratospheric Cooling  

Scientists are well aware of the complexities of global warming, but they are confident that the recent refinements in measuring changes in the atmosphere, the new computer models being developed to predict long-range changes in climate, and the new experiments that these developments have inspired will soon combine to yield a detailed understanding of the global environment. It is this understanding that will in turn produce a successful strategy for solving the problems we face.

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