17.9: The Global Greenhouse Effect

Essential to Know

• The average temperature of the Earth’s atmosphere is determined by the balance between solar energy that penetrates the Earth’s atmosphere and is absorbed by gases, liquids, and solids, and energy that is lost to space by the Earth’s radiation.
• The sun’s radiated energy is concentrated in the visible portion of the electromagnetic spectrum, whereas the Earth’s radiated energy is concentrated at longer wavelengths in the infrared portion of the spectrum.
• The Earth’s atmosphere absorbs energy in the infrared and ultraviolet portions of the spectrum more effectively than it does in the visible portion of the spectrum.
• Because the Earth’s atmosphere absorbs incoming solar radiation less effectively than it absorbs energy reradiated to space by the Earth, the Earth’s climate is warmer than it would otherwise be. This is the greenhouse effect.
• The efficiency of absorption of infrared energy by the atmosphere is increased by higher concentrations of certain gases, including carbon dioxide, methane, chlorofluorocarbons, nitrogen oxides, and ozone, which are called “greenhouse gases.”
• Concentrations of greenhouse gases in the atmosphere have increased and continue to increase as a result of human activities. Carbon dioxide concentrations have risen more than 50% since 1800.
• Radiation of heat from the Earth to space has probably been reduced and will continue to be reduced as a result of the absorption of radiated infrared energy by the increased greenhouse gas concentrations. In contrast, the amount of solar radiation passing through the Earth’s atmosphere has not been reduced by such absorption. If this were the only change occurring, the average temperature of the Earth’s atmosphere would increase. This is the global climate change that occurs as a result of the enhancement of the greenhouse effect by greenhouse gas releases.
• The Earth’s average atmospheric temperature has increased by between 1.1 and 1.3°C since 1880 (the industrial era).
• The increase in greenhouse gas concentrations in the atmosphere and the consequent reduction in the Earth’s radiative energy loss to space have many complex effects on the atmosphere, oceans, and biological systems. Many of these changes also have consequences that, on their own, would lead to warming or cooling of the Earth’s atmosphere. These positive and negative feedbacks would, respectively, enhance or offset predicted global warming.
• Most analyses and models of this system predict that the net effect of greenhouse gas emissions will be a global warming of an additional 2°C to 6°C in the next century, but the magnitude of such warming is not certain. Indeed, the ultimate net climate change may be global or regional cooling. The only certainty is that the increases in greenhouse gases will cause global climate changes of currently unknown magnitude.
• The consequences of global warming would be damaging to human civilization. Possible changes include flooding of coastal cities due to rising sea level and drastic changes in rainfall patterns that would lead to the loss of agricultural land.

Understanding the Concept

We experience temporal and spatial fluctuations of temperature in the Earth’s atmosphere as weather and seasonal change. However, the average temperature of the Earth’s atmosphere remains almost constant from year to year. Changes in average temperature of several degrees have occurred in the geological past (Fig. CC9-1). These changes, although generally small, profoundly affected the Earth’s climate. For example, they caused ice ages to begin and end.

Figure CC9-1. Diagram showing the close relationship between variations in average air temperature and atmospheric carbon dioxide concentration during the past 160,000 years. These data were obtained from an ice core drilled into the Antarctic ice sheet. Notice that the Earth’s average temperature at present (in geological time), is approximately at its highest level since more than 120,000 years ago. The diagram would show the post-industrial temperature change of about 1°C as only a thin line at the edge of the plot, and if the 50% CO2 concentration increase were shown, it would plot above this figure- off the top of this page. 

Changes in the chemistry of the Earth’s atmosphere due to human activity have already caused about a 1°C increase in the Earth’s average temperature and may cause it to increase by as much as 1°C to 3°C more within the next decade or so. This predicted change is usually called the greenhouse effect or “global warming.” The fear is that the Earth’s average temperature could change faster than it has ever changed during human history, and that the extent of the predicted change could be great enough to have major effects on the Earth’s climate. Rapid climate changes would have numerous damaging effects on nature and on humans. Consequently, there is considerable interest in determining how much of the post-industrial atmospheric temperature increase is caused by anthropogenic greenhouse gas emissions, if the increase is likely to continue and how fast, and whether the problem can be reduced by altering human activities.

The only significant source of heat energy to the atmosphere is solar radiation. Several other sources, including heat conducted from the Earth’s interior, tidal friction, and heat released by burning fossil fuels, are all negligibly small in comparison with solar input. Part of the solar radiation reaching the Earth is reflected directly back to space (primarily by clouds or snow). The rest is absorbed by the land, oceans, or atmosphere. Heat is transferred between oceans and atmosphere and between land and atmosphere, and it is radiated back to space by land, ocean, and atmosphere (Fig. 7-6). This system of energy transfer is discussed in Chapter 7. One important feature is that the amount of solar energy that reaches the Earth must equal the amount of energy reflected or radiated back to space. If the amount of incoming energy does not equal the amount of outgoing energy, the Earth’s atmosphere will either gain or lose heat continuously, and the atmospheric temperature will increase or decrease.

To understand how the chemistry of the atmosphere can affect the balance between incoming and outgoing radiation, we must understand the nature of electromagnetic radiation (Fig. 5-17). All objects radiate electromagnetic energy, but the wavelength range emitted by an object depends on its temperature. The hotter the object, the shorter the wavelengths of electromagnetic energy that it radiates. The sun, which is very hot, radiates much of its energy in the visible and ultraviolet wavelengths, whereas the Earth and oceans, which are much cooler, radiate energy in the much longer wavelengths of the infrared region of the spectrum.

We can understand this relationship of radiation wavelength to temperature from our everyday experience. If you turn the burner of an electric stove on at full power and hold your hand close to it, you quickly feel the heat that it radiates. You see no immediate change in the metal of the burner, because the burner is radiating all of its energy in long wavelengths in the (infrared) part of the electromagnetic spectrum, not visible to humans. As the burner heats further, it begins to glow red. At this higher temperature, it is radiating energy at wavelengths in the visible red and near-infrared part of the spectrum. If you could heat the burner even further, it would radiate at progressively shorter wavelengths until it appeared white, like the sun or the filament of an incandescent lightbulb.

Electromagnetic radiation from the sun must pass through the atmosphere before reaching the Earth’s surface. Similarly, longer-wavelength radiation emitted by the Earth, ocean, and clouds must pass through the atmosphere as it is radiated to space. You can feel the radiated heat by placing your hand near a rock or paved surface, especially immediately after dark in summer. As the longer wavelength radiation from the Earth passes through the atmosphere, it is partially absorbed by atmospheric gases, but these gases do not absorb all wavelengths of electromagnetic radiation equally. Each gas absorbs some wavelengths more effectively than others.

The absorption spectrum of the atmosphere and the gases that contribute the strongest absorption in parts of that spectrum are shown in Figure CC9-2. Compare this absorption spectrum with the emission spectra of the sun and the Earth (Fig. CC9-2). We see that the sun’s radiated energy is strongly absorbed by the atmosphere at ultraviolet wavelengths, but there is little absorption in the visible region of the spectrum where the sun’s radiant energy is concentrated. In contrast, there are strong absorption bands in the infrared portion of the spectrum in which most of the Earth’s radiation is emitted.

Figure CC9-2. The upper part of this figure shows the emission spectra of the Sun and the Earth. The lower part shows the absorption spectrum of the Earth’s atmosphere. The sun emits most of its energy in the visible, ultraviolet, and near-infrared parts of the spectrum, whereas the much cooler Earth emits radiation at much longer, infrared wavelengths. The amount of energy that enters the atmosphere from the sun equals the amount lost from the Earth to space by radiation. The sun’s radiation is partially absorbed by the Earth’s atmosphere, particularly in the ultraviolet region. The Earth’s radiation is also partially absorbed by the atmosphere, but at the much longer infrared wavelengths. Carbon dioxide and water vapor strongly absorb radiation at these longer wavelengths, but they do not absorb strongly at most wavelengths of the incoming solar radiation. Other greenhouse gases also absorb radiation primarily in the far-infrared wavelengths.

The atmosphere does not absorb all radiation that passes through it, even at wavelengths where the atmosphere is shown to have an absorption band (Fig. CC9-2). Only a fraction of the energy at such a wavelength is absorbed as it passes through the atmosphere. The rest is transmitted either to the Earth’s surface or to space. The fraction absorbed is a function of the number of absorbing molecules of the gas that lie in the radiation’s path as it passes through the atmosphere. If the atmospheric concentration of a gas or gases that absorb at a particular wavelength is increased, the fraction of the energy absorbed at this wavelength will increase. Consequently, the fraction transmitted to the Earth’s surface or to space is decreased.

Carbon dioxide does not absorb strongly in most of the visible wavelengths that correspond to the sun’s energy spectrum (Fig. CC9-2). However, it does absorb strongly throughout almost the entire wavelength range of the Earth’s radiation. If the carbon dioxide concentration in the atmosphere is increased, there will be little effect on the quantity of solar energy passing through the atmosphere to reach the Earth’s surface. However, the quantity of energy that is radiated by the Earth and atmosphere and transmitted to space will decrease. Because less energy will now escape to space, but the same amount will be received from the sun, excess energy will be retained in the atmosphere, and the global climate will warm. This process is known as the “greenhouse effect.” The name greenhouse effect refers to the similarity of this phenomenon to the effects of the glass of a greenhouse window. Glass transmits visible light but absorbs or reflects infrared energy radiated by objects within the greenhouse, keeping the greenhouse warmer than the surrounding air.

What we normally mean when we say the greenhouse effect is actually incorrect. Anthropogenic addition of greenhouse gases should be called the “enhanced greenhouse effect” because carbon dioxide and other gases that occur naturally in the atmosphere already cause a natural greenhouse effect. The temperature at the Earth’s surface would be much lower than it is now if there were no natural greenhouse effect.

We focus on carbon dioxide as a greenhouse gas because its concentration in the atmosphere has increased more than 50% since 1800, and is now increasing at a faster rate each year (Fig. 1-2). This increase is the result of burning fossil fuels and deforestation (live vegetation absorbs atmospheric carbon dioxide and releases oxygen). Several other greenhouse gases may also be important, including methane, chlorofluorocarbons (freon and similar compounds, often called “CFCs”), nitrogen oxides, and ozone. Each of these gases preferentially absorbs the Earth’s long-wavelength radiation more than the sun’s shorter-wavelength radiation. In addition, each of these gases is released to the atmosphere in substantial quantities as a result of various human activities, including fossil fuel burning, industry, and farming. Although the quantities of carbon dioxide released to the atmosphere are much larger than the quantities of these other gases released, the other gases absorb long-wavelength radiation more effectively. Therefore, global climate change studies must also take into account these other gases.

We know that the carbon dioxide concentration in the atmosphere has increased in the past two centuries. The concentrations of other greenhouse gases have undoubtedly also increased, although we have only limited information about how much. Because more of the Earth’s radiated heat energy is trapped and prevented from escaping to space while the amount of solar radiation reaching the Earth’s surface has not been substantially changed, the Earth’s atmosphere should be heating up. This is why the expected climate change has been called the “global warming problem.” However, the atmospheric heat balance is more complicated than the simple input and output of radiated energy. Many other processes affect the balance between heat absorption and heat loss by the atmosphere, and most of these processes are, in turn, affected by changes in atmospheric temperature or chemistry.

These secondary effects can either add to the warming that would be expected from the greenhouse effect or act in the opposite direction and cause a tendency for the Earth’s atmosphere to cool. Hence, they provide what are known as positive or negative feedbacks (often called “feedback loops”) to the primary greenhouse effect. Positive feedbacks would cause faster atmospheric warming than would occur if just the simple enhanced greenhouse effect were operating. Negative feedbacks would cause slower atmospheric warming. If negative feedbacks were great enough, the net effect of the release of greenhouse gases could actually be a colder climate.

Most models of the complicated atmospheric heat balance predict that negative feedbacks will not dominate, and the net result of the enhanced greenhouse effect will be global warming. This conclusion is supported by the very good correlation in the Earth’s past between high atmospheric carbon dioxide concentrations and high average climatic temperatures (Fig. CC9-1). However, there is considerable scientific uncertainty about the many feedback mechanisms, their magnitude, and how quickly they will respond to the extremely rapid (in geological time) increase of carbon dioxide concentrations in the atmosphere. Although most scientists believe that the release of greenhouse gases will cause global warming (estimated by models to be an increase of an additional 1°C to 6°C during the next century), this outcome is not certain. What is certain is that the release of greenhouse gases has altered the atmospheric heat balance and that the alterations will alter the Earth’s climate. For this reason, scientists now refer to the issue not as the “greenhouse effect” or “global warming,” but instead as “global climate change.”

To see the complexity of the feedback mechanisms involving ocean-atmosphere interactions, we can briefly examine two such mechanisms, neither of which is currently well understood. First, consider a positive feedback. The oceans contribute the greenhouse gas methane to the atmosphere (Chap. 5). The methane is primarily a by-product of the biological activity in the oceans. The rate of methane production and its release to the atmosphere could increase if biological productivity in the oceans increased. About one-quarter of all the carbon dioxide released by human fossil fuel burning has now become dissolved in the oceans. Higher dissolved carbon dioxide concentrations may result in higher rates of photosynthetic growth and production in the oceans, which may lead to an increase in methane release from the oceans. Because methane is a greenhouse gas, it would add to the greenhouse effect, trapping more heat.

Second, consider a negative feedback. Clouds reflect a fraction of the sunlight that hits their upper surfaces back to space. The extent of the Earth’s cloud cover is determined largely by the rate of evaporation of water from the ocean surface. The primary effect of greenhouse gases is to increase atmospheric and thus ocean surface temperatures. Higher water temperatures would lead to increased evaporation. Consequently, increasing global greenhouse gases may cause the extent of cloud cover to increase, and more solar radiation may be reflected to space to offset the additional heat trapped in the atmosphere by the greenhouse gases. This negative feedback is a particularly good example of the complexity of the climate change question, because additional clouds will not necessarily result in a greater reflection of solar heat to space. Certain types of clouds at different levels in the atmosphere are better reflectors than others. In addition, clouds not only reflect but also absorb incoming solar energy, and they both absorb and reflect the radiation outgoing from the Earth. Each of these factors may vary continuously as the rate and location of evaporation at the ocean surface and the ocean and atmospheric circulation change continuously in response to each other.

Of the many positive and negative feedbacks, some may respond almost immediately to greenhouse gas concentration changes in the atmosphere, and others may take centuries or millennia to respond to such changes. A number of the more important feedbacks involve interactions between the atmosphere and the oceans, and they are subject to intensive research because they must be understood if we are to successfully model and predict global climate change.

None of the mathematical models that were used up to about 1991 were able to include any of the relevant interactions between the atmosphere and oceans. The reasons were that too little was known about these interactions to include them in a meaningful way, and even the supercomputers that were available would have been overwhelmed by the additional computations needed to include them. As computational power and knowledge of ocean-atmosphere interactions continue to improve, and as more information about these feedbacks is included in the global climate models, the models may dramatically change predictions of the climate changes that we can expect as a result of the enhanced greenhouse effect. Nevertheless, it will be many years, or even decades, before these complex processes will be understood well enough that we can expect there to be a high degree of probability in model-based predictions of the future of Earth’s climate.

Fortunately, evidence is strong that the presence of many feedbacks within a complex system tends to produce stable situations that are difficult to change. The feedbacks generally tend to return the system toward a stable configuration. However, most of the feedbacks are nonlinear responses (in other words, the response does not vary in exact proportion to the change in the stimulant). Hence, the ocean-atmosphere is a chaotic system (CC11).

One important characteristic of chaotic systems is that they tend to remain relatively stable when disturbed, until the disturbance passes a critical point at which the system suddenly shifts to a completely different, but also relatively stable, state. Possibly the beginnings and endings of ice ages are driven by such chaotic dynamics. Could it be that the enhancement of the greenhouse effect will not significantly affect the Earth’s climate until a critical point is reached at which the climate will change abruptly to something as different as an ice age? Only decades of research or time can provide an answer.

The consequences of global climate change caused by the release of greenhouse gases are mostly unknown and difficult to predict. However, they are likely to be severe. For example, one effect will be the thermal expansion of ocean water as its temperature increases. The resultant sea-level rise will likely inundate many coastal cities in low-lying areas, such as Florida and some island nations. Partial melting of the polar ice sheets is likely to also contribute to sea-level rise. In addition, rainfall patterns might be changed. The bands of rainfall that now sustain the breadbaskets that feed much of the global population (Chap. 7) are likely to move northward, causing these regions to become deserts like those of North Africa. Because hurricanes are fueled by the heat energy of ocean water, another effect of a warmer atmosphere and ocean might be an increase in the intensity of hurricanes. Hurricanes might also be able to sustain damaging winds while traveling farther away from the equator than they do at present.