Global Warming and the Greenhouse Effect

Chapter 16 Global Warming and the Greenhouse Effect

Review Questions

Global warming is the term used to describe the rapid change in the Earth’s climate.

The phenomena that indicate that global warming is occurring include increased global air temperatures, increased annual precipitation in most areas, shorter winters, a shrinking ice cover, warming water, the presence of mosquito-borne diseases at higher altitudes, rising sea levels, and an increase in extreme weather conditions.

Global warming is believed to be caused primarily by the greenhouse effect.

Infrared light is associated with wavelengths greater than 750 nm to about a million nm. Thermal infrared radiation is associated with wavelengths from 4000 nm (4 m) to 50,000 nm (50 m).

The greenhouse effect is a phenomenon in which the temperature of the Earth’s surface and the nearby air is increased as a result of the absorption of thermal infrared radiation by molecules of certain gases in the air. The greenhouse effect refers to the absorption of energy and its conversion to heat, and the subsequent reemission of some of this trapped energy back toward Earth. As a consequence, the Earth’s surface and lower atmosphere become warmed.

A greenhouse gas is a gaseous component of the atmosphere that efficiently absorbs thermal infrared radiation that is emitted by the Earth. Important greenhouse gases include water vapor, carbon dioxide, methane, nitrous oxide, and ozone.

Nitrogen gas is not a greenhouse gas because diatomic molecules containing two identical atoms can not absorb infrared light.

Atmospheric window refers to the region of infrared radiation with wavelengths between 8 and 13 m. The atmosphere in this region acts like a glass window transmitting most of the light because there are no natural greenhouse gases that absorb light between 8 and 13 m.

The composition of today’s atmosphere differs from that of the pre-industrial world primarily in the concentration of atmospheric CO2. From the year 1750 to 2000, the concentration of atmospheric CO2 has increased about 30%.

a) The primary anthropogenic sources of carbon dioxide include the combustion of fossil fuels (coal, oil, and natural gas), the production of calcium oxide from calcium carbonate, and the clearing and burning of forests. The major natural source of carbon dioxide is the decay of vegetation.

b) Most of the methane naturally produced results from the decay of living matter in the absence of air, or anaerobic decomposition. Wetlands are the largest natural source of methane emissions. Anthropogenic activities that increase methane production include the expansion of wetlands, increasing the population of ruminant animals, the anaerobic decomposition of organic matter in landfills, the release of natural gas from coal mines, and leaks in gas pipelines.

c) Nitrous oxide is released naturally from the oceans and from the soils of tropical regions due to biological denitrification in aerobic environments and nitrification in anaerobic environments. The increased use of nitrogen-based fertilizers accounts for most of the anthropogenic emissions. Other anthropogenic sources include nylon production and automobile catalytic converters.

d) Natural sources of chlorine producing species include methyl chloride. CFCs are synthetically produced compounds that are used as propellants and refrigerants.

There is a direct relationship between per capita energy use and economic development. Not only is energy used to personal transportation vehicles and for heating and cooling homes, but energy is also used to produce and transport goods, heat and cool factories, classrooms, and offices, and to accomplish just about every task in a modern industrialized society.

a) The main temporary sinks for carbon dioxide are the ocean and increased plant growth. The permanent sink for carbon dioxide is calcium carbonate in sediments.

b) The major sink for methane is its oxidation to carbon dioxide in air.

c) The sinks for nitrous oxide include the atmosphere, stratosphere, and soils.

d) The stratosphere serves as a sink for CFCs.

a) Carbon dioxide that accumulates in the atmosphere absorbs thermal infrared radiation thereby increasing global warming.

b) Methane has 21 times the warming effect per molecule when compared to CO2, as CH4 molecules individually absorb a larger fraction of the thermal infrared radiation that passes through them.

c) As a result of its absorption characteristics, nitrous oxide is 206 times more effective per molecule than CO2 in increasing global warming.

d) Each molecule of CFC is tens of thousands of times more effective than CO2 in increasing global warming.

e) Global warming increases water vapor concentrations. Since water vapor is itself a greenhouse gas, it further contributes to global warming.

f) In general, aerosols exert a cooling effect on the environment. These particles tend to reflect incoming sunlight back into space thereby rendering it unavailable for absorption.

Clathrate compounds are structures in which a small molecule occupies vacant spaces or holes in a cage-like structure formed by other molecules.

Per unit of heat energy, natural gas generates less carbon dioxide than oil by a ratio of approximately 3 to 4. Oil generates less carbon dioxide than coal by a ratio of approximately 2 to 3.

Positive feedback refers to a phenomenon that produces a result that itself further amplifies the result.

Volcanic eruptions generally decrease atmospheric temperatures. These eruptions illustrate the short-term effects of atmospheric aerosols. The SO2 produced by volcanic eruptions creates a sulfuric acid aerosol that remains in the stratosphere for several years, effectively reflecting sunlight back into space and cooling the atmosphere.

Clouds potentially have two effects on global warming. Clouds contain water droplets and crystals that can reflect sunlight back into space, thereby cooling the Earth’s surface. Clouds also have the potential, however, to reflect outgoing infrared radiation back to Earth, thereby further heating its surface.

Carbon dioxide accumulates in the air over time because it has such a long lifetime in the atmosphere, a century or more on average.

Based on computer-based climate simulations of the Earth, scientists predict that the average global air temperature will increase between 1.9 and 2.9oC from the year 1990 to 2010. This is only an estimate, as the accuracy of the predictions is relatively uncertain given the complexity of the models of the Earth’s climate.

Global warming could be dramatically accelerated by the phenomenon known as feedback. Increased water vapor in the atmosphere, increased release of methane into the air resulting from accelerated anaerobic biomass decay, and changes in the circulation patterns of ocean currents could result in a dramatic rise in global temperature.

Per capita emissions are based upon both the total emissions of a country and its population. Due to the drastically different populations of different countries, per capita emissions are usually not an indication of total emissions. Total emissions of a highly populated country with modest per capital emissions can exceed those of a less populated country with greater per capita emissions.

The Kyoto Agreement was an agreement reached in 1997 wherein 39 industrialized nations agreed to collectively reduce greenhouse gas emissions by 5.2%, compared to 1990 levels, by the year 2012.

Four ways to control carbon dioxide emissions include switching to fossil fuels that produce less carbon dioxide per unit of heat energy produced, burying carbon dioxide in the oceans or on land, using iron fertilization of the oceans to remove CO2 from the air, and increasing plant growth to remove carbon dioxide from the atmosphere.

Understanding Concepts

The greenhouse effect refers to the absorption of infrared thermal energy by atmospheric gases thereby warming the lower level of the Earth’s atmosphere and its surface. There are naturally occurring gases in the atmosphere that allow for this beneficial warming effect. As a direct result of human activity, however, the concentrations of all of the natural greenhouse gases have increased appreciably. These and further increases will likely increase the Earth’s average air temperature. This human “enhancement” of natural greenhouse gases is called the enhanced greenhouse effect.

It has been estimated that the concentration of carbon dioxide in the atmosphere before 1750 was about 280 ppm. After the industrial revolution, atmospheric CO2 concentrations have increased due to the burning of fossil fuels to about 369 ppm in the year 2000. This concentration continues to increase at an annual rate of approximately 1.6 ppm.

Carbon dioxide is produced by the combustion of hydrocarbons, such as coal, oil, and natural gas. The process of combustion is represented by the reaction given below.

Hydrocarbons + O2 CO2 + H2O

Carbon dioxide is also released into the atmosphere when calcium carbonate is heated to produce calcium oxide.

CaCO3 (s) + heat  CaO (s) + CO2 (g)

Methane has 21 times the warming effect of CO2 per molecule. Similarly, nitrous oxide is 206 times more effective per molecule than CO2 in increasing global warming. Due to the large amounts of carbon dioxide that are released into the atmosphere annually, however, methane and nitrous oxide are less important than CO2in producing global warming.

In the process of nitrification, nitrogen (in the form of ammonia or ammonium) is converted to nitrate (NO3-) and nitrite (NO2-). In denitrification, nitrogen (in the form of the nitrate ion) is converted to molecular nitrogen, N2. In both processes, nitrous oxide is a by-product.

Several catalytic converters in use in automobiles today convert the NO produced from atmospheric nitrogen during the combustion of gasoline into N2O.

In terms of its impact on global warming, each CFC molecule is equivalent to tens of thousands of CO2 molecules. However, far more carbon dioxide has been released into the environment than CFCs. Thus, on a per molecule basis, CFCs have the greatest impact, but overall, CO2 emissions account for a greater portion of the global warming problem.

Through a feedback mechanism, water vapor contributes to global warming. Water vapor is a greenhouse gas. As other greenhouse gases increase the temperature of the atmosphere, the rate at which water on the surface of the Earth evaporates and the amount of water vapor that the air can hold both increase. The increase in water vapor concentration results in additional global warming. This phenomenon is generally referred to as positive feedback.

Aerosols consist of tiny particles suspended in air that are able to both absorb and reflect light. Depending upon whether they absorb incoming sunlight or reflect this light out to space determines whether these aerosols contribute to or decrease the effect of global warming. If these particles reflect incoming sunlight back into space, this energy is unavailable for absorption and heat production. If they absorb the light, the air immediately surrounding them is warmed.

None of the constituents of sulfate aerosols absorb sunlight. Only when these aerosols incorporate some tropospheric soot will they absorb significant amounts of sunlight. Thus, these aerosols generally reflect sunlight back into space, thereby cooling the air near the Earth’s surface and offsetting some of the effects of global warming. Indirectly, sulfate aerosols also cool the air by acting as centers for the formation of water droplets. In the presence of these aerosols, the water droplets are smaller and are able to reflect more light and last longer than larger water droplets.

Increased carbon dioxide concentrations account for the greatest increase in greenhouse gas related heating of the atmosphere. It is estimated that the average global temperature will be between 1.9 and 2.9oC higher in 2100 than it was in 1990. This increase in global temperature corresponds to an expected increase in carbon dioxide concentration to 560-820 ppm in 2100 from its current concentration of approximately 369 ppm (in 2000).

Large amounts of methane gas exist as methane hydrates on the ocean floor. These clathrate compounds are stable under conditions of high pressure and low temperature. As the ocean temperatures are warmed by the enhanced greenhouse effect, these clathrate compounds could decompose and release large amounts of methane gas, itself a greenhouse gas, into the atmosphere.

a) Global warming could produce a shift in ocean currents that could result in significant climate changes for several regions of the Earth. This could have a profound impact on atmospheric temperature, growing seasons, and marine life.

b)As a result of both the melting of the glaciers and the thermal expansion of seawater, global warming is likely to produce rising sea levels.

c)Precipitation is likely to increase as the concentration of water vapor in the air increases in response to increased atmospheric temperatures.

d)Higher levels of CO2 will result in a higher rate of photosynthesis and longer frost-free growing seasons at northern latitudes. These potentially beneficial effects will likely be offset by decreased soil moisture and increased insect infestations.

e)Insect-borne diseases will likely increase as global warming will extend the range and accelerate the life cycles of insects that carry diseases such as malaria, yellow fever, and encephalitis.

The density of water decreases gradually as water is warmed.

Rising sea levels could flood land in low-lying coastal regions, submerge small island countries, increase tropical storm damage, and contaminate fresh ground water sources.

Because different countries have drastically different populations, per capita emissions are usually not an indication of total emissions. Total emissions of a highly populated country with modest per capital emissions can exceed those of a less populated country with greater per capita emissions.

Developing countries did not agree to limit their future greenhouse gas emissions primarily because most of the growth to date in greenhouse gas concentration, and the accompanying global changes resulting from this increase, has been due to emissions from developed countries.

On a per-molecule basis, CFCs contribute more. However, since the amount of CO2 emissions far exceeds CFC emissions, CO2 emissions account for most of the global warming experienced to date. Since CFC molecules have a greenhouse factor of 25,000, one molecule of CFC has the same impact as 25,000 molecules of CO2.

Synthesizing Ideas

Yes, the replacements proposed for CFCs are themselves greenhouse gases. Their emission is considered less of a problem in enhancing the greenhouse effect because they have shorter atmospheric lifetimes and they absorb thermal infrared radiation less efficiently than do the CFCs. Thus, on a molecular basis, they pose less of a greenhouse threat.

Atmospheric particles can either reflect or absorb light. When these particles reflect sunlight back to space, the light energy is unavailable to be absorbed. Thus, reflection causes atmospheric cooling. When these particles absorb light energy, it is almost immediately converted to heat energy. Consequently the temperature of the air surrounding the particles increases. The net effect of energy absorption is atmospheric heating.

When aerosol particles absorb certain wavelengths of light, the energy associated with this light is almost immediately converted into heat. When these “heated” aerosol particles collide with nearby molecules of air, they impart some of this “heat” energy to the air molecules. Thus, the absorption of light energy by an aerosol particle results in the warming of the air immediately surrounding it.

Volcanic eruptions produce massive amounts of SO2 gas that are ultimately transformed into a sulfuric acid aerosol that remains in the stratosphere for several years. Sulfate aerosols to not absorb sunlight since their constituents do not absorb it. Rather, these aerosols efficiently reflect incoming sunlight back into space, thereby lower the air temperature. The climatic effects associated with volcanic eruptions are generally short-term and dramatic.

Sulfate aerosols in the troposphere reflect sunlight back into space more efficiently than they absorb it. As a result, less sunlight is absorbed by the surface of the Earth and by gases in the troposphere.

The trends in global temperature changes from the pre-industrial era to the present include an increase in average global temperature of several degrees, shorter winters, a shrinking ice cover, ocean warming, and a rise in sea levels.

Atmospheric substances can either reflect or absorb light. When these substances reflect sunlight back to space, the light energy is not available to be absorbed. Thus, reflection causes atmospheric cooling. When these substances absorb light energy, it is almost immediately converted to heat energy. Consequently the temperature of the air surrounding the substance increases. Energy absorption results in atmospheric heating. Since average global temperatures are increasing, the net effect is an increase in atmospheric heating.

50. The increase in CO2 concentrations in the air is mainly determined by the amount of CO2 emissions. Since carbon dioxide has atmospheric lifetimes of a century or more, the gas that does not find a temporary sink will accumulate in the air and remain there for a significant period of time.

The potential impact of global warming on weather patterns include an increase in global atmospheric temperature of 1.4 to 5.8oC, a rise in global sea level of 9 to 88 centimeters (3.5 to 34.6 inches), a reduction in northern hemisphere snow cover and sea-ice content, a global increase in average water vapor and precipitation, an increase in the number of “hot” days coupled with a decrease in the number of cold waves and frost days, and an increase in high-intensity precipitation events.

Due to the limited adaptive abilities of living organisms, a rapid rise in global temperatures poses a greater threat. More gradual transitions are easier for humans and other organisms to handle.

The density of seawater gradually decreases as water warms beyond 4oC. Since density is defined as the mass of a substance divided by the volume it occupies, and the mass of a substance is always constant, its volume must increase as the density decreases. Thus, as seawater warms, the volume it occupies increases and sea level increases.

Both air and land surfaces warm quickly when exposed to an increase in average global temperatures. The response of seawater, however, is considerably slower. It may take centuries for an increase in air temperature to be translated into increased water temperatures deep in the ocean. As a result, a rise in sea level due to global warming patterns may take many years to occur.

Since more water will evaporate from rivers, lakes, oceans, and the soil as surface temperatures increase, global rainfall should increase as a result of global warming. However, it is anticipated that the increased rate of evaporation of soil moisture, due to the elevated temperature, will be greater than the rate at which the moisture is replaced by increased rainfall.

Decreased soil moisture, increased insect populations, destabilization of ecosystems, increased summer mortality rates, and increased levels of insect-carried diseases are all negative effects of global warming. Potentially beneficial effects of global warming include higher rates of photosynthesis due to increased atmospheric CO2 levels, longer frost-free growing seasons in northern latitudes, decreased winter mortality rates, and increased disruption of insect life-cycles due to increased rainfall.