The Greenhouse Effect and Climate Change

Earth's climate is a delicate balance of energy inputs, chemical processes, and physical phenomena. Temperatures on Venus are too hot for the human body; on Mars, they are too cold. This difference in temperature is due to the varying composition of each planet's atmosphere. All three planets receive huge quantities of solar energy, but the amount radiated back into space as heat depends on the atmospheric composition of the particular planet. Some gases, such as carbon dioxide (CO₂) and methane, absorb and maintain heat in the same way that glass traps heat in a greenhouse. These gases in Earth's atmosphere allow temperatures to build up, keeping our planet warm and habitable. That is why the increased buildup of these and other gases caused by pollution is often called the "greenhouse effect." (See Figure 2.1.)

It is necessary, however, to distinguish between the "natural" and a possible "enhanced" greenhouse effect. The natural greenhouse effect creates a climate in which life can exist, causing the mean temperature of Earth's surface to be about 33 degrees Celsius (91 degrees Fahrenheit) warmer than it would be if natural greenhouse gases were not present. Without this process, Earth would be frigid and uninhabitable. An enhanced greenhouse effect, sometimes called the anthropogenic effect, refers to the possible increase in the temperature of Earth's surface due to human activity.

The scorching heat of Venus is the result of an atmosphere that is composed largely of CO₂. The atmosphere of Earth, however, is nitrogen- and oxygen-based and contains only 0.03 percent CO₂. This percentage has varied little over the past million years, permitting a stable climate favorable to life. The blanket of air enveloping Earth moderates its temperature and sustains life.

A Revolutionary Idea

Earth's atmosphere was first compared to a glass vessel in 1827 by the French mathematician Jean-Baptiste Fourier. In the 1850s British physicist John Tyndall tried to measure the heat-trapping properties of various components of the atmosphere. By the 1890s scientists had concluded that the great increase in combustion in the Industrial Revolution had the potential to change the atmosphere's load of CO₂. In 1896 the Swedish chemist Svante Arrhenius made the revolutionary suggestion that human activities could actually disrupt this delicate balance. He theorized that the rapid increase in the use of coal that came with the Industrial Revolution could increase CO₂ concentrations and cause a gradual rise in FIGURE 2.1
Greenhouse effect temperatures. For almost six decades his theory stirred little interest.

In 1957 studies at the Scripps Institute of Oceanography in California suggested that, indeed, half the CO₂ released by industry was being permanently trapped in the atmosphere. The studies showed that atmospheric concentrations of CO₂ in the previous 30 years were greater than in the previous two centuries and that the gas had reached its highest level in 160,000 years.

Findings in the 1980s and 1990s provided more disturbing evidence. Scientists detected increases in other, even more potent gases that contribute to the greenhouse effect, notably chlorofluorocarbons (CFC-11 and -12), methane, nitrous oxide (N₂O), and halocarbons (CFCs, methyl chloroform, and hydrochlorofluorocarbons). Atmospheric concentrations of these gases from the 1700s through the 1900s have increased drastically. As shown in Table 2.1, the atmospheric concentration of CO₂ increased from 280 parts per million (ppm) in preindustrial times to 370.3 ppm in 2001.

Table 2.2 shows trends for the late 1990s and early 2000s in U.S. greenhouse gas emissions and sinks (repositories, such as forests, that absorb and store carbon). Major sources of greenhouse gas emissions are shown in Figure 2.2. In 2001 electricity generation at power plants continued to account for the largest share (33 percent) of these emissions, followed by transportation (27 percent), industry (19 percent), agriculture (8 percent), commercial sources (7 percent), and residences (5 percent).

Emissions from most sources have increased since 1995. During the late 1990s emissions from industry began to decline and continued that trend into the early 2000s. The Environmental Protection Agency (EPA) attributes the decline to a shift in the overall U.S. economy from a focus on manufacturing industries to service-based businesses. Agricultural emissions are predominantly nitrogen-based, rather than carbon-based. Residential emissions are mainly due to CO₂ generated from combustion of fossil fuels (such as oil) for heating purposes.

Scientists know that atmospheric levels of greenhouse gases are increasing. Figure 2.3 shows the dramatic increase in concentrations of CO₂ from 1981 through 2002 based on data collected by the National Oceanic and Atmospheric Administration (NOAA) from its Climate Monitoring and Diagnostics Laboratory (CMDL). This buildup of CO₂ and other gases could possibly trap energy from the Sun. No one is certain how this accumulation affects Earth's climate.

Is the Earth Getting Warmer?

As of 2004 even experts are not sure whether the world has already experienced human-induced climate change. Scientists have been unable to provide a definitive answer because they do not know how much the global climate has varied on its own in the relatively recent past (about 1,000 years). Temperature records based on thermometers go back only about 150 years. Investigators have turned, therefore, to "proxy" (indirect) means of

TABLE 2.1
Global atmospheric concentration, rate of concentration change, and atmospheric lifetime (in years) of selected greenhouse gases

Atmospheric variable	CO₂	CH₄	N₂O	SF₆¹	CF₄¹
Pre-industrial atmospheric concentration	280	0.722	0.270	0	40
Atmospheric concentration²	370.3	1.842	0.316	4.7	80
Rate of concentration change³	1.5⁴	0.007⁴	0.0008	0.24	1.0
Atmospheric lifetime	50–200⁵	12⁶	114⁶	3,200	>50,000
¹Concentrations in parts per trillion (ppt) and rate of concentration change in ppt/year.
²Concentration for CO₂ was measured in 2001. Concentrations for all other gases were measured in 2000.
³Rate is calculated over the period 1990 to 1999.
⁴Rate has fluctuated between 0.9 and 2.8 ppm per year for CO 2 and between 0 and 0.013 ppm per year for CH₄ over the period 1990 to 1999.
⁵No single lifetime can be defined for CO₂ because of the different rates of uptake by different removal processes.
⁶This lifetime has been defined as an "adjustment time" that takes into account the indirect effect of the gas on its own residence time.
Note: Atmospheric concentrations are in parts per million (ppm) and rate of concentration change is parts per billion (ppb) per year.
SOURCE: "Table 1-1: Global Atmospheric Concentration (ppm Unless Otherwise Specified), Rate of Concentration Change (ppb/year) and Atmospheric Lifetime (Years) of Selected Greenhouse Gases," in Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2001, U.S. Environmental Protection Agency, Office of Atmospheric Programs, Washington, DC, April 15, 2003

measuring past temperatures. These methods include chemical evidence of climatic change contained in fossils, corals, ancient ice, and growth rings in trees.

In 1998 Drs. Michael E. Mann and Raymond S. Bradley of the University of Massachusetts at Amherst and Dr. Malcolm K. Hughes of the University of Arizona at Tucson surveyed proxy evidence of temperatures in the Northern Hemisphere since 1400. They discovered that the twentieth century was the warmest century of the past 600 years. They concluded that the warming trend seems to be closely connected to the emission of greenhouse gases by humans. Some experts, however, question whether studies of proxy evidence will ever be reliable enough to yield valuable information on global warming.

Three international agencies have compiled long-term data on surface temperatures—the British Meteorological Office in Bracknell, United Kingdom, the National Climatic Data Center in Asheville, North Carolina, and the National Aeronautics and Space Administration (NASA) Goddard Institute for Space Studies in New York. Temperature measurements from these organizations reported that the 1990s were the warmest decade of the twentieth century and the warmest decade since humans began measuring temperatures in the mid-nineteenth century. The average global surface temperature was approximately 1 degree Fahrenheit warmer than at the turn of the twentieth century, and this rise increased more rapidly since 1980.

According to NASA, the 2002 meteorological year was the second warmest year recorded since the late 1800s. (See Figure 2.4.) A meteorological year runs from the beginning of winter to the end of autumn. Thus, the 2002 meteorological year ran from December 1, 2001, to November 30, 2002. The mean surface temperature for that year was 0.51 degrees Celsius (33 degrees Fahrenheit) warmer than the climatological mean (average for 1951–1980). The warmest temperature occurred in 1998. There has been a strong warming trend over the past three decades.

While some scientists are uncertain whether the greenhouse effect accounts for the change, most believe that it is the most likely explanation. Climate models suggest the potential for a warming from 2 to 6 degrees Fahrenheit over the next 100 years, warmer than Earth has been for millions of years.

In December 2003 the council of the American Geophysical Union (AGU) issued a statement declaring that Earth's climate was warming faster than expected and blamed the increase on anthropogenic (human-related) emissions of greenhouse gases. The statement from the AGU, an international scientific organization with more than 40,000 members, declared that "scientific evidence strongly indicates that natural influences cannot explain the rapid increase in global near-surface temperatures observed in the second half of the 20th century" (David Perlman, "Earth Warming at Faster Pace, Say Top Science Group's Leaders," San Francisco Chronicle, December 18, 2003).

However, some scientists observe that major climate events should be viewed in terms of thousands of years, not just a century. A record of only the past century may indicate, but not prove, that a major change has occurred. Is it caused by greenhouse gases, or is it natural variability? While some experts believe it is not possible to conclude that the warming is caused by greenhouse gases emitted by human activity, the rising temperature is roughly on track with that of computer models programmed to predict the course of greenhouse warming.

FURTHER SIGNS.

In 1990, at the first of several meetings of the Intergovernmental Panel on Climate Change (IPCC), several early signs of actual climate change were noted: the average warm-season temperature in Alaska had risen nearly three degrees Fahrenheit in the previous fifty years; glaciers had generally receded and become thinner on average by about thirty feet in the past forty years; there was about 5 percent less sea ice in the Bering Sea than in the 1950s; and permafrost was thawing, causing

TABLE 2.2
Recent trends in greenhouse gas emissions and sinks, in teragrams of carbon dioxide equivalents (Tg CO₂ Eq), 1990–2001

Gas/source	1990	1995	1996	1997	1998	1999	2000	2001
CO₂	5,003.7	5,334.4	5,514.8	5,595.4	5,614.2	5,680.7	5,883.1	5,794.8
Fossil fuel combustion	4,814.8	5,141.5	5,325.8	5,400.0	5,420.5	5,488.8	5,692.2	5,614.9
Iron and steel production	85.4	74.4	68.3	71.9	67.4	64.4	65.8	59.1
Cement manufacture	33.3	36.8	37.1	38.3	39.2	40.0	41.2	41.4
Waste combustion	14.1	18.5	19.4	21.2	22.5	23.9	25.4	26.9
Ammonia manufacture & urea application	19.3	20.5	20.3	20.7	21.9	20.6	19.6	16.6
Lime manufacture	11.2	12.8	13.5	13.7	13.9	13.5	13.3	12.9
Natural gas flaring	5.5	8.7	8.2	7.6	6.3	6.7	5.5	5.2
Limestone and dolomite use	5.5	7.0	7.6	7.1	7.3	7.7	5.8	5.3
Aluminum production	6.3	5.3	5.6	5.8	5.6	5.9	5.4	4.1
Soda ash manufacture and consumption	4.1	4.3	4.2	4.4	4.3	4.2	4.2	4.1
Titanium dioxide production	1.3	1.7	1.7	1.8	1.8	1.9	1.9	1.9
Carbon dioxide consumption	0.9	1.1	1.1	1.2	1.2	1.2	1.2	1.3
Ferroalloys	2.0	1.9	2.0	2.0	2.0	2.0	1.7	1.3
Land-use change and forestry (Sink)¹	(1,072.8)	(1,064.2)	(1,061.0)	(840.6)	(830.5)	(841.1)	(834.6)	(838.1)
International bunker fuels²	113.9	101.0	102.3	109.9	112.9	105.3	99.3	97.3
CH₄	644.0	650.0	636.8	629.5	622.7	615.5	613.4	605.9
Landfills	212.1	216.1	212.1	207.5	202.4	203.7	205.8	202.9
Natural gas systems	122.0	127.2	127.4	126.0	124.0	120.3	121.2	117.3
Enteric fermentation	117.9	123.0	120.5	118.3	116.7	116.6	115.7	114.8
Coal mining	87.1	73.5	68.4	68.1	67.9	63.7	60.9	60.7
Manure management	31.3	36.2	34.9	36.6	39.0	38.9	38.2	38.9
Wastewater treatment	24.1	26.6	26.8	27.3	27.7	28.2	28.3	28.3
Petroleum systems	27.5	24.2	23.9	23.6	22.9	21.6	21.2	21.2
Rice cultivation	7.1	7.6	7.0	7.5	7.9	8.3	7.5	7.6
Stationary sources	8.1	8.5	8.7	7.5	7.2	7.4	7.6	7.4
Mobile sources	5.0	4.9	4.8	4.7	4.6	4.5	4.4	4.3
Petrochemical production	1.2	1.5	1.6	1.6	1.6	1.7	1.7	1.5
Field burning of agricultural residues	0.7	0.7	0.7	0.8	0.8	0.8	0.8	0.8
Silicon carbide production	*	*	*	*	*	*	*	*
International bunker fuels²	0.2	0.1	0.1	0.1	0.1	0.1	0.1	0.1
N₂O	397.6	430.9	441.7	440.9	436.8	430.0	429.9	424.6
Agricultural soil management	267.5	284.1	293.2	298.2	299.2	297.0	294.6	294.3
Mobile sources	50.6	60.9	60.7	60.3	59.7	58.8	57.5	54.8
Manure management	16.2	16.6	17.0	17.3	17.3	17.4	17.9	18.0
Nitric acid	17.8	19.9	20.7	21.2	20.9	20.1	19.1	17.6
Human sewage	12.7	13.9	14.1	14.4	14.6	15.1	15.1	15.3
Stationary combustion	12.5	13.2	13.8	13.7	13.7	13.7	14.3	14.2
Adipic acid	15.2	17.2	17.0	10.3	6.0	5.5	6.0	4.9
N₂O product usage	4.3	4.5	4.5	4.8	4.8	4.8	4.8	4.8
Field burning of agricultural residues	0.4	0.4	0.4	0.4	0.5	0.4	0.5	0.5
Waste combustion	0.3	0.3	0.3	0.3	0.2	0.2	0.2	0.2
International bunker fuels²	1.0	0.9	0.9	1.0	1.0	0.9	0.9	0.9
HFCs, PFCs, and SF₆	94.4	99.5	113.6	116.8	127.6	120.3	121.0	111.0
Substitution of ozone depleting substances	0.9	21.7	30.4	37.7	44.5	50.9	57.3	63.7
HCFC-22 production	35.0	27.0	31.1	30.0	40.2	30.4	29.8	19.8
Electrical transmission and distribution	32.1	27.5	27.7	25.2	20.9	16.4	15.4	15.3
Semiconductor manufacture	2.9	5.9	5.4	6.5	7.3	7.7	7.4	5.5
Aluminum production	18.1	11.8	12.5	11.0	9.0	8.9	7.9	4.1
Magnesium production and processing	5.4	5.6	6.5	6.3	5.8	6.0	3.2	2.5
Total	6,139.6	6,514.9	6,707.0	6,782.6	6,801.3	6,849.5	7,047.4	6,936.2
Net emissions (sources and sinks)	5,066.8	5,450.7	5,646.0	5,942.0	5,970.9	6,008.5	6,212.7	6,098.1
*Does not exceed 0.05 Tg CO₂ Eq.
¹For the most recent years, a portion of the sink estimate is based on historical and projected data. Parentheses indicate negative values (or sequestration).
²Emissions from international bunker fuels are not included in totals.
Note: Totals may not sum due to independent rounding.
SOURCE: "Table ES-1: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Tg CO₂ Eq.)," in Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2001, U.S. Environmental Protection Agency, Office of Atmospheric Programs, Washington, DC, April 15, 2003

the ground to subside, opening holes in roads, producing landslides and erosion, threatening roads and bridges, and causing local floods. Ice cellars in northern villages have thawed and become useless. More precipitation now falls as rain than snow, and the snow melts faster, causing more running and standing water. While this could be natural variability, it is the kind of change expected of global warming, that is, the Arctic will warm more than the global average.

According to the EPA, global sea levels have risen by six to eight inches over the last century. The increase is attributed to melting mountain glaciers, expansion of ocean water in response to rising temperatures, melting of the polar ice sheets, and surface discharge of groundwater FIGURE 2.2
U.S. greenhouse gas emissions allocated to economic sectors, 1990–2001 that has been pumped out of the ground. The sea level along much of the U.S. coast has risen by ten to twelve inches per century, although the rate varies by location.

Melting glaciers and collapsing ice shelves in Antarctica have been well publicized, particularly the breakup of the Larsen B Ice Shelf in early 2002. Scientists at the U.S. Geological Survey estimate that melting of Antarctica's entire ice sheet will increase the sea level by 73 meters (approximately 240 feet). However, it is unknown whether the ice sheet is actually shrinking or growing.

The Effects of Volcanic Activity on Climate

Volcanic activity, such as the 1991 eruption of the Mount Pinatubo volcano in the Philippines, can temporarily offset recent global warming trends. Volcanoes spew vast quantities of particles and gases into the atmosphere, including sulfur dioxide (SO₂) that combines with water to form tiny supercooled droplets. The droplets create a long-lasting global haze that reflects and scatters sunlight, reducing energy from the Sun and preventing its rays from heating the Earth, thereby causing the planet to cool.

This also occurred in 1982, when the El Chichon volcano in Mexico depressed global temperatures for about four years. NASA reported that satellite sensors measured the SO₂ cloud from Mount Pinatubo at 15 million tons, about twice the size of the one emitted by El Chichon. FIGURE 2.3
Global average atmospheric carbon dioxide concentration, 1981–2002 NASA found that the haze of sulfur from the eruption reflected enough sunlight to cool the Earth by about 1 degree Fahrenheit, as was predicted by computer models.

In 1815 a major eruption of the Tambora volcano in Indonesia produced serious weather-related disruptions, such as crop-killing summer frosts in the United States and Canada. It became known as the "year without a summer." On the other hand, for several years following the Tambora eruption people around the world commented about the beautiful sunsets, which were caused by the suspension of volcano-related particulate matter in the atmosphere.

The Effects of Clouds on Climate

Clouds may hold a key to understanding climate change. Although we see clouds virtually every day, surprisingly little is known about them—where they occur, their role in energy and water transfer, and their ability to reflect solar heat. Earth's climate maintains a balance between the energy that reaches Earth from the Sun and the energy that radiates back from Earth into space. Scientists refer to this as Earth's "radiation budget." The components of Earth's system are the planet's surface, atmosphere, and clouds.

Different parts of Earth have different capacities to reflect solar energy. Oceans and rain forests reflect only a small portion of the Sun's energy. Deserts and clouds, on the other hand, reflect a large portion of solar energy. A cloud reflects more radiation back into space than the surface would in the absence of clouds. An increase in cloudiness can also act like the panels on a greenhouse roof.

FIGURE 2.4
Mean global temperature changes, 1880–2000

NASA's Earth Science Enterprise is a satellite-based program that includes numerous scientific studies of clouds. These studies have revealed that:

The effect of clouds on climate depends on the balance between the incoming solar radiation and the absorption of Earth's outgoing radiation.
Low clouds have a cooling effect because they are optically thicker and reflect much of the incoming solar radiation out to space.
High thin cirrus clouds have a warming effect because they transmit most of the incoming solar radiation while also trapping some of Earth's radiation and radiating it back to the surface.
Deep convective clouds have neither a warming nor a cooling effect because their reflective and absorptive abilities cancel one another.

Another Possible Climate Culprit—Solar Cycles

Scientists have known for centuries that the Sun goes through cycles; it has seasons, storms, and rhythms of activity with sunspots and flares appearing in cycles of roughly 11 years. Some scientists contend that these factors play a role in climate change on Earth. Some research, though sketchy and controversial, suggests that the Sun's variability could account for some, if not all, of global warming to date. The biggest correlation occurred centuries ago—between 1640 and 1720—when sunspot activity fell sharply and Earth cooled about 2 degrees Fahrenheit. (The Sun is brighter when sunspots appear and dimmer when they disappear.)

The Effects of the Oceans on Climate

Oceans have a profound effect on climate, because of their huge capacity to store heat and because they can moderate levels of atmospheric gases that regulate global temperatures. Covering more than 70 percent of Earth and holding 97 percent of the water on the planet's surface, oceans function as huge reservoirs of heat. Ocean currents transport this stored heat and dissolved gases so that different areas of the world serve as either sources or sinks (repositories) for these components. While scientists know a great deal about oceanic and air circulation, they are less certain about the ocean's ability to store additional CO₂ or about how much heat it will absorb.

The top eight feet of the oceans hold as much heat as Earth's entire atmosphere. As ocean waters circulate globally, heat is transferred from low altitudes to high altitudes, from north to south, and vertically from surface to deep oceans and back. But how is this heat apportioned? If heat is able to circulate through the entire oceanic depth range, the process could take centuries and the world's oceans could serve to buffer or delay global warming. Researchers are working to determine that possibility, but it remains one of many unanswered questions.

The Effects of El Niño and La Niña on Climate

For centuries fishermen in the Pacific Ocean off the coast of South America have known about the phenomenon called El Niño. Every three to five years, during December and January, fish in those waters virtually vanish, bringing fishing to a standstill. Fishermen gave this occurrence the name "El Niño," which means "the Child," because it occurs around the celebration of the birth of Jesus, the Christ child. Although originating in the Pacific, the effects of El Niño are felt around the world. Computers, satellites, and improved data gathering have found that the El Niño phenomenon has been responsible for drastic climate change.

An El Niño occurs because of interactions between atmospheric winds and sea surfaces. In normal years, trade winds blow from east to west across the eastern Pacific. They drag the surface waters westward across the ocean, causing deeper, cold waters to rise to the surface. This upwelling of deep ocean waters carries nutrients from the bottom of the ocean that feed fish populations in the upper waters.

In an El Niño the westward movement of waters weakens, causing the upwelling of deep waters to cease. The resulting warming of the ocean waters further weakens trade winds and strengthens El Niño. Without upwelling, the nutrient content of deep waters is diminished, which in turn causes the depletion of fish populations. The warm waters that normally lie in the western waters of the Pacific shift eastward. This turbulence creates eastward weather conditions, in which towering cumulus clouds reach high into the atmosphere with strong vertical forces and the weakening of normal east-to-west trade winds. An El Niño is the warm phase of a phenomenon known as ENSO (El Niño/Southern Oscillation), which can also include a cold phase known as a La Niña.

The worldwide effects of El Niño can include torrential rains, tornadoes, hurricanes, mud slides, flash flooding, beach and cliff erosion, sewage spills, drought, increased snowfall, and disruption in the marine food chain. Such weather events often affect regional energy and economic markets. The El Niño of 1982–83 is estimated to have caused $13.6 billion in damage and killed 2,000 people around the world. In 1997 and 1998 El Niño–related storms in California caused more than 9,000 people to seek federal disaster assistance for property losses. An estimated $3.6 million in aid was distributed to approximately 1,689 people. The American Fisheries Society estimated that 1990s storms were the most devastating for marine life in more than a century.

The Greenhouse Effect and Climate Change - The World Climate