The Enhanced Greenhouse Effect and Global Warming

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Climate and weather are not the same thing. Both describe conditions in the lower atmospherefor example, wet or dry, cold or warm, stormy or fair, and cloudy or clear. Weather is the short-term local state of the atmosphere. Weather conditions can change from moment to moment and can differ in two places that are relatively close together. Climate describes the average pattern of weather conditions experienced by a region over a long period. For example, Florida has a warm climate but can experience days and even weeks of cold weather.

Earth's climate as a whole has not changed much for a few thousand years. In general, most of the planet has been warm enough for humans, animals, and plants to thrive. This was not so in the distant past, when the climate fluctuated between long periods of cold and warmth, each lasting for many thousands of years. Scientists are not sure what triggered these major climate changes. A variety of factors are believed to be involved, including movement of the tectonic plates, changes in Earth's orbit around the sun, and variations in atmospheric gases.

Scientists believe that the last ice age occurred about eighteen thousand years ago. During this period ice sheets and glaciers spread to cover vast regions of the Earth, including most of North America and Europe.

Earth's temperature depends on a delicate balance of energy inputs and outputs, chemical processes, and physical phenomena. As shown in Figure 3.1, solar radiation passes through Earth's atmosphere and warms the Earth. The Earth emits infrared radiation. Some outgoing infrared radiation is not allowed to escape into outer space but is trapped beneath the atmosphere. The amount of energy that is trapped depends on many variables. One major factor is atmospheric composition. Some gases, such as water vapor, carbon dioxide, and methane, act to trap heat beneath the atmosphere in the same way that glass panels trap heat in a greenhouse. The panels allow sunlight into the greenhouse, but prevent heat from escaping.

Earth's surface temperature is about sixty degrees Fahrenheit warmer than it would be if natural greenhouse gases were not present. Without this natural warming process, Earth would be much colder and could not sustain life as it now exists.

It is necessary, however, to distinguish between the "natural" and an "enhanced" greenhouse effect. The natural greenhouse effect provides a warm atmosphere for Earth that is necessary for life. The theory behind the enhanced greenhouse effect is that human activities can load the atmosphere with too much carbon dioxide and other heat-trapping gases. This could increase Earth's temperature above that expected from the natural greenhouse effect, an effect known as global warming. Such a temperature increase would be accompanied by major climatic changes.

The primary human activities linked to the enhanced greenhouse effect are the burning of fossil fuels (mainly coal and oil) and their derivatives (such as gasoline), and destruction of large amounts of vegetation that normally absorb carbon dioxide. (See Figure 3.2.)


Greenhouse gases are gases in the atmosphere that allow shortwave radiation (sunlight) from the sun to pass through to the Earth but absorb and reradiate long wave infrared radiation (heat) coming from the Earth's surface. This process serves to warm the lower atmosphere (the troposphere). The troposphere extends from the Earth's surface to approximately eight miles above the surface, as shown in Figure 3.3.


Water Vapor

Scientists know that water vapor is the most prevalent greenhouse gas in the atmosphere. According to the National Safety Council's Environmental Health Center, in "A 'Wet Blanket' Greenhouse Gas" (January 24, 2000,, water vapor makes up as much as 2% of the atmosphere and is responsible for approximately two-thirds of the natural greenhouse effect. Scientists believe, however, that humans have little to no influence on the amount of water vapor in the atmosphere. Water vapor is part of the natural water cycle that takes place on and around the Earth. Water evaporates from the surface, condenses into clouds, and then returns to the surface as precipitation. The water cycle is also a heat cycle, transferring heat around the Earth and back and forth between the surface and the atmosphere. Water vapor cycles quickly through the atmosphere, lingering for a few days at most.

Carbon Dioxide

Carbon dioxide (CO2) is a heavy colorless gas that, according to the Environmental Health Center, makes up approximately 0.037% of the atmosphere. CO2 is a respiration product from all living things (plants, animals, and humans). It is also released during the decay or combustion of organic materials. Huge amounts of CO2 are cycled back and forth between the oceans and the atmosphere. Likewise, vegetation absorbs CO2 from the air. The result of all these processes is a global carbon cycle that maintains CO2 at suitable levels in the atmosphere to sustain a natural greenhouse effect.

Before the 1800s humans had little impact on atmospheric CO2 levels. The Industrial Revolution ushered in widespread use of fossil fuels, primarily coal, oil, and natural gas. Combustion of these carbon-loaded fuels releases large amounts of CO2. The burning of fossil



fuels by industry and motor vehicles is, by far, the leading source of CO2 emissions in the United States, accounting for 5,751.2 teragrams (or 94%) of the nation's emission of greenhouse gases in 2005. (See Table 3.1.) Other anthropogenic (human-caused) sources include deforestation, burning of biomass (combustible organic materials, such as wood scraps and crop residues), and certain industrial processes. The atmospheric lifetime (how long a gas stays in the atmosphere) of CO2 is estimated to be fifty to two hundred years.


Methane (CH4) is a colorless gas found in trace (extremely small) amounts in the atmosphere. It is the primary component of natural gasthe gas trapped beneath the Earth's crust that is mined and burned for energy. Methane is an important component of greenhouse emissions, second only to CO2.(See Figure 3.4.) Even though there is less methane in the atmosphere, scientists believe that it may be much more effective at trapping heat in the atmosphere than CO2. During the 1900s methane's concentration in the atmosphere more than doubled. Scientists generally attribute these increases to human sources, such as landfills, natural gas systems, agricultural activities, coal mining, and wastewater treatment.

As shown in Table 3.1, methane emissions from landfills, enteric fermentation, and natural gas systems accounted for 356.2 teragrams (or 66%) of total U.S. emissions in 2005. Enteric fermentation is a natural digestive process that occurs in domestic animals, such as cattle and sheep, and releases methane. Growing markets for beef and milk products are driving a booming livestock business.

Humans contribute to atmospheric methane levels through activities that concentrate and magnify biological decomposition. This includes landfilling organic materials, raising livestock, cultivating rice in paddies, collecting sewage for treatment, and constructing artificial wetlands. In addition, methane is a by-product of the combustion of biomass and is vented (intentionally and unintentionally) during the extraction and processing of fossil fuels. It also results from incomplete combustion of fossil fuels. Methane is believed to break down in the atmosphere after approximately nine to fifteen years.


Ozone (O3) is a blue-tinted gas naturally found in Earth's atmosphere. In Reporting and Climate Change: Understanding the Science (June 2000,, the National Safety Council's Environmental Health Center notes that approximately 90% of the ozone is in the stratosphere, the atmospheric layer lying above the troposphere. The so-called ozone layer absorbs harmful ultraviolet radiation from the sun to prevent it from reaching the ground. Scientists believe that stratospheric ozone is being depleted by the introduction of certain industrial chemicals, primarily chlorine and bromine. This depletion has serious consequences in terms of ultraviolet radiation effects and probably lessens the warmth-trapping capability of ozone at this level.

Tropospheric ozone is the primary component in smog, a potent air pollutant. It is not emitted directly into the air but forms because of complex reactions that occur when other air pollutants, primarily volatile organic compounds and nitrogen oxides, are present. The primary sources of these ozone precursors include industrial chemical processes and fossil fuel combustion. The atmospheric lifetime of ozone ranges from weeks to months.

Nitrous Oxide

Nitrous oxide (N2O) is a colorless gas found in trace amounts in the atmosphere. Soils naturally release the gas as a result of bacterial processes called nitrification and denitrification. Soils found in tropical areas and moist forests are believed to be the largest contributors. Oxygen-poor waters and sediments in oceans and estuaries are also natural sources. Even though nitrous oxide makes up a much smaller portion of greenhouse gases than CO2, it is much more (perhaps 310 times more) powerful than CO2 at trapping heat. Agriculture is and has been the major source of nitrous oxide emissions in the United States, followed by energy and industrial sources. As shown in Table 3.1, agricultural soil management accounted for 365.1 teragrams (or 78%) of total nitrous oxide emissions during 2005.


Trends in U.S. greenhouse gas emissions and sinks, in teragrams of carbon dioxide equivalents, selected years 19902005
[Teragrams CO2 equivalent]
CO 25,061.65,384.65,940.05,843.05,892.75,952.56,064.36,089.5
Fossil fuel combustion4,724.15,030.05,584.95,511.75,557.25,624.55,713.05,751.2
Non-energy use of fuels117.3133.2141.0131.4135.3131.3150.2142.4
Cement manufacture33.336.841.241.442.943.145.645.9
Iron and steel production84.973.365.157.954.653.451.345.2
Natural gas systems33.733.829.428.829.628.428.228.2
Municipal solid waste combustion10.915.717.918.318.519.520.120.9
Ammonia production and urea application19.320.519.616.717.816.216.916.3
Lime manufacture11.312.813.312.912.313.013.713.7
Limestone and dolomite use5.
Soda ash manufacture and consumption4.
Aluminum production6.
Petrochemical production2.
Titanium dioxide production1.
Ferroalloy production2.
Phosphoric acid production1.
CO2 consumption1.
Zinc production0.
Lead production0.
Silicon carbide production and consumption0.
Land-use change and forestry (sink) a(712.8)(828.8)(756.7)(767.5)(811.9)(811.9)(824.8)(828.5)
International bunker fuels b113.7100.6101.197.689.183.797.297.2
Wood biomass and ethanol consumption b219.3236.8228.3203.2204.4209.6224.8206.5
CH 4609.1598.7563.7547.7549.7549.2540.3539.3
Enteric fermentation115.7120.6113.5112.5112.6113.0110.5112.1
Natural gas systems124.5128.1126.6125.4125.0123.7119.0111.1
Coal mining81.966.555.955.552.052.154.552.4
Manure management30.935.138.740.141.140.539.741.3
Petroleum systems34.431.127.827.426.825.825.428.5
Wastewater treatment24.825.126.425.925.825.625.725.4
Forest land remaining forest land7.
Stationary combustion8.
Rice cultivation7.
Abandoned coal mines6.
Mobile combustion4.
Petrochemical production0.
Iron and steel production1.
Field burning of agricultural residues0.
Ferroalloy production++++++++
Silicon carbide production and consumption++++++++
International bunker fuels b0.
N 2O482.0484.2499.8502.5479.2459.8445.2468.6
Agricultural soil management366.9353.4376.8389.0366.1350.2338.8365.1
Mobile combustion43.753.753.249.747.143.841.238.0
Nitric acid production17.819.919.615.917.216.716.015.7
Stationary combustion12.312.814.013.513.413.713.913.8
Manure management8.
Wastewater treatment6.
Adipic acid production15.
Settlements remaining settlements5.

Humans have significantly increased the release of nitrous oxide from soils through use of nitrogen-rich fertilizers. Other anthropogenic sources include combustion of fossil fuels and biomass, wastewater treatment, and certain manufacturing processes, particularly the production of nylon and nitric acid. Nitrous oxide has an atmospheric lifetime of approximately 120 years.

Engineered Gases

Engineered gases are synthetic gases specially designed for modern industrial and commercial purposes.

+Does not exceed 0.05 Tg CO2Eq.
aParentheses indicate negative values or sequestration. The net CO2 flux total includes both emissions and sequestration, and constitutes a sink in the United States. Sinks are only included in net emissions total.
bEmissions from International bunker fuels and biomass combustion are not included in totals.
Totals may not sum due to independent rounding
CO2Carbon dioxide
N20Nitrogen oxides
SF6Sulfur hexafluoride
SOURCE: "Table ES-2. Recent Trends in U.S. Greenhouse Gas Emissions and Sinks," in Inventory of U.S. Greenhouse Gas Emissions and Sinks: 19902005, U.S. Environmental Protection Agency, April 2007, (accessed June 19, 2007)
N2O product usage4.
Forest land remaining forest land0.
Field burning of agricultural residues0.
Municipal solid waste combustion0.
International bunker fuels b1.
HFCs, PFCs, and SF 689.3103.5143.8133.8143.0142.7153.9163.0
Substitution of ozonedepleting substances0.332.280.988.696.9105.5114.5123.3
HCFC-22 production35.027.029.819.819.812.315.616.5
Electrical transmission and distribution27.121.815.215.114.313.813.613.2
Semiconductor manufacture2.
Aluminum production18.511.
Magnesium production and processing5.
Net emissions (sources and sinks)5,529.25,742.26,390.56,259.56,252.76,292.36,378.96,431.9


They are also known as "high GWP gases," because they have a high global warming potential when compared with CO2. They include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6).

HFCs are chemicals that contain hydrogen, fluorine, and carbon. They are popular substitutes in industrial applications for chlorofluorocarbons (CFCs). CFCs are commonly used in cooling equipment, fire extinguishers, as propellants, and for other uses. They are one of the culprits blamed for the depletion of stratospheric ozone.

PFCs are a class of chemicals containing fluorine and carbon. They are also increasingly used by industry as substitutes for ozone-depleting CFCs. SF6 is a colorless, odorless gas commonly used as an insulating medium in electrical equipment and as an etchant (an etching agent) in the semiconductor industry.

Even though emissions of these chemicals are small in comparison with other greenhouse gases, they are of particular concern because of their long life in the atmosphere. PFCs and SF6 have atmospheric lifetimes of thousands of years and are actually far more potent greenhouse gases than CO2 per unit of molecular weight.


Indirect Greenhouse Gases

There are several gases considered indirect greenhouse gases because of their effects on the chemical environment of the atmosphere. These gases include reactive nitrogen oxides, carbon monoxide, and volatile organic compounds. Most of their emissions are from anthropogenic sources, primarily combustion and industrial processes.

U.S. Greenhouse Gases and Sources

The U.S. Environmental Protection Agency (EPA) reports that CO2 accounted for 84% of greenhouse gas emissions in the United States in 2005. (See Figure 3.4.) Methane was second with 7% of the total, followed by nitrous oxide with 7% and other greenhouse gases, such as HFCs, PFCs, and SF6, with 2%.

Figure 3.5 shows the major U.S. sectors that have produced energy-related emissions of CO2 since 1996. In Inventory of U.S. Greenhouse Gas Emissions and Sinks: 19902005 (April 2007,, the EPA reports that the contributions of these major sectors in 2005 were:

  • Transportation (33%)
  • Industry (27%)
  • Residences (21%)
  • Commercial (18%)

Emissions from most sources have increased since 1996. During the late 1990s emissions from industry began to decline and continued that trend into the early 2000s. The EPA attributes the decline to a shift in the overall U.S. economy from a focus on manufacturing industries to service-based businesses. Residential emissions are mainly because of CO2 generated from the combustion of fossil fuels (such as oil) for heating purposes.

International Emissions of Greenhouse Gases

In International Energy Annual 2004 (July 2006,, the Energy Information Administration presents data collected on CO2 emissions related to fossil fuel use around the world. As shown in Figure 3.6, the United States was responsible for the largest portion (23%) of


such emissions in 2004, followed by Europe (17%) and China (17%). The five countries with the largest CO2 emissions in 2004 in units of million metric tons were:

In its energy and emissions analyses the U.S. Department of Energy (DOE) assesses countries based on whether or not they are members of the Organization for Economic Cooperation and Development (OECD). The OECD was founded in 1961 and is an international organization of democratic countries with free market economies (such as the United States). OECD countries are developed nations characterized by mature economies and industries and relatively slow rates of growth in population and fuel usage. Non-OECD countries include developing nations in which population, industrial base, and fuel usage are growing quickly. Examples include China, India, Russia, and most other areas of Central and South America, Asia, the Middle East, and Africa.

As shown in Figure 3.7 and Figure 3.8, non-OECD economies are expected to experience the highest average annual growth in energy-related CO2 emissions between




2004 and 2030. U.S. CO2 emissions are expected to grow at a relatively low rate of 1.1% per year over this period. By comparison, China's average CO2 emissions are predicted to increase by 3.4% annually and India's by 2.6% annually. At these rates China's CO2 emissions will surpass those of the United States by 2015. (See Figure 3.9.) China and other developing nations are heavily reliant on coal to fuel their industrial development and electricity production, whereas developed nations, such as the United States, rely more on cleaner burning fuels such as natural gas. Environmentalism as a social and political force is also much more mature in developed nations.


Earth's atmosphere was first compared to a glass vessel in 1827 by the French mathematician Jean-Baptiste Fourier (17681830). In the 1850s the British physicist John Tyndall (18201893) 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 (18591927) 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 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 thirty years were greater than in the previous two centuries and that the gas had reached its highest level in 160,000 years. Scientists can estimate the makeup of Earth's atmosphere long ago by testing air pockets in ice sheets believed to have formed around the same time.

Findings in the 1980s and 1990s provided more disturbing evidence of atmospheric changes. Scientists detected increases in other, even more potent gases that contribute to the greenhouse effect, notably CFC-11 and CFC-12, methane, nitrous oxide, and halocarbons (CFCs, methyl chloroform, and hydrochlorofluorocarbons).

T. J. Blasing and Karmen Smith of the Carbon Dioxide Information Analysis Center estimate in "Recent Greenhouse Gas Concentrations" (July 2006, that the average "natural" background atmospheric concentration of CO2 before 1750 was 280 parts per million. Likewise, the average methane level before 1750 was around 730 parts per billion. Increases in these gases in recent years are shown in Figure 3.10 for CO2 and in Figure 3.11 for methane. These data were collected by the National Oceanic and Atmospheric Administration (NOAA) from its Earth System Research Laboratory (ESRL). The ESRL is headquartered in Boulder, Colorado, and operates observatories in Point Barrow, Alaska; Trinidad Head, California; Mauna Loa, Hawaii; American Samoa; and the South Pole. The agency compiles long-term records on air quality and solar radiation data. In 2006 the average global CO2 concentration exceeded 380 parts per million. The average global methane concentration was around 1,775 parts per billion.

Scientists agree that atmospheric concentrations of gases known to play a role in the natural greenhouse effect are increasing. There is scientific consensus that this increase is driving up Earth's temperature.


Scientists do not know how much the global temperature has varied on its own in the last one thousand years. Temperature records based on thermometers go back only about 150 years. Therefore, investigators have turned to


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

In 1998 Michael E. Mann, Raymond S. Bradley, and Malcolm K. Hughes surveyed proxy evidence of temperatures in the Northern Hemisphere since 1400 and reported their findings in "Northern Hemisphere Temperatures during the Past Millennium: Inferences, Uncertainties, and Limitations" (Geophysical Research Letters, vol. 26, no. 6, 1999). They note that the twentieth century was the warmest century of the past six hundred years. Mann, Bradley, and Hughes conclude 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 report 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 one degree Fahrenheit warmer than at the turn of the twentieth century, and this rise increased more rapidly since 1980.

NASA reports in "2006 Was Earth's Fifth Warmest Year" (February 8, 2007, that the five warmest meteorological years recorded since the 1890s are:

  • 2005
  • 1998
  • 2002
  • 2003
  • 2006

A meteorological year runs from the beginning of winter to the end of autumn. Figure 3.12 shows the anomaly (deviation from the normal) for global mean (average) surface temperatures from 1880 through 2006, compared with the mean from 1951 to 1980. There has been a strong warming trend over the past three decades with the Earth warming by approximately 0.6 degrees Celsius (or nearly 1.1 degrees Fahrenheit). Examination of the worldwide anomalies for 2006 indicate that a few parts of the Earth have cooled, whereas most others have warmed. The most dramatic warming trend is seen over the Arctic (North Pole), Alaska, Siberia, and Antarctica (South Pole) regions.


The World Meteorology Organization Speaks Up

At the 1972 Stockholm Conference, the world's first ecological summit, climate change was not even listed among the threats to the environment. Many Earth scientists and meteorologists, however, were becoming alarmed about the growing evidence supporting the notion of an enhanced greenhouse effect. In 1979 the World Meteorological Organization (WMO) established its World Climate Program to collect data and research the complex components of the Earth's climate system. The WMO is a nongovernmental agency under the United Nations Environment Program (UNEP).

According to the Intergovernmental Panel on Climate Change, in 16 Years of Scientific Assessment in Support of the Climate Convention (December 2004,, at its 1979 conference the WMO acknowledged that "man's activities on Earth may cause significant extended regional and even global changes of climate." This was the first major step in the response of the international community to the threat of global warming.

In 1985 representatives of the WMO, the UNEP, and the International Council for Science met in Austria to


discuss the role of CO2 and other greenhouse gases in climate change. They predicted that rising levels of these gases would cause an increase in the global temperature during the first half of the twenty-first century.

The Intergovernmental Panel on Climate Change

In 1988 the WMO and the UNEP established the Inter-governmental Panel on Climate Change (IPCC). The IPCC set up three working groups to assess available scientific information on climate change, estimate the expected impacts of climate change, and formulate strategies for responding to the problem. The first IPCC assessment report was issued in 1990.

Several signs of climate change were noted by the IPCC in its report:

  • 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 previous forty years.


  • There was about 5% less sea ice in the Bering Sea than in the 1950s.
  • Permafrost was thawing, causing 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 had thawed and become useless.
  • More precipitation was falling as rain than snow in northern areas, and the snow was melting faster, causing more running and standing water.

The IPCC report was the most comprehensive summary of climate-change science to date. It represented the input of approximately four hundred international scientists and acknowledged that global warming was a real threat to the Earth's climate.

Using computer models, IPCC researchers predicted that the global mean temperature would increase by 0.5 degrees Fahrenheit each decade during the twenty-first century. They also predicted that the global mean sea level would rise by 2.4 inches per decade. However, the scientists noted that there were a number of uncertainties in their assumptions based on lack of data.

The United Nations Framework Convention on Climate Change

In 1992 the United Nations (UN) adopted the UN Framework Convention on Climate Change (UNFCCC). The UNFCCC was an international agreement presented for signatures at the 1992 Earth Summit in Rio de Janeiro, Brazil. The stated objective of the agreement was: "Stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system. Such a level should be achieved within a timeframe sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner."

The agreement set specific goals for developed countries to track and publish detailed inventories of their greenhouse gas emissions. However, it did not include specific emissions targets that countries had to meet. The UNFCCC was signed by more than one hundred countries, including the United States. Many environmental-ists criticized the treaty as too weak because it did not establish specific targets that governments must meet. The treaty did not include specific targets mainly because the United States refused to accept them. The U.S. Senate ratified (formally approved into law) the UNFCCC. The treaty went into effect in 1994.

In 1995, 120 parties to the global warming treaty met in Berlin, Germany, in what is known as the Berlin Mandate to determine the success of existing treaties and to embark on discussions of emissions after 2000. Differences persisted along North-South lines, with developing countries making essentially a moral argument for requiring more of the richer nations. They pointed out that the richer nations are responsible for most of the pollution. The Berlin talks essentially failed to endorse binding timetables for reductions in greenhouse gases.

A Landmark JudgmentThe 1995 IPCC Report

In 1995 the IPCC reassessed the state of knowledge about climate change and published its findings in its second assessment report, Climate Change 1995 ( The panel reaffirmed its earlier conclusions and updated its forecasts, predicting that, if no further action is taken to curb emissions of greenhouse gases, temperatures will increase 1.4 degrees to 6.3 degrees Fahrenheit by 2100. The panel concluded that the evidence suggests a human influence on global climate. The cautiously worded statement was a compromise following intense discussions. Nonetheless, it was a landmark conclusion because the panel, until then, had maintained that global warming and climate changes could have been the result of natural variability.

The Kyoto Protocol

In 1997 delegates from 166 countries met in Kyoto, Japan, at the UN Climate Change Conference to negotiate actions to reduce global warming. Some developed nations, including the United States, wanted to require all countries to reduce their emissions. Developing countries, however, felt the industrialized nations had caused, and were still causing, most global warming and therefore should bear the brunt of economic sacrifices to clean up the environment. The conference developed an agreement known as the


Greenhouse gas emission reduction targets under the Kyoto Protocol
CountryReduction target (percent)
(R)=Country has ratified, accepted, approved, or acceded to the Kyoto Protocol.
*European Union member countries renegotiated their individual targets under the EU Shared Burden Agreement, which was agreed to in 1998 and reaffirmed in the ratification of the Kyoto Protocol in 2002.
SOURCE: "Table 17. Quantified Emissions Reduction Targets Under the Kyoto Protocol by Country," in International Energy Outlook: 2004, U.S. Department of Energy, Energy Information Administration, Office of Integrated Analysis and Forecasting, April 2004, (accessed July 20, 2007)
Austria (R)13.0
Belgium (R)7.5
Bulgaria (R)8.0
Canada (R)6.0
Czech Republic (R)8.0
Denmark (R)21.0
Estonia (R)8.0
European Community (R)*8.0
Finland (R)0.0
France (R)0.0
Germany (R)21.0
Greece (R)+25.0
Hungary (R)6.0
Iceland (R)+10.0
Ireland (R)+13.0
Italy (R)6.5
Japan (R)6.0
Latvia (R)8.0
Lithuania (R)8.0
Luxembourg (R)28.0
Netherlands (R)6.0
New Zealand (R)0.0
Norway (R)+1.0
Poland (R)6.0
Portugal (R)+27.0
Romania (R)8.0
Slovakia (R)8.0
Slovenia (R)8.0
Spain (R)+15.0
Sweden (R)+4.0
Switzerland (R)8.0
United Kingdom (R)12.5
United States7.0

Kyoto Protocol to the UNFCCC. Different targets were set for different countries to meet specific economic and social circumstances. (See Table 3.2.) Even though China and India were not required to commit to specific limits, they did have to pledge to develop national programs for dealing with climate change. Overall, the Kyoto Protocol is expected to effect a total reduction in greenhouse gas emissions of at least 5% by 2012, compared with 1990 levels.

The treaty also set up an emission trading system that allowed countries exceeding their pollution limits to purchase on an open market credits from countries that pollute less. This provision was viewed as necessary to U.S. congressional approval. The developing nations feared that such a trading system would allow rich countries to buy their way into compliance rather than make unpopular emissions cuts. Enforcement mechanisms were not agreed to, nor did developing nations commit to binding participation. Regardless, Vice President Al Gore Jr. (1948) signed the Kyoto Protocol on behalf of the United States. However, President Bill Clinton (1946) never submitted it to the Senate for ratification. The political climate at the time was unfavorable for a treaty that bound the United States to specific emissions limits but did not set limits on developing nations, such as China and India.

The Kyoto Protocol was set up to take effect when two conditions were met:

  • It was ratified by at least fifty-five countries.
  • The ratifying countries accounted for at least 55% of CO2 emissions based on 1990 levels.

In March 2001 President George W. Bush (1946) indicated that the United States would not ratify the treaty because it would cost an estimated $400 billion and 4.9 million jobs to comply. In 2002 the treaty was ratified by major entities including the European Union, Japan, China, Canada, and India. It came into effect in February 2005 following ratification by Russia in late 2004. The UNFCCC ( noted in June 2007 that the Kyoto Protocol had been ratified by more than 174 nations. The notable exceptions were the United States and Australia.

The IPCC's 2001 Assessment Report

In 2001 the IPCC released its third assessment report: Climate Change 2001 ( It actually consisted of four reports: Climate Change 2001: The Scientific Basis, Climate Change 2001: Impacts, Adaptation, and Vulnerability, Climate Change 2001: Mitigation, and Climate Change 2001: The Synthesis Report. The IPCC's assessment covered the adaptability and vulnerability of North America to climate change impacts likely to occur from global warming. Among the suggested possible effects of global warming were:

  • Expansion of some diseases in North America
  • Increased erosion, flooding, and loss of wetlands in coastal areas
  • Risk to "unique natural ecosystems"
  • Changes in seasonal snowmelts, which would have effects on water users and aquatic ecosystems
  • Some initial benefits for agriculture, but those benefits would decline over time and possibly "become a net loss"

The IPCC's 2007 Assessment Report

In 2007 the three working groups of the IPCC had each released their version of the assessment report. These reports were:

  • Working Group IClimate Change 2007: The Physical Science Basis (
  • Working Group IIClimate Change 2007: Impacts, Adaptation, and Vulnerability (
  • Working Group IIIClimate Change 2007: Mitigation (

The IPCC's so-called Synthesis Report (, which integrates information from all three working group reports, was expected to be released in November 2007.

The report from Working Group I concludes that there is "very high confidence that the global average net effect of human activities since 1750 has been one of warming." The report defines "very high confidence" as meaning that there is over a nine out of ten chance that a hypothesis is correct. This is the strongest wording yet from the IPCC indicting human activities for global warming. The report estimates that humans have driven a warming effect (technically known as radiative forcing) of 1.6 watts per square meter since 1750. Various emission þ modeling scenarios predict that the Earth will continue to warm by approximately 0.2 degrees Celsius (0.36 degrees Fahrenheit) per decade over the next twenty years.

Working Group II notes that "observational evidence from all continents and most oceans shows that many natural systems are being affected by regional climate changes, particularly temperature increases." These effects include an earlier occurrence of springtime events and a poleward movement in the ranges of plant and animal species. In addition, many changes are reported in ice and snow ecosystems near the Earth's poles. Warming water body temperatures are linked to range changes for algae, plankton, and fish species.

Estimated future impacts of global warming include the loss of freshwater store because of melting glaciers, greater extent of drought areas coupled with more frequent "heavy precipitation" events that tend to cause flooding, and acidification of the Earth's oceans because of greater carbon take-up. The latter effect is particularly troubling for ocean coral, which is sensitive to pH (potential hydrogen; the level of acidity; a lower value indicates more acid) changes. Approximately 20% to 30% of plant and animals species are deemed to be at increased risk of extinction if the global average temperature increases by more than 1.5 to 2.5 degrees Celsius (2.7 to 4.5 degrees Fahrenheit).

Sea level rises are expected to expose millions of people to increased risk of flooding, exacerbate coastal erosion, and endanger coastal ecosystems. Even though increasing temperatures should help lower the number of deaths caused by cold exposure in far northern and southern regions, this benefit is expected to be more than offset by higher death rates in the temperate regions of the Earth, particularly in developing countries.

Working Group III focuses on the mitigative (corrective) measures that have been and can be taken by governments to reduce greenhouse gas emissions in seven sectors: energy supply, transport, buildings (e.g., lighting and insulation decisions), industry, agriculture, forestry, and waste handling and treatment. The report describes technologies and practices that are currently commercially available and those that are projected to be commercialized before 2030. The report admits that most computer models predict losses in the gross domestic product (GDP; the total value of goods and services produced by a nation) associated with emission abatement measures; however, some models predict GDP gains because they assume that the creation of revenue-earning technological advancements will be spurred by mitigation policies. In addition, it is expected that overall improvements in air pollution will bring health benefits that may offset some mitigation costs.

For the energy sector, Working Group III recommends that industrialized countries upgrade their energy infrastructure, developing countries invest in new energy infrastructure, and both types of countries promote renewable energy resources and measures for energy efficiency improvement. Specific recommendations include eliminating fossil fuel subsidies and imposing carbon taxes on fossil fuels. The report notes that nations must balance the short-term economic costs of quickly achieving dramatic emission reductions against the medium- and long-term costs that will arise because of continued global warming if mitigation is delayed.

In October 2007 the IPCC and Gore were jointly awarded the 2007 Nobel Peace Prize for their work "to build up and disseminate greater knowledge about man-made climate change, and to lay the foundations for the measures that are needed to counteract such change." The prize committee noted that thousands of scientists from more than one hundred countries had collaborated through the IPCC to investigate the causes and effects of climate warming.


The U.S. Constitution grants the president the power to make treaties with foreign powers but only with the consent of two-thirds of the Senate. In other words, the president or a designee (such as the vice president) can sign treaties, but they do not become binding under U.S. law until they are approved by the Senate. The administration of President George H. W. Bush (1924) supported and signed the UNFCCC, which was ratified by the Senate, but opposed precise deadlines for CO2 limits, arguing that the extent of the problem was too uncertain to justify painful economic measures. In 1989 President Bush established the U.S. Global Change Research Program (USGCRP), which was authorized by Congress in the Global Change Research Act of 1990.

In 1993 President Clinton took office. Later that year the United States released, in accordance with the UNFCCC, The Climate Change Action Plan (, which detailed the nation's response to climate change. The plan included a set of measures by both government and the private sector to lay a foundation for the nation's participation in world response to the climate challenge.

The plan called for measures to reduce emissions for all greenhouse gases to 1990 levels by 2000. However, the U.S. economy grew at a more robust rate than anticipated, which led to increased emissions. Furthermore, Congress did not provide full funding for the actions contained in the plan.

Even though the United States had a comprehensive global warming program in place, Congress was reluctant to take steps to reduce emissions. However, the Clinton administration implemented some policies that did not require congressional approval. These included tax incentives and investments focusing on improving energy efficiency and renewable energy technologies, coordinating federal efforts to develop renewable fuels technology, and requiring all federal government agencies to reduce greenhouse gas emissions below 1990 levels by 2010. President Clinton also established the U.S. Climate Change Research Initiative to study areas of uncertainty about global climate change science and identify priorities for public investments.

The United States and the Kyoto Protocol

As noted earlier, the United States is a party to the UNFCCC but has never ratified the Kyoto Protocol to the UNFCCC. Vice President Gore signed the Kyoto Protocol in 1998 on behalf of the Clinton administration, but this was largely a symbolic gesture. At the time Gore noted, "We will not submit the Protocol for ratification without the meaningful participation of key developing countries in efforts to address climate change." The Senate had already made clear through a nonbinding, but unanimous, resolution passed in 1997 that it would not ratify the Kyoto Protocol as written because some nations were excluded from emissions limits and the treaty was deemed damaging to U.S. economic interests. The resolution was sponsored by Senators Robert Byrd (D-WV; 1917) and Chuck Hagel (R-N; 1946) and is often referred to as the Byrd-Hagel Resolution.

When President George W. Bush took office in 2001, he affirmed his administration's steadfast opposition to the Kyoto Protocol. He established a new cabinet-level management structure to oversee government investments in climate change science and technology. Both the U.S. Climate Change Research Initiative and the USGCRP were placed under the oversight (supervision) of the inter-agency Climate Change Science Program (CCSP), which reports integrated research that is sponsored by thirteen federal agencies. The CCSP is overseen by the Office of Science and Technology Policy, the Council on Environmental Quality, and the Office of Management and Budget.

According to the article "California Greenhouse Gas Bill Approved by State Senate" (, August 31, 2006), more than one hundred bills dealing with greenhouse gases and climate change have been introduced in Congress but have failed to move forward because of lack of consensus among legislators about how to deal with the issue. The exclusion of China from the Kyoto Protocol's emission limits is a major sticking point for many U.S. politicians, who fear that the United States would be put at an economic disadvantage if it had to meet specific limits. The United States' continued refusal to ratify the Kyoto Protocol or develop a similar control plan at the national level has elicited strong criticism from foreign and domestic sources. The editorial "Warming and Global Security" (New York Times, April 20, 2007) complains that "in an alliance of denial, China and the United States are using each other's inaction as an excuse to do nothing."

Greenhouse Gas Intensity

In 2002 Bush advocated his own plan for the U.S. response to the greenhouse gas problem. Rather than specific emission limits, he called for a reduction in greenhouse gas intensity (GGI), that is, greenhouse gas emissions per unit of economic growth. GGI has come to be defined primarily as metric tons of greenhouse gases emitted per million dollars of the GDP (assuming each dollar has the same purchasing value as it did in 2000). In this way, greenhouse gas emissions are tied to an economic indicator.

Critics complain that the GGI can be reduced without necessarily decreasing greenhouse gas emissions. This can occur if the U.S. economy (as measured by the GDP) continues to grow robustly as it has in recent years. If the GDP increases at a faster rate than emissions, the calculated intensity value will decrease; however, actual emissions will still be increasing. In fact, the DOE


expects U.S. emissions to increase. Figure 3.13 shows historical emissions and projects future emissions for three economic scenarios: low, reference (medium), and high growth rates over the coming decades.

Recent Reports and Updates

In May 2002 the U.S. Global Change Research Information Office released the U.S. Climate Action Report2002 ( The report acknowledges that greenhouse gases resulting from human activities are accumulating in the atmosphere and that they are causing air and ocean temperatures to increase. It does not rule out, however, the still-unknown role of natural variability in global warming. In addition, the report reiterates that the Bush administration plans to reduce the nation's GGI by 18% over the following decade through a combination of existing regulations and voluntary, incentive-based measures.

In July 2003 the CCSP published two major reports: Strategic Plan for the U.S. Climate Change Science Program ( and The U.S. Climate Change Science Program: Vision for the Program and Highlights of the Scientific Strategic Plan ( Together, these documents outline the approach the CCSP plans to take to achieve its five main scientific goals:

  • Improve knowledge of the Earth's past and present climate and environment, including its natural variability, and improve understanding of the causes of observed variability and change
  • Improve quantification of the forces bringing about changes in the Earth's climate and related systems
  • Reduce uncertainty in projections of how the Earth's climate and related systems may change in the future
  • Understand the sensitivity and adaptability of different natural and managed ecosystems and human systems to climate and related global changes
  • Explore the uses and identify the limits of evolving knowledge to manage risks and opportunities related to climate variability and change

In May 2007 the Climate Action Report2006 ( was released. It indicates that the United States is on track to meet Bush's GGI targets by 2012. Figure 3.14 shows that U.S. GGI and CO2 intensity (CO2 emissions per dollar of GDP) have declined steadily since 1990. In a White House press statement (May 23, 2007,, Bush noted that from 2005 to 2006 U.S. CO2 emissions declined by seventy-eight million metric tons (a 1.3% decrease), whereas the U.S. economy grew by 3.3%. This resulted in a reduction in CO2 intensity of 4.5%, the largest annual improvement achieved since 1990 and a significant step toward the president's goal of reducing GGI by 18% by 2012.

California Goes Its Own Way

As described in Chapter 2, the Clean Air Act allows states to set stricter air pollution standards than those imposed by the federal government if the EPA grants permission to do so. During the early 1990s California used this process to establish emission standards for vehicles more stringent than those set by the EPA for priority pollutants and air toxics. CO2 was not included. Growing concern about global warming and the federal government's lack of enthusiasm for the Kyoto Protocol has spurred the state to launch its own campaign against greenhouse gases.

In 2005 Governor Arnold Schwarzenegger (1947) issued an executive order establishing emissions targets for greenhouse gas emissions. He also created a Climate Action Team under the direction of the California Environmental Protection Agency to coordinate the state's climate policy. In August 2006 the California legislature passed AB 32, which adopted Schwarzenegger's plans into law. It calls for the state to reduce its greenhouse gas emissions by 2020 to 1990 levels. This represents an


approximate 25% reduction, compared with what emissions would be without the program. (See Figure 3.15.) The plan includes a market-based trading program in which industries emitting greenhouse gases can buy or sell credits among themselves to meet the limits.

Opponents to the plan worry about its economic consequences, fearing that it will drive businesses from California and raise consumer prices, particularly for gasoline. Advocates counter that California companies will create profitable technological innovations for the world marketplace that will ultimately offset and even surpass the short-term costs of tighter emission limits.

The Fight over Vehicle Emission Limits

Because transportation is the state's largest single source of greenhouse gas emissions, California proposes to achieve the reductions, in part, through strict new standards on the emissions of CO2 from vehicles. This plan has encountered stiff resistance from the federal government, because CO2 is neither a priority or toxic pollutant under federal standards. Eleven other states, all


clustered in the northeastern United States, have also asked the EPA for permission to establish their own vehicle emission limits on CO2. At first, the EPA argued that it did not have the authority to act, because greenhouse gas emissions do not fall under the Clean Air Act. In April 2007 this argument was ruled invalid by the U.S. Supreme Court in Commonwealth of Massachusetts v. U.S. Environmental Protection Agency (415 F. 3d 50).

As of September 2007 the EPA was still considering the states' requests; however, political analysts believe the agency will delay making a decision until after the 2008 national elections. Automakers are staunchly opposed to the prospect of differing vehicle emission standards from state to state. They have lobbied the EPA to deny the states' petitions.

In early 2007 Schwarzenegger also proposed a new standard for low-carbon fuels in California. If adopted, this would force petroleum refiners to reduce the carbon content of fuels by 10% by 2020. It would also encourage development and investment in alternative fuels other than gasoline.

Major U.S. Cities Embrace the Kyoto Protocol

In February 2002 Seattle Mayor Greg Nickels (1955) launched the U.S. Mayors Climate Protection Agreement (2007,, in which the mayors of U.S. cities agree to abide by the provisions of the Kyoto Protocol in an attempt to lower greenhouse gas emissions in their cities by 2012. As of 2007, more than six hundred mayors representing sixty-seven million people had joined this initiative.


Besides the greenhouse effect, there are many other natural and anthropogenic factors believed to affect Earth's climate. The following sections describe some of these factors and the scientific knowledge and uncertainties about them.

The Effects of the Forests

Forests act as sinks, or repositories, absorbing and storing carbon. This is an example of carbon sequestration (long-term storage of carbon that keeps it out of the atmosphere). Living trees naturally absorb and neutralize


CO2, although scientists do not agree on the extent to which forests can soak up excess amounts. The increasing levels of CO2 in the atmosphere might conceivably be tolerated in Earth's normal CO2 cycle if not for the additional complicating factor of deforestation. The burning of the Amazon rain forest and other forests has had a twofold effect: the immediate release of large amounts of CO2 into the atmosphere from the fires and the loss of trees to neutralize the CO2 in the atmosphere. (See Figure 3.16.)

In 2006 the scientific community was stunned when Frank Keppler et al. showed in "Methane Emissions from Terrestrial Plants under Aerobic Conditions" (Nature, no. 439, January 12, 2006) that live vegetation releases methanea potent greenhouse gas. Keppler and his colleagues estimated that on a global scale live plants (such as trees) could be emitting up to 236 million metric tons of methane per year. Some observers used the revelation to cast doubt on the conventional wisdom that forests clean the air of greenhouse gases. Other researchers questioned the methods used by Keppler and his colleagues to arrive at their conclusions, particularly the assumption that small-scale laboratory and field experiment results can be extrapolated (scaled up) to a global basis. Some scientists believed that Keppler and his cohorts' experimental methods were flawed and that living plants produce no or virtually no methane emissions. The so-called methane mystery has spurred a fierce controversy among scientists that is likely to continue for some time.

The Effects of the Oceans

The oceans are, by far, the largest reservoir of carbon in the carbon cycle. Oceanographers and ecologists disagree over the carbon cycleclimate connection and over the ocean's capacity to absorb CO2. Some scientists believe that the oceans can absorb one billion to two billion tons of CO2 per year, about the amount the world emitted in 1950. Until scientists can more accurately determine how much CO2 can be buffered by ocean processes, the extent and speed of disruption in the carbon supply remains unclear.

Oceans have a profound effect on global temperature because of their huge capacity to store heat and because they can moderate levels of atmospheric gases. Covering more than 70% of Earth and holding 97% 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 for these components. Even though 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 Effects of Clouds

Surprisingly little is known about cloudswhere 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. These are known as albedo effects. Albedo comes from the Latin word for "whiteness." Oceans and forests have relatively low albedo values, because they reflect only a small portion of the sun's energy. By contrast, deserts and snow reflect a large portion of solar energy.

NASA's Earth Science Enterprise is a satellite-based program that includes many scientific studies of clouds. These studies reveal 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.

The Effects of 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 eleven 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 agobetween 1640 and 1720when sunspot activity fell sharply and Earth cooled about two degrees Fahrenheit. (The sun is brighter when sunspots appear and dimmer when they disappear.) Other scientists believe that sun variability could indeed be a component of recent global warming, but only in combination with anthropogenic factors.

The Effects of El Niño and La Niña

For centuries fishermen in the Pacific Ocean off the coast of South America have known about the phenomenon called El Niño. At irregular yearly intervals, during December and January, fish in those waters virtually vanish, bringing fishing to a standstill. Fishermen called this occurrence El Niño, which means "the Child," because it occurs around the celebration of the birth of Jesus. Even though it originates in the Pacific, El Niño's effects are felt around the world. Computers, satellites, and improved data gathering find that the El Niño phenomenon is 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 the 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 part 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 El Niño/Southern Oscillation, which can also include a cold phase known as a La Niña.

According to NOAA (August 7, 2007,, El Niños occurred off and on between 1991 and 1995 and then during 199798, 200203, 200405, and 200607. These events disrupted the ocean-atmosphere system in the Pacific Ocean with subsequent effects on weather around the planet. Some scientists believe that the sudden and unexpected El Niño that formed during the summer of 2006 lessened hurricane formation during a season that had been predicted to be active. The El Niño caused increased wind shear over the Caribbean region, which tended to suppress hurricane formation. As an additional advantage, a strong low pressure system that dominated over the eastern United States pushed hurricanes away from the mainland and out into sea. The occurrence of more frequent and stronger hurricanes is a prediction widely associated by the public with global warming, because hurricanes thrive in warm waters. However, as this example shows, climate events are difficult to predict because of a number of variables.

The Effects of Aerosols

Aerosols are extremely tiny particles and/or liquid droplets that disperse in the atmosphere. Primary natural sources include volcanoes, forest fires, soil, sand, dust, sea salt, and scores of biological organisms and refuse particles (bacteria, pollen, dead skin cells, dander, spores, fungi, marine plankton, etc.). Aerosols are also produced by many fuel combustion and industrial processes and are a component of soot and smoke.

Aerosols in the atmosphere have direct and indirect effects on climate. A direct effect is that they scatter and absorb radiation. Indirectly they also modify the formation and water content of clouds. Aerosols can linger for several years in the troposphere and are returned to Earth in precipitation. Even though their exact effects on climate are not well understood, it is believed that they have a temporary cooling effect on the atmosphere.

volcanic activity

volcanic activity. Volcanic activity, such as the 1991 eruption of the Mount Pinatubo volcano in the Philippines, can temporarily reduce the amount of solar radiation reaching the Earth. Volcanoes spew vast quantities of particles and gases into the atmosphere, including sulfur dioxide, which 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. Scientists report that the Pinatubo eruption reduced solar radiation by more than 3%, compared with the baseline amount occurring in 1958.

This effect was previously seen in 1982, when the eruption of the El Chichon volcano in Mexico depressed global temperatures for about four years. 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." Furthermore, for several years following the Tambora eruption, people around the world commented on the beautiful sunsets, which were caused by the suspension of volcano-related particulate matter in the atmosphere.

The IPCC Rates Scientific Understanding about Climate Factors

In each of its assessments the IPCC rates the level of scientific understanding about climate factors that affect radiative forcingthe heating and cooling of Earth. The fourth assessment report rates as "high" the level of scientific understanding about the heating effect of long-lived greenhouse gases, such as CO2. Less is known about the heating effect of tropospheric ozone and the believed cooling effect of stratospheric ozone. Likewise, there is still much to learn about surface albedo and its contribution to cooling and heating. The IPCC rates as "low" the level of scientific understanding about some forcing components, including solar irradiance, airplane vapor contrails, and aerosols in the atmosphere.


Even though there is widespread agreement among scientists that Earth's temperature has warmed in recent years, there is lingering debate over the causes of this warming. Some scientists believe 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 anthropogenic greenhouse gases, or is it natural variability?

Among the claims of critics of global climate warming are:

  • Climate has been known to change dramatically within a relatively short period without any human influence.
  • Temperature readings already showed increased temperatures before CO2 levels rose significantly (before 1940).
  • Natural variations in climate may exceed any human-caused climate change.
  • Some of the increase in temperatures can be attributed to sunspot activity.
  • If warming should occur, it will not stress Earth; it may even have benefits, such as for agriculture, and may delay the next ice age.
  • Reducing emissions will raise energy prices, reduce the GDP, and produce job losses in the United States.
  • Even though clouds are crucial to climate predictions, so little is known about them that computer models cannot produce accurate predictions.

There are a handful of scientists notably known for their criticism of the IPCC and its conclusions about anthropogenic causes of global warming. They believe that modeling results exaggerate the role of CO2 emissions on climate and attack what they see as environmental hysteria on a subject about which much is still unknown by the scientific community. In the op-ed "Climate of Fear: Global-Warming Alarmists Intimidate Dissenting Scientists into Silence" (Wall Street Journal, April 12, 2006), Richard Lindzen of the Massachusetts Institute of Technology complains that "scientists who dissent from the alarmism have seen their grant funds disappear, their work derided, and themselves libeled as industry stooges, scientific hacks or worse."


In March 2007 the Gallup Organization conducted its annual poll on topics related to the environment. Participants were asked several questions about the greenhouse effect and global warming.

As shown in Figure 3.17, only 22% of those asked thought they understood very well the "greenhouse effect" issue. Nearly one out of five admitted either that they did not understand it very well (19%) or not at all (4%). Just over half (55%) said they understood the issue fairly well. When asked about the causes of global warming, a majority (60%) said that the temperature increase was due to the effects of pollution from human activities. (See Figure 3.18.) More than a third (35%) blamed natural causes.

Opinion was evenly split on whether the seriousness of global warming has been exaggerated or not. (See Figure 3.19.) Gallup found that 34% of those responding felt that the seriousness had been generally underestimated. Another 29% thought it was generally correct, and 33% felt that the seriousness was generally exaggerated. As shown in Figure 3.20, nearly two-thirds (63%) of those asked stated that global warming will not pose a "serious threat" to their way of life during their lifetime. Thirty five percent of respondents believed that it will pose a "serious threat."


Gallup also asked people about specific measures that individuals and the U.S. government could take to reduce global warming. The results indicate that large majorities of people support steps such as using fluorescent lightbulbs and solar panels to make their homes more energy efficient and riding mass transit whenever possible. There was also widespread support for government research and usage of renewable energy sources, even if it means increasing taxes.


Even though Gallup's 2007 poll did not specifically ask poll participants about the Kyoto Protocol, this topic has been addressed in previous Gallup polls. In 2005 pollsters found that more people support the treaty than oppose it. Figure 3.21 illustrates that 42% of poll respondents indicated that the United States should agree to abide by the provisions of the treaty, compared with 23% who disagreed. A sizable percentage (35%) of those asked had no opinion on the matter.




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The Enhanced Greenhouse Effect and Global Warming

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