Units:

Energy Through Our Lives

Section B. Energy Flow in Ecosystems

2. Greenhouse Gas Emissions and Climate Change

Hotter summers, stronger hurricanes, melting ice in the polar regions - are these events related to changes in climate due to the increase in greenhouse gas emissions from human activity? The evidence gathered by scientists around the world seems to point to this conclusion, one that is becoming more convincing over time. Nowadays, most scientists agree that human activity such as burning fossil fuels has lead to an increase in the concentration of carbon dioxide (CO₂) and other gases that are causing changes to Earth's climate.

Climate change, or more specifically global warming, refers to the warming of Earth's atmosphere due to the greenhouse effect. Incoming solar energy, including certain wavelengths of infrared radiation, passes through Earth's atmosphere and reaches the surface of Earth, thus warming it. Some of the infrared radiation emitted from Earth's surface returns back to the atmosphere where it is absorbed by certain atmospheric gases called greenhouse gases. The absorption of infrared by these greenhouse gases warms the atmosphere of Earth in a way similar to the way a greenhouse captures solar energy and warms the air inside it, thus helping plants to grow.

Natural levels of greenhouse gases keep Earth at an average temperature of 59°F (15°C). Without their presence, the average temperature of Earth would be about 0.4°F (18°C)¹. However, increases in greenhouse gas emissions due to burning fossil fuels, certain manufacturing processes, changes in agricultural practices, and deforestation have led to increased levels of these gases in the atmosphere. This, in turn, strongly suggests a cause for the observed increase in average global air temperature of 0.5°F to 1.1°F (0.3°C to 0.6°C) over the past century².

The major greenhouse gases include carbon dioxide, water vapor (H₂O), and methane (CH₄), as well as nitrous oxide (N₂O), fluorocarbons, and ozone³. Carbon dioxide is the greenhouse gas that is of greatest concern due to its large atmospheric concentration with respect to other greenhouse gases and the large amounts emitted. Worldwide concentrations of CO₂ have increased from 280 parts per million (ppm) by volume before 1750, the pre-industrial period, to 377.3 ppm in 2004, a 35% increase. More recently, CO₂ concentrations have increased by 19.4% from 1959 to 2004⁴.

Click here (.pdf) to see a graph showing CO₂ concentrations from 1958 to 2004

The United States is the largest emitter of carbon dioxide, emitting 5,692 million metric tons in 2002⁵. In 2003, CO₂ accounted for 89% of all greenhouse gas emissions⁶. After the U.S., the nations with the largest emissions for 2002 were: China (3,050 million metric tons), followed by Japan (1,158 million metric tons), India (917 million metric tons), and Germany (819 million metric tons)⁷. However, China is expected to pass the U.S. in CO₂ emissions by 2009 due primarily to the extensive use of coal to meet its increasing energy demand⁸.

Although CO₂ is the most prominent greenhouse gas, other greenhouse gases are either more common in Earth's atmosphere (such as water vapor), or are more effective in absorbing infrared radiation from Earth's surface (such as methane, nitrous oxide, and fluorocarbons). On the other hand, nearly all other greenhouse gases are present in the atmosphere in much lower concentrations than CO₂. For example, methane is 23 times more effective in absorbing infrared radiation compared to CO₂, but is present at concentrations that are 0.5% of those of CO₂ (see table titled Global Warming Potential of Selected Greenhouse Gases).

Global Warming Potential of Selected Greenhouse Gases^a

Greenhouse Gas

Global Warming Potential Relative to CO₂

Atmospheric
Concentration
(2004)^b

Carbon Dioxide (CO₂)

377.3 ppm

Methane (CH₄)

~1,789 ppb

Nitrous Oxide (N₂O)

296

318 ppb

CFC (Freon)-12 (CCl₂F₂)

10,600

542 ppt

HFC-23 (CHF₃)

12,000

14 ppt (1998)

^aBlasing, T.J., and Karmen Smith. Recent Greenhouse Gas Concentrations. Oak Ridge, Tenn. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratories, Oak Ridge, Tenn. http://cdiac.ornl.gov/pns/current_ghg.html Accessed August 5, 2007
^bparts per million (ppm), parts per billion (ppb), parts per trillion (ppt) by volume of gas

Because many different greenhouse gases are emitted due to human activities, many scientific and policy organizations report greenhouse gas emissions in terms of carbon dioxide equivalents (CO₂e), which are developed by combining the global warming potentials and the concentrations of the different greenhouse gases into a single value. This summary primarily addresses CO₂ emissions, although CO₂ equivalent values for greenhouse gases are noted in the text as well.

Does the destruction of the ozone layer cause global warming?⁹

Some believe that the destruction of the ozone layer is directly responsible for global warming because reduced amounts of ozone allows increased amounts of solar radiation to reach Earth's surface, thus heating it. However the two processes are different. Global warming results when increased concentrations of greenhouse gases more effectively trap infrared radiation coming from the surface of Earth after the surface has been warmed by visible and infrared radiation.

The stratospheric ozone (O₃) layer, located about 13 to 25 miles (20 to 40 Kilometers) above Earth's surface is diminished when chlorofluorocarbon compounds (CFCs), a subset of fluorocarbons, are released into the atmosphere from discarded air conditioners and manufacturing processes that use these compounds. Once released, CFCs break down, freeing chlorine atoms to attack ozone molecules. Reduced ozone concentrations at these altitudes allow increased levels of ultraviolet radiation to reach Earth's surface that would otherwise be absorbed by the ozone. While increased ultraviolet can damage plants and lead to elevated levels of skin cancer in humans, it is not the form of radiation that results in global warming.

Despite being different processes, the CFCs that cause damage to the ozone layer are also potent greenhouse gases that are about 4,000¹⁰ to 20,000¹¹ times more effective at absorbing infrared radiation from Earth's surface than CO₂. Hence, CFC emissions affect both global warming and the ozone layer.

The major causes of increased greenhouse gas emissions into the atmosphere are from fossil fuel combustion, agricultural practices, and industrial processes. Deforestation also affects the balance of atmospheric CO₂ since trees absorb this gas during photosynthesis.

Fossil fuels such as coal, oil, and natural gas are burned to produce energy for transportation, heat for buildings, electricity generation, and industrial processes and account for 95% of CO₂ emissions in the United States¹². The amount of CO₂ produced by fossil fuels varies with respect to the amount of carbon in the fuel and its energy content. Coal produces the most CO₂ per unit of energy, followed by petroleum and its products, and by natural gas, which produces the lowest amount of CO₂ per unit of energy.

Biomass such as wood, along with biofuels such as ethanol, methanol, and biodiesel also emit CO₂ when burned. The amount of CO₂ they produce per unit of energy is comparable to corresponding figures for petroleum products. However, unlike fossil fuels, biomass and biofuels are renewable energy resources, which form a complete carbon cycle provided they are grown sustainably. For example, growing a new corn crop will absorb the CO₂ emitted when the ethanol produced from a previously harvested corn crop is burned in a car engine.

How much CO₂ is absorbed by trees?

The amount of CO₂ absorbed by trees varies widely depending on the age, type of tree, climate, and the characteristics of the surrounding ecosystem. Hence figures which state the amount of CO₂ absorbed per tree or per acre of trees may also vary widely. According to one source, "a healthy, growing tree absorbs between 13 and 48 pounds of CO₂ per year. At the 13 pound rate, one acre of trees absorbs 2.6 tons of CO₂ per year."¹³ Another source states that "a forest absorbs about 3 tons of CO₂ per acre of trees per year."¹⁴ From these figures, an acceptable estimate for CO₂ absorption by trees would be 13 pounds of CO₂ per tree per year and 3 tons of CO₂ per acre of trees per year.

The following table provides a list of CO₂ emissions factors for various fuels in terms of pounds of CO₂ produced per a selected unit such as tons of coal or gallons of liquid fuel and per Million Btu of energy produced.

Type of Fuel

Pounds of CO₂ per Selected Unit

Pounds of CO₂ per Million Btu of Energy

COAL^a

Anthracite

5,685 per Ton

227

Bituminous

4931 per Ton

205

Subbituminous

3716 per Ton

213

Lignite

2792 per Ton

215

NATURAL GAS^a

121 per 1000 cubic feet

12.1 per hundred cubic feet (CCF)

11.7 per Therm

117

PETROLEUM PRODUCTS^a

Diesel; No. 1 and

No. 2 Fuel Oil

22.3 per Gallon

161

Gasoline

19.6 per Gallon

156

Jet Fuel

21.1 per Gallon

153

Kerosene

21.5 per Gallon

160

LPG

(Liquified Petroleum Gases)

12.8 per Gallon

139

Propane (C₃H₈)

12.7 per Gallon

139

BIOMASS FUELS

Methanol (CH₃OH)^b

9.1 per Gallon

157

Ethanol (C₂H₅OH)^b

12.6 per Gallon

166

Biodiesel (B100)^c

see Ethanol

Wood^a,d

(approximate values)

1.70⁴ to 1.91¹ per Pound

3,400⁴ to 3,812¹ per Ton

195¹

^aU.S. Department of Energy, Energy Information Administration. Voluntary Reporting of Greenhouse Gases Program - Fuel and Energy Source Codes and Emission Coefficients. Washington, DC., U.S. Department of Energy, Energy Information Administration. http://www.eia.doe.gov/oiaf/1605/coefficients.html Accessed August 7, 2007.
^bBased on stoichiometric calculations under ideal combustion conditions.
^cThe IPCC report on 2006 guidelines for greenhouse gas emissions inventories recommends using ethanol emissions factors for biodiesel. Gregg Marland, Carbon dioxide Information Analysis Center, Oak Ridge National Laboratories, Oak Ridge, TN; personal communication.
^dU.S. Environmental Protection Agency. Technology Transfer Network Clearinghouse for Inventories and Emissions Factors. Washington, DC., U.S. Environmental Protection Agency. http://www.epa.gov/ttn/chief/ap42/index.html Accessed October 29, 2006.

Residential CO₂ Emissions in Wisconsin
According to the Energy Center of Wisconsin, energy used in Wisconsin homes is responsible for about a quarter of all CO₂ emissions in the state. In addition, the average single-family home is responsible for about 18 tons of CO₂ emissions per year. Further information on residential CO₂ emissions, including a summary of approaches for reducing emissions, can be found at the Energy Center of Wisconsin's Web site at http://www.ecw.org/carbhome.php

Reducing residential CO₂ emissions often goes hand in hand with most all energy conservation and efficiency practices. Driving a more efficient vehicle, insulating a home, replacing incandescent light bulbs with compact fluorescent (CFL) bulbs, and purchasing energy efficient appliances will reduce CO₂ along with other air pollutant emissions.

The last section of the Energy Bill provides information on CO₂ emissions from using electricity and natural gas in the home. You can use the Energy Bill to see how CO₂ emissions are reduced by putting in values for electricity and natural gas use based on the energy conservation measures you have adopted.

Note that the CO₂ emissions factor of 1.64 lbs/kWh for electricity is based on an average for Wisconsin utilities for 2002¹⁵. The CO₂ emissions factor for electricity produced by your utility may vary somewhat from this figure.

CO₂and Greenhouse Gas Emissions Calculators

While the Energy Bill calculates CO₂ emissions produced when using electricity and natural gas, other CO₂ and greenhouse gas emissions calculators on the web can determine the amounts of these emissions due to a wide range residential energy uses. Some calculators calculate the users carbon footprint, an overall impact from greenhouse gases produced by the user due to their energy and resource use.

The U.S. Environmental Protection Agency (US EPA) Web site includes a set of links to CO₂ / greenhouse gas emissions calculators. The complete list of links to the calculators is found at

http://yosemite.epa.gov/oar/globalwarming.nsf/Content/
ResourceCenterToolsCalculators.html

Four of the calculators found at the US EPA Web site are included here, with each focusing on particular energy uses and environmental effects associated with CO₂/ greenhouse gas emissions and suggesting ways that users can reduce their emissions.

American Forests Climate Change Calculator
http://www.americanforests.org/resources/ccc/index.php

American Forests, an organization that aims to protect the nation's forest resources, provides a calculator that determines CO₂ emissions from residential energy use and transportation. The calculator also calculates the number of trees that need to be planted to absorb these CO₂ emissions. American Forests also encourages users offset their CO₂emissions by donating money to plant trees in one of their Global ReLeaf forest restoration programs.

Fueleconomy.gov (U.S. Dept of Energy - Energy Efficiency and Renewable Energy)
http://www.fueleconomy.gov

A Web site provided by the U.S. Dept of Energy that is devoted to providing information about vehicle fuel efficiency for specific vehicle makes and models. The section titled "Find Your Car's Energy Impacts" includes a greenhouse gas emissions calculator that reports emissions in tons per year of CO₂ equivalents, as well as miles per gallon (mpg) results, number of barrels of petroleum used, an air pollution score, and annual fuel costs. Click on "Find Your Car's Energy Impacts" in the main menu. When there, be sure that you select the "Side-by-Side Comparison" option. You will also need to enter the year, make, and model of your car to calculate CO₂ equivalents.

The U.S. Environmental Protection Agency's Personal Online Greenhouse Gas Calculator
http://www.epa.gov/climatechange/emissions/ind_calculator.html

The U.S. Environmental Protection Agency's provides a two-part greenhouse gas calculator. The first part calculates greenhouse gas emissions in CO₂ equivalents for residential and transportation activity, and waste disposal. The second part calculates emissions reductions from driving more efficient vehicles, using more efficient appliances, and recycling waste. Another section of the Web site titled "What You Can Do" identifies and describes twenty-five steps individuals can take to reduce greenhouse gases.

World Resources Institute (WRI) SafeClimate Carbon Footprint Calculator
http://www.safeclimate.net/calculator

World Resources Institutes SafeClimate Carbon Footprint Calculator determines greenhouse gas emissions in CO₂ equivalents for residential energy use and travel by car or plane. Users can select calculator inputs in English units or metric units by choosing options titled "calculator for individuals in the U.S./Canada" or "calculator for individuals in other countries".

ee-factors.html Accessed august 7, 2007.

Next reading: Energy Through Our Lives - Section B.3. Home Energy Use

Return to Energy Through Our Lives - Section B.1. Energy Use in Ecosystems.

To return to What is Energy? index page.

To return to the Course Overview page.