Carbon Cycle Sources Future Trends Links

EMISSIONS     Most scientists agree that fossil fuel combustion and other human activities have contributed to increases in atmospheric greenhouse gas concentrations.  While the CO2 emitted by plant respiration and the decomposition of organic matter is ten times that of anthropogenic emissions, these natural sources had been in balance with the global carbon cycle for thousands of years.1  The burning of fossil fuels which began during the industrial revolution upset this balance by removing carbon stored in the earth and emitting it to the atmosphere.  This page provides a brief overview of the carbon cycle, examines the anthropogenic sources of greenhouse gases in the United States and internationally, and looks at future emission trends.

Carbon Cycle

The amount of CO₂ in the atmosphere is regulated by processes collectively known as the carbon cycle. While the great majority of carbon is in long term storage deep within the oceans and the earth, that which is found within the atmosphere or at the earth's surface is in constant flux.  The atmosphere contains about 735 Gt (10⁹ metric tons) of carbon, which corresponds to a concentration of about 350 parts per billion.  Plants and other organisms utilize the energy of sunlight, CO₂ from the atmosphere and water to form biomass.  Photosynthesis on land, which occurs mostly in the leaves and needles of trees, removes about 60 Gt of CO₂ per year.²  Carbon is then returned to the atmosphere during decomposition and respiration of plants and animals.  Atmospheric carbon dioxide levels tend to rise during the spring and summer, when photosynthesis occurs, and fall during the fall and winter, when leaves fall from trees and decay.  Because trees absorb more carbon than they emit, they are considered carbon sinks.  Deforestation results in fewer carbon sinks and cycles CO₂ from the earth into the atmosphere as cut trees decay or are burned.

Diffusion to and from the oceans is a critical component of the global carbon cycle.  The oceans hold about fifty times more carbon than the atmosphere.  Oceans remove carbon from the atmosphere when CO2 dissolves in cold polar waters and sinks to the deep ocean. CO2 is recycled to the atmosphere by deep water upwells near the equator.  Ocean plankton also remove CO2 from the atmosphere by using it for growth.  Oceanic diffusion removes 104 Gt and emits 100 Gt of CO2 to and from the atmosphere annually.3

The feedbacks between fossil fuel emissions and the global carbon cycle have important implications for climate change. The rate of increase in atmospheric CO₂ depends not only upon fossil fuel use, but also upon the ability of oceans and forests to act as sinks. At this time, carbon cycle models can account for only 70-85 percent of the carbon emitted to the atmosphere by anthropogenic sources. We know that of the seven billion tons of carbon emitted each year, three billion remain in the atmosphere, and two billion are absorbed by the oceans.⁴  The fate of two billion tons of carbon remains a mystery. 

Sulfate aerosols, which derive in part from sulfur emissions, have the ability to reflect the sun's radiation. The effect these aerosols will have on global temperature is also not fully understood. (More)

Anthropogenic Sources of Greenhouse Gases

UNITED STATES

While carbon dioxide, methane (CH₄), and nitrous oxide (N₂0) all occur naturally in the atmosphere, their concentrations have risen by 30, 145, and 15 percent since 1800.⁵  Chlorofluorocarbons (CFCs), which are also a greenhouse gas, had also increased from the 1950's until they were banned by the Montreal Protocol in the mid-1980s. The use of hydrofluorocarbons (HCHCs), perfluorocarbons (PFCs), and sulfer hexafluoride (SF₆), which are CFC substitutes, increased as the Protocol took effect.  

The overwhelming majority of anthropogenic carbon dioxide emissions come from fossil fuel combustion or wood burning.  Fuel production and processing and cement, lime, and limestone production also contribute a small percentage of global CO₂ emissions.⁶   Methane emissions derive 30% naturally occurring and 70% as a result of human activities. Natural sources of methane include wetlands, gas hydrates and permafrost, termites, oceans, freshwater bodies, non-wetland soils, and other sources such as wildfires.  The primary anthropogenic causes of methane are fossil fuel production, animal husbandry, rice cultivation, biomass burning, and waste management activities.⁷ (For information on other greenhouse gases, click here.)

The transportation and industrial sectors are each responsible for about one-third of U.S. CO₂ emissions.  Ninety-eight percent of transportation sector emissions result from petroleum product use, 61% of which is gasoline. Passenger cars were responsible for 37% of U.S. CO₂ emissions in 1996, while light trucks (sports utility vehicles, pickup trucks and minivans) were responsible for 24%. Between 1990 and 1998, the carbon dioxide emissions from the transportation sector rose by 12.3%.

The industrial sector consists of agriculture, construction, mining, and manufacturing industries. Of these, manufacturing accounts for about 80% of industrial energy consumption. Electricity consumption accounted for about 37% percent of total industrial sector emissions, natural gas for 29%, petroleum for 21%, and coal for 12% of total emissions. Generally, industrial sector emissions are strongly affected by the growth of the economy.¹⁰

The residential and commercial sector are both heavily influence by the weather. Two-thirds of residential and three-fourths of commercial carbon dioxide emissions derive from electricity use for heating and air conditioning.  Residential carbon dioxide emissions are 19% of the U.S. total and rose 12.4% between 1990 and 1998.  Much of this rise can be attributed to increases in population in the Sunbelt, where the use of air conditioning is widespread. (Too calculte how much CO₂ you emit, click here.)  Commercial sector CO₂ emissions are 16% of the U.S. total and rose 15.3% during the same period.¹¹

INTERNATIONAL

Future Trends

China is expected to surpass the United States as the worlds largest emitter of greenhouse gases by the year 2015.  Although developing countries currently only contribute 23% global CO2 emissions, their contribution will rise in the coming decades.   Developing countries are likely to surpass developed countries in greenhouse gas emissions by the year 2035, in large part because of predicted increases in population and income.14 Depending upon how fast these variables grow and the mitigation actions taken, the IPCC estimates that global CO2 emissions will increase from between 19% to 104% by 2025.  Under the IPCC's middle scenario (see IS92a Scenario at right), emissions will increase by approximately 70%.15 (For more on this and expected increases in other GHGs, click here.)   The impacts of climate change are discussed in the next section.

LINKS

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                                                 Last Edited December 17, 1999