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Carbon Dioxide (CO2) Emissions

Carbon Dioxide (CO2) Emissions


Carbon dioxide (CO2) is released into Earth’s atmosphere mostly by the burning of carbon-containing fuels and the decay of wood and other plant matter. Under all conditions found naturally on Earth, CO2 is an invisible, odorless gas. It is removed from the atmosphere mostly by plants, which extract carbon from CO2 to build their tissues, and by the oceans, in which CO2 dissolves.

Because CO2 is opaque to infrared radiation (the electromagnetic waves emitted by warm objects) in the atmosphere, it acts as a blanket to slow the loss of heat from Earth into space. Although other gases are also causing Earth’s climate to warm, CO2 alone is responsible for about three-fourths of global warming.

The amount of CO2 in the atmosphere has increased greatly since human beings began burning large amounts of coal and petroleum in the nineteenth century. In more recent times, this source of CO2 emissions has increased rapidly, while destruction of forests has also become a major source of CO2. Atmospheric concentrations of several other gases, including methane (CH4) and nitrous oxide (N2O), have also been increased recently by human activities and are contributing to greenhouse warming of the planet.

Historical Background and Scientific Foundations

Emissions of CO2 predate the human race by billions of years and are essential to life on Earth, since the natural greenhouse effect keeps Earth’s average surface temperature above freezing. In the deep geological past, atmospheric CO2 has sometimes been much higher than today; for example, about 500 million years ago there was over 15 times as much atmospheric CO2 as there is now. On the other hand, until human beings began to burn large amounts of fossil fuel in the late eighteenth century, CO2 had been stable for about 20 million years. Due to anthropogenic (human-caused) emissions, atmospheric CO2 is now significantly higher than at any time in the last 800,000 years and probably in the last 20 million. This change has happened in a mere 200 years, which is instantaneous by geological standards.

The atmospheric CO2 record of the last 800,000 years is known precisely because layers of snowfall in Greenland and Antarctica can be counted like tree rings, and air bubbles in the layers preserve samples of the ancient air. For even more ancient times, geologists must rely on various chemical traces that atmospheric processes have left in the rocks and fossils. These traces do not provide as high-resolution a record as do ice cores.

In 1750, CO2 was present in the atmosphere at about 280 parts per million or ppm (that is, 280 out of every 1 million molecules in a typical volume of air were CO2 molecules). By 2008, atmospheric CO2 was up to about 385 ppm, an increase of 37.5% over pre-industrial times.

Atmospheric CO2 has been measured steadily ever since 1958, when American geochemist Charles David Keeling (1928–2005) made the first such measurements at Mauna Loa Observatory in Hawaii. Keeling found that, month by month, atmospheric CO2 tracks the growing season in the Northern Hemisphere, which contains most of the world’s land area and therefore most of its plants. In the spring and summer, as green plants grow, they remove CO2 from the air; in the winter, plant decay continues to release CO2 while growth absorbs relatively little. The result is a series of peaks and valleys in atmospheric CO2. From the top of each winter peak to the bottom of each summer valley, CO2 concentration decreases by about 5 ppm; from summer to winter, it increases by about the same amount.


ANTHROPOGENIC: Made by humans or resulting from human activities.

DEFORESTATION: A reduction in the area of a forest resulting from human activity.

INFRARED RADIATION: Electromagnetic radiation of a wave-length shorter than radio waves but longer than visible light that takes the form of heat.

KEELING CURVE: Plot of data showing the steady rise of atmospheric carbon dioxide from 1958 to the present, overlaid with annual sawtooth variations due to the growth of Northern Hemisphere plants in summer.

Not counting these seasonal ups and downs, the average CO2 concentration is increasing by about 2 ppm per year. A chart of these changes shows an upward-tilted wavy line called the Keeling curve. In 1958, atmospheric CO2, apart from seasonal changes, was 315 ppm; by 2008 it was 385 ppm.

Atmospheric CO2 is increased by emissions, but does not track them directly. Rather, the CO2 in the atmosphere is what is left after CO2 has been emitted by its various sources and absorbed by its various sinks. Carbon dioxide sinks (absorbers) include the oceans,which become more acidic as they absorb CO2, and green plants, including not only the familiar grasses, trees, and shrubs that grow on land but the trillions of single-celled phytoplankton that float in the top few meters of the world’s oceans. Atmospheric CO2 has been increasing since the beginning of the Industrial Revolution, when fuel-burning and forest-clearing began to accelerate, because absorption by sinks has not kept pace with anthropogenic emissions. About half of the CO2 released by human activities is absorbed each year by sinks.

Anthropogenic CO2 emissions, as of 2006, were about 30 million tons (27 million metric tons) per year. Volcanoes were the second-largest source of CO2, emitting about 145–255 million tons (130–230 million metric tons) per year. Thus, as of the early 2000s human beings were emitting CO2 about 130 times faster than all the world’s volcanoes put together.

Anthropogenic CO2 emissions increased by 1.1% per year during the 1990s and by more than 3% per year from 2000 to 2004. In 2004, 71% of CO2 emissions came from developed countries such as the United States. Of the roughly 315 billion tons (286 billion metric tons) of CO2 that have been emitted since 1750, about 77% have been emitted by the industrialized countries. However, poorer countries have recently been increasing their emissions more rapidly: 73% of 2004 growth in CO2 emissions occurred in rapidly developing countries such as China, which probably surpassed the United States as the largest single CO2 emitter in 2007. Nevertheless, per-person emissions remained far lower in the developing world. For example, in 2004 the average Chinese person emitted 3.8 tons of CO2 and the average U.S. person emitted 20.4 tons.

Anthropogenic emissions of CO2 come from three major sources. The first is the burning of fossil fuels such as oil, gasoline, natural gas, and coal. Fossil fuels account for about 77% of global anthropogenic CO2 emissions. The second major source is deforestation, about 20% of global CO2 emissions. The third is industrial processes other than fuel-burning, such as the release of CO2 during cement manufacture, where calcium carbonate (CaCO3) is heated to drive off CO2 and leave CaO.

Impacts and Issues

Emissions of carbon dioxide and other greenhouse gases are contributing to global climate warming and are causing the oceans to become more acidic. About three-quarters of global warming is due to CO2 alone. Climate change threatens agriculture, is causing sea levels to rise, and is likely to cause the extinction of hundreds of species of plants and animals. Ocean acidification threatens coral reefs and ocean food chains that depend on calcifers, organisms that build shells of calcium carbonate (mollusks, crustaceans, foraminifera, etc.).

The United Nations Framework Convention on Climate Change (UNFCCC; 1992) is an international treaty that commits countries to monitoring and, in theory, eventually reducing their CO2 emissions. An addendum to the treaty, the Kyoto Protocol, was negotiated in 1997. The Kyoto Protocol is the main international instrument for reducing CO2 emissions. It commits developed (richer) nations to reducing their CO2 emissions and requires developing nations such as China to monitor their emissions with a view to later reductions.

The Kyoto Protocol has been controversial. Overall, it has not reduced CO2 emissions, which continued to climb rapidly throughout the early 2000s, though it might be argued that Kyoto has kept emissions lower than they would have been otherwise. The United States, alone of industrialized nations, has never agreed to the protocol. Its position has been that requiring industrialized nations to reduce CO2 emissions while allowing developing nations such as China to increase theirs gives the developing nations an unfair advantage in the global marketplace. China and others counter that the United States and other industrialized countries have enriched themselves by emitting most of the CO2 in the atmosphere to date, and that it is unfair to forbid other countries to do whatever is necessary to establish their own industrial economies. Defenders of Kyoto tend to acknowledge that the treaty has been ineffectual so far while arguing that it is at least a start and lays out principles and precedents that can be developed into a truly effective system for reducing CO2 emissions.

Within the United States, apart from the Kyoto process, there has been debate and legal conflict over whether CO2 emissions should be regulated by the Environmental Protection Agency (EPA) as a form of pollution. Environmental groups have sought to force the EPA to acknowledge CO2 as a pollutant and regulate its emission in a legally binding way: industry, supported by the administration of President George W. Bush, has pushed instead for voluntary reduction targets.

Primary Source Connection

The following news article reports that the state of Vermont’s pollution laws are following the example of the recently enacted tough California law that states that carbon dioxide (CO2) emissions from cars sold in the state must be reduced by 30% by 2016. According to legal experts, the Environmental Protection Agency has been recognized as having the right to control greenhouse-gas emissions. The consensus on this ruling is that it will provoke other states with similar law styles to also amend their vehicle emissions laws, though much of the automobile industry claims that current funding and science is not yet adequate for such a change.


When historians finally take stock, Vermont may look like the mouse that roared—the tiny state that brought the nation’s mighty auto industry to heel by requiring cars that emit fewer greenhouse-gases.

This is one scenario that could unfold following a federal judge’s ruling Wednesday, which upheld a Vermont law patterned after California’s mandate that the carbondioxide emissions of cars sold in the state must be slashed 30 percent by 2016.

The judge’s finding—that federal fuel-economy laws are not in conflict with state emissions laws—is particularly significant, coming on the heels of a U.S. Supreme Court decision in April. That ruling found that the Environmental Protection Agency has the authority to regulate greenhouse-gas emissions, legal experts say.

On the one hand, Wednesday’s decision strengthens the hand of states that want to take action against global warming. But in the longer term, the impact from the ruling could lead to one nationwide standard, which is already expected by many.

In addition to the 12 states with California-style laws on the books, another six are close to acting.

The ruling this week could start dominoes falling by:

  • Prompting the U.S. EPA to grant California a waiver from the Clean Air Act allowing it, along with Vermont and the 10 other states with identical laws, to begin enforcing greenhouse-gas requirements for cars sold within their borders.
  • Causing six additional states—Arizona, Florida, New Mexico, Utah, Illinois, and Minnesota—to proceed with their own similar emissions requirements. Altogether, the 18 states that have such laws—or are leaning toward them—make up about half the US auto market.
  • Spurring Congress to reconsider the new fuel-efficiency standards it is currently weighing, which are not as demanding as Vermont’s, and mandate a tougher federal requirement that would also reduce greenhouse-gas emissions.
  • Causing federal judges in two similar cases brought by the auto industry—one in California, the other in Rhode Island—to dismiss those cases if they determine the industry has had its day in court and further proceedings would be redundant, according to environmental lawyers.


Carbon dioxide is the most important anthropogenic greenhouse gas. The global atmospheric concentration of carbon dioxide has increased from a pre-industrial value of about 280 ppm to 379 ppm in 2005. The atmospheric concentration of carbon dioxide in 2005 exceeds, by far, the natural range over the last 650,000 years (180-300 ppm) as determined from ice cores. The annual carbon dioxide concentration growth rate was larger during the last 10 years (1995–2005 average: 1.9 ppm per year), than it has been since the beginning of continuous direct atmospheric measurements (1960–2005 average: 1.4 ppm per year) although there is year-to-year variability in growth rates.

SOURCE: Solomon, S.D., et al. Technical Summary. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change Cambridge (UK) and New York: Cambridge University Press, 2007.

The efforts by the 12 states with laws in place could cut emissions by 100 million tons annually. By comparison, however, US cars and light trucks emit 1.5 billion tons annually.

Still, this would be “the most significant step so far” on vehicle emissions and pave the way for broader action, says Michelle Robinson, director of the clean vehicle program at the Union of Concerned Scientists, a Washington environmental group not party to the lawsuit.

Environmental groups, who joined Vermont as defendants in the current case, have been exultant. “This extremely important ruling makes clear that the US EPA and states acting under the Clean Air Act do have the power to set more stringent emissions limits on cars and can also regulate greenhouse gases,” says attorney Matt Pawa, who represented the Natural Resources Defense Council, the Sierra Club, and Environmental Defense.

Following the three-week trial, it was quite clear that US district judge William Sessions found less than convincing the arguments of auto-industry experts, who testified that the industry lacks the technology to make such vehicles and cannot afford to do so. The companies, he wrote, “have not carried their burden to show that compliance with the regulation is not feasible; nor have they demonstrated that it will limit consumer choice, create economic hardship for the automobile industry, cause significant job loss or undermine safety.” Auto-industry officials sounded a defiant note and promised to use what influence they could to try to block the EPA waiver to California, as well as potentially launch a court appeal.

Concerning EPA’s key decision on whether to grant the waiver requested by California, Dave McCurdy, president and CEO of the Alliance of Automobile Manufacturers, said in a statement that “the Alliance remains committed to working with policy makers to make certain that the EPA’s judgment is based on credible, sound scientific data as to what policies truly impact California, its citizens and global climate concerns.”

Yet some analysts see a familiar pattern being played out in which state mandates could be followed by federal requirements—which happened with the adoption of seat belts, air bags, and higher mileage standards. “We’ve got a similar dynamic here to what was happening in the 1990s with states leading with strict standards on tailpipe emissions of nitrous oxide and other pollutants—and the EPA and federal government finally following,” Ms. Robinson says.

The practical impact of the ruling could be more-efficient vehicles as soon as the 2009 model year, Mr. Pawa predicts—based on the idea that the industry could not afford to wait and see what Congress does. But veteran auto-industry analysts aren’t so sure. “You can wish for 100 miles per gallon or 200 m.p.g., but that doesn’t mean you can make it happen,” says David Cole, chairman of the Center for Automotive Research in Ann Arbor, Mich.

Others, however, say the ruling makes such arguments moot. “The court is looking carefully at the industry’s argument that this will bankrupt us and drive us to ruin,” says Steve Hinchman, an attorney for the Conservation Law Foundation. “The judge found the so-called obstacles to be overstated and that the industry has the financial resources. It’s ironic because this is a step that’s going to help the U.S. auto industry. They should fire their lawyers and promote their engineers.”

Mark Clayton


See Also Carbon Dioxide (CO2); Climate Change; Global Warming; Greenhouse Effect; Greenhouse Gases; Kyoto Protocol



Solomon. S., et al., eds. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.


Caldeira, Ken, et al. “Climate Sensitivity, Uncertainty and the Need for Energy without CO2 Emissions.” Science 299 (2003): 2052–2054.

Raupach, Michael R., et al. “Global and Regional Drivers of Accelerating CO2 Emissions.” Proceedings of the National Academy of Sciences 104 (2007): 10,288–10,293.

Web Sites

Oak Ridge National Laboratory (U.S.). “Global, Regional, and National Fossil Fuel CO2 Emissions.” (accessed April 23, 2008). “Beetle Adds to Canada’s CO2 Emissions.” (accessed April 23, 2008)

United Nations Statistics Division. “CO2 Emissions.” (accessed April 23, 2008).

U.S. Environmental Protection Agency. “Carbon Dioxide: Greenhouse Gas Emissions.” (accessed April 23, 2008).

Larry Gilman

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