Carbon Dioxide Concentrations

Introduction

Carbon dioxide, symbolized by CO₂, is a compound in which each carbon atom binds with two oxygen atoms. CO₂ is released into Earth's atmosphere primarily by volcanoes, the burning of fuels containing carbon, and the decay of plant matter. It is removed from the atmosphere mostly by plants, which take carbon from CO₂ to build their tissues, and by the oceans, in which CO₂ dissolves.

CO₂ is found in the atmospheres of Earth, Mars, and Venus. On Earth, it is normally an invisible, odorless gas. Because it is opaque to some infrared radiation (the electromagnetic waves emitted by warm objects), carbon dioxide in the atmosphere slows the loss of heat energy from Earth into space.

CO₂ is the most important of the greenhouse gases that are causing Earth to warm, changing climate and weather patterns. Atmospheric CO₂ has increased greatly

since humans began burning large amounts of coal and petroleum in the nineteenth century. As of mid-2007, CO₂ comprised about 383 parts per million (ppm) or0.0383% of the atmosphere, an increase of more than 36% over its pre-industrial level of about 280 ppm.

Historical Background and Scientific Foundations

One source of information about atmospheric carbon dioxide concentration is direct measurement of the air. Such data have been gathered steadily since 1958, when American geochemist Charles David Keeling (1928– 2005) made the first atmospheric CO₂ measurements at Mauna Loa Observatory in Hawaii. Keeling found that atmospheric CO₂ tracks the growing season in the Northern Hemisphere, which holds most of the world's vegetation. In the spring and summer, as green plants grow, they remove CO₂ from the air; in the winter, fuel burning and plant decay continue to release CO₂, while plants absorb little.

The result is a series of peaks and valleys in atmospheric CO₂ concentration. From the top of each winter peak to the bottom of each summer valley, CO₂ concentration decreases by about 5 ppm. The terminology “N parts per million” means that out of every 1,000,000 gas molecules in a sample of air, N are CO₂ molecules. Most of the others are nitrogen (N₂) and oxygen (O₂).

On average, not counting these seasonal variations, the average CO₂ concentration is slowly increasing by about 2 ppm per year. A chart of these changes, plotting time horizontally and CO₂ concentration vertically, is called a Keeling curve and looks somewhat like an upward-curved saw-blade. In 1958, the CO₂ concentration, apart from seasonal ups and downs, was about 315 ppm; today it is over 380 ppm.

In Antarctica and Greenland, annual snowfall has been packing down into thin layers of ice for many years. Small air bubbles trapped in this ice are samples of ancient air. By counting ice layers like tree rings, scientists know how old these samples are. Thus, cylinders of ice (ice cores) drilled out of such deposits reveal the amount of CO₂ in the air over long periods of time. The Vostok ice core, drilled in East Antarctica from 1990 to 1994, supplied a continuous series of CO₂ samples from 420,000 years ago to the present. In 2004, scientists announced results drilled from an Antarctic location called Dome C that pushed the record back to 740,000 years ago. In 2006, the Dome C ice-core record was increased to 800,000 years—a cylinder of ice some 2 mi (3.2 km) long.

These and other data show that human activity over the last 200 years has raised atmospheric CO₂ to a level substantially higher than any seen in the last 800,000 years. CO₂ is also rapidly increasing at an unprecedented rate. The most rapid rate of increase observed in the last 800,000 years was 30 ppm over 1,000 years, while the most recent increase of 30 ppm has occurred in only the past 17 years.

Impacts and Issues

Most scientists agree that human activity is causing most of the observed increase in atmospheric CO₂ concentrations. They also agree that increased CO₂ is the primary cause of the recent warming of the world's climate, which they predict will continue.

International gatherings of scientists such as the Intergovernmental Panel on Climate Change (IPCC) advise that reducing the amount of CO₂ that humans add to the atmosphere from this time forward, will result in smaller changes in global climate. There is political disagreement over what steps should be taken to reduce human CO₂ emissions. Burning less fossil fuel would reduce emissions, but as modern industrial economies are still deeply dependent on relatively cheap and abundant coal and petroleum, this is a difficult step to take.

Primary Source Connection

The Intergovernmental Panel on Climate Change (IPCC) is not a research organization, but a scientific body established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP). The IPCC was awarded the Nobel Peace Prize in 2007 for its efforts to educate global policymakers about human-made climate change. The following source from the IPCC's 2007 report highlights scientific data on the increase over time of atmospheric CO₂,CH₄, and other greenhouse gases (GHGs).

IPCC SCIENCE BASIS TECHNICAL SUMMARY

Current concentrations of atmospheric CO₂ and CH₄ far exceed pre-industrial values found in polar ice core records of atmospheric composition dating back 650,000 years. Multiple lines of evidence confirm that the post-industrial rise in these gases does not stem from natural mechanisms .

WORDS TO KNOW

FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

ICE CORE: A cylindrical section of ice removed from a glacier or an ice sheet in order to study climate patterns of the past. By performing chemical analyses on the air trapped in the ice, scientists can estimate the percentage of carbon dioxide and other trace gases in the atmosphere at that time.

INFRARED: Wavelengths slightly longer than visible light, often used in astronomy to study distant objects.

INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE (IPCC): Panel of scientists established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) in 1988 to assess the science, technology, and socioeconomic information needed to understand the risk of human-induced climate change.

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. Carbon dioxide began rising due to human activities in the 1800s, but direct, continuous measurements of atmospheric carbon dioxide were first made by U.S. oceanographer Charles David Keeling (1928–2005) starting in 1958.

The total radiative forcing of the Earth's climate due to increases in the concentrations of the LLGHGs CO₂, CH₄ and N₂O, and very likely the rate of increase in the total forcing due to these gases over the period since 1750, are unprecedented in more than 10,000 years . It is very likely that the sustained rate of increase in the combined radiative forcing from these greenhouse gases of about +1 W m^-2 over the past four decades is at least six times faster than at any time during the two millennia before the Industrial Era, the period for which ice core data have the required temporal resolution. The radiative forcing due to these LLGHGs has the highest level of confidence of any forcing agent.

The concentration of atmospheric CO₂ has increased from a pre-industrial value of about 280 ppm to 379 ppm in 2005 . Atmospheric CO₂ concentration increased by only 20 ppm over the 8000 years prior to industrialisation; multi-decadal to centennial-scale variations were less than 10 ppm and likely due mostly to natural processes. However, since 1750, the CO₂ concentration has risen by nearly 100 ppm. The annual CO₂ growth rate was larger during the last 10 years (1995–2005 average: 1.9 ppm yr^-1) than it has been since continuous direct atmospheric measurements began (1960–2005 average: 1.4 ppm yr^-1).

Increases in atmospheric CO₂ since pre-industrial times are responsible for a radiative forcing of +1.66 ± 0.17 W m^-2; a contribution which dominates all other radiative forcing agents considered in this report . For the decade from 1995 to 2005, the growth rate of CO₂ in the atmosphere led to a 20% increase in its radiative forcing.

Emissions of CO₂ from fossil fuel use and from the effects of land use change on plant and soil carbon are the primary sources of increased atmospheric CO₂ . Since 1750, it is estimated that about 2/3rds of anthropogenic CO₂ emissions have come from fossil fuel burning and about 1/3rd from land use change. About 45% of this CO₂ has remained in the atmosphere, while about 30% has been taken up by the oceans and the remainder has been taken up by the terrestrial biosphere. About half of a CO₂ pulse to the atmosphere is removed over a time scale of 30 years; a further 30% is removed within a few centuries; and the remaining 20% will typically stay in the atmosphere for many thousands of years.

In recent decades, emissions of CO₂ have continued to increase . Global annual fossil CO₂ emissions increased from an average of 6.4 ± 0.4 GtC yr^-1 in the 1990s to 7.2 ± 0.3 GtC yr^-1 in the period 2000 to 2005. Estimated CO₂ emissions associated with land use change, averaged over the 1990s, were 0.5 to 2.7 GtC yr^-1, with a central estimate of 1.6 Gt yr^-1.

Since the 1980s, natural processes of CO₂ uptake by the terrestrial biosphere and by the oceans have removed about 50% of anthropogenic emissions. These removal processes are influenced by the atmospheric CO₂ concentration and by changes in climate . Uptake by the oceans and the terrestrial biosphere have been similar in magnitude but the terrestrial biosphere uptake is more variable and was higher in the 1990s than in the 1980s by about 1 GtC yr^-1. Observations demonstrate that dissolved CO₂ concentrations in the surface ocean have been increasing nearly everywhere, roughly following the atmospheric CO₂ increase but with large regional and temporal variability.

Carbon uptake and storage in the terrestrial biosphere arise from the net difference between uptake due to vegetation growth, changes in reforestation and sequestration, and emissions due to heterotrophic respiration, harvest, deforestation, fire, damage by pollution and other disturbance factors affecting biomass and soils . Increases and decreases in fire frequency in different regions have affected net carbon uptake, and in boreal regions, emissions due to fires appear to have increased over recent decades. Estimates of net CO₂ surface fluxes from inverse studies using networks of atmospheric data demonstrate significant land uptake in the mid-latitudes of the Northern Hemisphere (NH) and near-zero land-atmosphere fluxes in the tropics, implying that tropical deforestation is approximately balanced by regrowth.

Short-term (interannual) variations observed in the atmospheric CO₂ growth rate are primarily controlled by changes in the flux of CO₂ between the atmosphere and the terrestrial biosphere, with a smaller but significant fraction due to variability in ocean fluxes . Variability in the terrestrial biosphere flux is driven by climatic fluctuations, which affect the uptake of CO₂ by plant growth and the return of CO₂ to the atmosphere by the decay of organic material through heterotrophic respiration and fires. El Niño-Southern Oscillation (ENSO) events are a major source of inter-annual variability in atmospheric CO₂ growth rate, due to their effects on fluxes through land and sea surface temperatures, precipitation and the incidence of fires.

Susan Solomon et al .

solomon, susan, dahe qin, and martin manning, et al. “ipcc science basis technical summary.” ipcc report 4th technical summary. united nations, 2007.

See Also Carbon Cycle; Climate Change; Greenhouse Effect; Greenhouse Gases; Intergovernmental Panel on Climate Change (IPCC).

BIBLIOGRAPHY

Books

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.

Periodicals

European Project for Ice Coring in Antarctica. “Eight Glacial Cycles from an Antarctic Ice Core.” Nature 429 (June 10, 2004): 623–628.

Maseh, Betsy. “The Hot Hand of History.” Nature 427 (February 12, 2004): 582–583.

Web Sites

Amos, Jonathan. “Deep Ice Tells Long Climate Story.” BBC News, September 4, 2006. < http://news.bbc.co.uk/2/hi/science/nature/5314592.stm> (accessed August 5, 2006).

“Climate Change Affecting Earth's Outermost Atmosphere.” University Corporation for Atmospheric Research, December 11, 2006. < http://www.ucar.edu/news/releases/2006/thermosphere.shtml> (accessed August 5, 2007).

“Trends in Atmospheric Carbon Dioxide—Mauna Loa; Trends in Atmospheric Carbon Dioxide—Global.” U.S. National Oceanic and Atmospheric Administration (NOAA). < http://www.esrl.noaa.gov/gmd/ccgg/trends/> (accessed August 5, 2007).

Larry Gilman