Since 1751 about 329 petagrams of carbon (PgC) have been released to the atmosphere from the combustion of fossil fuels and the production of cement (a petagram of carbon (Pg), also known as a Gigaton (Gt), is equal to 10¹⁵ grams or one billion tonnes).Half of these emissions have occurred since the mid 1970s. The 2006 global fossil-fuel carbon emission estimate, 7.5 PgC, represents an all-time high and a 3.2% increase from 2005. Coal and oil account for 77% of carbon emissions, while natural gas accounts for another 19%. 

Atmospheric concentrations

The long run temperature and CO2 record as revealed by Antarctic ice core data. Source: Luthi et al. (2008)

Deep ice cores drilled in Antarctica reveal Earth's long run history of green house gas concentrations. Scientists have drilled 3,270m into the Antarctic ice, which equates to drilling nearly 900,000 years back in time. Gas bubbles trapped as the ice formed yield important evidence of the mixture of gases present in the atmosphere at that time, and of temperature. The ice core data reveal that CO₂ levels are substantially higher now than at any time in the last 800,000 years. Carbon dioxide concentrations over the 800,000 range from 172 to 300 ppm. The current value of about 390 ppmv is well beyond any level observed in the pre-industrial period.

The same ice cores reveal that that the concentration of methane (CH₄), another important greenhouse gas, is also substantially higher now than at any time in the last 800,000 years. In recent decades, however, the the rate of increase in CH₄ emissions has slowed considerably.

The ice core data also reveal the Earth's natural climate rhythm over the last 800,000 years. When carbon dioxide changed there was always an accompanying climate change. Higher concentrations of greenhouse gases are associated with high temperatures, and vice versa. This connection is one reason that scientists conclude that the recent warming of the Earth is due in large part to the record high level of green gas concentrations.

The Mauna Loa or "Keeling" curve. Credit: Robert A. Rhode, Global Warming Art

The recent historical trend in CO₂ concentration is revealed in what is known as the Mauna Loa curve.  The curve is also known as the "Keeling curve", named for Charles D. Keeling (1928-2005), an American pioneer in the monitoring of carbon dioxide concentrations in the atmosphere.  Since 1958, the concentration of CO₂ in the atmosphere has been measured daily at Mauna Loa Observatory, Hawaii. Mauna Loa Observatory is located on the Island of Hawaii at an elevation of 3,397 meters above  sea level on the northern flank of Mauna Loa volcano. The Mauna Loa record shows a 23% increase in the mean annual concentration, from 316 parts per million by volume (ppmv) of dry air in 1959 to nearly 390 ppmv in 2009.

Scientists are certain that the increasing concentration of CO₂is due to human activity, and fossil fuels in particular, for a number of reasons:

• The known sources of carbon are adequate to explain the observed increase.
•  For thousands of years preceding 1850 (approximately the start of the industrial revolution and fossil fuel use), the concentration of CO₂ varied between 260 and 280 ppm. Since 1850, concentrations have increased by 35%. The timing of the increase is coincident with the rising emissions of carbon from fossil fuel combustion.
• The latitudinal gradient in CO₂ concentrations is highest at northern mid-latitudes and lower at higher and lower latitudes, consistent with the fact that most of the emissions of fossil fuel are located in northern mid-latitudes.
• The increase in concentrations over the period 1850 to 2000 was accompanied by a decrease in the ¹⁴C content of CO₂. The decrease is what would be expected if the CO₂ added to the system were fossil carbon depleted in ¹⁴C through radioactive decay. This dilution of ¹⁴CO₂ is called the Suess effect, named for the Austrian chemist Hans Suess, who noted the influence of this effect on the accuracy of radiocarbon dating.

Causes of increasing emissions

What's behind these trends?  A number of factors have contributed to the increase in CO₂emissions:

• Population growth: After growing very slowly for thousands of years, the human population increased very rapidly over the past several centuries, particularly in the 20th century. In most nations, every person consumes some fossil fuel energy to meet basic needs such as heating and cooking, so more people on the planet means more energy use. Many people also consume fossil fuels to support lifestyles above bare subsistence, which increases CO₂emissions. In 2006, the "average person" in the world was associated with the release of about 4.5 metric tons of CO₂from the use of fossil fuels. However, there are very large differences in per capita CO₂ emissions across nations due to differences in lifestyles, technologies, and policies.
•  Affluence. Higher per capita incomes are typically associated with higher per capita CO₂ emissions. As income rise, most people purchase more goods and services, use some energy-consuming devices more intensively, and change attitudes towards energy use. These forces combine to produce higher per capita CO₂ emissions in rich nations compared to poor nations. With a per capita GDP of more than $37,000, the United States releases about 20 metric tons of CO₂ per person, while India ($1,822 per capita GDP) releases just 1.2 metric tons of CO₂ per person.
• Technology.  Technology affects the combination of capital, labor, energy and materials used to produce a good or service, and thus affects CO₂ emissions. For example, Sweden generates most of its electricity with hydropower and nuclear power, and thus releases relatively small amounts of CO₂ per kilowatt-hour.  On the other hand, Poland uses a lot of coal to generate its power, and has a much higher release of CO₂ per kilowatt-hour.  Technological advances that reduce energy use by cars, light bulbs, appliances and other energy-consuming devices trend to reduce CO₂ emissions.
Kyoto Protocol participation map-2009. Green=signed & ratified, Red=signed, not ratified; Gray=non-signatory. Credit: Wikipedia.
• Policies. Policies. Government policy can dramatically affect energy use and CO₂ emissions. Many European nations have heavily taxed motor gasoline, resulting in driving habits and vehicle fuel efficiency that reduce CO₂ emissions relative to the United States where gasoline is much cheaper.  Signatories to the Kyoto Protocol (a protocol to the United Nations Framework Convention on Climate Change) have agreed to reduce to reduce their greenhouse gas emissions compared to the level of emissions in the year 1990.  Government policy can also increase emissions.  For example, many governments provide energy subsidies--measures that keep energy prices for consumers below market levels or for energy producers above market levels, or reduce costs for consumers and producers of energy. Such subsidies promote energy use and thus increase CO₂ emissions.
•  Attitudes and behaviors. The use of energy, and hence CO₂ emissions, is influenced by a society's attitudes towards material consumption, energy use, and energy environmental policy. A society that places a high value on material consumption will tend to have higher levels of energy use and CO₂ emissions.  On the other hand, a society that places a high value on environmental quality and international cooperation might be more likely to adopt policies to reduce greenhouse gas emissions.

Sources

• Boden, T.A., G. Marland, and R.J. Andres. 2009. Global, Regional, and National Fossil-Fuel CO2 Emissions. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A. doi 10.3334/CDIAC/00001.
• Denman, K.L., G. Brasseur, A. Chidthaisong, P. Ciais, P.M. Cox, R.E. Dickinson, D. Hauglustaine, C. Heinze, E. Holland, D. Jacob, U. Lohmann, S Ramachandran, P.L. da Silva Dias, S.C. Wofsy and X. Zhang, 2007: Couplings Between Changes in the Climate System and Biogeochemistry. 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 [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
• Kaufmann, Robert K. and Cleveland, Cutler J. 2007. Environmental Science (McGraw-Hill, Debuke, IA), ISBN-10: 0073311863.
• Lüthi, Dieter , Martine Le Floch, Bernhard Bereiter, Thomas Blunier, Jean-Marc Barnola, Urs Siegenthaler, Dominique Raynaud, Jean Jouzel, Hubertus Fischer, Kenji Kawamura & Thomas F. Stocker, High-resolution carbon dioxide concentration record 650,000–800,000 years before present, Nature 453, 379-382 (15 May 2008), doi:10.1038/nature06949.