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Probably the major concern about increased atmospheric carbon dioxide (CO2) is the climate change phenomenon of global warming. While there is little disagreement within the scientific community on the warming trend over the past century and a half, there have been different interpretations on the extent to which this can be attributed to anthropogenic (from human activities) increases in CO2. The most recent international scientific committee report (IPCC, 2007) concludes that the CO2 increase as a result of human activities is the principal reason for the warming. This section will briefly address global warming.
The process of global warming is linked to an increase in the “greenhouse effect” of the Earth. A simple explanation of the greenhouse effect: the atmosphere allows solar radiation to enter but blocks part of the return radiation from passing back out into space. This “greenhouse effect” is due to gases in the atmosphere; the major greenhouse gas in the atmosphere is water vapor and without this greenhouse effect, the Earth would be so cold it would not be habitable. The concern is about a recent increase in concentrations of some greenhouse gases. Among the gases other than water that can have greenhouse roles, the main variable ones are CO2, methane, and nitrous oxide. Methane and nitrous oxide are less abundant in the atmosphere than is CO2. All three of these gases have had large increases in the past century (see “Increasing Atmospheric CO2” section).
Global Warming Over Geological Time Scales
Paleoceanographers study the history of the ocean over long time periods (geological time). Tools they use to reconstruct geological time include studies of rock minerals in exposed layers of rock on land and cores of bottom sediments or glacial ice - cylindrical cuts made by pushing tubes into the sediment or ice. Rock strata and cores can be dated using a variety of chemical methods. Paleoceanographers have used these reconstructions recently to estimate trends of temperature, atmospheric oxygen, and atmospheric CO2 over time periods ranging from several hundreds of years to hundreds of millions of years. From these records, it can be seen that large differences in the temperature of the Earth and in atmospheric CO2 occurred over geological time. The time periods of most interest are the past hundreds of thousands of years and the past few thousand years. Human beings, in what we consider civilization, have only been around for the past few thousand years; a small fraction of geological time. Figure 1 shows records of the atmospheric CO2 levels and air temperatures at the Earth surface for the past 420,000 years. The unit for measurement of atmospheric CO2 is parts per million; 1 part per million is 1 molecule of CO2 in a total of 1 million molecules of gas. The unit ppmv is parts per million by volume; for example, 1 milliliter of CO2 in 1 million milliliters of total gas.
Figure 1. Records of atmospheric CO2 levels and air temperatures over the past 420,000 years reconstructed from ice cores taken from the Vostock ice station in Antarctica. Time is shown on the x-axis (horizontal) as years before present (BP); atmospheric concentration of CO2 is shown on the left-hand y-axis (vertical) as parts per million by volume (ppmv); temperature on right hand y-axis in degrees Celsius. Figure derived from Petit et al, 1999 (see References section for information).
As Figure 1 shows, the patterns of CO2 and temperature track together; leading to the conclusion that heat retention and warming of the Earth surface and atmospheric CO2 are linked. The geological record covered by this time period is one encompassing four glacial cycles with colder temperatures when the global glacial coverage was maximal and warmer temperatures when the glaciers retreated. We are in a period of glacial minimum and a glacial advance should occur naturally in the next several thousand years.
Recent Temperature Records
Looking at the current level of atmospheric CO2 in Figure 1, it could be suggested that the Earth should be considerably warmer today than it is. There is reason to suggest that climate is not yet in equilibrium with atmospheric CO2 and that more warming is already underway. Many attempts have been made in recent years to determine the temperature trend in a time period where good measurements are available. It is difficult to estimate a global average temperature because of the necessity for a representative average over the entire globe, recognizing seasonal and small scale variability. In addition, care must be made to avoid “heat island effects” of warming from industrial and municipal activities; trends should represent the global picture not local changes from local effects. Recognizing the complexities, multiple independent efforts give similar pictures. Figure 2 shows the global temperature trend from 1856-2005.
Figure 2. Global annual temperature anomaly, 1856-2005. The temperature anomaly (y-axis) is the deviation of temperature (in degrees Celsius) from an average value calculated for the period of 1961-1990. The x-axis is date. Plotted using data from the British Hadley Centre for Climate Prediction and Research found on the CDIAC website (see References section for information).
The figure shows a general warming over the 150-year period with a large increase since the 1970s. Records also show significant increases in the temperature of surface ocean water. Many driving forces influence temperature trends; as a result, it is not easy to see a simple consistent trend. The warmest year in history occurred in 1998; 2005 was the second warmest year in the record; and overall, 11 of the last 12 years (1995-2006) rank among the 12 warmest years in the instrumental record of surface temperature (since 1850); from IPCC, 2007.
There is little disagreement within the scientific community on the warming trend of the past century and a half. However, there have been different interpretations on the extent to which this can be attributed to anthropogenic (human-caused) increases in CO2. The evaluation of the previous large international committee report (IPCC, 2001) was that the CO2 increase is the principal reason for the warming. The just released 2007 IPCC report attributes and even greater certainty with the statement: “..leading to very high confidence (..at least a 9 out of 10 chance of being correct..) that the globally averaged net effect of human activities since 1750 has been one of warming.
Surface Temperature Over the Past Millenium
A number of different approaches have used to reconstruct the temperature history of the last 1000 to 2000 years. A dramatic picture showing the recent warming as a sharp change from the previous 500 – 1000 years was produced by Mann and colleagues (see References section for more information). The US National Academy of Sciences this past summer published a thorough analysis that confirmed the general shape which had also been shown independently by a number of other scientists. A number of independent “proxies” have been used for the temperature reconstructions which include ice cores, tree rings, marine sediments, historical sources from Europe and Asia, borehole temperature measurements, and retreat of glaciers. Figure 3 shows the temperature reconstruction from the National Academy of Sciences assessment.
Figure 3. Smoothed temperature records for the Earth surface over the past 1100 years. The colored lines represent six independent reconstructions and the black line shows actual instrument measurements (1850-present). Each curve is based on a different “proxy” method, each shows a slightly different history of temperature variations with its individual uncertainty. This set of reconstructions conveys a qualitatively consistent picture of temperature changes over the last 1,100 years, and especially over the last 400. Picture from NRC 2006 (see References section for information).
In the past century, the average global air temperature has increased by 0.74 degrees Celcius, much of this increase has occurred in the past 25 years. Increased atmospheric CO2 may cause an even larger temperature increase in the near future. Various projections (scenarios) have been made to estimate future atmospheric CO2 levels; scenarios are based on inputs ranging from “business as usual” with increasing CO2 emission rates, to stabilized rates, to decreased emission rates. Many of these scenarios predict that the atmospheric CO2 concentration will be double (over 500 ppm) that of the pre-industrial levels by the end of the current century. By convention, the “industrial” period is considered to be since 1750. Scenarios also give estimates of future temperatures, of changes in ocean chemistry (see “Ocean Acidification” section), and of sea level (see “Sea Level Rise” section).
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References for Background and Further Information on Climate Change
Crowley, T.J. 2005. Raising the ante on the climate debate. EOS 86 (12 July 2005).
Crowley, T.J. and Berner, R. A. 2001. Paleoclimate: Enhanced: CO2 and climate change. Science 292: 870-872.
IPCC. 2001. Climate Change 2001: The scientific basis. Intergovernmental Panel on Climate Change. Cambridge University Press.
IPCC, 2007. Climate Change 2007: The Physical Science Basis, Summary for Policymakers. IPCC Secretariat, WMO, Geneva. [On the web at www.ipcc.ch/SPM2feb07.pdf].
Jones, P.D., M. New, D.E. Parker, S. Martin, and I.G. Rigor. 1999. Surface air temperature and its changes over the past 150 years. Reviews of Geophysics 37:173-199.
Mann, M.E., R.S. Bradley, and M.K. Hughes. 1998. Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392: 779-787.
Mann, M.E. and P.D. Jones. 2003. Global surface temperatures over the past two millennia. Geophysical Research Letters 30: 1820.
NRC. 2006. Surface temperature reconstructions for the last 2000 years. National Academy of Sciences. 196p.
The Oak Ridge CDIAC data can be found on the website:
Petit, J.R., J. Jouzel, D. Raynaud, N.I. Barkov, J.-M. Barnola, I. Basile, M. Benders, J. Chappellaz, M. Davis, G. Delayque, M. Delmotte, V.M. Kotlyakov, M. Legrand, V.Y. Lipenkov, C. Lorius, L. Pépin, C. Ritz, E. Saltzman, and M. Stievenard. 1999. Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399: 429-436.
Prepared by Jonathan H. Sharp with assistance from Ferris Webster, Joseph Farrell, and John Wehmiller and Douglas White.