Just to review, the “greenhouse effect” occurs when incoming solar ultraviolet (UV) radiation penetrates the atmosphere, is reflected by the earth’s surface, clouds, etc., but is prevented from completely escaping the atmosphere as infrared (IR) thermal energy by intervening gases that absorb the energy and re-radiate it back toward the Earth’s surface, similar to what occurs in a glass greenhouse (Figure 1). The greenhouse gases can include carbon dioxide (CO2), methane (CH4), water vapor (H2O), and man-made chlorofluorocarbons. The greenhouse effect is essential to our existence. Without it, the Earth would be nearly uninhabitable, resulting in an average temperature below freezing. The question is can an imbalance caused by excessive greenhouse gases cause a rise in the surface temperatures to dangerous levels? As to the specific role of CO2, there is even some conjecture as to whether increases in CO2 alone can even produce a significant increase in the Earth’s average temperature. This was discussed in more detail in the “Hockey Stick” blog.
Figure 1. A schematic depiction of the “Greenhouse Effect.” (Source: Smithsonian Institute)
While CO2 is the most publicized greenhouse gas, it is not the most prevalent (that is water vapor), nor is it the most efficient absorber (that is methane). Fortunately, methane (CH4) is not nearly as abundant as water vapor and CO2. Methane is less than 2 parts per million (ppm), whereas CO2 is more than 400 ppm. Water vapor is highly variable but can range from 10 ppm in deserts to 50,000 ppm in the tropics. Other significant absorbing gases in the atmosphere are nitrous oxide (N2O) and man-made chemicals such as chlorofluorocarbons (CFCs) that are used in refrigerant and fire suppression systems (their use in aerosol cans was banned by the Montreal Protocol in the late 1970’s).
Another important factor to consider is the longevity of the greenhouse gases. Airborne CO2 and N2O can persist for many years (30 to more than 100) whereas water vapor has an average residence time of nine days. Thus, the effects of CO2 are longer-lasting. Man-made chemicals such as CFCs are extremely long-lasting (100 to thousands of years).
The efficiency of greenhouse gases has been combined with their lifetimes to form a Global Warming Potential (GWP) which has been calculated for various time periods.  While CO2 is much less important than other gases at short time scales (~20 years), it comes closer to CH4 over periods greater than 500 years. Due to its persistence and greater radiative efficiency, N2O has GWP significantly greater than CH4 and CO2 at all time periods. 
TABLE I. Global Warming Potential (GWP) of well know natural and man-made greenhouse gases relative to CO2 from the IPCC 2007 report. The GWP of CO2 is set to one for comparison. (Source: U. S. Dept. of Energy)
It is important to note that CO2 is produced by both natural and man-made causes. The latter include emissions from motor vehicles and other forms of fossil fuel burning (especially coal burning power plants that account for more than 40% of emissions). Natural emissions can come from volcanoes, decaying vegetation and the oceans. The rise in CO2 is partly alleviated by absorption from plant photosynthesis and the oceans, a sort of checks and balances system. Plants thrive in a CO2-rich environment.
It appears that CH4 has been increasing globally over the past decade, with a 30% increase in the United States, according to a recent study based on satellite data and surface observations.  A third of the U.S. increase is from oil and gas emissions, another third from livestock (yes, cow flatulence), while the remainder comes from landfill emissions and coal burning.
Although man-made emissions of CO2 are relatively small compared to other sources in the carbon cycle, not all of it can be absorbed, leaving a net increase in atmospheric CO2. Current levels (around 400 ppm) are believed to be the highest in more than 600,000 years.  This is not that surprising, since the Earth’s population is continuing to increase in an exponential fashion. Figure 2 below clearly shows that the trends of CO2 emissions and population match almost exactly. But as the many variations in global temperature over the last century show (Figure 3), CO2 increases do not always result directly in a rise in mean temperature, as a portion of the excessive heat can be absorbed by “sinks” such as the oceans, or incoming solar radiation can be blocked by increased aerosol concentrations.
Figure 2. Trend of CO2 emissions (millions of tons) versus global population (millions) from 1850-2005 (Source: Steve Easterling, U. of Toronto)
Figure 3. Trend of CO2 concentration (ppm) since 1880 (solid black line) is compared with annual mean global temperature (°F) for the same period. Horizontal reference line is mean temperature for the period (57.6°F). (Source: NOAA/National Climate Data Center)
The role of CO2 in significantly warming the Earth’s climate has been questioned by some. One researcher, the late Dr. William Gray, argued that the expected increase in CO2, even a doubling of CO2, will not bring the anticipated increase in global temperatures. Instead, he believed that warming of only half a degree C or less will occur, not the 2-4°C or more expected by the IPCC. This claim has historical basis. Around 1900, Swedish physicist Knut Angstrom determined that CO2 concentration beyond about 50 ppm has little effect on the Earth’s temperature, although the results of that experiment were disputed. Dr. Gray believed that changes in the strength of the inter-ocean circulation (such as the Atlantic Thermohaline Current) which controls the salinity of the oceans has led to the 0.8°C increase we have seen in the past century, not man-made greenhouse gases.
- Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report Climate Change, 2007, Working Group 1: The Physical Science Basis, Table 2.14, Section 2.10.2 Direct Global Warming Potentials
- Turner, A. J. and co-authors, 2016: A large increase in U. S. methane emissions over the last decade inferred from satellite data and surface observations. Geophysical Research Letters, Vol. 43, pages 2218-2224.
- Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report Climate Change, 2007, Synthesis
- Gray, William M., 2012: Physical Flaws of the Global Warming Theory and Deep Ocean Circulation Changes as the Primary Climate Driver. Unpublished manuscript prepared for the Heartland Institute’s 7th Int’l Conference on Climate Change, Chicago, May 21-23, 2012.