Why does CO2 get most of the attention when there are so many other heat-trapping gases (greenhouse gases)?

Global warming is primarily a problem of too much carbon dioxide in the atmosphere.  This carbon overload is caused mainly when we burn fossil fuels like coal, oil and gas or cut down and burn forests. There are many heat-trapping gases (from methane to water vapor), but CO2 puts us at the greatest risk of irreversible changes if it continues to accumulate unabated in the atmosphere. There are two key reasons why.

CO2 has caused most of the warming and its influence is expected to continue

CO2, more than any other cf driver, has contributed the most to climate change between 1750 and 2005.[1, 2, 3] The Intergovernmental Panel on Climate Change (IPCC) issued a global climate assessment in 2007 that compared the relative influence exerted by key heat-trapping gases, tiny particles known as aerosols, and land use change of human origin on our climate between 1750 and 2005.[3] By measuring the abundance of heat-trapping gases in ice cores, the atmosphere, and other climate drivers along with models, the IPCC calculated the “radiative forcing” (RF) of each climate driver—in other words, the net increase (or decrease) in the amount of energy reaching Earth’s surface attributable to that climate driver. Positive RF values represent average surface warming and negative values represent average surface cooling. CO2 has the highest positive RF (see Figure 1) of all the human-influenced climate drivers compared by the IPCC. Other gases have more potent heat-trapping ability molecule per molecule than CO2 (e.g. methane), but are simply far less abundant in the atmosphere and being added more slowly.

Figure 1. How Does CO2 Compare To Other Climate Drivers?


 
 Carbon dioxide (CO2), more than any other climate driver, has contributed the most to climate change between 1750 and 2005.[4]

CO2 sticks around

CO2 remains in the atmosphere longer than the other major heat-trapping gases emitted as a result of human activities. It takes about a decade for methane (CH4) emissions to leave the atmosphere (it converts into CO2) and about a century for nitrous oxide (N2O).[3] In the case of CO2, much of today’s emissions will be gone in a century, but about 20 percent will still exist in the atmosphere approximately 800 years from now.[3] This literally means that the heat-trapping emissions we release today from our cars and power plants are setting the climate our children and grandchildren will inherit. CO2’s long life in the atmosphere provides the clearest possible rationale for reducing our CO2 emissions without delay.

What about water vapor?

Water vapor is the most abundant heat-trapping gas, but rarely discussed when considering human-induced climate change. The principal reason is that water vapor has a short cycle in the atmosphere (a few days) before it is incorporated into weather events and falls to Earth, so it cannot build up in the atmosphere in the same way as carbon dioxide does.[1, 2]

Too much of a good thing: the carbon overload

Earth receives energy that travels from the sun in a variety of wavelengths, some of which we see as sunlight and others that are invisible to the naked eye, such as shorter- wavelength ultraviolet radiation and longer-wavelength infrared radiation. As this energy passes through Earth’s atmosphere, some is reflected back into space by clouds and small particles such as sulfates; some is reflected by Earth’s surface; and some is absorbed into the atmosphere by substances such as soot, stratospheric ozone, and water vapor (see Figure 2 for relative proportions).[4] The remaining solar energy is absorbed by the earth itself, warming the planet’s surface. 

Figure 2. Heat-trapping Gases in the Atmosphere


 
The molecules depicted in the inset box represent heat-trapping gases, such as water vapor, carbon dioxide, methane, nitrous oxide. The number of incoming and outgoing arrows are proportional to the balance between incoming and outgoing energy.[2] Data source: IPCC 2007; Figure: Union of Concerned Scientists.

If all of the energy emitted from the Earth’s surface escaped into space, the planet would be too cold to sustain human life. Fortunately, as depicted in Figure 2, some of this energy does stay in the atmosphere, where it is sent back toward Earth by clouds, released by clouds as they condense to form rain or snow, or absorbed by atmospheric gases composed of three or more atoms, such as water vapor (H2O), carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4).

Long-wave radiation absorbed by these gases in turn is re-emitted in all directions, including back toward Earth, and some of this re-emitted energy is absorbed again by these gases and re-emitted in all directions. The net effect is that most of the outgoing radiation is kept within the atmosphere instead of escaping into space. Heat-trapping gases, in balanced proportions, act like a blanket surrounding Earth, keeping temperatures within a range that enables life to thrive on a planet with liquid water.  Unfortunately, these gases—especially carbon dioxide—are accumulating in the atmosphere at increasing concentrations due to human activities such as the burning of fossil fuel in cars and power plants, the clearing of forests for agriculture or development, and agricultural practices.[4] As a result, the insulating blanket is getting too thick and overheating the Earth as less energy (heat) escapes into space.

Long-term perspective

Antarctic ice core records vividly illustrate that atmospheric CO2 levels today are higher than levels recorded over the past 800,000 years (see Figure 3).[5] Atmospheric CO2 levels have risen 36 percent in the last 250 years, with half of that rise occurring only in the last three decades.[4] CO2 (and other gases emitted from industrial and agricultural sources) trap heat in the atmosphere, so it is no surprise that we are now witnessing an increase in global average temperature. In the same way that CO2 emitted long ago is now contributing to the changes in climate we are already experiencing today, the emissions we are currently releasing will help determine the climate future our children and grandchildren experience.

Figure 3. CO2 History Over the Ages

Composite record of Antarctic ice core carbon dioxide concentrations and the modern (1959-2007) atmospheric carbon dioxide record from Mauna Loa Hawaii. Data Sources: Supplementary information two for Luthi et al (2008) [5] and Mauna Loa data from [6]; Figure Union of Concerned Scientists.

References

[1] Note that human activities provide a small indirect influence on water vapor through methane (CH4) emissions that undergo chemical transformation in the stratosphere, thus creating the byproduct of water vapor. Another indirect influence is that human-induced global warming has increased the tropospheric air temperature, which can now hold more water vapor (around 4% increase since 1970) which in turn traps even more heat amplifying the warming.[2]

[2] Trenberth, K.E., P.D. Jones, P. Ambenje, R. Bojariu, D. Easterling, A. Klein Tank, D. Parker, F. Rahimzadeh, J.A. Renwick, M. Rusticucci, B. Soden and P. Zhai, 2007: Observations: Surface and Atmospheric Climate Change. 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.

[3] Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. Lowe, G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz and R. Van Dorland, 2007: Changes in Atmospheric Constituents and in Radiative Forcing. 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.

[4] Somerville, R., H. Le Treut, U. Cubasch, Y. Ding, C. Mauritzen, A. Mokssit, T. Peterson and M. Prather, 2007: Historical Overview of Climate Change. 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.

[5] Keeling, C.D. and T.P. Whorf. 2004. Atmospheric CO2 records from sites in the SIO air sampling network. In Trends: A Compendium of Data on Global Change. Oak Ridge, TN: Carbon Dioxide Information Analysis Center; Petit J.R. et al. 1999. Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399:429–436; Siegenthaler, U. et al. 2005. Stable carbon cycle-climate relationship during the late Pleistocene. Science 310:1313–1317; Lüthi, D.,  M. Le Floch, B. Bereiter, T. Blunier, J-M. Barnola, U. Siegenthaler, D. Raynaud, J. Jouzel, H. Fischer, K. Kawamura, and T. F. Stocker. 2008. High-resolution carbon dioxide concentration record 650,000–800,000 years before present. Nature 453:379-382, doi:10.1038/nature06949.

[6] Keeling, R.F., S.C. Piper, A.F. Bollenbacher and J.S. Walker. 2008. Atmospheric CO2 records from sites in the SIO air sampling network. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A. Also accessed online September 2008 at >The Carbon Dioxide Information Analysis Center&website.

Summary prepared by B. Ekwurzel (UCS) and reviewed by M. Baker (University of Washington), N. Cole, P. Frumhoff, and S. Shaw (UCS).

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