Global climate change results from the emissions into the atmosphere of greenhouse gases (GHGs), such as carbon dioxide (CO2), methane, water vapor, nitrous oxide and ozone.  To evaluate the relative contribution of each of these GHGs to global warming, a calculation is made as to the ability of each GHG to trap heat in the atmosphere, including how much of the gas is removed from the atmosphere over a given number of years.  The calculation is called the global warming potential (GWP) and it uses CO2 as the standard to which the others are compared.  

There is generally more CO2 discharged to the atmosphere than other GHGs and it remains for decades, while methane releases may be less in volume and remain for shorter periods, but methane is more powerful in trapping heat.  So methane is determined to have a GWP of 21 compared to the GWP of 1 of CO2; in other words, methane is approximately 21 times more heat-absorptive than CO2 per unit of weight.  Such comparisons provide useful information for policy makers to determine which GHGs require what kind of mitigation or adaptation.

Some further ideas to explore on Global Warming Potential:

Since methane traps more heat than CO2 but lasts for a shorter period of time, what different policies would make sense for mitigating effects from methane in comparison to policies for CO2.

What different policies would help mitigate the impacts from the other GHGs.

Sources:

“Global Warming Potential,”  Global Greenhouse Warming www.global-greenhouse-warming.com/global-warming-potential.html

European Environment Agency, “Global warming potential” glossary.en.eea.europa.eu/terminology/concept_html?term=global%20warming%20potential

“Global Warming Potentials and Other Metrics for Comparing Different Emissions,” Climate Change 2007: Working Group I: The Physical Science Basis (Section 2.10), IPCC Fourth Assessment Report: Climate Change 2007.  www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-10.html

US Environmental Protection Agency, “Global warming potential.” iaspub.epa.gov/sor_internet/registry/termreg/searchandretrieve/termsandacronyms/search.do;jsessionid=8541177390de0d7fa1f321a4fe908881b00ad415d844766ba27825ec2e9bc86f