Methane is a stronger greenhouse gas than carbon dioxide because it has much higher heat trapping ability. Methane on a weight basis has 21 times the global warming potential (GWP) of carbon dioxide.

Methane emissions are lower than carbon dioxide emissions, it is considered a major greenhouse gas because each methane molecule has 25 times the global warming potential of a carbon dioxide molecule.

Great question. It comes from the fact that the radiative forcing on the atmosphere is approximately logarithmic dependent on concentration. Mathematically, the radiative forcing (RF) can be written as RF = a*log(C/C0), where C is the new concentration and C0 is the baseline concentration and a is some gas dependent constant. Therefore, if you double the concentration of a gas in the atmosphere you get the same radiative forcing, regardless of the starting concentration. That is, the radiative forcing is the same to increase CO2 from 300 ppm to 600 ppm as it is to increase it from 600 ppm to 1200 ppm, even though the absolute change is twice as large. Now, methane has a much smaller atmospheric concentration (~2 ppm) in comparison to CO2 (400 ppm). Therefore, the absolute concentration change to double methane is much smaller than for CO2. This is described in more detail in Sec. 6.3 of the IPCC third assesment report (https://www.ipcc.ch/report/ar3/wg1/). For very large changes in gas concentration, the logrithmic relationship between concentration and radiative forcing breaks down. For more information, take a look at this study (by me) describing this in more detail: Article Radiative forcing at high concentrations of well mixed green...

Note that the relationship between radiative forcing and concentration for methane is closer to square root dependence than logarithmic. However, square root and log are both less than linear and the same general argument described above follows.

Dear Dr. Brendan Byrne

Thanks a lot for your elegant answers.

I greatly appreciate your valuable response.

Best regards,

Dear

Arvind Singh,

Abderrahim Benkhaled

Your comments are welcome.

Thank you very much to everyone who shared his sincere and wonderful opinion.

Wish you all the best in your career,

Understanding Global Warming Potentials (by the EPA)

Greenhouse gases (GHGs) warm the Earth by absorbing energy and slowing the rate at which the energy escapes to space; they act like a blanket insulating the Earth. Different GHGs can have different effects on the Earth's warming. Two key ways in which these gases differ from each other are their ability to absorb energy (their "radiative efficiency"), and how long they stay in the atmosphere (also known as their "lifetime").

The Global Warming Potential (GWP) was developed to allow comparisons of the global warming impacts of different gases. Specifically, it is a measure of how much energy the emissions of 1 ton of a gas will absorb over a given period of time, relative to the emissions of 1 ton of carbon dioxide (CO2). The larger the GWP, the more that a given gas warms the Earth compared to CO2 over that time period. The time period usually used for GWPs is 100 years. GWPs provide a common unit of measure, which allows analysts to add up emissions estimates of different gases (e.g., to compile a national GHG inventory), and allows policymakers to compare emissions reduction opportunities across sectors and gases.

• CO2, by definition, has a GWP of 1 regardless of the time period used, because it is the gas being used as the reference. CO2 remains in the climate system for a very long time: CO2 emissions cause increases in atmospheric concentrations of CO2 that will last thousands of years.
• Methane (CH4) is estimated to have a GWP of 28–36 over 100 years (Learn why EPA's U.S. Inventory of Greenhouse Gas Emissions and Sinks uses a different value.). CH4 emitted today lasts about a decade on average, which is much less time than CO2. But CH4 also absorbs much more energy than CO2. The net effect of the shorter lifetime and higher energy absorption is reflected in the GWP. The CH4 GWP also accounts for some indirect effects, such as the fact that CH4 is a precursor to ozone, and ozone is itself a GHG.
• Nitrous Oxide (N2O) has a GWP 265–298 times that of CO2 for a 100-year timescale. N2O emitted today remains in the atmosphere for more than 100 years, on average.
• Chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6) are sometimes called high-GWP gases because, for a given amount of mass, they trap substantially more heat than CO2. (The GWPs for these gases can be in the thousands or tens of thousands.)
• Conclusion: EPA rates methane to have a GWP of 28 -36 times higher than that of CO2 because it absorbs equally more solar radiation than CO2. It's that simple. Als mind the impact of N20 with a GWP of 265 to 298 that of CO2. Nitrous oxide emissions are produced by both natural and human sources. Important natural sources include soils under natural vegetation and the oceans. Natural sources create 62% of total emissions. Important human sources come from agriculture, fossil fuel combustion and industrial processes. Human-related sources are responsible for 38% of total emissions. Nitrous oxide emissions.
• Have a nice day and be happy.
• Frank

Dear Frank Veroustraete

Thanks a lot for your elegant answers.

With my best regards!

Frank Veroustraete left a good answer but there are probably some details worth mentioning.

1. A gas can only be a greenhouse gas if it can absorb IR. Typically to absorb IR a molecule must have a vibrational motion that changes its dipole (all 3+ atom molecules do). Having more such stretches increase the molecules ability to absorb heat radiated from the surface of the earth.

2. The stronger a molecule's absorption at a given wavelength, the more energy it will add to the atmosphere.

3. Absorbing in regions that don't overlap with CO2 or H2O will be more important in adding energy (heat) to the atmosphere.

4. If the molecule has a longer lifetime in the atmosphere it will be able to add more heat over a period of time (for equal weight, not sure why equal moles wasn't chosen as the standard). Because CH4 has a lifetime shorter than CO2 its GWP goes down as the timeline increases. Because SF6 has a lifetime longer than CO2 its GWP increases as the timeline increases.

5. Any corrections to mistakes or oversites I made are welcome.