Some molecules will last in the atmosphere longer than others, thus increasing their potential. Sulfur hexafluoride for example has a GWP of 25000 because it will last longer.
As with any statistic you need to know how it is worked out to understand it.
My understanding is that methane is a "less powerful" GHG than CO2 on a concentration basis. That is, 400 ppm of both would show that CO2 absorbed more radiation.
However, think of the number of times you hear that a given company has the fastest growth rate. Straightaway you suspect that the company in question is small. If you have a turnover of $1 it is easier to double it than if you have a turnover of $1billion.
Same with methane. Current concentrations are low and in the range of parts per billion and it is nowhere near being saturated so a small increase in the amount results in a large increase in ratio. CO2, on the other hand, is measured in parts per million and is on the way to being saturated. So a small increase has little effect in ratio terms.
It has to be one of these possibilities, or a combination:
1) Methane's resonance is at a frequency that is a stronger blackbody frequency. The blackbody spectrum of the Earth is a spectrum, with different frequencies having different amplitudes. While infrared region is the strongest, there is a difference for different frequencies in infrared.
2) The interaction is intrinsically stronger for methane. You can have to substances with the same resonance frequency, but one will interact more strongly with that radiation. The strength of the interaction can be calculated from the rules of quantum mechanics, specifically Fermi's golden Rule, and there is no reason at all to think different substances must have the same "affinity" to a resonance frequency.
I don't actually know, but it wouldn't surprise me if it's both, plus a third factor. Since most of them are more complicated molecules than CO2, it seems like there would be more frequencies that they could absorb, though some of what they can absorb may be "masked" by another greenhouse gas, stratospheric ozone, or the like.
Since they are present in such small quantities, it's likely that they are not as close to saturated. I believe "basic" saturation (there's probably a term for this, but I don't know it) happens at something like 10 ppm for CO2 (that is, saturation of the core emissions spectrum, not the shoulders of the band), it wouldn't surprise me if other greenhouse gasses are similar. Since trace GHGs are a lot closer to 10 ppm, they're not relying as much on "shoulder" absorption for their effects.
And, of course, since they are present in much smaller quantities, a smaller total emission is a much larger percentage emission.
It doesn't as long as there is any amount of water vapor in the atmosphere methane is completely benign and water vapor more than soaks up all IR energy bands that methane could react with. And methane's dwell time in the atmosphere is only a few years before UV breaks it down. It is pretty much a mute point and a straw that only the most desperate alarmists will reach for.
http://upload.wikimedia.org/wikipedia/co...
It is not that methane and other trace gases are necessarily intrinsically stronger greenhouse gases than carbon dioxide. Some gases are intrinsically stronger, meaning that they would have a stronger effect in the same concentration and others would have a weaker effect at the same concentration. But global warming potential is the ratio of the amount of carbon dioxide that would have to be added to have the same effect on temperature as a given amount of the other greenhouse gas.
The effect of a greenhouse gas on temperatue is logarithmic with respect to concentration. If the is very little of a given greenhouse gas in the atmosphere, adding more will have a strong effect. If there is a lot of the given greenhouse gas in the atmosphere, adding the same quantity will have little effect. Most greenhouse gases, including methane are present in the atmosphere in much smaller amounts than carbon dioxide, so adding a given amount will have a much stronger effect than adding the same amount of carbon dioxide.
It is not, it is has a smaller bandwidth than CO2, the reason why it is considered dangerous is because it is in much smaller concentrations, it is not saturated like CO2 is, and therefore can soak more heat if it increases.
google "logarithmically diminishes with concentration"
http://upload.wikimedia.org/wikipedia/co...
Hmm what did I say to deserve 3 thumbs down my answer is pretty much the same as Climate Realist
Wikipedia explain this quite well http://en.wikipedia.org/wiki/Global-warm...
Just wondering what it is about trace gasses which have relatively high GWP. Is it due to band saturation or something to do with the range of IR frequencies they absorb?
