Mr. T wrote:But at the same time, methane is removed from the atmosphere in 8 years while CO2 remains in the atmosphere for much, much longer (up to hundreds of years).
Cutting down on methane emissions would reduce GHG related warming very rapidly in the short term, but in the long-term it's much more preferable to cut CO2 emissions due to there longer atmoshperic lifecycle, even though methanes emissions have 26 times the heat trapping of carbon.
Cutting long lasting GHG's like CO2 and SF6 is a better long-term strategy.
The commonly stated figure of methane having 23 times the effect of carbon dioxide per unit mass is based on the average effect over a long 100-year time period, e.g.:
usinfo.state.gov wrote:Methane is considered a potent greenhouse gas because, kilogram for kilogram, it is 23 times more effective than carbon dioxide at trapping heat in the atmosphere over a 100-year time period.
From here.
The IPCC also makes use of considering such a long time period. Methane has a much shorter half-life in the atmosphere of around 7 years due to gradual oxidation. In a far shorter time period than 100 years like just the first decade instead, methane has literally *hundreds* of times greater effect per unit mass than carbon dioxide.
For example, consider melting of sea ice in the north pole region over the coming couple of decades. Short of unpopular geoengineering proposals, reduction in methane emissions would be one of the most cost-effective measures to somewhat reduce such. In that short time period, a single cow's methane emissions can cause as much radiative forcing as multiple times future CO2 emissions per person.
Only after one considers a relatively long-term timespan of 100 years does the equivalent GHG effect for methane decline from hundreds of times as much as CO2 per unit mass to "only" 23 times as much.
If one considered a timeframe much greater than one century, then average methane effect relative to CO2 would decline well below even the 100-yr ratio of 23 to 1 per unit mass. However, if talking about a timeframe of many centuries, excess CO2 itself tends to be removed from the atmosphere eventually even by natural means (slow oceanic absorption, etc). Besides, if fourth-millennial human or posthuman civilization couldn't deal with such, they'd suck anyway.
In the most relevant time period like the 100-year illustration, reduction of methane emissions can provide relatively high benefits per dollar spent. Indeed, unlike CO2 causing the beneficial effect of carbon fertilization on plants in addition to mostly-undesirable warming, methane is worse per unit of GHG potential because it does nothing of much significance in the atmosphere aside from the primarily-harmful warming.
Of course, reduction of both CO2 and CH4 emissions would have more effect than either alone.
And, of course, methods involved in reducing CO2 often have benefits well beyond reducing global warming alone. For example, replacing gasoline from fossil fuels would be desirable for improving energy security, aside from the secondary benefit of also reducing CO2 emissions.
However, for environmental measures, the least benefit-to-cost ratio can occur with those that reduce CO2 emissions alone without doing much else, e.g. a proposal to build clean coal power plants with the carbon dioxide sequestered underground (receiving relatively significant amounts of available funding by governments from the U.S. to Australia).
As silly as it superficially seems, the quantitative benefit to cost ratio of measures to reduce methane emissions from cows and their manure is relatively good.
