The sting in the tail

The sting in the tail...

As mentioned last time, methane is a powerful GHG and highly able at trapping radiation. We've also learnt that methane has a relatively short residence time in the atmosphere of about 8.9 years (according to the AR4 report). Its warming ability however is not just linked to the time methane exists as methane within the atmosphere - the majority of methane metabolises into other harmful GHGs (including CO2) and consumes free radical OH. This is something the AR4 report as with others did not fully recognise...

"We found that gas-aerosol interactions substantially alter the relative importance of the various emissions. In particular, methane emissions have a larger impact than that used in the current carbon-trading schemes or in the Kyoto Protocol.

and

"...studies including the Inter-Governmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), provide estimates of RF [radiative forcing] and GWPs ... however, except for the indirect effect of NOx emissions on nitrate aerosols, gas-aerosol interactions were not included." [...] ...any separate efforts to mitigate warming from short-lived pollutants, should include gas-aerosol interactions." 

 [both: Shindell et al, Improved Attribution of Climate Forcing to Emissions, 2006]

Why is this? Because when these interactions are not considered the overall image for the warming potential of the GHG is underestimated, particularly so with methane. Let's look into this with the following figure again published by Shindell et al:

When looking at the emissions based diagram, methane's contribution to radiative forcing as methane (the yellow brick within the methane column) is little over half the total radiative forcing of methane when considering the effect its metabolites and interactions with other aerosols has on radiative forcing.

When these are not considered the ease to underestimate methane's total contribution to radiative forcing is significant: this can be seen clearly below - look how great the difference between AR4's figure for methane's 100-year GWP is and Shindell et al's calculation, incorporating the gas's direct and indirect interaction with aerosols is...

We've talked in previous weeks about how the GWP of methane has been edged up through the years:

The IPCC published in 1995 that the 100 year GWP for methane was 21. In the AR4 report as shown in these diagrams it was published as being nearer to 25. In the AR5 report it was published as being either 28 or 34 depending on whether climate-carbon feedback is included or not. 

Shindell et al's 2009 paper did not incorporate climate carbon feedbacks and they calculate as the diagram shows methane to have a 100 year GWP of 33 when direct and indirect aerosols are considered. If they considered climate-carbon feedbacks as well (which AR5 showed to raise the figure by 6 the 100 year GWP of methane would be close to 40, possibly greater if the climate carbon feedback is non-linear (something that could quite reasonably be assumed?). 

Is then even now the real extent and importance of methane in climate change still being underestimated? Or masked even in major climate reports and in carbon trading schemes? That's up to you to decide...