Lack of methane growth explained?

I’ve said before (and correctly sourced the original observation to JA) that atmospheric methane is way below its IPCC scenarios (which of course leads to a lower forcing). There is a recent thing in Nature that may explain this:

Atmospheric methane (CH4) increased through much of the twentieth century, but this trend gradually weakened until a stable state was temporarily reached around the turn of the millennium1, 2, after which levels increased once more3. The reasons for the slowdown are incompletely understood, with past work identifying changes in fossil fuel, wetland and agricultural sources and hydroxyl (OH) sinks as important causal factors1, 4, 5, 6, 7, 8. Here we show that the late-twentieth-century changes in the CH4 growth rates are best explained by reduced microbial sources in the Northern Hemisphere. Our results, based on synchronous time series of atmospheric CH4 mixing and 13C/12C ratios and a two-box atmospheric model, indicate that the evolution of the mixing ratio requires no significant change in Southern Hemisphere sources between 1984 and 2005. Observed changes in the interhemispheric difference of 13C effectively exclude reduced fossil fuel emissions as the primary cause of the slowdown. The 13C observations are consistent with long-term reductions in agricultural emissions or another microbial source within the Northern Hemisphere. Approximately half (51 ± 18%) of the decrease in Northern Hemisphere CH4 emissions can be explained by reduced emissions from rice agriculture in Asia over the past three decades associated with increases in fertilizer application9 and reductions in water use10, 11.

(disclaimer: that is all I’ve read, not the actual text). But then again, perhaps you prefer a different answer, again from Nature:

Methane and ethane are the most abundant hydrocarbons in the atmosphere and they affect both atmospheric chemistry and climate. Both gases are emitted from fossil fuels and biomass burning, whereas methane (CH4) alone has large sources from wetlands, agriculture, landfills and waste water. Here we use measurements in firn (perennial snowpack) air from Greenland and Antarctica to reconstruct the atmospheric variability of ethane (C2H6) during the twentieth century. Ethane levels rose from early in the century until the 1980s, when the trend reversed, with a period of decline over the next 20 years. We find that this variability was primarily driven by changes in ethane emissions from fossil fuels; these emissions peaked in the 1960s and 1970s at 14-16 teragrams per year (1 Tg = 1012 g) and dropped to 8-10 Tg yr−1 by the turn of the century. The reduction in fossil-fuel sources is probably related to changes in light hydrocarbon emissions associated with petroleum production and use. The ethane-based fossil-fuel emission history is strikingly different from bottom-up estimates of methane emissions from fossil-fuel use1, 2, and implies that the fossil-fuel source of methane started to decline in the 1980s and probably caused the late twentieth century slow-down in the growth rate of atmospheric methane3, 4.

This science stuff is confusing, isn’t it? Good evidence says its due to changes in Agricultural emissions; but other good evidence says it is fossil fuel changes. Oh well, I suppose they will fight it out and we’ll know in a year or two.

[Update: this post made it to the main page. That might draw in a few folk from outside my usual in-crowd, so perhaps I’ll amplify that last paragraph a bit: as Eli says in the comments, likely the decrease is due to a variety of factors. My best guess would be that the two “competing” analyses here reflect people tending to see results in the mirror of their own tools.]

Another interesting one is C4 grasses prosper as carbon dioxide eliminates desiccation in warmed semi-arid grassland on the eternal problem of will CO2 fertilisation help, or will it get wiped out by encouraging the wrong sort of grass?

Oh yes, and there is another contribution to the Antarctic warming question: We use measured firn temperatures down to depths of 80 to 90 m at four locations in the interior of Dronning Maud Land, East Antarctica to derive surface temperature histories spanning the past few decades using two different inverse methods. We find that the mean surface temperatures near the ice divide (the highest-elevation ridge of East Antarctic Ice Sheet) have increased approximately 1 to 1.5 K within the past ∼50 years, although the onset and rate of this warming vary by site. Histories at two locations, NUS07-5 (78.65°S, 35.64°E) and NUS07-7 (82.07°S, 54.89°E), suggest that the majority of this warming took place in the past one or two decades. Slight cooling to no change was indicated at one location, NUS08-5 (82.63°S, 17.87°E), off the divide near the Recovery Lakes region. In the most recent decade, inversion results indicate both cooler and warmer periods at different sites due to high interannual variability and relatively high resolution of the inverted surface temperature histories. The overall results of our analysis fit a pattern of recent climate trends emerging from several sources of the Antarctic temperature reconstructions: there is a contrast in surface temperature trends possibly related to altitude in this part of East Antarctica.

Good grief, the world is full of new science all of a sudden:

The ongoing increase in extremely warm temperature events across large areas of the globe is generally thought to be a signature of a more extreme climate. However, it is still unclear whether global warming is accompanied by changes in statistical properties beyond the mean, such as an increasing temperature variability. Here we shed light on this issue by uncovering the way probabilities of extremes are being influenced by temperature evolution. Focusing on Europe, we show how the behavior of warm and cold extremes can be determined to a high accuracy by statistically modeling daily temperatures and their changes. Detailed comparison with observations over the past decades puts forward the dominant role of the mean in explaining exceptionally hot events, and rules out contributions from potential changes in second and higher moments.

(Evolution of extreme temperatures in a warming climate, C. Simolo et al.. Translation: no, things are not getting more extreme, it is just that the mean is increasing. Roger will be happy).


* Reduced methane growth rate explained by decreased Northern Hemisphere microbial sources Fuu Ming Kai, Stanley C. Tyler, James T. Randerson & Donald R. Blake, Nature 476, 194-197 (11 August 2011) doi:10.1038/nature10259
* Recent decreases in fossil-fuel emissions of ethane and methane derived from firn air Murat Aydin, Kristal R. Verhulst, Eric S. Saltzman, Mark O. Battle, Stephen A. Montzka, Donald R. Blake, Qi Tang & Michael J. Prather, Nature 476, 198-201 (11 August 2011) doi:10.1038/nature10352