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 (51â±â18%) 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 20âyears. 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 (1âTg = 1012âg) and dropped to 8-10âTgâyrâ1 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 â¼50 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

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The methane thing is everyone is looking for a single smoking gun, but it is a bunch of buckshot, so the answer is yes. Very common.

Eli's entry is that since natural gas has become more expensive, folk are tightening up the wells and transmission lines which leak something fierce. It would be cute to study methane concentrations as a function of time near cities for example.

"Translation: no, things are not getting more extreme, it is just that the mean is increasing."

I wouldn't have tranlated it quite that way -- as the mean increases, extremely high values (by any fixed definition) become more probable, because the whole distribution is shifting up. It is just that the spread and shape of the distribution are not changing *relative* to that rising mean.

Put another way, the warmest 5% of days in an average year will tend to be warmer (and the coolest 5% of days warmer too). And so on to more extreme values: record-setting highs become more likely as the mean rises.

[Yes, that is what I meant; unfortunately the language doesn't correspond to anything we commonly use. But the point is, that the commonly touted idea of "more extremes, as in more wildness" isn't there -W]

By L Hamilton (not verified) on 22 Aug 2011 #permalink

Yeah, well it is there because the extreme high values are extremely high. The interesting question is that with more energy in the system are the absolute excursions from the new means higher on a percentage or an absolute basis.

This science stuff is confusing, isn't it?
      -- William Connolley, August, 2011

That's a keeper! :)

But more importantly, isn't this entire topic just one more data point supporting the skeptic's position that climate science is so ill understood at this point that making any sort of forecast is simply whimsical folly?

[I'm sure people will find their own mottoes in this. Yours was the obvious skeptic talking-point, which is wrong for the obvious reasons (methane isn't terribly important at this point); if you think a bit harder you might find mine -W]

The question of climate variability extends beyond temperature to wind, drought and desertification, and precipitation amounts and timing (snowpack, melting, run-off, water storage, and flooding). The article did not address whether or not the higher moments of these other weather variables have changed.

More, climte instability is a flipping among multiple equilibria, not just a steady rise in the mean temperature but the potential for flipping back and forth of the mean, conditions that occurred in the past when Earth shifted toward the Holocene.

> that making any sort of forecast is simply whimsical folly?

Yep, especially the forecast that, nah, not to worry, spoken with calm conviction, an expensive business suit, and a smile

By Martin Vermeer (not verified) on 23 Aug 2011 #permalink

Just to add to the debate, here is a recent paper from Science (… )

Turns out that there is a lot more terrestrial surface water than originally thought -- and that these ponds are very productive. They sequester a lot of carbon, but produce a lot of methane. The surface water has a comparable impact on the carbon and methane cycles as the entire land mass or the entire ocean.

A random ice question plausibly related to the Antarctic ice abstract: doesn't ice get more plastic/less rigid at temps closer to 0C than lower temps? Could warming speed up outflow into the oceans for that reason? I've never seen that mentioned, if it's true.

[You want a glacio for that, in detail. But yes: ice definitely has a temperature-dependent rheology, and that is taken into account in all the credible models (not in GCMs, of course, since they don't have moving-ice-sheet-models). Warmer ice moving more sounds quite plausible, and even familiar -W]