Wednesday, December 28, 2011

Methane Talk-Down: Partial

One of the true joys of learning about science – as opposed to, say, economics – is that eventually you can usually get to a scientific summary that clears up many of the distortions that popular reports create. In the midst of wading through yet another cherry-picked-evidence blog post (this one on methane) by Andrew Revkin of the NY Times, it suddenly occurred to me that I should check out Justin Gillis of the Times, whose posts have been praised iirc by Joe Romm of climateprogress fame. Gillis’ reporting still seemed a little superficial to me, but he had a link to a 2006 scientific summary of the research about methane and climate change, an oldie but goodie where I found the answers to many of my questions. My recent blog post on methane laid out the doomsday scenario that I fear; Chapter 6 of this summary, as Rachel Maddow would say, talked me down – but only partially.

Because the broad scenario that I laid out is not drastically affected by the information in the summary, it is easier to lay out the summary’s picture of methane and then, at the end, note how this may affect my scenario. I will focus on methane clathrates, since the changes to everything else are less substantial. And, of course, I am sure that more misconceptions remain – because a summary article of ongoing research can’t be expected to answer everything. Anyway, let’s begin.

Methane Clathrates, Water Methane

Last time, I presented a very summarized picture of natural-source methane as coming from three sources: methane clathrates under the sea, permafrost on land at high latitudes, and peat bogs next to the permafrost or in the tropics. It turns out that the picture is a bit more complicated, and the complications matter.

To start with, methane clathrates are formed and remain stable in sea-floor sediment in particular combinations of sea temperature and pressure from the sea above that limit them to the sea floor somewhere between 200 meters and 1000 meters below sea level. In other words, the water has to be near zero F, and the clathrate has to be lower than 200 meters below sea level and higher than 1000 meters below sea level. Between those two limits, the deeper the sea floor, the wider the zone in the sediment where it can exist. Guesstimates for a typical clathrate “stability zone depth” might be 250-300 meters. Btw, a confusing part of the scientific lingo apparently refers to Arctic clathrates as “subsea permafrost.”

What happens to melt the clathrates? The water next to the sea floor warms up, or warmer temps further up the sea slope cause the equivalent of a mudslide on the sea floor that basically slices through the clathrate, stirs up everything above the slice as a cloud of sediment, and melts all the clathrate above the new sea floor. That is what they think happened at Storegg, a place near Norway where there is a “crater” 30 km across that may have released a gigaton of carbon, all at once (methane is CH4).

Now here’s an odd part. We are used to thinking of gas coming up to the surface in bubbles and releasing itself into the atmosphere when the bubble pops. Not so with clathrate methane – most bubbles pop long before they rise the 200 meters or more to the surface, according to the models. Instead, one of several things happens: the methane rises to the surface but not as bubbles (it is “buoyant”) and then releases into the atmosphere, or it is eaten by methane-eating bacteria, or it converts (typically to carbon dioxide) en route. Initial indications are that a small percentage of melted clathrate should rise to the surface combined with water and is released into the atmosphere as methane, which happens effectively immediately; a large percentage should be eaten by bacteria, who convert it into carbon dioxide on the surface of the sea, and the carbon dioxide is released into the atmosphere in order to equalize atmospheric and oceanic CO2; and a medium-sized percentage should convert to carbon dioxide without going through the bacteria, to be released into the atmosphere as carbon dioxide in the same way.

The methane clathrates in the Arctic seas contain perhaps 50%-80% of all clathrates. They are also by far the most likely to be affected by global warming, since water temperature variation due to increased sunlight on the water and increased temps of sun-warmed currents from the south are widest there.

Other Methane Sources

The picture of land-based methane sources also needs amendment. It appears that much of the methane stored in permafrost is stored in peat within the permafrost – which can extend as far down as 200 meters or so. Meanwhile, wetlands at whatever latitude are generators of methane, the Amazon as much as Ireland. When the permafrost melts, the water plus peat turns into a bog that (under global warming) is maintained by increased precipitation: that’s what often drives increased methane production.

Here, the translation to the atmosphere is more clear. Melting of permafrost releases any methane locked in the ice (but not in clathrates), and also creates new constantly-emitting sources of methane. Likewise, wetlands inject methane directly into the atmosphere.

Now we come to the tricky part. We are accustomed to thinking of methane in the atmosphere as separate from carbon dioxide. Not so. What often happens to methane in the atmosphere is that it "oxidizes”, which typically means that one of the hydrogen atoms is broken off to help form H2O (water), while the rest forms a methyl group (CH3) which eventually breaks down to carbon dioxide. In other words, much of the methane tossed into the atmosphere actually winds up as the major greenhouse gas, and stays up there for 150-250 years.

What’s the Effect? Um …

OK, so now the scientist wants to figure out what the global-warming effect of unlocking all that methane is going to be. The problem is that we have two sources of comparison, and neither of them is great.

The first is to use what happens over 10-20,000 years immediately after a Milankovitch-cycle minimum (a “glaciation”) as a model. Using that model, scientists have pretty well determined that in such times of rising global temperatures, the amount of methane in the atmosphere probably doesn’t vary by a heck of a lot, and the effects on global temps compared to atmospheric carbon are pretty minimal. Methane melt in general might have a role in things like sea-ice melting near Greenland, which has been shown to have surprisingly wide effects on global climate, but most of the good candidates for that type of melt (subsea, permafrost, wetlands) just don’t make a strong case for themselves.

The problem with this type of analysis is that it looks only at periods when most of the ice remains – because that’s what happens at the peak temps of a Milankovitch cycle. We have almost certainly moved above those peak temps in the last couple of decades, and so we are in much less charted waters. For a period much more comparable, you have to go back to the PETM – 55 million years ago.

OK, in the PETM, temps were 5-10 degrees C warmer than now. Increases in carbon in the atmosphere just don’t seem to be enough to justify those warmer temps. So for a while, there were theories floating around that methane was the complete reason for that kind of warming – no carbon needed. That would have been nice, since figuring out why carbon suddenly spiked in the first place, not to mention why the time period of this rapid warming was around 20,000 years as the latest research suggests, has been a headache. Bad news: there simply doesn’t seem to be a natural source of methane that comes near to explaining the whole temperature rise, not to mention keeping going for 20,000 years. So it looks like we have a choice between carbon emissions plus “unknown”, and carbon plus methane. Tentatively, the scientists are voting for carbon plus methane.

But the PETM isn’t great as a model, either. The problem there is that things happened slowly compared to today. If we say that the carbon atmospheric-concentration rise then happened over the course of 20,000 years, well, our carbon rise appears to be happening over 350 years – and it may very well double the rise of the PETM over the course of those 350 years. In other words, this is happening at least a hundred times faster. And, as we’ve seen in the case of carbon, that can mean that the positive follow-on effects happen well before the negative “stabilizer” effects. So, for example, don’t necessarily expect the magical munching methane sea bacteria to appear in the Arctic and save the day.

OK, so the models we have aren’t great. Can we at least use them for some guesstimates?

Preliminary Guesses

Well, the scientists have done the guessing for me. The key sentences I find in Chapter 6 say, more or less (with the usual caveats about my understanding), that the amount of atmospheric methane from natural sources pre-Industrial Revolution equals the amount of methane added from human sources since then, which equals the likely amount of methane to be added at some point due to all natural sources except subsea methane, which equals the potential amount of methane from subsea methane. In other words, in a worst-case scenario with 2006 models, at some point in the next 300 years, we might expect atmospheric methane four times what it was in 1850.

How much added heating would that translate to? Again, reading between the lines, perhaps 1 degree C from the methane alone. However, if we take the PETM as a model, it might be more like 2 degrees C. And that’s the maximum, so we can all semi-relax, right?

Well, no. You see, there are two problems. First of all there’s the fact that much of that methane is going to convert to carbon dioxide when it’s up there. Second, there’s the fact that the more methane gets into the atmosphere from now on, the longer it sits there. The 2006 estimate was that methane hangs up in the atmosphere an average of 9 years. But at twice the concentration, I think we can count on it sitting up there for 12-18 years on average. So those two things should add another ½-1 degrees C to the “additive effect” of methane in the atmosphere.

And then, of course, there’s the question of methane that converts to carbon dioxide before it gets into the atmosphere. Here, the summary didn’t really have much to add in the long term. Even by their time-frame estimates, all that methane-to-carbon-dioxide, even if it doesn’t get there in the next 100 years, will almost certainly show up in the next 1000 years. So it’s a more extreme version of my “pay me now or pay me later” scenario – except that we can at least hope that by the time the methane-turned-carbon-dioxide shows up, we will have managed to cut down on our human-caused carbon emissions and the amount in the atmosphere will have begun to go downhill significantly.

All in all, not great, but not as bad as my full doomsday scenario. Instead of 6 degrees C from methane-turned-CO2, perhaps 2-3, although that increase will stick around for maybe twice as long; instead of 7-9 degrees C from methane-stayed-methane over the next 160 years, perhaps 1-2 degrees over the next 300-500 years. And it will happen more gradually, so it won’t be really noticeable, probably, for the next 30-40 years. Except …

I’m Not All the Way Down

Read carefully the interview with the head of the survey of methane releases in the Siberian Sea. He states, effectively, that the diameter of the “craters” I referred to earlier had increased by up to 100 times this year, and this methane was “bubbling to the surface.” If you look at the 2006 summary, neither is supposed to happen. Very little methane should arise to the surface in a bubble, as noted above, and the methane hydrates should not suddenly do a big jump in melting: and 5 degrees C increase in water temps (since 1984, a jump of 2.1 degrees C has been observed) should cause perhaps 1 meter’s worth or less of methane hydrates to melt over the next 40-80 years – and it can’t be explained as mudslides, since it has happened in quite a few places.

So why would scientists’ models be wrong? Well, in the first place, they assume that relative sea-water warming will only occur in a short space in the summer, when the ice is melted and the sun warms its top. However, the depth of the surface ice in winter is also less than before, and the water carried by currents from the south is warmer. Clearly, it’s very possible that scientists are underestimating the amount of melting going on the rest of the year. Add this to the known problems with the original model developed in 1995, and you have some, but maybe not all, of the increase in clathrate atmospheric methane release explained.

The second flaw may be the modeled prediction that very little methane melt will rise to the surface as bubbles. Why might this model be wrong? I don’t have a clear answer from the summary – it could be that the turbulence of the water keeps the bubble from popping, although that seems unlikely. One thing seems clear: the magical munching methane bacteria are nowhere to be seen.

And the third flaw, which also affects the land methane emissions rate, is a major underestimate in the models of the rate of global warming. The models implicitly assume that the Arctic sea ice won’t melt entirely in summer before somewhere between 2030 and 2100, and year-round perhaps never – that one seems clearly wrong. Therefore, they underestimate the speed of the follow-on effects, including much faster warming of water within 100 meters of the surface, which would inevitably mean much faster warming at the 200-500 meter level – sorry, that’s not “deep ocean.”

In other words, what the latest information is telling us is that the semi-comforting story I just gave you is almost certainly an underestimate. The “true” effect of methane is somewhere between my doomsday estimate and the one above – except that the roles of methane-stayed-methane and methane-turned-carbon-dioxide have switched, because we now know that much of that atmospheric methane is going to change to carbon dioxide while it’s up there.

I find the logic of the summary convincing as well as semi-comforting; so if I had to guess, I would say that the net effect is somewhere between 3-5 degrees C, mainly in carbon dioxide, and spiking over the next 40-150 years before leveling off. But that’s a complete guess. Until I understand just how the models went wrong, I’m only partially talked down from my panic. So here’s to the New Year: It will be a season of hope, it will be a season of despair, it will be a season of enormous impatience until the first scientific explanations come out.


Kevin O'Neill said...

Wayne, you might be interested in reading a couple of articles: the first is a a letter by Vasilii Petrenko to Science Magazine, pp1146 in rebuttal to an article on *previous* research by Shakhova and Arctic methane; the second is a paper by Petrenko - CH4 Measurements in Greenland Ice:
Investigating Last Glacial Termination CH4 Sources

Petrenko'a analysis indicates that clathrates had minimal effect on the most recent large methane 'burp' -- 11600 years ago during the last deglaciation. The isotopic ratios attribute most of the increase in methane to organic sources (wetlands). And as Petrenko points out in his letter to Science Magazine, we cannot differentiate between high latitude and low latitude sources.

Now, all of this has to be taken with a pretty large dose of caution; as you've pointed out, events in the Arctic appear to be occurring much faster than either models or paleo-history would predict.

Wayne Kernochan said...

Hi Kevin -

Actually, from what you're saying, Petrenko in no way contradicts my fears. He is studying Milankovitch-cycle methane fluctuations, when we know that major Arctic-ice melt (and therefore, probably, Arctic clathrate melt) did not occur. I'm not sure why the permafrost methane didn't spike, but I suppose it's possible that exposure of new permafrost areas locked in as much methane as melting of the old released. The question is, what happens when we venture into uncharted waters as global temps zoom above M-cycle limits and Arctic sea ice really does melt? There, the only guide is the PETM.