Mon Mar 6th, 2006 at 08:22:35 AM EST
Once more, I bring you some good news on the earth.
In this and the previous session on climate science, I report on remarkable discoveries in climate research which (so I suspect) are not broadly reported by the daily press. In a sea of press clippings spelling out the doom of humankind on nearly weekly basis, this news is actually somewhat comforting. But, as these findings do not have the proper "impact" factor, it won't get massively printed. To reiterate my catch phrase of last time: if it ain't doom, it won't boom.
In my previous diary, I described how a team of biologists from Colorado discovered a potential negative feedback loop on global warming, which could theoretically aid in slowing global warming. The protagonists in that story were microbial communities living in forest soil. In today's diary, microbes again play a part, but we've to go much, much deeper this time. This time, they're linked to one of the most fascinating features of our world's oceans: methane ice, or methane clathrates.
The existence of methane clathrates itself has been fruitful writer's material, witnessed in such books as "Mother of Storms" by John Barnes, or used with similar catastrophic effects in "The Swarm" by Frank Schätzing. (Both are good reads, BTW.)
And true enough, methane clathrates are in potential the stuff of nightmares. As always, the Wikipedia entry on Methane clathrates is a welcome starting point (but should never be the end point of one's curiosity). I will suffice here with the cliffs notes: The major component (up to 99%) of the clathrates is methane (CH4), a greenhouse gas with a ten times higher capacity to capture heat compared to the nefarious CO2. The compression of methane gas in clathrates is enormous: 1 cubical meter of clathrates brought to the ocean's surface releases up to 164 cubical meters of CH4. And lower (but inaccurate) estimates on size of the methane clathrates in the oceans are simply gargantuan, making them hands down the largest concentration of methane found on earth. Possibly there is 10 times more methane present in the oceans than in the gas reservoirs we now tap.
Today, methane is a word freighted with concerns on climate change. There is no doubt that free methane, methane in gaseous form in the atmosphere, enhances global warming. Logically, when the enormity of the methane clathrates was discovered, immediately there were worries what would happen if this reservoir would be released in its entirety into the atmosphere. And interestingly, there is strong evidence that this indeed once happened in our earth's history: during the Paleocene - Eocene extinction. Eocene was possibly the warmest periods in the Cenozoic (the time after the dinosaurs) and global temperatures has since that period dropped dramatically, eventually culminating in the Pleistocene ice ages.
But there have been found indicators that even during the Pleistocene the methane clathrates can become instable and degas, influencing the climate. This scenario of violent degassing is coined the clathrate gun hypothesis. Some of these degassing events are triggered by natural instability, perhaps occurring on a cyclic frequency, as speculated in a 2000 paper in Science. There is even a theory afloat on the Internet that mammoths are extinct because of a dramatic clathrate burst, triggered by an oceanic landslide. The most dramatic report that I know of a clathrate gun was presented in a paper published in Geology (among others), and suggests that ocean's bottom waters can warm up to 8 degrees C, setting off massive clathrate destabilisation, as recently as the late Pleistocene (85.000 years ago). All in all, this has led to frightful scenarios where global warming makes the methane clathrates a ticking time bomb. In a nut shell: Doom.
The violent degassing of clathrates is used to explain the historic spikes visible in the historic CH4 record, which shows a fair number of CH4 spikes. There is, however, an opposite view, which explains those spikes by the increased release of methane from wetlands and bogs, under the influence of changes in the hydrologic cycle, the water household of the earth.
Phrased differently, these two theories approach both sides of the chicken and the egg. The first theory is saying: massive CH4 degassing caused increased global temperatures. The second theory is saying: increased global temperatures caused increased CH4 degassing. Both have something to say for.
Now geoscientist Todd Sowers connected to Pennsylvania State University enters the fray. In his article, published in Science on 10 February, Sowers describes his analysis on ice samples from the GISP II ice-core, drilled out of the Greenland ice sheet. Particularly novel in Sower's study is the choice of his stable isotopes, his "tool" to make his assessment.
First, the basics. To form the methane captured in the clathrates both CO2 and hydrogen (H2) are needed. The source of the CO2 used in the methane clathrates is, through a long process, organic in nature and involves our beloved microbes. Carbon (C) comes in several isotopes (14C, 13C, 12C, where the number correspond with the atomic "weight" of the element.) Organisms prefer lighter atoms above heavier atoms, and this causes a natural selection for the 12C atom above 13C. For that reason, methane clathrates have a fingerprint of increased 12C. Expressed as a ratio, the 13C/12C ratio of methane is smaller compared to a standard 13C/12C ratio.
The methane produced in the wetlands also has an organic (microbes!) source, similarly resulting in a smaller 13C/12 ratio compared to the standard. This makes interpreting the ice core data and determining which CH4 comes from where pretty hard. The two opposing theories sketched out above were largely based on measurements on the carbon isotopes from the CO2, by attaining the 13C/12C ratio from ice samples.
The critical new point in Sowers' work is that he leaves the carbon alone and moves toward that other component of CH4: hydrogen. Like carbon, hydrogen also comes in stable isotopes. The most abundant isotope is hydrogen (H), with one proton, and the second isotope is deuterium (D), with one proton and one neutron in its atomic core. This makes deuterium almost twice as heavy, so even although there is not much deuterium around in natural form, its heaviness can still produce a traceable mark.
What the deuterium/hydrogen ratio sets apart from the carbon isotopes is that there is a clear distinction between deuterium in clathrates methane and deuterium in wetlands methane. The problem, however, was in measuring the ratio because of the low detection levels. A new measuring technique was developed to work around that and Sowers is one of the firsts to pick the fruits of it.
In wetlands, part of the hydrogen to form methane comes from the groundwater, precipitated in rain. Sowers used the estimation that about a quarter of the hydrogen is originally from precipitated rain water, the rest is from an acetate substrate. Yet rain water heavily samples light hydrogen (which, being lighter, evaporates quicker), and very little deuterium. This creates methane with a very small D/H ratio and hence, a large negative deviation from the standard D/H ratio. This negative deviation from the standard is expressed in per thousand, or per mil. See in this introduction to stable isotopes for those who can't stop reading. For wetlands the D/H deviation from the standard ranges from -250 to -380 per mil.
The hydrogen used for methane clathrates, after passing through a biological process, also has a negative deviation from the standard. But that negative deviation, in contrast to the wetlands CH4, is smaller: -189 per mil. This difference is significant enough to make a distinction between the two D/H signals in the CH4 stored in the ice.
In today's atmosphere, CH4 has a D/H deviation of about -100 per mil. Most contributing CH4 sources are far, far more depleted in deuterium. Methane clathrates, with a -189 per mil deviation is one of the least depleted CH4 sources. Now for the tricky but important part: if clathrates would heavily contribute to CH4 in the atmosphere, the D/H deviation becomes less negative (increases) .
Back to Sowers. He took an ice core going back up to 30.000 years, measured the oxygen isotopes (as global temperature indicator), the CH4 concentrations and determined the D/H deviations in the CH4. That's easy to put within one sentence, but a lot of work in total. The resulting graphs, however, are immediately telling and show two important things:
Firstly, the graphs show that there is an increasing CH4 concentration with time, together with a progressive decrease in the D/H deviation (mind the scale bars!!). Meaning, the D/H deviation becomes more and more negative. As stressed above, this is the exact opposite what we would expect if clathrates would heavily contribute to the increasing CH4 concentration: that would cause an increase in its deviation.
Sowers attempted to explain this increase of CH4 by the increased contribution of different CH4 sources (none of which methane clathrates), which I will not go into at this point.
To fully make sure that methane clathrates are not the source, Sowers focussed on a number of already identified warming events and sampled specifically around these time periods.
Here, even more clearly, it shows that with an increasing temperature there is an increasing amount of CH4 in the atmosphere. But in one case the D/H deviation does not change at all, or in the posted graph, it lags behind the CH4 increase and when it does change, the deviation decreases. In a final devastating blow to the clathrate gun, Sowers made a model that simulated what would happen if all the CH4 released in the atmosphere would be from the clathrates with a -189 per mil D/H signature. This gives the black line in the graph and shows that the simulation resulted in an increase (less negative) of the D/H deviation, the precise opposite of what the record is showing.
In one stroke, Sowers delivers damning evidence that the clathrate gun hypothesis in the earth's most recent history is not even smoking. In fact, it provides the question whether it was loaded in the first place. And this is good news, since it indicates that the methane stored in the oceans is a LOT more stable than what many people were speculating. It shows that even with abrupt warming events in our recent history, the clathrates contribute insignificantly to increasing global CH4 concentrations. For the greater part, they remain undisturbed on the bottom. With his findings, Sowers gives us a reason to sleep a little more comfortably - at least on this issue. And the theory that mammoths were knocked off this sphere by a catastrophic clathrate degassing can be carried for certain to the grave.
A final note. Of course, the results of Sowers' work does not rule out cataclysmic clathrate degassing from happening. Yet one conclusion that I draw from his work is that on a warming Earth there should be more attention directed to other methane sources, which are quicker to degas than clathrates. In that respect, I find reports of degassing marshes and bogs in Siberia releasing their CH4 of far greater concern. But in a diary on the good news, that subject is clearly out of place.
More internet articles on Sowers' paper: