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Staring Down the Double-Barrel Climate Shotgun

by Bill Chameides | April 29th, 2009
posted by Erica Rowell (Editor)

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Huge amounts of methane can come from melting calthrates from the bottom of the ocean or from permafrost. What if that were to happen? It would throw our climate predictions out the window. (NOAA)

Surprises in the climate system can very quickly make global warming a whole lot worse than predicted. Scientists have known that one such surprise could come from a sudden release of methane, from wetlands or marine clathrates frozen on the ocean bottom, or both. Thanks to a new paper, we probably know which barrel of that double-barrel shotgun to worry about first.

Our understanding of the climate system is imperfect — no argument there. And so, it is possible that this global warming thing won’t be so bad, that the models have over-predicted the amount of warming and climate disruption that will occur in the coming decades.

Risky Business: Underestimating Climate Change

The kicker is that uncertainty cuts both ways. It is just as likely (some argue even more likely) that the models have under-predicted the change and things will be a lot worse than we now think.

The major worry is a “climate surprise,” an unforeseen rapid climate change. For example, the climate could pass a tipping point that, like a runaway truck barreling down a hill without brakes, triggers a much more rapid warming that leads to catastrophic climate disruption. It’s a worry that has some grounding in reality. The geologic record tells us that such shifts in climate (either warming or cooling) have occurred in the past, and sometimes over the space of just a few years closely followed by a decades-long temperature response.

Big Methane Burp Wreaked Climate Havoc in the Past

One such event occurred about 11,600 years ago (the so-called Younger Dryas-Preboreal transition), when the northern hemisphere’s climate rapidly moved from ice age to non-ice age conditions in a short period of time (over the course of several years to several decades). Analysis of air bubbles trapped in ice from that time period show that the transition was accompanied by a rapid rise in the concentration of atmospheric methane. Because methane is a very powerful greenhouse gas (more than 20 times more effective than carbon dioxide as a global warmer), the obvious inference is that much of that rapid warming was caused by methane emissions.

Such a large increase in methane could have come from two sources: melting of frozen clathrates (also called methane hydrates) that are buried on the ocean’s bottom and/or volatilaztion of organic carbon from wetlands as methane. (This latter source includes wetlands formed from melting permafrost.) But which one? Scientists have been unsure. Last week, a paper published in Science by Vasilii Petrenko of the Scripps Institution of Oceanography and colleagues suggests an answer — wetlands.

Through what has been described as a “heroic” effort Petrenko and colleagues mined tons of ice from west Greenland. Yes, that’s right, tons of ice — one ton for each time sample collected. They then analyzed the methane trapped in the ice’s bubbles for its radiocarbon abundance. Why radiocarbon? To understand, let’s digress a moment to review.

What Is Radiocarbon? (Hint: It’s Not a Hard Rock Station on Satellite Radio, I Don’t Think)

Radiocarbon is the isotope of carbon that has six protons and eight neutrons (thus, C-14). Most carbon on the Earth has six protons and six neutrons (C-12). Radiocarbon is produced by cosmic rays; fast neutrons entering the upper atmosphere slam into nitrogen atoms and convert the nitrogen (N-14) to C-14. Called radiocarbon because it is radioactive, over a period of tens of thousands of years C-14 decays back into N-14. (For those of you up on your nuclear physics, C-14 has a half-life of about 6,000 years. Thus, if you started with 100 atoms of C-14, you would expect to find about half or 50 left after 6,000 years, 25 after 12,000 years, and so on.)

Because radiocarbon decays on time scales of tens of thousands of years, it provides a great way to distinguish between methane from wetlands and methane from clathrates. You see, the methane in clathrates was formed millions of years ago and so has no radiocarbon left, not so for wetlands methane.

Source of the Vintage Methane?

And so, Petrenko et al. realized they could figure out where the methane blip from the Youger Dryas-Preboreal transition came from by looking for radiocarbon in the methane in the bubbles trapped in the ice from that period. The problem is, such a measurement requires a lot of methane, and to get that much methane requires a lot of air bubbles, which in turn requires a lot of ice — about a ton per sample, to be exact.

Undaunted by the challenge, Petrenko and his colleagues mined their tons of ice, isolated the methane, and did the radiocarbon analysis. They found that the major source of increased methane during the Younger Dryas-Preboreal transition was from wetlands.

What Does What Happened ~12,000 Years Ago Tell Us About Today?

That was then and this is now. Why should we care?

Huge amounts of carbon currently reside in clathrates and permafrost, more so in clathrates than permafrost, but both are substantial. If either of those reservoirs should give up their carbon as methane, the event would cause a major increase in the g
lobal warming from greenhouse gases (what we call an increase in the equivalent carbon dioxide concentration) — and one of the climate surprises that scientists are not sure about but worry about.

The Petrenko work is one of those good news/bad news things. It suggests that clathrates may not be a problem, which is good. But it confirms that melting permafrost may very well be not so good. And here is the kicker: the permafrost is melting already and methane has already been found to be bubbling up from lakes formed from melting permafrost.

filed under: carbon dioxide emissions, climate change, faculty, global warming, methane, oceans, Planetary Watch, science
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