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Of farming, methane bubbles, and Antarctic glaciers

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But that methane freezing zone seems to be retreating deeper, say the authors. Thirty years ago, methane hydrates were stable at depths greater than 360 meters. Now, it's necessary to descend beyond 400 meters to keep methane hydrates solid.

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These observations are troublesome for a number of reasons. A warming ocean, scientists have long hypothesized, may cause methane hydrates to melt around the world. And a pulse of methane entering the atmosphere could, in turn, trigger more warming.

Indeed, scientists have also long surmised that just such a positive feedback led to dramatic warming episodes in Earth's past — such as, perhaps, the end-Permian extinction 251 million years ago, the largest of Earth's five mass extinctions, or the Paleocene-Eocene thermal maximum some 55 million years ago.

In this case, the (relatively) good news is: that the plumes of methane bubbles seem to be dissolving in the sea water before reaching the surface; they're not entering the atmosphere directly. The bad news: Methane contributes to ocean acidification, which, among other things, negatively impacts shell-forming creatures' ability to grow their shells.

Finally, a study reported in the journal Quaternary Science Reviews concludes that humans have been altering climate for as long as we've practiced slash-and-burn agriculture — that is, since agriculture was invented.

That's not a new hypothesis. William Ruddiman, one of the study's authors, first proposed the idea five years ago. By cutting and burning forests, humans have been altering climate throughout much of the Holocene, the past 10,000 years or so, he argues.

But when he first proposed it, some countered that, even if humans had been altering the landscape, our numbers were so small for so much of our history that our impact on climate was likely negligible.

But in this new study, Ruddiman says that, when it comes to agriculture's impact on climate, the current human per capita footprint is now far less than that of our slashing-and-burning forebears. People used to burn down forests or savannas, plant crops, and replant until the soil was exhausted. Then they'd clear a new patch and repeat.

The end result: People cleared five times the land they actually farmed at any given time, he says. Now, with modern agricultural techniques, we get about 90 percent more out of the same plot of land than the farmers of yore would have.

That's good news in one sense; it means we've greatly improved efficiency. But in another sense, it shifts more responsibility for climate change to our farming ancestors than we usually give them. They may deserve more credit for initiating the ongoing — and inadvertent — experiment in climate engineering we call "global warming" than they usually get.

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