A paper in Nature Geoscience published early this month was much derided by the usual suspects in the pseudoskeptic community. Contrary to what many critics of “Methane emissions from extinct megafauna” claim, the research does not lead to the conclusion that humans are solely responsible for a global cooling event known as the Younger Dryas, which saw a brief reversal in the warming trend that brought the last ice age to an end. But it does remind us of just how interconnected are all the elements of the planetary ecosystem, and how dangerous it is to tinker with one of them.

The authors, Felisa A. Smith of the University of New Mexico, Scott M. Elliott of Los Alamos National Laboratory and S. Kathleen Lyons of the Smithsonian Institution, tried to estimate how much methane, a potent greenhouse gas, was being produced by the herbivorous mammoths, camels and giant sloths that used to inhabit the Americas before humans came on the scene about 13,000 years ago.

Then they looked at atmospheric methane levels covering the time around the extinction of all those species, as recorded in ice cores. And they concluded that “that the loss of megafauna could explain 12.5 to 100% of the atmospheric decrease in methane observed at the onset of the Younger Dryas.”

There’s a large margin for error, here. The authors estimate that the now-extinct herbivores were responsible for about 9.6 teragrams of methane, but that number could be as low as 2.3 and as high as 25.5. Still, it makes a fair bit of sense. If, as has been speculated, the megafauna of the period in the Americas enjoyed higher densities than modern African counterparts, then their disappearance would have significant consequences for atmospheric methane levels.

This is not the same as saying humans caused the Younger Dryas. It’s not even saying that falling methane levels, brought on by whatever ultimate cause, cooled the planet. As noted in the paper, the Younger Dryas is a singular event, and one that might have been the result of a rare combination of factors. Here’s what can be said, according to Smith et al:

The attribution and magnitude of the Younger Dryas temperature shift, however, remain unclear. Nevertheless, our calculations suggest that decreased methane emissions caused by the extinction of the New World megafauna could have played a role in the Younger Dryas cooling event.

We are not the first to suggest that human-mediated activities influenced the planet prior to the industrial age. Although still controversial, the megafaunal extinction is the earliest catastrophic event attributed to human activities.

As far as policy implications go, the paper is relatively modest:

…we propose that the onset of the ‘Anthropocene’ should be recalibrated to 13,400 years before present, coincident with the first large-scale migrations of humans into the Americas.

Which is perhaps a little premature, given just how controversial is the notion that humans killed off the American megafauna. As I wrote a couple of years ago, just because we’ve screwed up the climate doesn’t mean we’re responsible for everything. But this kind of research should give us pause. It could be that prehistoric humans contributed to a unique change in the Earth’s climate, by indirectly fiddling with levels of a single greenhouse gas. Today, by comparison, we’re directly and single-handedly responsible for changes in a several GHGs, and making those changes at an unprecedented pace.

Comments

  1. #1 klem
    May 31, 2010

    “Although still controversial, the megafaunal extinction is the earliest catastrophic event attributed to human activities.”

    What?!!! Still controversial is a serious understatement! The evidence to support this claim is weak as usual. And it is based on the notion that humans are bad and so when we blame human activity for something, intuitivly people believe it. Once again it starts as children in church where were taught that we are all born sinners. There is an inate beleif that no matter what humans do it leads to disaster. Bullshit. No Nobel Prize for you!

  2. #2 Mac
    May 31, 2010

    If those first human arrivals were related to my former roommate, there could have been a significant INCREASE in methane production from North America, around that time…

  3. #3 Wazza
    May 31, 2010

    klem, we have the spearpoints with which the people were killing mammoth, and we know that humans have wiped out entire species before, for instance the Passenger Pigeon, so why is it so implausible that we killed a slow-breeding species like mammoths?

  4. #4 fix it pro
    June 1, 2010

    The evidence to support this claim is weak as usual. And it is based on the notion that humans are bad and so when we blame human activity for something, intuitivly people believe it.

  5. #5 Jeremy
    June 1, 2010

    In the 1870s humans hunted approximately 30 million American bison nearly to extinction.

    Do you think it’s more plausible that the millions of individuals of American Pleistocene megafauna just dropped dead on their own at a time when humans just happened to be entering their range?

    Use your head.

  6. #6 Erasmussimo
    June 1, 2010

    There seems to be some skepticism regarding the likelihood that humans are responsible for the mass extinctions on the North American continent after their arrival. I’d like to remind everybody of a small bit of evidence in support of the hypothesis. We already have many test cases of humans arriving at an isolated biome and wiping out many species: the human expansion across Polynesia. In every such case, the humans quickly destroyed a number of indigenous species. Of course, islands don’t have megafauna, so we don’t have any examples of that particular action. But humans proved to be quite ruthless in exploiting the environment to levels way beyond sustainability. Indeed, there is now a fairly well-established sequence describing the human experience arriving in new biomes:

    1. Arrival, bringing along a few favored species
    2. Kill all easily-taken prey, which comprise most of the indigenous fauna.
    3. Enjoy big population increase because of surplus food.
    4. Newly-introduced species complete the extinction process.
    5. Exploit remaining resources, such as timber, to the fullest.
    6. Exhaust these resources.
    7. Fight internecine wars over insufficient resources.
    8. Population contraction.
    9. Stabilize at low population in an impoverished biome, fighting continuous small-scale wars.

    The Americas were just a really big island, so it took longer to reach Step 9, but that’s pretty much where the Americas were when the Europeans arrived.

  7. #7 Wazza
    June 2, 2010

    Now, let’s be fair, at stage 9 the culture usually includes a lot of sustainable practises which can be quite admirable…

  8. #8 Thomas R. Holtz, Jr.
    June 3, 2010

    The case for anthropogenic ultimate causes in these and other Quaternary extinctions is very strong, protests to the contrary. For example, the Pleistocene megafaunal extinctions differ greatly in specificity and timing than earlier mass extinctions: they do not include marine invertebrates, for instance.

    They are not synchronous around the world the way other mass extinctions seem to be; instead, the correlate strongly with the arrival of humans (or sometimes, a particular human technology) to a region.

    They are weakest in areas that had long contact with Homo sapiens and our immediate precursors (Africa, western Eurasia), stronger in regions near these without long human contact (SE Asia, boreal Eurasia), and strongest by far in regions that never had humans (or, indeed, anthropoid primates) previously (Australiasia, the Americas).

    Alternative hypotheses, in contrast, are very weak: the post-Pleistocene interglacial is not markedly stronger than any of the many previous interglacials the very same species survived with no decline; the intensity of the Last Glacial Maximum was not more severe than previous GMs; etc.

    So, no: this is not a “blame humans first” approach, but rather the reasoned sorting of the available evidence.

  9. #9 michael
    July 6, 2010

    What happened in North America circa 13,000-11,000 YA was the massacre of all massacres. Native Americans were supposed to look at animals like their brothers or something….Well, let me tell ya something. They must not have gotten that memo until after they slaughtered every last tasty Ground Sloth, Mammoth, Mastodon Short Faced Bear, Camel, Stag Moose, Horse, American Lion, Saber Toothed Cat or Camel they could get their spears into.

    There’s no question what happened to anyone who thinks about it. We’re not evil. We’re greedy and we don’t know when to stop. We killed or caused the extinction of every massive animal that was too slow or too obvious to hide from us. We came upon a defenseless continent that had never seen such a vicious, relentless and intelligent (well, not that smart, since we destroyed forever some of the most amazing animals the world has ever known) predator.

    It’s sad. I wish I could go back just time travel a few thousand years….With a video camera

  10. #10 Ned
    January 16, 2011

    Ah yes, the Overkill Myth seems to keep going, like a cult. Doesn’t anyone do any reading post-1967?

    It’s now been confirmed that humans were in North America long before Clovis. The Paisley Cave findings of human coprolites (14,300 BP) are not in serious dispute (Jenkins, 2008). Ditto for Monte Verde in Chile (13,000). There are lots of other sites that are pushing back initial human occupation in the Americas even further, possibly 50,000 years (Goodyear, 2004).

    The overkillers want us to believe that 14,300 years ago humans came down the Bering Strait and spread throughout North and South America (appearing almost simultaneously in both continents!) killing everything in their path armed only with spears and atlatls. And somehow the giant short faced bear and giant ground sloths and camels all just stood still in amazement while humans carved them up, despite having over 2,500 years (at least) to get used to human predation. Moreover, since several species of birds also went extinct, pre-clovis humans must have had superpowers to chuck their spears 5,000 feet into the air with incredible accuracy!

    Not only do overkillers want us to believe this just-so story, they want us to believe all this without a shred of tangible evidence. There are only 11 sites in North America associated in any way with megafaunal kills by humans (and most of those are pretty sketchy), and all of those are mammoths. Yet 35 genera of animals went extinct.

    wow.

    So much for science education in America.

  11. #11 Houston Lawyer
    March 30, 2011

    What?!!! Still controversial is a serious understatement! The evidence to support this claim is weak as usual. And it is based on the notion that humans are bad and so when we blame human activity for something, intuitivly people believe it. Once again it starts as children in church where were taught that we are all born sinners. There is an inate beleif that no matter what humans do it leads to disaster. Bullshit. No Nobel Prize for you!

  12. #12 doğal taş
    May 16, 2011

    It’s sad. I wish I could go back just time travel a few thousand years….With a video camera

  13. #13 mantolama
    July 5, 2011

    Duplicate comment detected; it looks as though you’ve already said that!

  14. #14 Wow
    July 5, 2011

    spamlink detected from a spambot at post 13 above.