Dark Age America: The Rising Oceans | John Michael Greer

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Aug. 6, 2014 (Archdruid Report) -- The vagaries of global climate set in motion by our species’ frankly brainless maltreatment of the only atmosphere we’ve got, the subject of last week’s post here, have another dimension that bears close watching.

History, as I suggested last week, can be seen as human ecology in its transformations over time, and every ecosystem depends in the final analysis on the available habitat. For human beings, the habitat that matters is dry land with adequate rainfall and moderate temperatures; we’ve talked about the way that anthropogenic climate change is interfering with the latter two, but it promises to have significant impacts on the first of those requirements as well.

It’s helpful to put all this in the context of deep time. For most of the last billion years or so, the Earth has been a swampy jungle planet where ice and snow were theoretical possibilities only. Four times in that vast span, though, something -- scientists are still arguing about what -- turned the planet’s thermostat down sharply, resulting in ice ages millions of years in length. The most recent of these downturns began cooling the planet maybe ten million years ago, in the Miocene epoch; a little less than two million years ago, at the beginning of the Pleistocene epoch, the first of the great continental ice sheets began to spread across the Northern Hemisphere, and the ice age was on.

We’re still in it. During an ice age, a complex interplay of the Earth’s rotational and orbital wobbles drives the Milankovich cycle, a cyclical warming and cooling of the planet that takes around 100,000 years to complete, with long glaciations broken by much shorter interglacials. We’re approaching the end of the current interglacial, and it’s estimated that the current ice age has maybe another ten million years to go; one consequence is that at some point a few millennia in the future, we can pretty much count on the arrival of a new glaciation. In the meantime, we’ve still got continental ice sheets covering Antarctica and Greenland, and a significant amount of year-round ice in mountains in various corners of the world. That’s normal for an interglacial, though not for most of the planet’s history.

The back-and-forth flipflop between glaciations and interglacials has a galaxy of impacts on the climate and ecology of the planet, but one of the most obvious comes from the simple fact that all the frozen water needed to form a continental ice sheet have to come from somewhere, and the only available “somewhere” on this planet is the oceans. As glaciers spread, sea level drops accordingly; 18,000 years ago, when the most recent glaciation hit its final peak, sea level was more than 400 feet lower than today, and roaming tribal hunters could walk all the way from Holland to Ireland and keep going, following reindeer herds a good distance into what’s now the northeast Atlantic.

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