Cutting off the circulation

Around 12,900 years ago, the world was warming up after the last major ice age.  Climatologists call it the "Late Glacial Interstadial," a natural warm-up due to the combined effects of the slow, gradual alterations in the Earth's orbit and precession cycle.  But then...

... something happened, and within only a few decades, the Northern Hemisphere -- especially what are now North America and western Europe -- were plunged back into the deep freeze.

The episode is called the "Younger Dryas" event, because the way scientists figured out it had happened was finding traces of pollen in ice cores from a plant, Dryas octopetala, which is only found in cold, dry, windswept habitats.  Areas that had been progressing toward boreal forest, or even temperate hardwood forest, suddenly reverted to tundra.

[Image licensed under the Creative Commons Steinsplitter, Weiße Silberwurz (Dryas octopetala) 2, CC BY-SA 3.0]

There are a number of curious features of the Younger Dryas event.  First, its speed -- climate shifts ordinarily take place on the scale of centuries or millennia, not decades.  Second, the fact that its effects were huge (the average temperature in Greenland dropped by something on the order of 7 C), but were limited in range; in fact, the Southern Hemisphere appears to have continued warming.  And third, after the initial plunge, the system righted itself over the next twelve hundred years -- by 11,700 years ago, the Northern Hemisphere was back on its warming track, and caught up with the rest of the world.

What could have caused such a strange, sudden, and catastrophic event is still a matter of some debate, but the leading candidate is that something halted the Atlantic Meridional Overturning Circulation, sometimes nicknamed "the Atlantic conveyor."  This is the massive ocean current of which the Gulf Stream is only a small part, and which is powered by warm water evaporating and cooling as it moves north, finally becoming cold and salty (and thus dense) enough to sink, somewhere south of Iceland.  This draws more warm water up from near the equator.  But as the Earth was warming during the Interstadial, the ice in the north was melting, eventually making the water in the North Atlantic too fresh to sink, and thus halting the entire circulation.  Some researchers think the process was sent into overdrive by the collapse of an ice dam holding back a massive freshwater lake called Lake Agassiz (encompassing what are now all five Great Lakes and the surrounding region), causing it to drain down the Saint Lawrence Seaway and into the North Atlantic, stopping the AMOC dead in its tracks.  (This point is still being debated.)

What's certain is that the AMOC stopped, suddenly, and took over a thousand years to get started again, plunging the Northern Hemisphere back into an ice age.

Why does this come up today?

Because a new study out of the University of Utrecht has found that our out-of-control fossil fuel use, and consequent boosting of the global temperature and melting of polar ice, is hurtling the AMOC toward the same situation it faced 12,900 years ago.  One of the consequences of anthropogenic global warming might be sending eastern Canada, the northeastern United States, and western Europe into the freezer.

One of the most alarming findings of the study is that climatologists have been systematically overestimating the stability of the AMOC.  It's an easy mistake to make; the current is absolutely enormous, amounting to a hundred million cubic meters of water per second, which is nearly a hundred times the combined flow of all the rivers in the world.  The idea of anything perturbing something that massive is a little hard to imagine.

But that's what happened during the Younger Dryas, and it happened fast.  The new study suggests that if the AMOC does collapse, within twenty years the temperature of Great Britain, Scandinavia, and the rest of northern Europe could see winter temperatures ten to thirty degrees Celsius colder than they are now, which would completely alter the ecosystems of the region (including agriculture).  It would also change precipitation patterns drastically, and in ways we are currently unable to predict.

If you're not already alarmed enough, here's how climatologist Stefan Rahmstorf put it, writing for the site RealClimate:

Given the impacts, the risk of an AMOC collapse is something to be avoided at all cost.  As I’ve said before: the issue is not whether we’re sure this is going to happen.  The issue is that we need to rule this out at 99.9 % probability.  Once we have a definite warning signal it will be too late to do anything about it, given the inertia in the system...  We will continue to ignore this risk at our peril.

The problem is that last bit -- we don't have a very good history of addressing problems ahead of time.  We're much more prone to waiting until things are really awful, at which point they're harder (if not impossible) to fix.  We've let the corporate interests and short-term expediency drive policy for too long; it's increasingly looking like we're close to hitting the now-or-never point.

We need to start electing candidates who take this whole thing seriously.  It is the most important issue of our time.  I try not to be a one-issue voter, but if someone's answer to "What do you intend to do to remediate climate change" is "Nothing" -- or, worse, "Climate change is't real" -- they've lost my vote.

And they should lose yours, too.  For the good of the planet.

****************************************