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.

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The tipping point

There's a common comment from climate change deniers that I'm sick unto death of, and it usually takes the form of something like "the Earth's climate has had ups and downs in the past, and life has gone along just fine."

First, it's the pure ignorance of this attitude that gets me.  If you did ten minutes' worth of research online, you'd find that a number of these "ups and downs" coincided with mass extinctions.  Life survived, yes, but dramatic losses in biodiversity and overall population numbers hardly constitutes "getting along just fine."  (In fact, the largest mass extinction ever -- the Permian-Triassic Extinction, which wiped out an estimated 96% of marine species and 65% of terrestrial species -- was coincident with a huge spike in temperature and atmospheric carbon dioxide, thought to have been triggered by a colossal volcanic eruption.)

But second, and honestly worse, is the blithe attitude that we can continue doing whatever we want to the environment and face no consequences whatsoever.  This raises entitlement to the level of an art form; apparently, we humans occupy such an exalted niche that no matter what we do, the rest of nature is magically going to make sure we not only survive, but thrive.

Even the most diehard deniers, however, are getting some serious hints that their determination to ignore science is not working out so well.  First of all, we have the fires in the Amazon, which have reached unprecedented levels in Brazil, with 74,000 acres burned to the ground already.  The cause is twofold -- the natural dry season, and the incomprehensible policy by Brazilian president Jair Bolsonaro of recommending large-scale slash-and-burn to "encourage agriculture."  The problem is larger in scale than just South America, however.  Environmental scientist Jonathan Foley explained why in an interview with Science News:

Some computer models... show a hypothetical scenario that when we clear rainforest, it starts to almost immediately warm up and dry out the atmosphere nearby.  When we stand in a forest, it feels cool and moist.  But when you clear-cut large areas of the forest, the air right around you gets hotter and drier, and it affects even rainfall patterns.  The worry is if you start clear-cutting more of the Amazon, in theory, a tipping point could be reached where the rest of the forest dries out, too. 

If that happens, the idea is that the Amazon could flip suddenly from being a rainforest to being a dry savanna-like ecosystem.  We’re not absolutely certain about it, but even that theoretical possibility is kind of terrifying...  Globally, about 10 to 15 percent of our CO2 emissions comes from deforestation.  If this is going back up again in Brazil, that’s going to make climate change even worse.

The only amendment that needs to be made to Foley's statement is his use of the word "if" in the last sentence.

Smoke from the 2019 Amazonian wildfires [Image is in the Public Domain, courtesy of NASA]

The other piece of alarming information this week comes from some research done at Stockholm University that firmed up a link between the salinity of the North Atlantic and climate throughout the world.  The team, made up of David K. Hutchinson, Helen K. Coxall, Matt OʹRegan, Johan Nilsson, Rodrigo Caballero, and Agatha M. de Boer, were studying the Eocene-Oligocene Transition (EOT), which occurred 34 million years ago and involved a shift from a "hothouse" to "icehouse" climate.  

The authors write:

The Eocene–Oligocene transition (EOT), ~34 Ma ago, marked a major shift in global climate towards colder and drier conditions and the formation of the first Antarctic ice sheets.  A gradual decrease in CO2 is thought to be the primary driver of the transition, causing long-term cooling and increasing seasonality through the Eocene, culminating in the glaciation of Antarctica.  Deep water circulation proxies suggest that the EOT, including the preceding 1 Myr, also marked either the onset or strengthening of an Atlantic meridional overturning circulation (AMOC).

Catch that?  An event occurring the North Atlantic is the probable trigger for the formation of the Antarctic Ice Sheet, and a radical change in Earth's climate -- and, unsurprisingly, a massive extinction that wiped out 20% of the Earth's species (large mammals were the hardest hit).

What Hutchinson and his team showed is that the likely cause of this transition was the closing of a oceanic channel between the Arctic Ocean and the North Atlantic.  The Arctic Ocean at that time had very low salinity (something the team demonstrated using the species of algae that show up in the fossil record from that time and place).  As long as the channel was open, fresh water flooded into the North Atlantic, keeping the salinity of the surface water low.  When the channel closed due to tectonic movement, the salinity of the surface of the North Atlantic spiked.  Saline water is more dense than fresh water, so this surface water began to sink, kicking off the "North Atlantic Thermohaline Circulation" and (more germane to our discussion here) drawing down dissolved carbon dioxide by the metric ton.

The result: a drastic drop in temperature, Antarctica becoming an ice-covered wasteland, and a mass extinction worldwide.

How anyone can read this and not be freaking out over the spike in carbon dioxide we've seen in the last hundred years -- that appears to be the fastest carbon dioxide concentration change anywhere in the geological record -- is beyond me.  There's no doubt that there's a tipping point, as the Eocene-Oligocene Transition shows.  The only question is where that tipping point is -- and whether we've already passed it.

What's driving it all is money.  Altering our lifestyle, protecting the rain forests, and divesting ourselves from fossil fuels all require that we make sacrifices ourselves, but more importantly, that we end the chokehold the fossil fuels industry has on our elected officials.  Of course the politicians don't want anyone to understand and accept climate science, because that would mean cutting off one of their biggest sources of donations -- the petrochemical industry.

It is fitting to end with a quote from Upton Sinclair: "It is difficult to get a man to understand something, when his salary depends upon his not understanding it."  The problem is, here what we're jeopardizing is not simply someone's salary, but the long-term habitability of the Earth.

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This week's Skeptophilia book recommendation is about a subject near and dear to my heart; the possibility of intelligent extraterrestrial life.  In The Three-Body Problem, Chinese science fiction writer Cixin Liu takes an interesting angle on this question; if intelligent life were discovered in the universe -- maybe if it even gave us a visit -- how would humans react?

Liu examines the impact of finding we're not alone in the cosmos from political, social, and religious perspectives, and doesn't engage in any pollyanna-ish assumptions that we'll all be hunky-dory and ascend to the next plane of existence.  What he does think might happen, though, makes for fascinating reading, and leaves you pondering our place in the universe for days after you turn over the last page.

[Note: if you purchase this book from the image/link below, part of the proceeds goes to support Skeptophilia!]