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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Written in the genes

Two years ago, I wrote about a mysterious plunge in global average temperature that occurred 12,800 years ago.  It's nicknamed the "Younger Dryas event," after the tundra wildflower Dryas octopetala, which showed a population explosion over the following millennium (as judged by pollen in ice core samples).  This plant only flourishes when the winters are extremely cold, and the pollen spike, along with various other lines of evidence, supports a rapid drop in temperature averaging around six degrees Celsius worldwide.

[Image licensed under the Creative Commons xulescu_g, Dryas octopetala (41907904865), CC BY-SA 2.0]

The obvious question, of course, is what could cause such a rapid and catastrophic drop in temperature.  There are three reasonably plausible answers that have been suggested: 

1. an impact by a comet or meteorite causing an ejection of ash into the atmosphere, blocking sunlight
2. the collapse of an ice dam across what is now the St. Lawrence Seaway -- the temperature had been warming prior to the event -- allowing the emptying of an enormous freshwater lake into the North Atlantic, shutting off the thermohaline circulation and propelling the Northern Hemisphere back into an ice age
3. a nearby supernova in the constellation Vela frying the ozone layer, causing a collapse of ecosystems worldwide and an atmospheric chain reaction resulting in a global drop in temperature

The discussion amongst the scientists is ongoing, but the weight of evidence seems to favor the impact hypothesis.  (The link I posted above has more details, if you're curious.)

What's more certain is that the Younger Dryas event had a massive effect.  A number of large mammal groups -- including mastodons, North American camels, dire wolves, and gomphotheres (a bizarre-looking elephant relative) -- all went extinct shortly after the event itself, whatever it was, occurred.  Humans very nearly bit the dust, too; two of the dominant cultures of the time, the Natufian culture of the Middle East and the Clovis culture of North America, both collapsed right around the same time.

It's the latter that brings the topic up, because of some fascinating new research that came out last week, led by Paula Paz Sepúlveda of the Universidad Nacional de La Plata (Argentina), which looks at the effects this wild climate reversal had on the human genome.

What the researchers did was look at the makeup of the Q Y-DNA haplogroup.  You probably already know that two bits of our genome, the Y chromosome and the mitochondrial DNA, are frequently used for analyzing ethnic group affiliations because they don't recombine each generation -- they're passed down intact through (respectively) the paternal and maternal line.  So your mtDNA is the same as your mother's mother's mother's (etc.), and if you're male, your Y DNA is the same as your father's father's father's (etc.).  This means that the only differences in either one are due to mutations, making them invaluable as a measure of the degree of relatedness of different ethnic groups, not to mention providing a way to track patterns of human migration.

The Q haplogroup is ubiquitous in indigenous people of North and South America, so it was a good place to start looking for clues that the climate shift might have written into the human genome.  And they found them; coincident with the Younger Dryas event there was a marked drop in genetic diversity in the Q haplogroup.  It looks like the climate calamity caused a bottleneck -- a severe reduction in population, resulting in a loss of entire genetic lineages:

The YD impact hypothesis states that fragments of a large disintegrating asteroid/comet hit North America, South America, Europe, and Western Asia at 12,800 cal BP.  Multiple airbursts/impacts produced the YD boundary layer (YDB, Younger Dryas boundary), depositing peak concentrations of a wide variety of impact markers.  The proposed impact event caused major changes in continental drainage patterns, ocean circulation, in temperature and precipitation, large-scale biomass burning, abrupt climate change, abrupt anomalous distribution of plants and animals, extinction of megafauna, as well as, cultural changes and human population decline.  The diversity of the set of markers related to the cosmic impact is found mainly in the Northern hemisphere, including Venezuela, but they have also been recorded in the Southern hemisphere, in Chilean Patagonia, and Antarctica.

It's fascinating to think of our own genomes, and (of course) the genomes of other species, as being a kind of proxy record for climate; that not only gradual fluctuations, but sudden and unexpected events like impacts and volcanic eruptions, can leave their marks on our DNA.  It brings home once again how interlocked everything is.  Our old perception of humans as being some kind of independent entity, separate from everything else on Earth, is profoundly wrong.  We were molded into what we are today by the same forces that created the entire biosphere, and we can't separate ourselves from those forces any more than we could disconnect from our own heartbeats.  As Chief Seattle famously put it, "Man did not weave the web of life, he is merely a strand in it.  Whatever he does to the web, he does to himself."

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Analysis of a cold snap

Almost exactly 12,800 years ago -- and yes, we know it to that degree of accuracy -- there was a sudden plunge in the global temperature.

It's known as the "Younger Dryas" event, after the steppe wildflower Dryas octopetala, which only grows well when the conditions are very cold in winter.  The proxy records (bubbles in ice cores, patterns of glaciation, and types of pollen found in ice and sediments -- such as the aforementioned Dryas) are all in good agreement that in only ten or twenty years, the temperature in the Northern Hemisphere plunged, in some places by as much as 6 C.

[Image licensed under the Creative Commons Bjoertvedt, Dryas octopetala IMG 5641 reinrose reinsdyrflya, CC BY-SA 3.0]

That may not seem like very much, but six degrees is huge.  In fact, the word that comes to mind is "catastrophic."  The glaciers that had been receding -- this is, or at least was, an interglacial period -- suddenly began to extend their reach.  The cold period didn't abate for over a thousand years, with enormous impact on the humans around at the time.  The Younger Dryas correlates with the collapse of two of the dominant cultures, the Clovis civilization of North America and the Natufian culture of the Middle East.

But what could cause such a sudden and calamitous change in the temperature?

For years, the culprit was thought to be Lake Agassiz, a colossal freshwater lake that encompassed all five of the Great Lakes (and a lot more square milage as well), which was held back by an ice dam across what is now the Saint Lawrence Seaway.  As the temperature warmed -- remember, interglacial period -- the dam became unstable and finally collapsed, causing a humongous (I'm running out of words for "big," here) outflow of cold fresh water into the North Atlantic.  The result was a drastic slowing of the North Atlantic Meridional Turnover, which powers the Gulf Stream and keeps the Northeastern United States, Great Britain, Iceland, and Scandinavia at least reasonably warm.  The Turnover is caused by saline water (which is denser) sinking south of Iceland, and when the ice dam collapsed and the lake drained, the entire North Atlantic was covered by a sheet of water that was too fresh to sink.  The result: a slowdown of the circulation, and a return of glacial conditions.

Another, more far-fetched possibility is that the Earth got blasted by the shock wave of a supernova in the constellation Vela.  There is good evidence that the Vela supernova was coincident with the beginning of the Younger Dryas -- but connecting this to the drop in temperature is a bit of a stretch for most climatologists.

Recently, a third option has been gaining strength, and that's the fallout from the impact of a comet or meteor.  Here, the idea is that the debris thrown skyward by the impact blocked sunlight and caused a drop in temperature.  The impact hypothesis just got a boost last week with a paper in Scientific Reports, about a microanalysis of sediments from a place called Abu Hureyra that show good evidence of being flash-fried 12,800 years ago.

The sediments were collected decades ago, because the site itself was drowned when the Taqba Dam was put in place in 1970.  Archaeologists figured they better get what they needed from Abu Hureyra before the waters rose, and that included samples of everything they could get their hands on.  And an analysis by a team led by Andrew Moore of the Rochester Institute of Technology found that the bits of rock and other debris from the site dating to -- you guessed it, 12,800 years ago -- were coated with melted glass, indicative of a temperature of at least 2,200 C.

"To help with perspective," said James Kennett of University of California-Santa Barbara, who co-authored the paper, "such high temperatures would completely melt an automobile in less than a minute."

So the impact hypothesis is sounding more and more plausible.  What this kind of research always brings home for me, though, is how fragile the Earth's climate balance is.  Climate change deniers like to point out that there have been climatic ups and downs in the past, and the Earth has recovered; what they seldom add is that those ups and downs often resulted in mass extinctions.  So sure, the temperature rebounded after the Abu Hureyra collision.

A fat lot of good that did for the Pleistocene megafauna, such as mastodons, dire wolves, North American camels, and gomphotheres -- a bizarre North American elephant relative.  The humans didn't do much better; the ones who didn't get cooked and/or flattened by the impact very likely starved to death because of the mass die-off of plants and animals in the years following the collision.  How many made it through the bottleneck, and became our direct ancestors, is unknown, but it probably wasn't many.

So that's our cheery scientific discovery of the day.  A meteor impact triggering temperatures hot enough to melt glass, followed by a shower of debris and a drastic drop in global temperature.  I'd like to think this would be a cautionary tale, showing the effect one event can have on the climate, but at this point I know better.  We've pretended that everything is hunky-dory and ignored the scientists so far, so we'll be fine, right?

Of course right.

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This week's Skeptophilia book-of-the-week is brand new: Brian Greene's wonderful Until the End of Time.

Greene is that wonderful combination, a brilliant scientist and a lucid, gifted writer for the scientifically-inclined layperson.  He'd already knocked my socks off with his awesome The Elegant Universe and The Fabric of the Cosmos (the latter was made into an equally good four-part miniseries).

Greene doesn't shy away from difficult topics, tackling such subjects as relativity, quantum mechanics, and the nature of time.  Here, Greene takes on the biggest questions of all -- where the universe came from, how it has evolved and is evolving, and how it's going to end.

He begins with an observation that as a species, we're obsessed with the ideas of mortality and eternity, and -- likely unique amongst known animals -- spend a good part of our mental energy outside of "the now," pondering the arrow of time and what its implications are.  Greene takes a lens to this obsession from the standpoint of physics, looking at what we know and what we've inferred about the universe from its beginnings in the Big Bang to its ultimate silent demise in the "Heat Death" some billions or trillions of years in the future.

It's definitely a book that takes a wide focus, very likely the widest focus an author could take.  And in Greene's deft hands, it's a voyage through time you don't want to miss.

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

Confidence, impacts, and ice ages

One of the most common misunderstandings about science by laypeople centers around the concept of degree of confidence.

This misunderstanding can be summed up that "all unproven hypotheses are equally likely."  You hear it with lots of loopy ideas -- that (for example) because we don't have strong evidence one way or the other regarding the existence of an afterlife, it's on the same footing as other phenomena for which we have no direct evidence, such as dark matter, time travel, and the claim we've been visited by extraterrestrials.

Another way this shows up is the dismissive, "all of this could be proven wrong tomorrow" attitude toward science.  The fact that new discoveries have on occasion overturned what we thought we understood is taken as evidence that all of science is on thin ice, that it's all equally tentative.  But this rests on a serious misapprehension about the reliability of evidence.  It's true that, as Einstein allegedly put it, "one experiment could prove wrong" either the Second Law of Thermodynamics or our understanding of the mechanisms of quantum entanglement; but the first is extremely unlikely (the Second Law is one of the most extensively-tested scientific principles known, and there has never been a single exception found to it) while even the physicists would admit the second is a possibility (we're still elucidating the idea of quantum entanglement, and new and intriguing data is being added to our understanding on nearly a daily basis).

This frustrates people who like to have certainty, or at least like to be able to say with confidence that something isn't possible.  I ran into an especially good example of this just yesterday when I was reading an article about the Younger Dryas, a mysterious climatological reversal that occurred 12,900 years ago and lasted only 1,200 years -- a mere blip on the geological time scale.  What happened was that during a period when the Earth was warming, in only a few decades the average temperature of the Earth dropped by an average of four degrees Celsius, enough to put most of the Northern Hemisphere back in the deep freeze.  (The event is named after a plant, Dryas octopetala, which only grows in extremely cold places, and which became common across Europe and North America through the duration of the temperature drop.)

[Image licensed under the Creative Commons Opioła jerzy, Dryas octopetala a4, CC BY-SA 3.0]

Of course, presented with such a conundrum, the first question that comes up is "Why did this happen?"  There are three main hypotheses:

• As the Earth was warming up after the last major glaciation, a huge freshwater lake that had piled up behind an ice dam was suddenly emptied when the dam collapsed.  This lake, nicknamed "Lake Agassiz," emptied out through what are now the St. Lawrence and Mackenzie Rivers, and caused a slowdown (or complete cessation) of the thermohaline circulation.  Put simply, this is the engine that powers the Gulf Stream, which brings warm water northward and keeps the northeastern United States and most of western Europe relatively temperate.  When the flood occurred, the north end of the thermohaline circulation became too fresh to sink, and the whole system ground to a halt, propelling us into another ice age.  It was only after a thousand years had passed, and the lake water had adequately mixed with the ocean water, that the circulation rebooted and things warmed back up.
• 12,900 years ago, the Earth was hit by an object from space -- probably either a comet or a meteorite -- and that collision flash-burned a significant fraction of the vegetation in northern North America.  The debris and ash blocked sunlight, cooling down the surface of the Earth and halting the warm-up we'd been experiencing in its tracks.  Eventually the ash settled out, the forests regrew, and the climate restabilized, but that took several centuries.
• A supernova in the constellation of Vela created a burst of radiation that destroyed the Earth's ozone layer and killed most of the Earth's megafauna, including mammoths, mastodons, dire wolves, and several species of temperate-climate camels, rhinos, and hippos.  The gamma radiation striking the atmosphere caused a cascade of chemical reactions that disrupted the balance of nitrogen-containing compounds (such as nitrous oxide and nitrogen dioxide), and this caused a sudden and drastic temperature drop.

Each has some points in its favor.  The ice-dam proponents argue that the temperature drop wasn't as fast as you'd expect from something catastrophic like a collision or supernova, and that in fact the extinctions that occurred were in species that had already been declining for millennia.  Scientists supporting the impact hypothesis were buoyed by the discovery of a previously-unknown crater in Greenland -- but they've been unable to pinpoint its age any more accurately than "some time between three million and twelve thousand years ago."  The supernova enthusiasts point to the existence of "black mats" -- thin layers of the remnants of anaerobic organisms -- as evidence that something drastic happened to the atmosphere at the beginning of the Younger Dryas, and samples taken from it do seem to have skewed nitrogen content.  (This same evidence is considered support for the impact hypothesis, because there have been "microspherules" -- tiny spheres of melted and refrozen metal -- found in some of those boundary layers.)  But the black mats in different locations seem to date from different time periods, with only three of the thirteen studied being coincident with the Younger Dryas event.  And most of the black mats studied don't contain microspherules.

So the argument is still out there.  As far as my own opinion, I can only say that I'm neither a paleoclimatologist nor an astrophysicist, so am unqualified to weigh in (and my opinion wouldn't mean much anyway).  It seems like the dam collapse model is the one that currently has the most support, but -- like all science -- new information could tilt us toward one of the others.

Why does this come up with regards to our confidence in scientific models?  Not only because it's a great example of competing explanations and the fact that good scientists are willing to entertain the possibility of alternate solutions to the conundrums they study.  The idea for this post came to me because of another twist on the Younger Dryas -- this one from noted wingnut Graham Hancock, who says that the Younger Dryas event not only inconvenienced the camels and dire wolves, it also wiped out an advanced technological civilization...

... which gave rise to the myth of Atlantis.

So this is what I mean about levels of confidence.  No, we haven't been able to rule out two of the three models for the cause of the Younger Dryas with any real certainty.  But the fourth idea -- that whatever caused the event also destroyed Atlantis -- has nothing, not a shred of evidence, to support it.  As the brilliant skeptic Jason Colavito put it:

[R]egardless of whether a comet hit, the existence (or non-existence) of the comet implies nothing about the existence of Atlantis any more than it would unicorns or leprechauns. 

It remains a point of astonishment that the bones of megafauna that supposedly died in the comet strike turn up with regularity, but every human being and all of the buildings, tools, and material possessions of the lost Atlantis-like civilization were blasted clean off the face of the Earth, without a single trace remaining.  I have trouble imagining that a sloth can manage to have its bones preserved for all time, but not a single outpost of Atlantis had even a single bolt or screw remain.

As do I.  All unproven assertions are not on an equal footing.  And that's really the point of Ockham's Razor, isn't it?  The fastest way to winnow down competing ideas is to see which ones require you to make the most ad hoc assumptions.  And I'd put any of the three scientific explanations I mentioned above ahead of Hancock's assertion that the Younger Dryas event destroyed the lost civilization of Atlantis.

I'm perfectly willing to stay in uncertainty, indefinitely if need be, in the absence of convincing evidence one way or the other.  But in the case of explanations that require us to stretch credulity to the snapping point, I have no problem saying, "Nope.  That one isn't true."

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This week's Skeptophilia book recommendation is an entertaining one -- Bad Astronomy by astronomer and blogger Phil Plait.  Covering everything from Moon landing "hoax" claims to astrology, Plait takes a look at how credulity and wishful thinking have given rise to loony ideas about the universe we live in, and how those ideas simply refuse to die.

Along the way, Plait makes sure to teach some good astronomy, explaining why you can't hear sounds in space, why stars twinkle but planets don't, and how we've used indirect evidence to create a persuasive explanation for how the universe began.  His lucid style is both informative and entertaining, and although you'll sometimes laugh at how goofy the human race can be, you'll come away impressed by how much we've figured out.

[If you purchase the book from Amazon using the image/link below, part of the proceeds goes to supporting Skeptophilia!]