The shape of past climate provides a warning: everything can change quickly

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Sawtooth wave

t’s the suddenness that feels like a betrayal, severing the casting line of continuity. That unthinkable call, the collapsing spire, a briefly unattended child. A geological timeline of events blocked out into before and afters, as if different earths altogether, separated by a single nightfall. The feeling is always the same: a futile search for what could have been done differently. Sometimes the answer is nothing, and sometimes, cruelly, the answer is a lot.

When Willi Dansgaard was making oxygen isotope measurements on the first deep ice core drilled at the Camp Century military base on Greenland, he thought something must be wrong with the measurements; they seemed to change to heavier values — an indication of warming — much too quickly. He kept measuring deeper in the core and the ice kept telling the same story: it appeared temperature in Greenland could change by 10–15ºC on timescales of years to decades.

When he first published these results in the late ’60s, many scientists thought there must be something wrong with the ice — perhaps the mile-high glacier had been folded somehow in its transit and the anomalously warm temperatures that appeared to pepper the last glacial period were really from relic ice of the last interglacial period. It would take a couple of decades and a few more deep ice cores to establish the undeniable truth of these rapid climate jumps — now known as Dansgaard-Oescheger events — of which there were 25 over the course of the last 120,000 years.

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Willi Dansgaard (photo: Niels Bohr Institute). It is no longer considered best practice to smoke a pipe while handling an ice core.

The thing about these events, is that they don’t just reflect local warming in Greenland; they represent global-scale climate reorganization. The Intertropical Convergence Zone shifted northward, the East Asian monsoons ramped up, ocean hypoxia set in suddenly across the Pacific, the Gulf Stream shifted position in the North Atlantic, terrestrial vegetation changed drastically, and with it, the production of methane gas, to name just a few of the strings plucked. The trigger of these events is often attributed to changes in North Atlantic Ocean circulation, but like the Uroborus eating its own tail, it’s hard to pinpoint the origins of a cascade in the Earth System.

What is clear — and perhaps most important — is the consistency of the pattern of these events. Past climate records trace out the same basic shape over and over again: a sawtooth wave pattern — warming is the abrupt edge, cooling the descending slope.

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Climate records showing a similar sawtooth wave pattern at different timescales, with warming as the abrupt near-90 degree slope, and cooling as the descending ramp (oxygen isotopes are a proxy for temperature). The upper plot shows longer glacial-interglacial cycles on the order of ~100,000 years, as recorded in ocean sediments, whereas the bottom plot shows the rapid Dansgaard-Oeschger cycles that occur on timescales of a few thousand years, recorded in a Greenland ice core.

takes much longer to build a Notre Dame than it does to destroy it. This is true with the climate system as well. Ice sheets take time to grow, but they can melt much faster. Carbon takes time to bury, but it can be burned quickly. Ecological systems take time to mature into complex webs of interconnectivity, and yet these systems can be destabilized rapidly when environmental conditions exceed their thresholds of tolerance. Human societies can develop for hundreds and thousands of years in evolving complexity only to be undone by a few years of environmental extremes.

This is the pervasive asymmetry of complex systems: stability requires time, whereas instability happens rapidly.

While collapse is inevitable to some extent, we spend a considerable portion of our lives trying to lower the probability of it happening in our immediate vicinity. Flying buttresses were designed to circumvent the structural instability that arose over time in cathedrals, as the heavy stone would spread down and outward under its own weight. Buttresses provided counter pressure to help shore up the walls — allowing cathedrals to be built taller with greater panels of stained glass to illuminate the interior.

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Large stained glass panels such as the rose window in Notre Dame were facilitated by the structural support of flying buttresses

Ice shelves are to ice sheets what flying buttresses are to gothic cathedrals, as the prominent glaciologist, Richard Alley, has pointed out. The West Antarctic Ice Sheet is a marine-based ice sheet, which means its grounding line is below sea level and the central ice mass flows outward into floating ice shelves. The ice shelves are like buttresses that provide a stabilizing force to the interior of the ice sheet. Because the ice shelves are in contact with the ocean, warming water and rising sea level can enhance melting, making them more susceptible to collapse. Once the buttresses are removed, the glaciers start accelerating into the ocean.

The melting of the West Antarctic Ice Sheet would raise sea level by about 10 feet, and collapse of some parts of it may already be irreversibly underway. Rather than shoring up these buttresses, however, we are the proxy agents actively eroding them. We are burning carbon deposits that took millions of years to bury, unleashing all of that slowly stored energy in one quick (century-scale) burst. To the best of our knowledge, the rates of current carbon release are 10 times faster than even the most rapid global carbon calamities in the geological record.

he memory — or inertia of ice sheets and oceans is what has given us some ‘safe operating space’ in relation to modern climate change. The same inertia, however, means that when we wake up to a world of discomfort, there is no thermostat to dial back; the climate system is already set on its trajectory.

An example of the ocean’s memory comes from a study looking at the temperatures of the deep Pacific Ocean, which still ‘remembers’ the chill of the Little Ice Age that turned Europe into an ice skating rink from the 14th to 18th centuries. It can take a thousand years for water to circulate through the ocean interior, retaining properties inherited from when it was near the surface, exchanging with the atmosphere. The abyssal waters of the deep Pacific are like my junkyard father, from an era long gone, seemingly unperturbed by the rapidly changing modern world.

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Dutch canals during the Little Ice Age. ‘On the ice’ by Hendrick Avercamp, circa 1610.

This lag benefits us on the short-term because the ocean is a large reservoir that has been absorbing atmospheric heat and carbon dioxide, but it will likely come back to haunt us on the long-term, as ocean warming and acidification dissolve another buttress of Earth system stability: marine ecosystems. Foundational species such as coral, kelp, and sea grasses that support entire trophic networks are already showing signs of massive failure in response to heat stress. Collapse of these systems can happen on the timescale of years.

ike the West Antarctic Ice Sheet, quietly being undercut by a rising and warming ocean, we are allowing the pillars of Earth system stability — on which all societal stability rests — to be eroded at the base. Maybe we take this foundation for granted because we didn’t have to put in any of the millions of years of work it took to get us here. Or maybe it’s our short memories. But like Notre Dame, instability can come unexpectedly, with shocking swiftness. When that instability comes, billionaires won’t be able to bail us out. We need to invest now, on this side of stability.

Rather than viewing this as a chore, we should embrace it as an opportunity to retrofit society’s top-heavy fossil fuel infrastructure with the elegant solutions of clean energy — like the flying buttresses — solutions that are not only more enduring and structurally sound, but utilize more of the natural light.

Paleoceanographer and climate scientist, pattern seeker, (micro)fossil hunter.

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