Abrupt climate change

• 1. Introduction

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  Abrupt climate change is not a gradual drift. It is the climate system lurching into a new state faster than the forces driving it. That distinction matters enormously, and it sits at the center of one of the most active areas of Earth science today.

  Consider what happened at the end of the Younger Dryas. Ice cores drilled from Greenland show that temperatures there jumped by plus ten degrees Celsius within a timescale of just a few years. Not centuries. Not decades. Years. A similar signal appears in Antarctica, where an abrupt warming of plus six degrees Celsius occurred around twenty-two thousand years ago.

  These are not outliers. Scientists have identified about twenty-five distinct climate shifts, called Dansgaard-Oeschger cycles, embedded in ice core records spanning the last one hundred thousand years. The Earth has done this before, repeatedly, and the question scientists are pressing now is whether it is poised to do it again.

  What drives these lurches? What has happened when they struck in the past? And what does modern greenhouse gas emissions have to do with a clock that has been ticking since long before humans arrived? Those are the questions this documentary will follow.

• 2. What Abrupt Actually Means

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  Physicists and ecologists define the word abrupt differently, and both definitions are useful. In physics, an abrupt climate change is a transition faster than the forcing that caused it. The climate system, rather than simply responding to an external push, accelerates on its own internal energy. In terms of impacts, the definition shifts: a change is abrupt if it arrives so rapidly and unexpectedly that human or natural systems cannot adapt in time.

  Those two framings complement each other. The physics definition explains the mechanism. The impacts definition explains why so much research is devoted to the subject.

  Timescales vary dramatically across the historical record. The Younger Dryas, which began twelve thousand eight hundred years ago and transitioned back to a warm-and-wet regime about eleven thousand six hundred years ago, ended in a matter of years by Greenland's ice core measurement. The Paleocene-Eocene Thermal Maximum, by contrast, may have taken anywhere from a few decades to several thousand years to get going.

  The word abrupt, then, is relative. It describes a pace that outstrips the pace of the forcing behind it, whether that plays out over a human lifetime or over geologic time. Climate models today project that under continuing greenhouse gas emissions, Earth's near-surface temperature could move outside the range of variability seen in the last one hundred fifty years as early as 2047.

• 3. Past Events, Vast Scales

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  Three hundred million years ago, tropical rainforests that had covered vast stretches of the globe were devastated by a rapidly cooling and drying climate. This Carboniferous Rainforest Collapse hit the biodiversity of amphibians especially hard. Amphibians were then the primary form of vertebrate life on land, and the fragmented forest patches that remained after the collapse reshaped evolutionary paths for millions of years.

  Fifty-five million years ago, the Paleocene-Eocene Thermal Maximum unfolded. One leading hypothesis traces it to a massive release of methane clathrates, though researchers have not ruled out alternative mechanisms. Whatever the trigger, it came with rapid ocean acidification that reshaped marine ecosystems.

  The Permian-Triassic Extinction Event stands in a category of its own. Up to ninety-five percent of all species were wiped out. Life on land took thirty million years to recover fully. The event has been hypothesized to connect to a rapid shift in global climate, though the causal chain remains a subject of active study.

  Abrupt changes have also been tied to the catastrophic draining of glacial lakes. The 8.2-kiloyear event is linked to the draining of Glacial Lake Agassiz. The Antarctic Cold Reversal, roughly fourteen thousand five hundred years before present, is believed to have been caused by a meltwater pulse from either the Antarctic ice sheet or the Laurentide Ice Sheet. Those rapid meltwater releases have themselves been proposed as a possible trigger for Dansgaard-Oeschger cycles.

• 4. Volcanoes, Ice, and Ozone

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  Isostatic rebound is what happens when a glacier retreats and the land beneath it, freed from the weight of ice, begins to rise. That rebound, along with changes in local salinity, has been linked to increased volcanic activity at the onset of the abrupt Bolling-Allerod warming. Volcanic particles falling onto glacier surfaces can alter the albedo of the ice, absorbing more heat and accelerating melt, which feeds back into further warming.

  A 2017 study added another dimension to this picture. Around seventeen thousand seven hundred years ago, conditions resembling today's Antarctic ozone hole contributed to abrupt deglaciation across the Southern Hemisphere. Stratospheric ozone depletion altered atmospheric circulation and the patterns of rainfall and snowfall. Coinciding with that ozone event was an estimated one hundred ninety-two-year series of massive volcanic eruptions attributed to Mount Takahe in West Antarctica.

  The Greenland Sea's main flushing site, located at 75 degrees North, shut down in 1978 and took the following decade to recover. Then the second-largest flushing site, the Labrador Sea, shut down in 1997 and remained offline for ten years. Whether two such sites could shut down simultaneously has not been observed in the fifty years of direct ocean monitoring. But the paleoclimatic record suggests that when overlapping shutdowns did occur in the past, the worldwide consequences were severe.

• 5. Feedback Loops and Tipping Points

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  Ice-albedo feedback is among the clearest examples of how warming amplifies itself. When ice retreats, the darker land or ocean surface that it exposes absorbs far more of the sun's energy than the reflective white ice did. The added heat causes more melting, which exposes more dark surface, which absorbs more heat. The loop is self-reinforcing.

  Soil carbon feedback works through a different pathway. As global temperatures rise, carbon locked in soils is released. That carbon enters the atmosphere and amplifies the warming that set the process in motion. The burning and dying of forests driven by warming adds another layer to the same dynamic.

  Scientists have identified a range of tipping elements in the climate system that could reach critical points within this century under anthropogenic climate change. The probability of an abrupt shift for any single feedback may be relatively low. But three factors raise the odds: a larger magnitude of warming, a faster rate of warming, and warming sustained over longer periods of time.

  Teleconnections, which are oceanic and atmospheric processes operating across different timescales, have been proposed as the mechanism linking both hemispheres during abrupt change. The several dozen shutdowns of the North Atlantic's Meridional Overturning Circulation during the last ice age are the best-known examples of sudden circulation shifts in action. Scientists describe these as analogous to a flood cutting a new river channel: once the new path opens, the old one may close for good.

• 6. Human History in the Frame

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  A five-year study that wrapped up in 2013 asked a pointed question: do major human development shifts tend to happen during or immediately after abrupt climate changes? The project, called RESET, stood for Response of Humans to Abrupt Environmental Transitions. It was led by the Oxford School of Archaeology and carried out in collaboration with Royal Holloway University of London, the Oxford University Museum of Natural History, and the National Oceanography Centre Southampton.

  The team drew on palaeoenvironmental records, prehistoric archaeology, oceanography, and volcanic geology reaching back one hundred thirty thousand years and spanning multiple continents. The goal was not only to test the hypothesis about human development but also to predict how humans might behave in the face of future climate change and to refine estimates of when such changes might arrive.

  The impacts catalogued from past abrupt events underscore why that question is urgent. Mass extinctions, biodiversity loss, changes to ocean circulation, increasing frequency of El Nino events, disruptions to the thermohaline circulation, shifts in the North Atlantic Oscillation, and changes to the Atlantic Meridional Overturning Circulation that could produce more severe weather patterns: these are not theoretical risks. They are outcomes the paleoclimatic record has already documented. What the RESET project sought to determine was how deeply those climate lurches shaped the human story alongside the ecological one.