Polar Ice Cores, Part 4: Abrupt Climate Change

During the last ice age, massive abrupt warming events in the Arctic occurred repeatedly, which altered the climate of the entire planet.

As paleoclimate records have taught us, abrupt climate change in Earth’s past is more the rule than the exception. For instance, as the planet warmed near the end of the last ice age, wild fluctuations in temperature occurred in the Arctic region. We will discuss these further below. For now, let’s be clear about what we mean by abrupt climate change. A very good definition was given decades ago in a Scientific American piece by Richard Alley, a famous glaciologist from Penn State:

A rapid shift occurs when a slow but steady force, such as global warming, moves a crucial component of the climate system past a point of no easy return. Crossing such a threshold triggers a sudden switch to a new state—much the way leaning over too far in a canoe suddenly dumps you in the lake. Knowing exactly how far you can tip the canoe without overturning is almost impossible, however, especially as wind and waves rock the boat. Similarly, it is exceedingly tough to recognize when an aspect of climate is approaching a critical threshold.

Ok, so abrupt climate change is about crossing a threshold from an old climate state to a new climate state in a short amount of time.

Before we go further into the science, I want to recognize that abrupt climate change has played an outsize role in storytelling for centuries and longer. I spent some time browsing through over fifty climate-fiction (‘cli-fi’) stories that I found on the ‘interwebs’ (seriously, what is the name of the internet these days?). I chose my favorite three, listed below, spanning three different centuries - enjoy! (note: there must be 10s of thousands of instances of cli-fi stories, they are everywhere! I assume they literally go back to the beginnings of civilization.)

1. The popular French author, Jules Verne, known for adventure stories like Twenty Thousand Leagues Under the Sea and Around the World in 80 Days, wrote the novel Sans Dessus Dessous in 1889. In the book, an international auction is held to purchase the North Pole above the 84th parallel, which is won by an anonymous United States buyer, later revealed to be Barbicane and Co. The buyer intends to use the recoil of a giant cannon to remove the tilt of the Earth, causing the loss of seasons, day and night to always be equal, and each place on Earth to have the same climate year round. Most importantly, to maximize profits, the Arctic would point directly towards the sun, causing ice to melt and allowing vast coal reserves under the ice to be extracted and sold.
   
   Note: I thought this story was extremely interesting because we currently live in a time period where Arctic ice is melting rather quickly compared to the past, and many nations, especially nations affiliated with the Arctic Council, are vying for position to capitalize on Arctic natural resources and new shipping lanes.

   

   Old school book cover for Sans Dessus Dessous.

2. Alright, my second choice is pretty ‘out there’, the movie Silent Running (1972). One of the original movie posters for this film from Universal Pictures stated, “The day the last flower died on Earth this incredible mission began”. That line really knocked my socks off - losing the last flower is so abhorrent in my mind that I felt a brief and powerful moment of existential dread. The movie tells the story of a fleet of eight "American Airlines Space Freighters" stationed outside the orbit of Saturn. The freighters hold the last of Earth’s forests because the planet can no longer sustain plant life due to some climate change. The hope is to one day repopulate the forests of Earth.
   
   Note: This story was a stark reminder that we face a a mass extinction event in the coming decades due to climate change and pollution. It also reminded me there is a weird space race going on right now, aimed at launching habitats into orbit, such as the Orbital Reef Project - a “premier mixed-use space station in low Earth orbit for commerce, research, and tourism by the end of this decade.”

   

   A movie poster for Silent Running. I didn’t notice the rabbits until much later. Amazing graphic design.
   

3. A more recent choice, the imagination and storytelling in the movie Beasts of the Southern Wild (2012) is breathtaking. Undertones of climate change are throughout the film. Six year old Hushpuppy is trying to understand the world in which she lives, a Louisiana bayou called the Bathtub. In the rustic schoolhouse, Miss Bathsheba suggests the universe will soon come unraveled, causing the ice sheets to melt, releasing prehistoric creatures known as aurochs, and rising waters in the bayou to eventually destroy communities. In the end, Hushpuppy confronts the aurochs as they approach the Bathtub, and convinces them to leave.

   
   Note: This beautiful film is filled with hidden meanings. Perhaps the aurochs are the forces that compel climate change closer and closer to our communities. And perhaps Hushpuppy is the future, the younger generations that will solve this challenge by standing strong.

   

   A movie poster for Beasts of the Southern Wild, which is probably one of the best movie titles I have ever come across. [Wikipedia, Fair Use]

Alright, moving on from popular culture to science. There are three things to know about the last 120,000 years.

1) For ~90,000 years the Earth was in a glacial period. During this time, the Earth was on average much colder, and ice sheets reached as far south as the British Isles and Great Lakes. So much ice was stored on land that the oceans were 120 feet lower than present.

2) Within the glacial period, about twenty-five abrupt climate change events occurred related in part to changes in ocean circulation. During these abrupt changes, known as Dansgaard-Oeschger (D-O) Events, the North Atlantic, Greenland, Eurasia, and parts of North America warmed very quickly, often in a decade or less. 

3) Around 12,000 years ago, the Earth entered an interglacial period. During this time, the Earth was on average warmer, the ice sheets receded, and sea level rose. Civilizations were built, and some civilizations collapsed due to abrupt climate changes that were much smaller in magnitude than the D-O Events of the glacial.

Take a look at the plot below which shows the long-term trend of climate in Greenland, as recorded in the North Greenland Ice Core Project (NGRIP) ice core. On the left is the warm Eemian period 120 thousand years ago, followed by cooling into a glacial period from about 115 to 20 thousand years ago, and a warming into the Holocene warm period that lasted from 12 thousand years ago until roughly the present. This is the general pattern of climate change across the last ice-age cycle. Now, compare this to the 2nd plot below, which shows the full climate record. Look at all the spiky-aspects of the record in the glacial period (labeled with numbers 2-25, BA, and end YD) - those are abrupt warming events in Greenland, the so called D-O Events. They are named for the Danish (Willi Dansgaard) and Swiss (Hans Oeschger) scientists who helped to discover and understand them.

A record of oxygen isotopes (δ18O) from the North Greenland Ice Core Project (NGRIP), extending over the last 120,000 years (the ‘ka’ on the x-axis refers to ‘thousands of years before present’). The δ18O measurement can be considered a proxy for local temperature. On the right side of the plot is year 0, corresponding to the current warm period in which we live, whereas from 115-12 ka (115,000 to 12,000 years before present) the Earth experienced a long ice age, interrupted about twenty-five times by abrupt warming events, known as Dansgaard-Oeschger Events, labeled with numbers 2-25, BA, and end YD on the plot. Some abrupt warmings are not traditionally labeled, such as a spike in temperature between events 19 and 18 (usually referred to as event 19a), or multiple warmings for event 15. [data: Centre for Ice and Climate, Niels Bohr Institute; plot: Tyler R. Jones, CC BY 4.0]

D-O cycles follow a characteristic evolution:

1. Abrupt initial warming in decades or less: In a study of NGRIP by Kindler et al. 2014, the authors state “The detected temperature rises at the onset of D-O events range from 5°C [9°F] (D-O 25) up to 16.5°C [30°F] (D-O 11) with an uncertainty of ±3°C [±5°F].”

Imagine being a creature who lived through climate changes in Greenland such as these! Hypothetically, it’s like getting transported from Banff, Canada to Sedona, Arizona. That’s a big move, especially if you weren’t planning on it, and especially if you prefer colder temperatures.

2. Gradual cooling: While the abrupt warming took decades or less, the cooling aspect of the cycle took hundreds to thousands of years to return to full glacial conditions.

Note: the exact cause, or trigger, of D-O Events remains debated, and will be the topic of a future newsletter. The general line of thinking is that dramatic shifts in ocean currents affect the transport of heat northward towards Greenland, and that large fluctuations in sea-ice extent combine to cause the large changes in temperature that we observe in Greenland ice cores. But other theories abound.

In the plot below, you can see the characteristic D-O abrupt warming for the iconic Bølling–Allerød, an event named after scientific study sites in Denmark, followed by cooling into the Younger Dryas cold period, named after the alpine-tundra wildflower Dryas octopetala. Not labeled, to avoid confusion, is the Older Dryas and Oldest Dryas. Just after the Younger Dryas, another abrupt warming brought us into the Holocene.

A record of oxygen isotopes (δ18O) from the North Greenland Ice Core Project (NGRIP). Two abrupt warming events are shown, one for the Bølling–Allerød (also known as D-O Event 1) and another at the end of the Younger Dryas (also known as D-O Event 0). The ‘ka’ on the x-axis refers to ‘thousands of years before present’. [data: Centre for Ice and Climate, Niels Bohr Institute; plot: Tyler R. Jones, CC BY 4.0]

The Dryas octopetala is an Arctic–alpine flowering plant that prefers cold temperatures. The Younger Dryas, Older Dryas, and Oldest Dryas cold periods of the last glacial period are named after this flower because its pollen was found in sediment cores dating to these times. [Wikipedia, CC BY-SA 3.0, Steinsplitter]

Now, you might wonder what happened outside of Greenland during these D-O Events? The full story is much too long for this post alone, but in short, the entire world changed. The tropical rain belts shifted, Asian monsoon patterns were altered, and ocean currents were rearranged - and so much more. Even Antarctica was affected. In the plot below, you can see that for every D-O Event in Greenland, a concurrent event is found in Antarctica - a 1:1 correlation! The peak of these Antarctic warmings are known as Antarctic Isotope Maxima (AIM). This was an amazing scientific finding a few decades ago. But, what physical mechanism links abrupt temperature changes in Greenland to temperature changes in Antarctica? The answer is ocean currents and heat redistribution, but that too is a story for another day. Long story short, when it comes to the Earth climate system, everything is interrelated.

(Top, in blue) A record of oxygen isotopes (δ18O) from the North Greenland Ice Core Project (NGRIP). (Bottom, in red) A record of hydrogen isotopes (δD) from the West Antarctic Ice Sheet (WAIS) Divide (WDC) ice core. The δ18O and δD measurements can be considered proxies for local temperature. A 1:1 correlation between abrupt warming events in Greenland and warming events in West Antarctica can be seen by the vertical lines. The ‘ka’ on the x-axis refers to ‘thousands of years before present’. [data: NGRIP, Centre for Ice and Climate, Niels Bohr Institute; WDC, USAP-DC; plot: Tyler R. Jones, CC BY 4.0]

I will end this newsletter with a quote from Andrew Dessler, Professor of Atmospheric Sciences at Texas A&M University. In a 2014 Senate testimony, Dessler stated:

“As a consequence of our understanding of the climate system, unchecked greenhouse-gas emissions would lead to warming over the 21st century of 4.7- 8.6°F (for the global average). Regionally, on land and in the Arctic, the warming is apt to be larger.

These warmings may not sound like much until you realize that the warming since the last ice age — a warming that completely reconfigured the planet — was 9°F-14°F (5-8°C). The upper limits of projected warming over the 21st century would therefore herald a literal remaking of the Earth’s environment and our place within it.”

Climate science “has led me to personally conclude that climate change is a clear and present danger.”

Thanks for reading. And please support this newsletter with a paid subscription, it would go a long way to helping this effort stay afloat.

Sincerely, 

TRJ, PaleoClimate Scientist