Our planet has posed a baffling puzzle for scientists ―one that contradicts established climate theories and has enormous implications for the future.
The Earth warms and cools every 100,000 years between ice ages.[i] A subset called the interglacial interval, the warmer period of time between ice ages when glaciers retreat and sea levels rise, lasts about 10,000 to 20,000 years.
We currently live in an interglacial, known as the Holocene, when the earth moved from the last ice age into the current warm period, and temperatures and oceans rose over the course of thousands of years. This transition took around 5,000 to 6,000 years.
Yet, during this period of warming, as massive ice-sheets receded and sea levels rose towards their current levels, scientists discovered an oddity. Past temperature records unearthed from Antarctica showed there was a 2,000 year pause in this warming, around 14,000 years ago, that is restricted just to the high-latitude Southern Hemisphere. The causes of this perplexing phenomenon, researchers believe, may help us to understand the Earth’s natural responses to rapid climate change and perhaps one day guide us in mitigating our current unprecedented rise in global temperatures.
The Earth is changing. The planet’s temperature rose 2 degrees Fahrenheit (1 degree Celsius) since the beginning of the 20th Century[iii]. Although warming and cooling trends occur naturally with the position of the Earth in relation to the sun over tens of thousands of years, scientists believe recent temperature increases are led by human influences and are beyond natural variability.[iv]
This change can have consequential effects, including more severe droughts and frequent wildfires, harsher tropical storms, declining water supplies, reduced agricultural yields, heat-related health impacts in cities, and flooding and erosion in coastal areas.[v]
Now Aussie researcher Matthew Harris is trying to understand the mechanisms of climate change in a different way ― by looking at microbial life in ancient cores of ice from Antarctica and reconstructing what happened in the past when climate change occurred naturally.
Harris is a paleoclimatologist. He looks at climate change in the past as a way to enhance our understanding of current and future climate change. His focus is on the Southern Ocean and the Antarctic region.
“The planet is warming,” he said. “That's pretty well established at this point. And in order to get an understanding of why that might be and how we can potentially mitigate this warming, we need to look for ways in which the planet has reacted in the past to warming and reacted to large amounts of carbon in the atmosphere.”
After all, increased levels of atmospheric carbon dioxide from human industry trap heat from the planet through the Greenhouse Effect, and raise temperatures. The more carbon we pump into the atmosphere, the more the Earth will convert solar radiation to heat, in turn warming the atmosphere. Understanding the ways in which the atmosphere and oceans work together to regulate global heat and carbon is vital to understanding how this process will cause changes to the earth’s climate.
Scientists reconstruct past changes in the Earth’s oceans in a litany of ways. One of the most important is the use of proxies ― through chemistry and microscopic organisms trapped within the Earth that can indicate changes in past conditions. These proxies can be found in a range of places, including layers of sediment on the sea floor.
But what fascinated Harris, who spent his childhood snorkeling and learning about ocean life in South-East Australia, was the idea of using old layers of ice from Antarctica to examine organisms that were blown in from the ocean in the distant past.
Why Antarctica? The Southern Ocean occupies fourteen percent of the Earth’s surface, and plays a central role in the global carbon cycle and climate.[vi]
The Earth started to emerge from the last ice age about 18,000 years ago.[vii] The planet was warming. The driving factor was increased solar irradiance, with rising CO2 levels playing an important role. It is during this period that human civilization as we know it has flourished.
Yet strangely, CO2 levels and warming plateaued for 2,000 years, between 14,600 and 12,700 years ago in Antarctica.[viii] This period is known as the Antarctic Cold Reversal (ACR).
The reasons behind this pause are uncertain. One theory is that at this time, a huge portion of the Greenland ice sheet collapsed, and a massive amount of melted ice or fresh water was pumped into the North Atlantic, which is thought to have triggered rapid warming in the Northern Hemisphere. Oddly, this period coincides with the ACR, which was a period of pronounced cooling for parts of the other hemisphere. The connection between these two events is the subject of heated scientific debate.
So while the temperature rose in the Northern hemisphere, and the Greenland ice sheets collapsed and sea level rise ramped up ― by potentially 10 meters over the course of a few hundred years ― it was cooling in the Southern Ocean and Antarctica.
“This implies that there is some kind of mechanism in the climate and ocean systems that as a result of increasing temperature, something kicked in that both stopped the increasing levels of carbon and of temperature in the atmosphere,” Harris said.
And this discovery presented opportunities for scientific breakthroughs.
“Thescientific community is really keen to find ways that we can potentially use the Earth's natural mechanisms to lower the amount of carbon in the atmosphere, and to slow down the warming,” he said. “We're also keen to find instances in the past where the Earth has reacted in chaotic and rapid ways to increased warming and carbon, because these incidents might indicate potential rapid climate change events that are in store for us as a species.”
That, according to Harris, implies there might be either mechanisms we can exploit in the natural system through geoengineering that would potentially allow us to slow climate change. However, he stressed that interfering in the Earth’s climate system is fraught with challenges and shouldn’t be approached lightly – especially given doing just that, unintentionally, is what caused the whole warming problem to begin with. Regardless, these past instances of “strange” climate change – where the earth appeared to partially cool despite warming elsewhere, indicate there are unknown dynamics in the earth systems that react unpredictably.
“And that's also a big worry, because it could mean the climate change could speed up or stop,” he said. ”And we're just not aware of that at the moment.”
During the 2,000-year temperature plateau, marine life flourished in the Southern Ocean. Plankton, microscopic green organisms that live in the near-surface ocean, are far more effective than trees and plants at photosynthesis.
In order to produce energy, plankton take up carbon dioxide and, using chlorophyll and sunlight, convert it to oxygen and energy. These microscopic tree-like organisms are abundant in the ocean, and produce 50 to 80 percent of the plant’s oxygen[x]. As a result of this process, carbon dioxide is removed from the atmosphere, and transported into the food chain (when the plankton is eaten) or the ocean floor when the plankton perish. In this way, plankton act as a ‘carbon sink’, removing carbon dioxide from the atmosphere.
It's believed that throughout history, plankton, more so than terrestrial trees and shrubs, have played a vital role in regulating the levels of atmospheric carbon dioxide.
This became the focus of Harris’s team’s study: to understand whether this marine plant life preserved in ice cores, and their role in regulating atmospheric carbon dioxide, could open the door to understanding the mechanisms of the Antarctic temperature plateau 14,000 years ago.
The Keele Ice Laboratory, in Staffordshire, UK, documented past changes in Southern Ocean marine productivity and sea ice through the analysis of fluorescent compounds preserved in ice cores from Antarctica using fluorescence spectroscopy, according to Harris.
“Marine life is full of tryptophan and tyrosine, essential amino acids,” he said. “So, all microbial life has this stuff in them. There's not much microbial life in ice. It’s a very unwelcoming microbiome to live in. Research has shown that typically when there's elevated levels of microbial life they generally have to come from the ocean. (This period in the) past when the temperature stopped rising, we had a huge increase in the presence of a microbial life in our ice cores.”
Harris uses a HORIBA Aqualog® spectrofluorometer at the University of New South Wales in Australia, and at the Keele University Ice Laboratory in the United Kingdom to measure changes in the intensity and type of fluorescent compounds in ancient Antarctica ice. The Aqualog quickly performs simultaneous absorbance and transmittance measurements along with the acquisition of an Excitation Emission Matrix (EEM). This proprietary Aqualog A-TEEM technology also automatically corrects EEMs for the Inner Filter Effect.
“(The Aqualog is) incredibly stable,” he said. “The signal-to-noise ratio is very high. And the ease of use is very good. So training students to use it is very straightforward. It has a high bandwidth range, so it goes from the UV spectrum all the way up to the high visible range. It’s just the most straightforward, reliable fluorometer we could find.”
“Our results show that the dominant fluorescent materials in ice are typically amino-acid-like substances bound to proteins within microbial life,” he said. “This life originates mostly from the Southern Ocean, and is blown onto the ice during storms and weather events. By measuring the fluorescence of the ice, we gain a means to measure the 'presence' of these microbes within our ice samples.”
His research group’s most significant finding this far was a major increase in this amino acid fluorescence over thousands of years during the time when the earth was moving out of the last ice age. This indicated that, despite the warming pause 14,000 years ago, microscopic marine life flourished during the ACR.
“Our samples of ice from this time show that amino-acid filled microbes were being deposited onto the ice in unusually large amounts when compared with other times in the ice core record,” he said. “A persistent millennial signal of marine-associated fluorescent organic compounds peaked during this period.”
This discovery has added a layer to the understanding of the Antarctic warming plateau.
“We think that means that this period was potentially driven by a change in ocean circulation, but also by a sweet spot that was reached in the Southern Ocean that caused, during this warming phase, life to flourish like it hadn't for a long, long time.”
“As a result, this life absorbed a huge amount of carbon and potentially helped put a pause on the rise of atmospheric carbon dioxide for about 2,000 years.”
Scientists normally drill straight down through the ice to collect their ice core samples. As you go deeper, and farther back in time, the layers are more compressed.
“Scientists have unearthed 800,000 year-old ice, and there's currently projects underway to get back to a million and a half years old,” Harris said.
One of the downsides of this approach is that, once you get back to these ancient layers of ice, you've only got a drill’s width of ice to work with. There's really only so much you can do with one of these incredibly old ice layers, and as a result there is some understandable reluctance to try out new analysis techniques.
Harris’s group takes a different sampling approach.
The Antarctic ice sheet is essentially a huge, frozen river that, because of gravity, is moving slowly towards the ocean. Harris’ group looked for a mountain range where the ice had been shifted onto its side. Instead of being layered vertically, the ice has been laid horizontally as it was forced up against the side of a valley.
This approach makes it more difficult to date the ice, but it allows researchers to walk along the surface of the ice, and gain access to an almost unlimited quantity of ancient ice.
“We look at ice dating back to about 140,000 years old through this nontraditional drilling method,” he said.
The Antarctica Cold Reversal counterintuitively involved a drop in Antarctic temperature at a time when the rest of the globe was warming. This period also saw a pause in the rise of atmospheric carbon in Antarctica, likely due to a proliferation of life in the parts of the Southern Ocean closest to Antarctica.
In simplistic terms, Harris’s group interprets this as indicating that these tiny ocean microbes were exceptionally active during this warming reprieve, and acted as a major carbon consumer ― drawing huge amounts of carbon from the atmosphere, preventing its rise, and so preventing temperature from rising.
Before his group’s recent research, there was no coherent theory as to why both carbon and temperature plateaued for about 2,000 years in the Southern Ocean. And because carbon is so crucial to current climate change, Harris’s group was really mostly interested in the carbon side of side of this project.
“Strange climate dynamics like this imply that there are natural mechanisms within the earth system that can act to stop or start atmospheric warming,” Harris said.
With global atmospheric carbon dioxide levels still rising rapidly, there are increasingly calls for rapid and drastic action to slow temperature rise. Harris’ work implies, for example, that “fertilizing” the Southern Ocean with nutrients, such as iron, could put the plankton into overdrive and sequester a huge amount of carbon, by taking up that carbon and storing it. This so-called ‘Iron Fertilization Hypothesis’ might, in effect, kick start a temperature reversal similar to the one that occurred in Antarctica 14,000 years ago. But like any interference in natural processes, there could be unintended consequences.
“What would be the secondary impact of that?” he reflected. “Geoengineering is a very sensitive subject. And for good reason, because we got into this current situation by playing with the Earth's climate systems. But to the same degree, if we don't know about these things, then there simply isn't the option. We’re going to be pretty desperate in the future, so maybe it this research will provide a means to intervene. And who's to know, in a hundred years or so, we might have come up with a way to influence the climate system using this type of knowledge.”
[i] Glacial-Interglacial Cycles, National Centers for Environmental Information, National Oceanic and Atmospheric Administration, 2020
[ii] - Jouzel, J., Masson-Delmotte, V., Cattani, O., Dreyfus, G., Falourd, S., Hoffmann, G., … Wolff, E. W. (2007). Orbital and Millennial Antarctic Climate Variability over the Past 800,000 Years. Science, 317(5839), 793 LP – 796.
[iii] Lindsey, Rebecca and Dahlman, LuAnn, Climate Change: Global Temperature, National Oceanic and Atmospheric Administration, August 14, 2020
[iv] Karl, Thomas R and Trenberth, Kevin E. Modern Global Climate Change –Science, Dec. 2003
[v] The Effects of Climate Change, NASA, 2020
[vi] Fogwill, C. J; Turney, C. S. M, Cooper, A, et al. Southern Ocean carbon sink enhanced by sea-ice feedbacks at the Antarctic Cold Reversal Nature Geoscience, June 2020
[vii] Harris, Matthew, Southern Ocean carbon dynamics: New insights from ancient ice, Nature Research, June 22, 2020
[ix] - Jouzel, J., Masson-Delmotte, V., Cattani, O., Dreyfus, G., Falourd, S., Hoffmann, G., … Wolff, E. W. (2007). Orbital and Millennial Antarctic Climate Variability over the Past 800,000 Years. Science, 317(5839), 793 LP – 796.
[x] How much oxygen comes from the ocean? National Ocean Service, National Oceanic and Atmospheric Administration, June 6, 2020
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