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ScienceJanuary 27, 2022

Returning to a green Antarctica


Antarctica wasn’t always covered in ice. But a return to a green continent would be bad news for us.

Comic by Simone Giovanardi. Words by Bella Duncan.

Antarctica conjures up images of a frozen, white continent, devoid of vegetation. This has been our narrative of Antarctica for 1,400 years, since explorer Hui Te Rangiora described the region as Te tai-uka-a-pia, depicting the frozen ocean. As more explorers and geologists started to study the rocks of the continent they discovered clues that Antarctica hasn’t always been an icy expanse. Rock samples of glossopteris leaf fossils, an ancient tree that lived 250 million years ago, were found alongside the bodies of Robert Falcon Scott and his companions, and told the story of a much greener, warmer past.

This collaboration began at the Drawing Science workshop, hosted by The Spinoff. A directory of illustrators who are eager to collaborate on science projects is available here, via the Science Media Centre.

Click here to read the accompanying text.

Returning to a green Antarctica

My Antarctic journey began on an expedition searching for younger plant fossils, preserved in rocks dragged up by the ice and deposited in huge stone fields. Stumbling through this rubble I broke open a brown, muddy rock and found a fossil of a Nothofagus leaf, a 40-million-year-old ancestor of our southern beech trees, currently widespread across Aotearoa New Zealand, Tasmania and Patagonia. It’s hard to put into words the feeling of standing on the frozen ground, looking at this leaf fossil, and imagining the forest that once covered Antarctica emerging from the ice around me, like lifting the veil to a long-forgotten world. What happened that turned this lush Aotearoa-like forest into the barren polar desert it is today?

It all comes back to CO2. While the continent of Antarctica has sat over the South Pole for around 250 million years, earth’s atmospheric CO2 level and intertwined temperature has vastly varied over this time, resulting in an Antarctic environment which hasn’t always been so hostile to plant life. 40 million years ago when my fossil was a living, growing leaf, CO2 levels were approximately 1000 parts per million (ppm) in the atmosphere. This blanketed the earth in a greenhouse effect that resulted in global temperatures about 10°C warmer than our present day average temperature of 15°C. Go back to 52 million years ago and temperatures were warm enough that near-tropical palms similar to those growing in modern Indonesia grew along the Antarctic coast. Intriguingly, the polar position of Antarctica meant these forests lived for six months of the year in darkness, spending the winter months essentially hibernating, waiting for the sun to return. 

Since the times of Antarctic palm trees and tropical beaches, global climate has gradually cooled with major ice sheets first growing on the continent 34 million years ago. This cooling has been primarily driven by declining atmospheric CO2 levels. Earth’s carbon is split between different reservoirs – the atmosphere, oceans, the biosphere (i.e. vegetation) and sediments (including fossil fuels). When my fossil leaf was alive, more carbon was stored in the atmosphere, leading to the greenhouse temperatures which enabled forests to flourish in Antarctica.

But over time, tectonic plate movements have resulted in changes in ocean circulation and driven mountain building in regions like the Himalaya. Tectonic rifting around Antarctica pushed away Australia and South America, isolating Antarctica and leading to the development of the Southern Ocean and the Antarctic Circumpolar Current. This windswept, vast area of ocean draws a huge amount of carbon out of the atmosphere, where little critters and plants use it to grow and build shells. When they die, they sink into the sediment at the bottom of the ocean, essentially locking carbon away. In some regions, mountain building, changing rainfall patterns, and erosion of the land may play a similar role. Rocks with a lot of minerals made of silicates can react with CO2 when they are eroded, removing this carbon from the atmosphere and transporting it through rivers into the ocean and deep ocean sediments.

The processes that move carbon between different reservoirs are called the Carbon Cycle, and humans have been carrying out a big experiment with it since the industrial revolution. The use of fossil fuels shifts carbon from one reservoir (sediments) into another (the atmosphere). We’ve increased the amount of CO2 in the atmosphere by about 140 ppm across less than 200 years, up to levels not experienced on earth for about three million years.

As researchers who study the past climate of the planet (known as paleoclimatologists), we look at times in the past when CO2 levels were similar to today, and investigate what different aspects of the environment looked like during those periods. We use methods like fossils, chemistry and changes in sediment deposition to reconstruct temperatures, rainfall, vegetation, ice extent and numerous other features. Together, these build a picture of the earth under various past climates, and give us important insights into what it might look like in future with different CO2 and temperature scenarios. 

It’s through this research that we can connect my fossil leaf to the higher CO2 and temperature levels that occurred when its host tree was alive. CO2 levels of 1000 ppm and temperatures 10°C above present meant very little ice could survive on Antarctica, leading to sea level potentially upwards of 60 metres higher than today. Even three million years ago when CO2 levels were the same as present, Antarctica had significantly less ice, and global sea level may have been more than 20 meters higher. While it takes a while for these sort of changes to happen, and for that much ice to melt, it tells us the sort of world we’re binding humanity to with inadequate action on climate change.

A forested Antarctica might sound like a pretty fascinating concept, but the consequences would be severe. Along with sea level rise, it would come with the disappearance of unique polar and subpolar ecosystems, as well as startling changes to global temperature, agriculture and the habitability of large, currently very populated, areas of the globe. As atmospheric CO2 levels continue to relentlessly creep up, the world of our future becomes more and more like worlds further back in our past. So what image of Antarctica will we commit to? A lower CO2 version where impacts of temperature and sea level rise are reduced? Or the version that will play out if we don’t act fast enough, the “greenhouse” world of vast ice loss, grave environmental change, and eventually, a return to a green Antarctica. 

– Bella Duncan

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