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A rainforest once grew near the South Pole
A green Antarctica would have been ice-free and the air rich in carbon dioxide
Millions of years ago, a rainforest flourished relatively close to the South Pole. This artist’s image shows what it might have looked like.
J. McKay/Alfred Wegener Institute (CC-BY 4.0)
May 11, 2020 at 6:30 am
Once upon a time, a rainforest grew near the bottom of the world.
To find remnants of it, researchers explored the seafloor near Antarctica. In buried sediment there, they found ancient pollen. They also turned up fossilized roots and chemical evidence of a diverse forest. These woods flourished less than a thousand kilometers from the South Pole. That’s only a little more than 600 miles. It’s roughly the distance from New York City to Knoxville, Tenn.
The sediment offers a glimpse of what Earth was like in the distant past. It shows just how warm the planet was during the mid-Cretaceous Period. That was between 92 million and 83 million years ago. From buried traces of vegetation, the scientists reconstructed what the climate must have been like back then.
Average annual temperatures in the forest would have been about 13° Celsius, or 55° Fahrenheit. Summertime temperatures could have reached 25 °C (77 °F). That’s quite warm compared to how frigid Antarctica is now.
The team described its findings April 2 in Nature.
The temperatures aren’t that big a surprise. The mid-Cretaceous was one of the warmest on Earth in the past 140 million years. That estimate is based on studying fossils and seafloor sediments collected closer to the equator. Carbon-dioxide levels in air back then were high. They were at least 1,000 parts per million. (Today’s levels are lower average around 407 ppm. That’s the highest they’ve been at any time in the past 800,000 years.)
But ancient Antarctica wasn’t just warm. It was a forested. And for trees to thrive so far south, something else had to be going on. There had to be even more potent greenhouse-gas conditions. The air would have held more gases, such as carbon dioxide — far more than previously thought. Carbon-dioxide levels could have been between 1,120 and 1,680 ppm, says Johann Klages. He is a marine geologist who works at the Alfred Wegener Institute in Bremerhaven, Germany.
Just like now, he explains, the South Pole millions of years ago got little light — almost none for four months a year. Even so, Antarctica back then "could still have a temperate climate," Klages says. "It shows us the extreme potency of carbon dioxide — what carbon dioxide can really do." The gas made it possible for a forest to grow.
Looking back in time
Klages was part of the team that figured out Antarctica had a forest. The group retrieved a core of soil-like material. It was 30 meters (98 feet) long. The researchers took the core from within the Amundsen Sea. That’s near where water melting off of Thwaites and Pine Island glaciers drain into the sea. The researchers knew the core was special before they even studied it closely, Klages says. It was what they saw in the bottom three meters of sediment. It comes from a time in the mid-Cretaceous. And it showed traces of roots.
"We’ve seen many cores from Antarctica," Klages says, "but we’ve never seen anything like that."
The core had pollen, too. All this suggested that Antarctica was once a forest full of conifers, ferns and flowering shrubs. There were also mats of bacteria. But the sediment showed no traces of salt. That means it would have been a freshwater swamp.
The forest data also suggest something else. They offer a clue that Antarctica was largely ice-free during the mid-Cretaceous, Klages says. High carbon dioxide in the atmosphere alone wouldn’t have been enough to keep the region balmy so close to the pole. And if it would have had an ice sheet, all that whiteness would have reflected much of the incoming sunlight back into space. That would keep the land cold.
Plant cover would have had the opposite effect. It absorbs and holds the sun’s heat. And that would have boosted greenhouse warming. With more plants and no ice, the atmosphere would have been warmer. That would have allowed trees to flourish.
Julia Wellner is a geologist at the University of Houston, in Texas, who was not involved in the study. The findings offer "an unambiguous record of not just warmer conditions," she says. They also point to "a diverse forest flora" at the South Pole, she says.
"This paper is a great reminder that, just because there [is] a continent sitting at the South Pole, [that] doesn’t mean it necessarily has to have ice everywhere," she says. It doesn’t even need to have been particularly cold, she adds.
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Past climate, present climate
What lessons might those ancient data tell about modern climate change, such as rising levels of atmospheric carbon dioxide and melting glaciers? That’s hard to know.
It is difficult to make direct parallels between then and now, Wellner points out. Carbon dioxide levels in today’s air are well below mid-Cretaceous levels. Still, they are climbing. And continental landmasses have moved over millions of years. They’ve been pushed and pulled along with the tectonic plates beneath them. Those plates are pieces of Earth’s crust that move. Their movements led, in part, to changes in the flow of ocean waters and air.
The study does highlight several features that are important to overall climate. Ice cover is one of them. Whether there’s a lot of ice or a little matters, Wellner says. What role such features might play in the future is not yet clear. Antarctica’s existing ice sheets, for example, could theoretically limit runaway greenhouse warming. And that could happen even as carbon dioxide levels in the atmosphere continue to rise.
Klages agrees. "Ice present on the planet is a big gift," he says. "And [we] should do everything we can to keep it."
Power Words
More About Power Wordsannual: Adjective for something that happens every year.
Antarctica: A continent mostly covered in ice, which sits in the southernmost part of the world.
atmosphere: The envelope of gases surrounding Earth or another planet.
average: (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.
bacteria: (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals). Bacteria are one of the three domains of life on Earth.
carbon: The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules. (in climate studies) The term carbon sometimes will be used almost interchangeably with carbon dioxide to connote the potential impacts that some action, product, policy or process may have on long-term atmospheric warming.
carbon dioxide (or CO2): A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.
chemical: A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
climate: The weather conditions that typically exist in one area, in general, or over a long period.
climate change: Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.
conifer: Cone-bearing trees or shrubs such as the pine, fir, spruce or yew. Conifers are usually evergreen and have either needle-shaped or scale-like leaves.
continent: (in geology) The huge land masses that sit upon tectonic plates. In modern times, there are six established geologic continents: North America, South America, Eurasia, Africa, Australia and Antarctica. In 2017, scientists also made the case for yet another: Zealandia.
core: Something — usually round-shaped — in the center of an object. (in geology) Earth’s innermost layer. Or, a long, tube-like sample drilled down into ice, soil or rock. Cores allow scientists to examine layers of sediment, dissolved chemicals, rock and fossils to see how the environment at one location changed through hundreds to thousands of years or more.
Earth’s crust: The outermost layer of Earth. It is relatively cold and brittle.
equator: An imaginary line around Earth that divides Earth into the Northern and Southern Hemispheres.
forest: An area of land covered mostly with trees and other woody plants.
fossil: Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called "body fossils." Things like footprints are called "trace fossils." Even specimens of dinosaur poop are fossils. The process of forming fossils is called fossilization.
freshwater: A noun or adjective that describes bodies of water with very low concentrations of salt. It’s the type of water used for drinking and making up most inland lakes, ponds, rivers and streams, as well as groundwater.
glacier: A slow-moving river of ice hundreds or thousands of meters deep. Glaciers are found in mountain valleys and also form parts of ice sheets.
greenhouse gas: A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.
ice sheet: A broad blanket of ice, often kilometers deep. Ice sheets currently cover most of Antarctica. An ice sheet also blankets most of Greenland. During the last glaciation, ice sheets also covered much of North America and Europe.
landmass: A continent, large island or other continuous body of land.
marine: Having to do with the ocean world or environment.
parallel: An adjective that describes two things that are side by side and have the same distance between their parts. (In the word "all," the final two letters are parallel lines.) Or two things, events or processes that have much in common if compared side by side.
planet: A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.
pollen: Powdery grains released by the male parts of flowers that can fertilize the female tissue in other flowers. Pollinating insects, such as bees, often pick up pollen that will later be eaten.
potent: An adjective for something (like a germ, poison, drug or acid) that is very strong or powerful.
rainforest: Dense forest rich in biodiversity found in tropical areas with consistent heavy rainfall.
remnant: Something that is leftover — from another piece of something, from another time or even some features from an earlier species.
salt: A compound made by combining an acid with a base (in a reaction that also creates water). The ocean contains many different salts — collectively called "sea salt." Common table salt is a made of sodium and chlorine.
sea: An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.
sediment: Material (such as stones and sand) deposited by water, wind or glaciers.
shrub: A perennial plant that grows in a generally low, bushy form.
tectonic: Surface activity on a large rocky body (such as a planet or moon) as liquid rock flows up to the surface where it solidifies, then slowly drifts atop molten rock, carrying surface features with it.
vegetation: Leafy, green plants. The term refers to the collective community of plants in some area. Typically these do not include tall trees, but instead plants that are shrub height or shorter.
Citations
Journal: J.P. Klages et al. Temperate rainforests near the South Pole during peak Cretaceous warmth. Nature. Vol. 580, April 2, 2020, p. 81. doi: 10.1038/s41586-020-2148-5.
About Carolyn Gramling
Carolyn Gramling is the earth & climate writer at Science News. She has bachelor’s degrees in geology and European history and a Ph.D. in marine geochemistry from MIT and the Woods Hole Oceanographic Institution.
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