For most of the year, the Arctic is covered by sheets of ice. In early summer, these sheets act like a big mirror, reflecting back a sun that never sets. During July, the heat finally eats away at the ice, illuminating algae growing 50 feet below on the seafloor. [Image Credit: Flickr user Christopher Michel | CC BY 2.0]
Fifteen feet beneath the surface of the Bering Sea, thick kelp ensnared Jochen Halfar as he tried to surface. He was diving in frigid waters off the tip of Alaska’s Aleutian Islands to chisel off pieces of a 700-year-old hardened pink plant, called coralline algae, coating the region’s ocean floor. The margin of error for field researchers collecting data in the Arctic, he knew, was slim — access to life-saving medical care is often thousands of miles away. “In the Arctic, an accident is fatal,” he says.
He made it out of the water that day in 2008, but Halfar no longer dives in remote, icy waters. He’s had one too many encounters with a snapping sea lion, heavier than a grizzly bear. Others from his research team still brave the Arctic seas to find and extract the oldest, best-preserved samples of the algae, Clathromorphum compactum, from the watery depths. This bubblegum crust deposits layer after layer of growth each summer, when the sun finally graces the polar sky after its long annual hiatus.
From these samples, scientists can reconstruct sea surface temperatures and ice cover over each year of the plant’s life — a degree of precision that was only guesswork until Halfar, now a geologist at the University of Toronto, started studying the plant almost two decades ago. In the past few years, divers have unearthed algae that started to accrete centuries before the Industrial Revolution. The full collection is providing a clearer picture of what the polar seas looked like prior to the impacts of human activity.
The climate data from these samples are a boon to scientists who aim to understand how the Arctic environment is changing today, and how global climate patterns will continue to shift in the coming century. With more precise climate records, forecasters can better calibrate their models to project changes in the height of sea-level rise in coastal cities, expected hurricane frequency, rainfall intensity, and fishery yields. “That’s what all the governments are interested in,” Halfar says.
Filling in the gaps
People have collected climate records for, at most, the last few hundred years, says Alberto Pérez-Huerta, a geologist at the University of Alabama. But in geological terms, that’s the blink of an eye. It’s not enough information for climate models to predict how weather patterns in different parts of the world will alter as the climate changes, he says.
To reconstruct older climatic history, scientists rely on records preserved in rocks, ice sheets, tree rings, fossils and long-lived creatures, like corals. These stand-ins are called proxies.
Since the 1950s, researchers have ventured aboard large research vessels to collect soil cores from the ocean floor. Buried in the old sediment are fossils of little shelled ameba called foraminifera. These creatures have inhabited the ocean for tens of millions of years. When they die and fall to the bottom of the ocean, they leave behind shells that contain biochemical information about the sea from which they came. Scientists also look at the abundance and species composition of shells to figure out past environmental conditions, since each species thrive under different settings.
“Foraminifera are the canonical approach,” says Fred Andrus, a sclerochronologist — or someone who studies time records from skeletons — also at the University of Alabama. But foraminifera only give a wide-angle view of climate trends that affect the ocean and its life — long-term fluctuations on vast geologic time scales that don’t provide finer details.
“Seasonal variation is really important for climate,” says Pérez-Huerta, “and it one of the most difficult components to reconstruct from the geological records.” To view the ocean’s climate history on a smaller scale, scientists look elsewhere.
Corals are the classic organism to study. “Coral tell a great story, but not over exceedingly deep time, and more or less confined to the tropical belt of the world,” says Andrus. “But most of climate happens outside of that, so there have been a lot of efforts trying to get records at higher latitudes over different time intervals.”
To fill in those gaps, Andrus looks at the shells of snails and oysters, bones from fish ears, and one of the most elderly animals in the world: a clam called Arctica islandica that can live for 500 years. “I study weird proxies,” he says. “A little unorthodox.”
These creatures are unusual to study because they’re so idiosyncratic, Andrus says. “My niche is to study very local records of climate focused on particular time intervals.” He has added coralline algae to his inventory in the last few years. “Where they live are places that are today really sensitive to climate change,” he says. “So, they can give some valuable baseline data to help us understand the magnitude of the changes we’re seeing now.”
From Mexico to the Northwest Passage
Halfar wasn’t anywhere near the Arctic when he first thought to see if coralline algae might provide clues about the region’s past. After a day of fieldwork in the waters off of an island in Baja California, he and a friend decided to investigate the mounds of hard deposits they had collected.
“We split them open with a hammer to see if there was any kind of internal structure,” Halfar says. Inside, they found layers of growth: an annual accrual. His friend, a geochemist, suggested they test the chemistry of these layers. They found that levels of magnesium varied with each layer of growth. When they combined this chemical information with historic climate data, they had a record of climate signals.
Others had studied coralline algae as a stand-in for temperature before — Walter Adey, an emeritus research botanist at the Smithsonian Institute, first began analyzing samples in the 1960s. But he didn’t have a good way of isolating layers of growth from individual years. He did, however, know where the oldest algae grew. That’s why Halfar first approached him, and the two teamed up to voyage to the remote Labrador Sea.
Now, results coming out from their research have reestablished baseline sea-surface temperatures in the Arctic. Halfar and his team extended temperatures from the North Atlantic back to the 1300s, in a paper published in September of last year in Scientific Reports, supplementing previous estimates generated using tree rings and tropical proxies. Another study, from Geophysical Research Letters in April of 2017, questioned what is taken to be normal climate patterns in the North Pacific, derived from instrument data beginning in the 1950s. It showed that these trends are not consistent with long-term natural variability prior to the impacts of carbon pollution.
Building a record of sea ice
Just as important as temperature variations, Halfar’s algae tell the story of ice cover in the Arctic.
When the sun finally melts the Arctic ice and light reaches the sea floor, coralline algae can photosynthesize. “They basically are woken up with a shock, like an alarm in the morning,” Halfar says. That kind of growth makes them unique. “There is no other proxy that reconstructs ice on an annual resolution.”
Halfar’s latest expedition took him off the coast of Svalbard, a series of small glacier-covered islands between Greenland and Russia, where polar bears outnumber human inhabitants. There, his team collected samples of algae to answer the question of just how much ice coverage has changed in that area. His goal is to build a network of sea ice reconstructions throughout the Arctic.
“I’d rather hang out in Mexico,” Halfar says. But he does research as close to the North Pole as he can for a reason — that’s where the algae have the most important story to tell. “Any climate change that’s happening,” he says, “it’s happening in the Arctic.”