The rapid warming and ice loss in the Arctic is among the indisputable facts of global warming, and the basic cause is clear: the buildup of atmospheric greenhouse gases from fossil-fuel burning. But scientists struggling to understand why the Arctic is heating up so much faster than other places have discovered what they believe is one piece of the puzzle.
Pink algae that blooms across the surface of Arctic glaciers and snowfields each summer is absorbing heat instead of reflecting it. And according to a new study published by scientists with the Helmholtz German Research Center for Geosciences, it is a more powerful contributor to the warming feedback loop than previously understood. The algae causes what is known as "watermelon snow" because the ice and snow around it turns pink. It could be responsible for reducing the reflectivity of the snow surface by as much as 13 percent on glaciers and snowfields studied.
The peer-reviewed study was published this week in the journal Nature Communications.
The study is the latest trying to explain what causes "amplification" of Arctic warming. For years, researchers have documented how the shrinking of bright, reflective Arctic ice is exposing more dark-colored ocean and land. Dark surfaces absorb more heat, spurring more Arctic warming. Sea ice, glaciers and ice sheets are also getting darker because of industrial pollution, soot, ship exhaust and smoke from wildfires.
According to the new research, the little-studied algae may be playing a large role as one of these critical darkening agents. The algae turn pink as they reach peak summer bloom. The color comes from pigments that protect the algal cells from ultraviolet radiation. The more the Arctic warms, the more algae grow, which in turn leads to even more darkening and melting.
The research team studied the algae at 40 sites on 16 glaciers and snowfields in different parts of the Arctic and subarctic, from Greenland to Iceland and Svalbard to the north of Sweden. They used light-sensitive instruments to measure how much the algae is reducing the reflectivity of the snow and ice, and found it has cut reflectivity by 13 percent during the melt season. They also analyzed the algae's genetics and found that that the communities are very similar across the Arctic. That enabled the scientists to extrapolate their findings across the region.
"At the moment this bio-albedo effect is not included in any albedo or climate models," said co-author of the study Steffi Lutz, a molecular ecologist at the Helmholtz German Research Center for Geosciences in Potsdam, Germany. "Up to now, climate models have only considered soot from forest fires and Saharan dust as external factors in the meltdown," Lutz said.
Albedo is the scientific name for the reflectivity of a particular surface. Bio-albedo refers to the role of algae and other organisms in reducing the reflectivity of the snow and ice, and Lutz said her study was the first to study bio-albedo on a broad scale.
"This study was focused on glaciers, but ice sheets will be included in the future. We can't see a reason why there wouldn't be a similar effect on ice sheets," she said.
"This is an intriguing finding, and suggests an additional avenue for exploration in understanding why Arctic melt is proceeding so much faster than the models predict," said Michael Mann, a Penn State University climate scientist.
The findings come as the UK's National Environmental Research Council launches a $4.4 million, five-year Black and Bloom research project aimed at quantifying how much algae and microbes speed up the melting of Arctic ice. The goal is to "improve our understanding of how the albedo or reflectivity of the surface of the Greenland ice sheet will change over time," said NERC associate director of research Ned Garnett. It's "critical to inform decisions like building defenses to sea level rise, and agreeing targets for future greenhouse gas emissions," he added.
All the impurities combined speed up melting of Arctic snow and ice, and understanding how they interact is important in projecting how fast sea levels will rise.
The Greenland ice sheet is currently losing about 200 billion tons of ice per year and the rate has doubled since the 1900s. Other factors potentially amplifying the meltdown include changes in the jet stream and the effect of different types of cloud cover above the ice sheets.
Joseph Cook, a climate scientist at the University of Sheffield who is part of the Black and Bloom project, said doing more studies on algae and other surface impurities on a large scale is important. Knowing more will change the equation used for calculating the melt rate of Arctic ice and projecting sea level rise.
"The algae is spatially extensive. It's ubiquitous across the melting surface of the ice," said Cook. His work will include taking detailed readings in algae-covered with a spectrometer to measure changes in the albedo.
Alex Gardner is a researcher with the NASA Jet Propulsion Laboratory who studies how melting ice affects sea level. He is working with the Black and Bloom project to coordinate data from aerial and space observations with information gathered on the ground.
"I think we can definitely expect an increase in biological activity, but how big an impact that will be on the overall reflectivity of the surface is still an open question," he said.
For now, climate experts aren't in agreement about how much of a factor algae are at a global scale of climate change, and some questioned whether the new study's findings could be applied across the Arctic.
The sites used in the study aren't necessarily representative of Arctic terrestrial regions or sea ice as a whole, said Robert Stone, a former researcher with the Cooperative Institute for Research in Environmental Sciences. Stone also said the way the researchers measured the drop in albedo—at a low angle and from just 12 inches above the surface—might skew the results.
"Algae has always been ubiquitous during the spring melt season, on snow fields, glaciers, and all over the Arctic. This is not a new phenomenon. What may be different now and have impact on the climate is an increase in algae growth over time and space in response to a warming climate," he said.
"With the caveat that this isn't my area, this is very unlikely to be that relevant at the global scale," said climatologist Gavin Schmidt, director of the NASA Goddard Institute for Space Studies. "Their results are specific to areas that are already melting and have these algae. That is a very small fraction of the snow cover. I'm all for improving climate models, but this is very niche," Schmidt said.
The big questions may be, how much algae is there and is it expanding over time, said Julienne Stroeve, a senior research scientist with the National Snow and Ice Data Center. "Obviously we do not have long-term data on this," Stroeve said. Existing satellite technology can't tease out the impact of algae and other surface impurities from overall readings of surface reflectivity.
"There's still a lot about the rapid rate of Arctic change that we don't understand," said Mark Serreze, director of the National Snow and Ice Data Center. "Sure, it is warming, which is consistent with sea ice loss, reductions in spring snow cover and strengthening summer melt of the Greenland ice sheet, but pieces of the puzzle are still missing. This could very well be one of those missing pieces."
Correction: An earlier version of this article said that Steffi Lutz is a hydrologist at the Helmholtz Center for Environmental Research. She is a molecular ecologist and works at the Helmholtz German Research Center for Geosciences in Potsdam, Germany.