Scientists say changes in the amounts of wildfire smoke mixing with clouds over the Arctic Ocean may play a key role in determining how fast the Arctic sea ice cover will shrink during the next few decades, and when it will disappear completely in summer.
A climate modeling study published Wednesday in Science Advances took a close look at the complex interaction of wildfire aerosol pollution, clouds and sea ice from 1997 to 2014, and its findings suggest that Arctic sea ice melts more during summers with relatively low fire activity.
The tiny aerosol particles generated by wildfires are carried around the northern hemisphere by winds and sometimes swirl into the Arctic, where they can seed water droplets and brighten clouds, increasing the amount of light and heat they reflect away from the Earth.
So clouds over the Arctic Ocean may be thinner and less reflective in years with fewer fires, leading to more surface heating and sea ice melt, said study co-author Alexandra Jahn, a polar climate researcher at the University of Colorado Boulder’s Institute of Alpine and Arctic Research.
In turn, what happens with the sea ice also affects wildfires, with research showing that ice loss causes regional warming of land and sea surfaces, leading to hotter, drier and more fire-prone conditions in the Western U.S.
“We can’t study or think about climate change in isolation because everything is interconnected,” Jahn said. “The warming that is affecting Arctic sea ice loss is due to the emission of greenhouse gases, and things don’t stay where they are emitted, they get transported around. That’s what we see here, in this new study.”
“Previous work has shown that the sea ice loss in the Arctic has a connection leading to more wildfires in the U.S. like here in Colorado, and in the Midwest,” she said, “so now we’re also having a connection from the fires here, as well as in Russia and Northern Europe and Asia, back into the Arctic.”
Jahn said the effect of wildfire aerosols on sea ice could be described as a negative feedback, “because when we have fewer fires, Arctic sea ice loss will be accelerated, and if we have more fires, it doesn’t accelerate it,” Jahn said.
The rate of sea ice loss was about 13.3 percent per decade in the early part of the study period, from 1993 to 2006, corresponding roughly with a period of relatively fewer fires. In the increasingly fiery 14 years after that, from 2007 to 2020, sea ice loss was about 4 percent per decade, less than one-third the rate of the earlier period.
Even with the new knowledge about the effects of wildfire aerosols, it’s not possible to predict the exact year that all the Arctic sea ice will melt. A 2018 study found that “there is a 20-year uncertainty in when that first ice-free Arctic will happen,” she said.
The Arctic Ocean is likely to have its first ice-free period by 2050, she said. But people still have a chance to determine what happens to the sea ice, even after that, with their choices about future greenhouse gas emissions determining how often and how long the Arctic Ocean will be ice free after 2050, she added.
With immediate steep emissions cuts, the ice-free time could be limited to just a few weeks in September. But if emissions keep increasing, “We could potentially have five months of ice-free Arctic at the end of the century,” she said. “That would be a huge, huge change to what we’re having right now.”
Once the Arctic sea ice fully melts, “I think it really shows we’ve kind of passed this point where we can kind of keep things normal,” said Alice DuVivier, a climate scientist with the National Center for Atmospheric Research who was not involved in the new study.
“It’s never happened in human experiences for tens of thousands of years,” she said. “We don’t really know what this looks like and what the effects will be. So honestly, I find it kind of terrifying, even though I know logically we’re on this trajectory.”
An ice-free Arctic Ocean will intensify the spiral of melting and warming.
“The problem is once these sorts of things happen, you get feedbacks in the climate system,” she said.
Once the ice melts entirely in the Arctic seas, it increases the amount of time the water can absorb sunlight to warm the upper ocean, she said. That makes it harder to grow sea ice the following winter. For many of the 4 million people who live in the Arctic, the decline of the sea ice is creating existential challenges now, she added.
“They’re dealing with things like eroding coastlines, because there is no sea ice along the coast to buffer waves,” she said.
That’s left Arctic communities asking, “‘Do we move our village or not,’” she said. “”And if we do, where do we move that’s safe?’”
Similar questions are going to come up in other places as changes in the Arctic ripple across the globe, she said. Recent research has shown that reduced sea ice can affect the paths of storms that affect North America and Europe, and contribute to deadly climate extremes like supercharged rainstorms, droughts and heat waves.
“Communities in the lower 48 are more distant to the changes in the Arctic,” she said, “but some of those same insecurities are going to come up about, like, ‘Is this a safe place to live? Where could we move that would be safe?’”
Sea ice decline also affects the entire Arctic food web, including polar cod, which drift with the ice, eating the algae that grows in its cracks. Polar bears, walruses, Arctic foxes and seals also all thrive in sea ice habitat, and are threatened by its decline.
Collaboration Fine-tunes the Models
DuVivier said the new study stands out for its interdisciplinary scientific collaboration.
“It’s a combination of sea ice, cloud, aerosol experts—that’s something that’s really good,” she said. “You really need that combination to understand how the climate is going to work. Climate modeling really takes a village.”
The new study can help scientists make more accurate projections about Arctic climate change. Now, most climate models use wildfire emissions scenarios that don’t accurately represent year-to-year variability, she said.
“We don’t look at how aerosols changed dramatically from year to year, and we know those things vary from year to year in the real world,” she said. “So I think this study highlights that we need to account for some of that.”
The study’s lead author, Patricia DeRepentigny, also with Institute of Arctic and Alpine Research at the University of Colorado, said the findings could help narrow down some of the uncertainties about the meltdown of Arctic sea ice. Other research has shown that sea ice has dwindled faster than would be expected from the global temperature increase alone, she said.
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Since 1979, September sea ice extent has dropped by 45 percent and its thickness has declined by 66 percent. “However this loss has not occurred at the same rate year after year,” the authors wrote. “September sea ice loss was largest in the early 21st century.”
The variability of the wildfire aerosol effect could be an important factor in explaining the change. But it’s also possible that it’s part of a natural cycle of variability, with some research showing that sea ice loss could be negligible for as long as 20 years, even as global temperatures keep rising, DeRepentigny said.
One of the things the study highlights, she said, is the uncertainty about how aerosol emissions are used in climate models.
“This, honestly, is something that has been overlooked in the past in the science community in terms of the type of uncertainty in climate projections,” she said. “The key here is that this is not a simple linear relationship.”