Warming trends bring more insects, extreme weather and wildfires that wipe out plants
U.S. Forest Service
The Chugach people of southern Alaska’s Kenai Peninsula have picked berries for generations. Tart blueberries and sweet, raspberry-like salmonberries — an Alaska favorite — are baked into pies and boiled into jams. But in the summer of 2009, the bushes stayed brown and the berries never came.
For three more years, harvests failed. “It hit the communities very hard,” says Nathan Lojewski, the forestry manager for Chugachmiut, a nonprofit tribal consortium for seven villages in the Chugach region.
The berry bushes had been ravaged by caterpillars of geometrid moths — the Bruce spanworm (Operophtera bruceata) and the autumnal moth (Epirrita autumnata). The insects had laid their eggs in the fall, and as soon as the leaf buds began growing in the spring, the eggs hatched and the inchworms nibbled the stalks bare.
Chugach elders had no traditional knowledge of an outbreak on this scale in the region, even though the insects were known in Alaska. “These berries were incredibly important. There would have been a story, something in the oral history,” Lojewski says. “As far as the tribe was concerned, this had not happened before.”
At the peak of the multiyear outbreak, the caterpillars climbed from the berry bushes into trees. The pests munched through foliage from Port Graham, at the tip of the Kenai Peninsula, to Wasilla, north of Anchorage, about 300 kilometers away. In summer, thick brown-gray layers of denuded willows, alders and birches lined the mountainsides above stretches of Sitka spruce.
For more than 35 years, satellites circling the Arctic have detected a “greening” trend in Earth’s northernmost landscapes. Scientists have attributed this verdant flush to more vigorous plant growth and a longer growing season, propelled by higher temperatures that come with climate change. But recently, satellites have been picking up a decline in tundra greenness in some parts of the Arctic. Those areas appear to be “browning.”
Like the salmonberry harvesters on the Kenai Peninsula, ecologists working on the ground have witnessed browning up close at field sites across the circumpolar Arctic, from Alaska to Greenland to northern Norway and Sweden. Yet the bushes bereft of berries and the tinder-dry heaths (low-growing shrubland) haven’t always been picked up by the satellites. The low-resolution sensors may have averaged out the mix of dead and living vegetation and failed to detect the browning.
Scientists are left to wonder what is and isn’t being detected, and they’re concerned about the potential impact of not knowing the extent of the browning. If it becomes widespread, Arctic browning could have far-reaching consequences for people and wildlife, affecting habitat and atmospheric carbon uptake and boosting wildfire risk.
The Arctic is warming two to three times as fast as the rest of the planet, with most of the temperature increase occurring in the winter. Alaska, for example, has warmed 2 degrees Celsius since 1949, and winters in some parts of the state, including southcentral Alaska and the Arctic interior, are on average 5 degrees C warmer.
An early effect of the warmer climate was a greener Arctic. More than 20 years ago, researchers used data from the National Oceanic and Atmospheric Administration’s weather satellites to assess a decade of northern plant growth after a century of warming. The team compared different wavelengths of light — red and near-infrared — reflecting off vegetation to calculate the NDVI, the normalized difference vegetation index. Higher NDVI values indicate a greener, more productive landscape. In a single decade — from 1981, when the first satellite was launched, to 1991 — the northern high latitudes had become about 8 percent greener, the researchers reported in 1997 in Nature.
The Arctic ecosystem, once constrained by cool conditions, was stretching beyond its limits. In 1999 and 2000, researchers cataloged the extent and types of vegetation change in parts of northern Alaska using archival photographs taken during oil exploration flyovers between 1948 and 1950. In new images of the same locations, such as the Kugururok River in the Noatak National Preserve, low-lying tundra plants that once grew along the riverside terraces had been replaced by stands of white spruce and green alder shrubs. At some of the study’s 66 locations, shrub-dominated vegetation had doubled its coverage from 10 to 20 percent. Not all areas showed a rise in shrub abundance, but none showed any decrease.
In 2003, Howard Epstein, a terrestrial ecologist at the University of Virginia in Charlottesville, and colleagues looked to the satellite record, which now held another decade of data. Focusing on Alaska’s North Slope, which lies just beyond the crown of the Brooks Range and extends to the Beaufort Sea, the researchers found that the highest NDVI values, or “peak greenness,” during the growing season had increased nearly 17 percent between 1981 and 2001, in line with the warming trend.
Brown among the green
As the Arctic warms up, it’s getting greener, but some pockets have been going brown instead. Satellite imagery and ecologists on the ground have observed browning in the circled areas on this map.
Earth-observing satellites have been monitoring the Arctic tundra for almost four decades. In that time, the North Slope, the Canadian low Arctic tundra and eastern Siberia have become especially green, with thicker and taller tundra vegetation and shrubs expanding northward. “If you look at the North Slope of Alaska, if you look at the overall trend, it’s greening like nobody’s business,” says Uma Bhatt, an atmospheric scientist at the University of Alaska Fairbanks.
Yet parts of the Arctic, including the Yukon-Kuskokwim Delta of western Alaska, the Canadian Arctic Archipelago (the islands north of the mainland that give Canada its pointed tip) and the northwestern Siberian tundra, show extensive browning over the length of the satellite record, from the early 1980s to 2016. “It could just be a reduction in green vegetation. It doesn’t necessarily mean the widespread death of plants,” Epstein says. Scientists don’t yet know why plant growth there has slowed or reversed — or whether the satellite signal is in some way misleading.
“All the models indicated for a long time that we would expect greening with warmer temperatures and higher productivity in the tundra, so long as it wasn’t limited in some other way, like [by lower] moisture,” says Scott Goetz, an ecologist and remote-sensing specialist at Northern Arizona University in Flagstaff. He is also the science team lead for ABoVE, NASA’s Arctic-Boreal Vulnerability Experiment, which is tracking ecosystem changes in Alaska and western Canada. “Many of us were quite surprised … that the Arctic was suddenly browning. It’s something we need to resolve.”
While global warming has propelled widespread trends in tundra greening, extreme winter weather can spur local browning events. In recent years, in some parts of the Arctic, extraordinary warm winter weather, sometimes paired with rainfall, has put tundra vegetation under enormous stress and caused plants to lose freeze resistance, dry up or die — and turn brown.
“As an ecologist, you’re thinking, ‘Whoa! Those plants would usually be nicely insulated under the snow,’ ” he says. “Suddenly, they’re being exposed because all the snow has melted. What are the consequences of that?”
Arctic plants survive frigid winters thanks to that blanket of snow and physiological changes, known as freeze resistance, that allow plants to freeze without damage. But once the plants awaken in response to physical cues of spring — warmer weather, longer days — and experience bud burst, they lose that ability to withstand frigid conditions.
Bjerke, Phoenix and other researchers continue to find brown vegetation in the wake of winter warming events. Long periods of mild winter weather have rolled over the Svalbard archipelago, the cluster of islands in the Arctic Ocean between Norway and the North Pole, in the last decade. The snow melted or blew away, exposing the ground-hugging plants. Some became encrusted in ice following a once-unheard-of midwinter rainfall. In 2015, the Arctic bell heather, whose small white flowers brighten Arctic ridges and heaths, were brown that summer, gray the next and then the leaves fell off. “It’s not new that plants can die during mild winters,” Bjerke says. “The new thing is that it is now happening several winters in a row.”
The weather needn’t always be extreme to harm plants in the Arctic. With warmer winters and summers, leaf-eating insects have thrived, defoliating bushes and trees beyond the insects’ usual range. “They’re very visual events,” says Rachael Treharne, an Arctic ecologist who completed her Ph.D. at the University of Sheffield and now works at ClimateCare, a company that helps organizations reduce their climate impact. She remembers being in the middle of an autumnal moth outbreak in northern Sweden one summer. “There were caterpillars crawling all over the plants — and us. We’d wake up with them in our beds.”
In northernmost Norway, Sweden and Finland in the mid-2000s, successive bursts of geometrid moths defoliated 10,000 square kilometers of mountain birch forest — an area roughly the size of Puerto Rico. The outbreak was one of Europe’s most abrupt and large-scale ecosystem disturbances linked to climate change, says Jane Jepsen, an Arctic ecologist at the Norwegian Institute for Nature Research.
The Arctic is warming more than twice as fast as the rest of the world. The higher temperatures have led to browning in some areas due to:
“These moth species benefit from a milder winter, spring and summer climate,” Jepsen says. Moth eggs usually die at around −30° C, but warmer winters have allowed more eggs of the native autumnal moth to survive. With warmer springs, the eggs hatch earlier in the year and keep up with the bud burst of the mountain birch trees. Another species — the winter moth (O. brumata), found in southern Norway, Sweden and Finland — expanded northward during the outbreak. The spring and summer warmth favored the larvae, which ate more and grew larger, and the resulting hardier female moths laid more eggs in the fall.
While forests that die off can grow back over several decades, some of these mountain birches may have been hammered too hard, Jepsen says. In some places, the forest has given way to heathland. Ecological transitions like this could be long-lasting or even permanent, she says.
Once rare, wildfires may be one of the north’s main causes of browning. As grasses, shrubs and trees across the region dry up, they are being set aflame with increasing frequency, with fires covering larger areas and leaving behind dark scars. For example, in early 2014 in the Norwegian coastal municipality of Flatanger, sparks from a power line ignited the dry tundra heath, destroying more than 100 wooden buildings in several coastal hamlets.
Sparsely populated places, where lightning is the primary cause of wildfires, are also seeing an uptick in wildfires. Scientists say lightning strikes are becoming more frequent as the planet warms. The number of lightning-sparked fires has risen 2 to 5 percent per year in Canada’s Northwest Territories and Alaska over the last four decades, earth system scientist Sander Veraverbeke of Vrije Universiteit Amsterdam and his colleagues reported in 2017 in Nature Climate Change.
In 2014, the Northwest Territories had 385 fires, which burned 34,000 square kilometers. The next year, 766 fires torched 20,600 square kilometers of the Alaskan interior — accounting for about half the total area burned in the entire United States in 2015.
In the last two years, wildfires sent plumes of smoke aloft in western Greenland (SN: 3/17/18, p. 20) and in the northern reaches of Sweden, Norway and Russia, places where wildfires are uncommon. Wildfire activity within a 30-year period could quadruple in Alaska by 2100, says a 2017 report in Ecography. Veraverbeke expects to see “more fires in the Arctic in the future.”
The loss of wide swaths of plants could have wide-ranging local effects. “These plants are the foundation of the terrestrial Arctic food webs,” says Isla Myers-Smith, a global change ecologist at the University of Edinburgh. The shriveled landscapes can leave rock ptarmigan, for example, which rely heavily on plants, without enough food to eat in the spring. The birds’ predators, such as the arctic fox, may feel the loss the following year.
The effects of browning may be felt beyond the Arctic, which holds about half of the planet’s terrestrial carbon. The boost in tundra greening allows the region to store, or “sink,” more carbon during the growing season. But carbon uptake may slow if browning events continue, as expected in some regions.
Treharne, Phoenix and colleagues reported in February in Global Change Biology that on the Lofoten Islands in northern Norway, extreme winter conditions cut in half the heathlands’ ability to trap carbon dioxide from the atmosphere during the growing season.
Yet there’s still some uncertainty about how these browned tundra ecosystems might change in the long-term. As the land darkens, the surface absorbs more heat and warms up, threatening to thaw the underlying permafrost and accelerate the release of methane and carbon dioxide. Some areas might switch from being carbon sinks to carbon sources, Phoenix warns.
On the other hand, other plant species — with more or less capacity to take up carbon — could move in. “I’m still of the view that [these areas] will go through these short-term events and continue on their trajectory of greater productivity,” Goetz says.
A better view
The phenomena that cause browning events — extreme winter warming, insect outbreaks, wildfires — are on the rise. But browning events are tough to study, especially in winter, because they’re unpredictable and often occur in hard-to-reach areas.
At a couple of recent workshops on Arctic browning, remote-sensing experts and ecologists tried to tackle the problem. “We’ve been talking about how to bring the two scales together,” Bhatt says. New sensors, more frequent snapshots, better data access and more computing power could help scientists zero in on the extent and severity of browning in the Arctic.
Researchers have begun using Google Earth Engine’s massive collection of satellite data, including Landsat images at a much better resolution of 30 meters by 30 meters per pixel. Improved computational capabilities also enable scientists to explore vegetation change close up. The European Space Agency’s recently launched Sentinel Earth-observing satellites can monitor vegetation growth with a pixel size of 10 meters by 10 meters. Says Myers-Smith: “That’s starting to get to a scale that an ecologist can grapple with.”
This article appears in the April 13, 2019 issue of Science News with the headline, "Arctic Browning: Warming trends bring insects, extreme weather and wildfires to land that had been going green."
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