Global warming is transforming the Arctic, and the changes have rippled so widely that the entire biophysical system is shifting toward an "unprecedented state," an international team of researchers concludes in a new analysis of nearly 50 years of temperature readings and changes across the ecosystems.
Arctic forests are turning into bogs as permafrost melts beneath their roots. The icy surface that reflects the sun's radiation back into space is darkening and sea ice cover is declining. Warmth and moisture trapped by greenhouse gases are pumping up the water cycle, swelling rivers that carry more sediment and nutrients to the sea, which can change ocean chemistry and affect the coastal marine food chain. And those are just a few of the changes.
The researchers describe how warming in the Arctic, which is heating up 2.4 times faster than the Northern Hemisphere average, is triggering a cascade of changes in everything from when plants flower to where fish and other animal populations can be found.
Together, the changes documented in the study suggest the effects on the region are more profound than previously understood.
"What stands out for me is an intensified hydrological system," said Jason Box, a climate scientist with the Geological Survey of Denmark and Greenland and lead author of the study, published today in the scientific journal Environmental Research Letters.
Warmer temperatures have shifted forest and tundra growing seasons, boosted rain and snowfall, increased melting, accelerated glaciers and possibly even increased the number of lightning strikes that could increase the risk of Arctic wildfires in the tundra and boreal forest, Box said. "I think this is a clear signal due to climate warming," he said.
Following are snapshots of some of the changes underway across the region.
Bering Sea 'In a State We've Never Seen Before'
Commercial fishers and the indigenous population of the Bering Sea region are feeling how Arctic change is spilling out of the polar region.
During two consecutive years of record-low sea ice, coastal communities lost the ice buffer that protects the land from winter storm surges. Pollock and cod, two valuable fish species, may be running out of spawning habitat in the Bering Sea, and it's not clear they've found a replacement area.
Less sea ice and warming farther north have a domino effect in the Bering Sea, said Jim Overland, a climate researcher with the National Oceanic and Atmospheric Administration.
"In the past, you had sea ice growing in the fall, with northerly winds that helped grow ice. Now, with the delay of Arctic-wide freeze-up, you don't have the pre-conditioning for the Bering freeze-up. Combined with unusual storm systems, you can get these off-the-charts changes in the Bering Sea," said Overland, a co-author of the study.
"Last year, with no sea ice and no pool of deep, cold water, pollock were found in the north Bering Sea where they don't usually go. The question was if they will they spawn in the new location or not, and it doesn't seem that they did," he said. "When this happens two years in a row, it becomes really important. The Bering Sea is now in a state we've never seen before."
The new paper helps to show how the Arctic is a connected system affected by global warming, said National Snow and Ice Data Center scientist Twila Moon, who was not involved in the study.
"It's causing coastal erosion that eats away at community land, and, in some cases, causes building and infrastructure loss," she said. "These Arctic changes are also affecting people and communities far from the Arctic." Coastal flooding in the U.S., for example, is worsened by sea level rise that is fed by melting Arctic ice sheets and glaciers.
Warm Winters Put Spring Closer to Melting Point
Looking at temperature changes across the seasons, the researchers documented an Arctic that is warming 2.8 times faster than the rest of the Northern Hemisphere in the cold season, and 1.7 times faster in the warmer months.
The higher rate of cold season warming can be traced to the delayed freeze-up of sea ice, Box said. The relatively warm (compared to ice) ocean water increases moisture in the atmosphere, forming clouds that trap warmth near the surface. The warming in the cold season reduces the overall "cold content" in the Arctic, like leaving the freezer door open. When spring starts, snow, ice and permafrost are already closer to the melting point, he said.
Thawing permafrost creates another climate risk: As long-frozen organic material starts to decompose, it releases methane, a potent short-lived climate pollutant, as well as CO2, both of which contribute to more warming.
In recent years, scientists have measured record-high annual average temperatures in the top 10 to 20 meters of permafrost at many measuring sites, with the biggest warmup in the coldest parts of the northern Arctic. At three sites on Alaska's North Slope, data in the study show that the freeze-up of the active permafrost layer (which thaws in summer and freezes in winter) now comes two months later than it did in the mid-1980s.
Changing Flowering Times and Snow Cover
The study found "strong evidence that the summer warming trend is causing an earlier and more condensed flowering period of key plant species," leading to mismatches between plants and pollinators, as well as making some plants more vulnerable to harmful insects.
Over time, that could fundamentally change the composition of Arctic vegetation, which in turn would affect animals that depend on those plants for food.
The data also contain widespread evidence that snow cover has been declining in the Arctic at a rate of two to four days per decade over the past 30 to 40 years. The trend is stronger the farther north and the higher up you go, Box said. Most of the decline is due to earlier snowmelt in spring, but a later start to the snow season is a factor in some areas, particularly in the eastern Canadian Arctic.
Overall, spring (May and June) snow cover extent has decreased by more than 30 percent since 1971, with evidence of increased ice-layer development in some parts of the Arctic because of more frequent winter thaw and rain events. A decline in the snow cover outside the growing season can make plants more vulnerable to extreme winter temperatures.
Loss of Sea Ice Also Has Ripple Effects
The decline of sea ice is one of the most closely tracked indicators of Arctic change. The new paper describes how, over the past half century, it has shifted "from an environment dominated by thick multi-year sea ice to one dominated by thinner first-year sea ice, with an earlier start to the melt season and a later start to the freeze-up."
A study published April 2 in Scientific Reports digs into one of the ways global warming is affecting sea ice formation and transport.
Off the coast of Russia, sea ice forms as cold winds blowing off the big landmass chill the water. At the same time, the winds push the newly formed ice near the shore northward toward the central Arctic. Over months, those drifting floes pile up to form thick ice that can last through the summer. But with a warmer atmosphere and ocean, more of that newly formed ice melts before it gets out of the formation region, said Thomas Krumpen, a sea ice physicist with the Alfred Wegener's Institute, who scoured satellite images to show changes in the transpolar drift current.
The breakdown of the ice transport will have impacts on Arctic Ocean ecosystems because the ice formed near shorelines carries with it minerals and tiny biological organisms, including plankton and algae, "like frozen spinach packed in ice," said Eva-Maria Nöthig, an AWI oceanographer who studies the biology of the Arctic Ocean.
"The ice floes with all these particles inside are getting thinner," she said. "All the organisms, from fish at the surface to benthic organisms 4,000 meters deep, who need the sea ice for their development will be gone."