Arctic sea ice shrinks to its second-lowest annual minimum extent ever


Arctic sea ice reached a minimum extent of 3.74 million square kilometers on September 15, likely the annual low, and the second lowest minimum on record. The orange line represents average extent of sea ice from 1981 to 2010 on that day. (NSIDC)

Arctic sea ice extent fell to the second-lowest annual minimum on record last week, a status that followed a summer of extreme heat in Siberia and accelerated melt even after summer’s end, the National Snow and Ice Data Center announced on Monday.

A report by Yereth Rosen said (Arctic Today, September 21, 2020):

Arctic sea ice extent bottomed out at 3.74 million square kilometers (1.44 million square miles) on September 15. It was only the second time in the satellite record that the minimum was below 4 million square kilometers; the record-low sea ice minimum was measured in 2012, when mid-September ice extent fell to 3.39 million square kilometers (1.13 million square miles).

Breaking that 4 million-square-kilometer threshold is likely to get attention, said Mark Serreze, director of the Colorado-based NSIDC.

“It’s only a number, but it’s only the second time it’s happened,” he said.

This year’s minimum was 2.51 million square kilometers (969,000 square miles) lower than the average annual minimums calculated from 1981 to 2010, the Colorado-based NSIDC said.

Extent is defined as the area of the ocean where there is at least 15 percent ice cover.

The 2020 ice retreat was part of a well-defined pattern. The 14 lowest minimums for sea ice extent have occurred in the last 14 years, according to NSIDC information.

“That is rather telling,” Serreze said. “The story is, we are in this new age.”

The “strong downward trend” in Arctic sea ice, despite some year-to-year variation, will ultimately result in an Arctic with no more summer ice, he said. That is expected to happen sometime in the next 10 to 20 years, he said.

Among the year-to-year variations is the changing location of extreme annual melt. Siberia turned out to be the hotspot this year, Serreze said; in other years different parts of the Arctic have the most extreme melt.

The Siberian heatwave, with temperatures hitting the first-recorded 100-degree Fahrenheit mark and massive Arctic wildfires burning, was part of a feedback cycle. Itself a product of Arctic climate change, according to scientists, it also contributed to Arctic climate change.

To some extent, this year’s extreme melt off Siberia was set up by winter conditions there, which made the ice there thin, Serreze said. “Once the melt started going, it had those self-perpetuating tendencies,” he said.

Those winter conditions were set up in part by a persistent positive Arctic Oscillation pattern that pushed ice from the Siberian coast, Serreze said.

The Arctic Oscillation is in a positive phase when there is lower-than-average air pressure over the Arctic. Recent research by an international team of scientists links a positive Arctic Oscillation to late-winter heat in Siberia — and increased risk of Siberian wildfires.

The Siberian Arctic was not the only place where ice melt was extreme.

In the Chukchi Sea, where early summer ice extent was not unusually low by recent years’ standards, melt was dramatic in late summer. That resulted in the earliest ice-free state in the Chukchi in the satellite record.

Retreat in the Chukchi was pushed along by a late-July storm that chewed up whatever freeze remained at the time, said Rick Thoman of the Alaska Center for Climate Assessment and Policy. “I don’t think we would have been that precipitous decline without the end-of-July storm,” he said.

Having so much open water there will again make it more difficult for the winter freeze to set in, Thoman said, though sea-surface temperatures are not as warm as they had been in the past. “It’s going to be a late freeze up in the Chukchi,” he said. “If the weather was to cooperate, it might not be super-late.”

That large amount of open water is also expected to continue a pattern of increasingly warm autumns off Alaska, he said.

The declaration of a minimum is preliminary, the NSIDC said. A shift in wind patterns or some sort of late melt could reduce the total ice extent again, the center said.

A green Arctic

Earth’s northern landscapes are greening.

Using satellite images to track global tundra ecosystems over decades, a new study found the Arctic region has become greener, as warmer air and soil temperatures lead to increased plant growth.

“The Arctic tundra is one of the coldest biomes on Earth, and it’s also one of the most rapidly warming,” said Logan Berner, a global change ecologist with Northern Arizona University in Flagstaff, who led the recent research. “This Arctic greening we see is really a bellwether of global climatic change – it’s a biome-scale response to rising air temperatures.”

The study (Logan T. Berner, Richard Massey, Patrick Jantz, Bruce C. Forbes, Marc Macias-Fauria, Isla Myers-Smith, Timo Kumpula, Gilles Gauthier, Laia Andreu-Hayles, Benjamin V. Gaglioti, Patrick Burns, Pentti Zetterberg, Rosanne D’Arrigo, Scott J. Goetz, Summer warming explains widespread but not uniform greening in the Arctic tundra biome, Nature Communications, 2020; 11 (1) DOI: 10.1038/s41467-020-18479-5), published this week in Nature Communications, is the first to measure vegetation changes spanning the entire Arctic tundra, from Alaska and Canada to Siberia, using satellite data from Landsat, a joint mission of NASA and the U.S. Geological Survey (USGS). Other studies have used the satellite data to look at smaller regions, since Landsat data can be used to determine how much actively growing vegetation is on the ground. Greening can represent plants growing more, becoming denser, and/or shrubs encroaching on typical tundra grasses and moss.

When the tundra vegetation changes, it impacts not only the wildlife that depend on certain plants, but also the people who live in the region and depend on local ecosystems for food. While active plants will absorb more carbon from the atmosphere, the warming temperatures could also be thawing permafrost, thereby releasing greenhouse gasses. The research is part of NASA’s Arctic Boreal Vulnerability Experiment (ABoVE), which aims to better understand how ecosystems are responding in these warming environments and the broader social implications.

Berner and his colleagues used the Landsat data and additional calculations to estimate the peak greenness for a given year for each of 50,000 randomly selected sites across the tundra. Between 1985 and 2016, about 38% of the tundra sites across Alaska, Canada, and western Eurasia showed greening. Only 3% showed the opposite browning effect, which would mean fewer actively growing plants. To include eastern Eurasian sites, they compared data starting in 2000, when Landsat satellites began regularly collecting images of that region. With this global view, 22% of sites greened between 2000 and 2016, while 4% browned.

“Whether it’s since 1985 or 2000, we see this greening of the Arctic evident in the Landsat record,” Berner said. “And we see this biome-scale greening at the same time and over the same period as we see really rapid increases in summer air temperatures.”

The scientists compared these greening patterns with other factors, and found that it’s also associated with higher soil temperatures and higher soil moisture. They confirmed these findings with plant growth measurements from field sites around the Arctic.

“Landsat is key for these kinds of measurements because it gathers data on a much finer scale than what was previously used,” said Scott Goetz, a professor at Northern Arizona University who also worked on the study and leads the ABoVE Science Team. This allows the researchers to investigate what is driving the changes to the tundra. “There’s a lot of microscale variability in the Arctic, so it’s important to work at finer resolution while also having a long data record,” Goetz said. “That’s why Landsat is so valuable.”

The study report said:

Arctic warming can influence tundra ecosystem function with consequences for climate feedbacks, wildlife and human communities. Yet ecological change across the Arctic tundra biome remains poorly quantified due to field measurement limitations and reliance on coarse-resolution satellite data. Here, we assess decadal changes in Arctic tundra greenness using time series from the 30 m resolution Landsat satellites. From 1985 to 2016 tundra greenness increased (greening) at ~37.3% of sampling sites and decreased (browning) at ~4.7% of sampling sites. Greening occurred most often at warm sampling sites with increased summer air temperature, soil temperature, and soil moisture, while browning occurred most often at cold sampling sites that cooled and dried. Tundra greenness was positively correlated with graminoid, shrub, and ecosystem productivity measured at field sites. Our results support the hypothesis that summer warming stimulated plant productivity across much, but not all, of the Arctic tundra biome during recent decades.

The scientists specifically asked:

  1. To what extent did tundra greenness change during recent decades in the Arctic?
  2. How closely did inter-annual variation in tundra greenness track summer temperatures?
  3. Were tundra greenness trends linked with climate, permafrost, topography, and/or fire?
  4. How closely did satellite observations of tundra greenness relate to temporal and spatial variation in plant productivity measured at field sites?

The report said:

Our analysis showed strong increases in average tundra greenness and summer air temperatures during the past three decades in the Arctic and constituent Arctic zones.

The scientists found widespread greening in recent decades that was linked with increasing summer air temperatures, annual soil temperatures, and summer soil moisture; however, tundra greenness had no significant trend in many areas and even declined in others.

The study and prior regional Landsat assessments show pronounced greening in northern Quebec.

Their analysis also indicated recent browning along the rugged southwestern coast of Greenland that is consistent with local declines in shrub growth.

The study report said:

Overall, satellites images show extensive greening and modest browning in the Arctic tundra biome during recent decades; however, regional discrepancies in greening and browning highlight the need for rigorous comparisons among satellites and between satellite and field measurements.

The report said:

We found no trend in tundra greenness at most locations, despite pervasive increases in summer air temperatures. It is possible that indirect drivers of vegetation change, such as permafrost thaw and nutrient release, are accumulating in response to warming of summer air temperatures, or that plants are limited by other environmental constrains. Low soil temperatures, nutrients, and moisture can limit plant response to rising air temperatures, as can strong genetic adaptation to prevailing environmental conditions. In other cases, warming might have stimulated plant growth, but led to no change in tundra greenness due to grazing, browsing, and trampling by herbivores. Field and modeling studies show that herbivory can significantly suppress tundra response to warming, although effects of vertebrate and invertebrate herbivores on Arctic greening and browning remain unclear. Last, tundra greenness could, in some areas, be confounded by patchy vegetation being interspersed with bare ground, surface water, or snow.

It said:

“Our results indicate Arctic plants did not universally benefit from warming in recent decades, highlighting diverse plant community responses to warming likely mediated by a combination of biotic and abiotic factors.”

“Our analysis showed that tundra browning occurred at a small percentage (~5%) of sampling sites during recent decades, and although uncommon, it was widely distributed in the Arctic.”

“Our analysis suggests that warming tended to promote rather than suppress plant productivity and biomass in the Arctic during recent decades, but increasing frequency of permafrost degradation, extreme weather events, pest outbreaks, and industrial development could contribute to future browning.”

“Tundra fires are another contributor to greening and browning in the Arctic; however, our results indicate that their contribution is currently quite small at a pan-Arctic extent due to their infrequent occurrence.”

“We found that 1.1% of sampling sites burned over the 16 years period, which suggests a current fire rotation of ~1450 years for the Arctic tundra biome. Regional fire rotation within the biome is strongly governed by summer climate and is considerably shorter (~425 years) in the warmest and driest tundra regions (e.g., Noatak and Seward, Alaska).”

“Our analysis further showed that fires recently occurred at ~1.0% of sampling sites that greened and ~2.4% of sampling sites that browned. Tundra fires can emit large amounts of carbon into the atmosphere and lead to temporary browning by burning off green plants, while subsequent increases in soil temperature and permafrost active layer depth can stimulate a long-term increase in plant growth and shrub dominance in some but not all cases. Continued warming will likely increase annual area burned in the tundra biome; thus, fires could become a more important driver of tundra greening and browning in the Arctic over the twenty-first century.”

“Our analysis contributes to a growing body of evidence showing recent widespread changes in the Arctic environment that can impact climate feedbacks. Rising temperatures are likely stimulating carbon uptake and storage by plants in areas that are greening (negative climate feedback), but also leading to soil carbon loss by thawing permafrost and enhancing microbial decomposition (positive climate feedback). Moreover, greening can reduce surface albedo as plants grow taller and leafier (positive climate feedback) while also affecting soil carbon release from permafrost thaw by altering canopy shading and snow-trapping (mixed climate feedbacks). The net climate feedback of these processes is currently uncertain; thus, our findings underscore the importance of future assessments with Earth system models that couple simulations of permafrost, vegetation, and atmospheric dynamics at moderately high spatial resolution.”

“Widespread tundra greening can also affect habitat suitability for wildlife and semi-domesticated reindeer, with consequences for northern subsistence and pastoral communities. As an example, moose and beavers recently colonized, or recolonized, increasingly shrubby riparian habitats in tundra ecosystems of northern Alaska and thus appear to be benefiting from recent tundra greening. Conversely, caribou populations in the North American Arctic could be adversely affected if warming stimulates vascular plant growth at the expense of lichens, an important winter forage. In the western Eurasian Arctic, indigenous herders (e.g., Sami, Nenets) manage about two million semi-domesticated reindeer on tundra rangelands. Shrub growth, height, and biomass significantly increased on these rangelands in recent decades, while lichen cover and biomass declined mostly due to trampling during the snow-free period.”

“Our analysis showed tundra greening in regions with potential moose, beaver, caribou, and reindeer habitat and demonstrated that variability in tundra greenness was often associated with annual shrub growth in these regions. Many northern communities rely on subsistence hunting or herding and thus changes in wildlife or herd populations can influence food security and dietary exposure to environmental contaminants. By documenting the extent of recent greening, analyses such as ours can help identify where wildlife and northern communities might be most impacted by ongoing changes in vegetation.”

“In summary, we assessed pan-Arctic changes in tundra greenness, and found evidence to support the hypothesis that recent summer warming contributed to increasing plant productivity and biomass across substantial portions of the Arctic tundra biome during the past three decades.”

“We also document summer warming in many areas that did not become greener. The lack of greening in these areas points towards lags in vegetation response and/or to the importance of other factors in mediating ecosystem response to warming. Sustained warming may not drive persistent greening in the Arctic over the twenty-first century for several reasons, particularly hydrological changes associated with permafrost thaw, drought, and fire. Overall, our high spatial resolution pan-Arctic assessment highlights tundra greening as a bellwether of global climatic change that has wide-ranging consequences for life in northern high-latitude ecosystems and beyond.”



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