Climate

By 2100, Greenland will be shedding ice faster than at any time in the past 12,000 years, scientists report October 1 in Nature.
Since the 1990s, Greenland has shed its ice at an increasing rate (SN: 8/2/19). Meltwater from the island’s ice sheet now contributes about 0.7 millimeters per year to global sea level rise (SN: 9/25/19). But how does this rapid loss stack up against the ice sheet’s recent history, including during a 3,000-year-long warm period?
Glacial geologist Jason Briner of the University at Buffalo in New York and colleagues created a master timeline of ice sheet changes spanning nearly 12,000 years, from the dawn of the Holocene Epoch 11,700 years ago and projected out to 2100.
The researchers combined climate and ice physics simulations with observations of the extent of past ice sheets, marked by moraines. Those rocky deposits denote the edges of ancient, bulldozing glaciers. New fine-tuned climate simulations that include spatial variations in temperature and precipitation across the island also improved on past temperature reconstructions.
During the past warm episode from about 10,000 to 7,000 years ago, Greenland lost ice at a rate of about 6,000 billion metric tons each century, the team estimates. That rate remained unmatched until the past two decades: From 2000 to 2018, the average rate of ice loss was similar, at about 6,100 billion tons per century.
Over the next century, that pace will accelerate, the team says. How much depends on future greenhouse gas emissions: Under a lower-emissions scenario, ice loss is projected to average around 8,800 billion tons per century by 2100. With higher emissions, the rate of loss could ramp up to 35,900 billion tons per century.
Lower emissions could slow the loss, but “no matter what humanity does, the ice will melt this century at a faster clip than it did during that warm period,” Briner says. More

How is melting a continent-sized ice sheet like stirring milk into coffee? Both are, for all practical purposes, irreversible.
In a new study published in the Sept. 24 Nature, researchers outline a series of temperature-related tipping points for the Antarctic Ice Sheet. Once each tipping point is reached, changes to the ice sheet and subsequent melting can’t be truly reversed, even if temperatures drop back down to current levels, the scientists say.
The full mass of ice sitting on top of Antarctica holds enough water to create about 58 meters of sea level rise. Although the ice sheet won’t fully collapse tomorrow or even in the next century, Antarctic ice loss is accelerating (SN: 6/13/18). So scientists are keen to understand the processes by which such a collapse might occur.
“What we’re really interested in is the long-term stability” of the ice, says Ricarda Winkelmann, a climate scientist at Potsdam Institute for Climate Impact Research in Germany. In the new study, Winkelmann and her colleagues simulated how future temperature increases can lead to changes across Antarctica in the interplay between ice, oceans, atmosphere and land.

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In addition to direct melting due to warming, numerous processes linked to climate change can speed up overall melting, called positive feedbacks, or slow it down, known as negative feedbacks.
For example, as the tops of the ice sheets slowly melt down to lower elevations, the air around them becomes progressively warmer, speeding up melting. Warming temperatures also soften the ice itself, so that it slides more quickly toward the sea. And ocean waters that have absorbed heat from the atmosphere can transfer that heat to the vulnerable underbellies of Antarctic glaciers jutting into the sea, eating away at the buttresses of ice that keep the glaciers from sliding into the sea (SN: 9/11/20). The West Antarctic Ice Sheet is particularly vulnerable to such ocean interactions — but warm waters are also threatening sections of the East Antarctic Ice Sheet, such as Totten Glacier (SN: 11/1/17).
In addition to these positive feedbacks, climate change can produce some negative feedbacks that delay the loss of ice. For example, warmer atmospheric temperatures also evaporate more ocean water, adding moisture to the air and producing increased snowfall (SN: 4/30/20).
The new study suggests that below 1 degree Celsius of warming relative to preindustrial times, increased snowfall slightly increases the mass of ice on the continent, briefly outpacing overall losses. But that’s where the good news ends. Simulations suggest that after about 2 degrees Celsius of warming, the West Antarctic Ice Sheet will become unstable and collapse, primarily due to its interactions with warm ocean waters, increasing sea levels by more than 2 meters. That’s a warming target that the signatories to the 2015 Paris Agreement pledged not to exceed, but which the world is on track to surpass by 2100 (SN: 11/26/2019).
As the planet continues to warm, some East Antarctic glaciers will follow suit. At 6 degrees Celsius of warming, “we reach a point where surface processes become dominant,” Winkelmann says. In other words, the ice surface is now at low enough elevation to accelerate melting. Between 6 and 9 degrees of warming, more than 70 percent of the total ice mass in Antarctica is loss, corresponding to an eventual sea level rise of more than 40 meters, the team found.
Those losses in ice can’t be regained, even if temperatures return to preindustrial levels, the study suggests. The simulations indicate that for the West Antarctic Ice Sheet to regrow to its modern extent, temperatures would need to drop to at least 1 degree Celsius below preindustrial times.
“What we lose might be lost forever,” Winkelmann says.
There are other possible feedback mechanisms, both positive and negative, that weren’t included in these simulations, Winklemann adds — either because the mechanisms are negligible or because their impacts aren’t yet well understood. These include interactions with ocean-climate patterns such as the El Niño Southern Oscillation and with ocean circulation patterns, including the Atlantic Meridional Overturning Circulation.
Previous research suggested that meltwater from the Greenland and Antarctic ice sheets might also play complicated feedback roles. Nicholas Golledge, a climate scientist with Victoria University of Wellington in New Zealand, reported in Nature in 2019 that flows of Greenland meltwater can slow ocean circulation in the Atlantic, while cold, fresh Antarctic meltwater can act like a seal on the surface ocean around the continent, trapping warmer, saltier waters below, where they can continue to eat away at the underbelly of glaciers.
In a separate study published Sept. 23 in Science Advances, Shaina Sadai, a climate scientist at the University of Massachusetts Amherst, and her colleagues also examined the impact of Antarctic meltwater. In simulations that look out to the year 2250, the researchers found that in addition to a cool meltwater layer trapping warm water below it, that surface layer of freshwater would exert a strong cooling effect that could boost the volume of sea ice around Antarctica, which would in turn also keep the air there colder.
A large plug of such meltwater, such as due to the West Antarctic Ice Sheet’s sudden collapse, could even briefly slow global warming, the researchers found. But that boon would come at a terrible price: rapid sea level rise, Sadai says. “This is not good news,” she adds. “We do not want a delayed surface temperature rise at the cost of coastal communities.”
Because the volume and impact of meltwater is still uncertain, Winkelmann’s team didn’t include this factor. Robert DeConto, an atmospheric scientist also at the University of Massachusetts Amherst and a coauthor on the Science Advances study, notes that the effect depends on how scientists choose to simulate how the ice breaks apart. The study’s large meltwater volumes are the result of a controversial idea known as the marine ice-cliff hypothesis, which suggests that in a few centuries, tall ice cliffs in Antarctica might become brittle enough to suddenly crumble into the ocean like dominoes, raising sea levels catastrophically (SN: 2/6/19).
Despite lingering uncertainties over the magnitude of feedbacks, one emerging theme — highlighted by the Nature paper — is consistent, DeConto says: Once the ice is lost, we can’t go back.
“Even if we get our act together and reduce emissions dramatically, we will have already put a lot of heat into the ocean,” he adds. For ice to begin to grow back, “we’ll have to go back to a climate that’s colder than at the beginning of the Industrial Revolution, sort of like the next ice age. And that’s sobering.” More

New seafloor maps reveal the first clear view of a system of channels that may be helping to hasten the demise of West Antarctica’s vulnerable Thwaites Glacier. The channels are deeper and more complex than previously thought, and may be funneling warm ocean water all the way to the underside of the glacier, melting it from below, the researchers found.
Scientists estimate that meltwater from Florida-sized Thwaites Glacier is currently responsible for about 4 percent of global sea level rise (SN: 1/7/20). A complete collapse of the glacier, which some researchers estimate could happen within the next few decades, could increase sea levels by about 65 centimeters. How and when that collapse might occur is the subject of a five-year international collaborative research effort.
Glaciers like Thwaites are held back from sliding seaward both by buttressing ice shelves — tongues of floating ice that jut out into the sea — and by the shape of the seafloor itself, which can help pin the glacier’s ice in place (SN: 4/3/18). But in two new studies, published online September 9 in The Cryosphere, the researchers show how the relatively warm ocean waters may have a pathway straight to the glacier’s underbelly.
Channels carved into the seafloor, extending several kilometers wide and hundreds of meters deep, may act as pathways (red line with yellow arrows as seen in this 3-D illustration) to bring relatively warm ocean waters to the edges of vulnerable Thwaites Glacier, hastening its melting.International Thwaites Glacier Collaboration
From January to March 2019 researchers used a variety of airborne and ship-based methods — including radar, sonar and gravity measurements — to examine the seafloor around the glacier and two neighboring ice shelves. From those data, the team was able to estimate how the seafloor is shaped beneath the ice itself.
These efforts revealed a rugged series of high ridges and deep troughs on the seafloor, varying between about 250 meters and 1,000 meters deep. In particular, one major channel, more than 800 meters deep, could be funneling warm water all the way from Pine Island Bay to the submerged edge of the glacier, the team found.

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Sea ice in the Bering Sea, on the southern margin of the Arctic Ocean, dwindled to its smallest wintertime expanse in 5,500 years in 2018, new data show.  
Summertime sea ice loss due to climate change has captured headlines, but winter ice in the region has also shown recent signs of decline. In both February 2018 and February 2019, the extent was 60 to 70 percent lower than the average February-to-May extent from 1979 to 2017. However, researchers thought that those declines might be linked to unusual short-term atmospheric conditions.
Instead, the new study suggests that human-caused climate change is also helping to shrink Bering Sea ice during the winter. The findings, by geologist Miriam Jones of the U.S. Geological Survey in Reston, Va., and colleagues, were published September 2 in Science Advances.
Jones and her team collected cores of peat from St. Matthew Island, a remote spot in the Bering Sea west of Alaska. Within the peat — packed remains of partially decomposed plants — oxygen-bearing organic compounds called cellulose contain clues to the climate history of the region.

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Rain falling on the island contains two different isotopes, or forms, of oxygen: oxygen-18 and oxygen-16. The relative values of those isotopes in the rainfall change depending on atmospheric conditions, and as plants take up that oxygen from the air, they record those changes. By analyzing the amounts of those isotopes in the cellulose over time, the team was able to track changes in precipitation and atmospheric circulation going back 5,500 years.
Then, the team established the link between this oxygen isotope record and sea ice extent.
Bering Sea ice is known to be directly tied to shifts in wind direction. So the researchers created a computer simulation that included climate conditions from 1979 to 2018, oxygen isotope values from cellulose during that time and satellite observations of sea ice. When winds were strongly blowing from the south, and there was less sea ice, the relative amount of oxygen-18 increased. When winds from the north dominated, and there was more sea ice, there was less oxygen-18 in the cellulose.
Next, the researchers used the oxygen isotopes in the peat to track the waxing and waning of the region’s sea ice over thousands of years. Most of the area’s rainfall occurs in winter and spring, so those oxygen isotopes are indicative of conditions between February and May, rather than summer. The peat cellulose oxygen-18 values recorded in winter 2018 were the highest, and the sea ice extent the smallest, in the last 5,500 years, the team found. 

In preindustrial times, the researchers found, wintertime sea ice was gradually decreasing, largely due to natural changes in incoming sunlight during winter, related to changes in Earth’s orbit. But the team also found that atmospheric carbon dioxide concentrations, compiled from previous studies, were closely correlated to ice volume. As CO2 levels began to climb past 280 parts per million following the onset of the Industrial Revolution in the mid-1700s, the oxygen-18 values also began to rise, with corresponding sea ice decreases.
How exactly increasing CO2 might be linked to winter ice volume is less clear. The losses may be directly due to greenhouse gas warming. Or more indirectly, changes to atmospheric circulation patterns due to increasing CO2 might also lead to those losses.
The study demonstrates just how exceptional the recent winter sea ice losses in the region are, says Benjamin Gaglioti, an environmental scientist at the University of Alaska Fairbanks who was not involved in the study. “Although there [was] an overall trend towards less sea ice prior to anthropogenic warming, recent increases in human-derived greenhouse gases have enhanced this trend,” he says. And that is not good news for the region’s denizens.
“Winter sea ice in this region serves as a critical habitat for unique marine wildlife like Pacific walrus and kittiwakes,” Gaglioti says. The ice also helps dampen the impacts of intense winter storms and flooding on coastal communities, he adds.
Climate change due to CO2 and other climate-warming gases has already taken a visible toll on summertime sea ice in and around the Arctic; within 10 to 15 years, the region may be ice-free during the warmer months. Arctic sea ice in September 2019 tied for second-lowest on record with 2007 and 2016; first place still goes to 2012 (SN: 9/25/19). The loss of that ice is not just a bellwether for climate change in the Arctic, but is also speeding up the rate of warming in the region, a process called Arctic amplification (SN: 7/1/20). And the missing summer ice is also triggering a cascade of changes through Arctic ecosystems, including within the Bering Sea (SN: 3/14/19).
But the new study suggests that winter sea ice losses might lag behind CO2 changes by decades, perhaps even a century — and that could mean a year-round ice-free Bering Sea by 2100. More

Hurricane Maria and Heat Wave Henrietta?
For decades, meteorologists have named hurricanes and ranked them according to severity. Naming and categorizing heat waves too could increase public awareness of the extreme weather events and their dangers, contends a newly formed group that includes public health and climate experts. Developing such a system is one of the first priorities of the international coalition, called the Extreme Heat Resilience Alliance.
Hurricanes get attention because they cause obvious physical damage, says Jennifer Marlon, a climate scientist at Yale University who is not involved in the alliance. Heat waves, however, have less visible effects, since the primary damage is to human health.
Heat waves kill more people in the United States than any other weather-related disaster (SN: 4/3/18). Data from the National Weather Service show that from 1986 to 2019, there were 4,257 deaths as a result of heat. By comparison, there were fewer deaths by floods (2,907), tornadoes (2,203) or hurricanes (1,405) over the same period.
What’s more, climate change is amplifying the dangers of heat waves by increasing the likelihood of high temperature events worldwide. Heat waves linked to climate change include the powerful event that scorched Europe during June 2019 (SN: 7/2/19) and sweltering heat in Siberia during the first half of 2020 (SN: 7/15/20).
Some populations are particularly vulnerable to health problems as a result of high heat, including people over 65 and those with chronic medical conditions, such as neurodegenerative diseases and diabetes. Historical racial discrimination also places minority communities at disproportionately higher risk, says Aaron Bernstein, a pediatrician at Boston Children’s Hospital and a member of the new alliance. Due to housing policies, communities of color are more likely to live in urban areas, heat islands which lack the green spaces that help cool down neighborhoods (SN: 3/27/09).
Aaron Bernstein, a pediatrician at Boston Children’s Hospital, says giving heat waves names and severity rankings may help save lives.John Wilcox for Coverage, a BCBS of MA news service
Part of the naming and ranking process will involve defining exactly what a heat wave is. No single definition currently exists. The National Weather Service issues an excessive heat warning when the maximum heat index — which reflects how hot it feels by taking humidity into account — is forecasted to exceed about 41° Celsius (105° Fahrenheit) for at least two days and nighttime air temperatures stay above roughly 24° C (75° F). The World Meteorological Organization and World Health Organization more broadly describe heat waves as periods of excessively hot weather that cause health problems.
Without a universally accepted definition of a heat wave, “we don’t have a common understanding of the threat we face,” Bernstein says. He has been studying the health effects of global environmental changes for nearly 20 years and is interim director of the Center for Climate, Health and the Global Environment at the Harvard T.H. Chan School of Public Health.
Defined categories for heat waves could help local officials better prepare to address potential health problems in the face of rising temperatures. And naming and categorizing heat waves could increase public awareness of the health risks posed by these silent killers.
“Naming [heat waves] will make something invisible more visible,” says climate communicator Susan Joy Hassol of Climate Communication, a project of the Aspen Global Change Institute, a nonprofit organization based in Colorado that’s not part of the new alliance. “It also makes it more real and concrete, rather than abstract.”
The alliance is in ongoing conversations with the National Oceanic and Atmospheric Administration, the World Meteorological Organization and other institutions to develop a standard naming and ranking practice.
“People know when a hurricane’s coming,” Hassol says. “It’s been named and it’s been categorized, and they’re taking steps to prepare. And that’s what we need people to do with heat waves.”

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To curb the spread of COVID-19, much of the globe hunkered down. That inactivity helped slow the spread of the virus and, as a side effect, kept some climate-warming gases out of the air.
New estimates based on people’s movements suggest that global greenhouse gas emissions fell roughly 10 to 30 percent, on average, during April 2020 as people and businesses reduced activity. But those massive drops, even in a scenario in which the pandemic lasts through 2021, won’t have much of a lasting effect on climate change, unless countries incorporate “green” policy measures in their economic recovery packages, researchers report August 7 in Nature Climate Change.
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“The fall in emissions we experienced during COVID-19 is temporary, and therefore it will do nothing to slow down climate change,” says Corinne Le Quéré, a climate scientist at the University of East Anglia in Norwich, England. But how governments respond could be “a turning point if they focus on a green recovery, helping to avoid severe impacts from climate change.” 
Carbon dioxide lingers in the atmosphere for a long time, making month-to-month changes in CO2 levels difficult to measure as they happen. Instead, the researchers looked at what drives some of those emissions — people’s movements. Using anonymized cell phone mobility data released by Google and Apple, Le Quéré and colleagues tracked changes in energy-consuming activities, like driving or shopping, to estimate changes in 10 greenhouse gases and air pollutants. 

“Mobility data have big advantages” for estimating short-term changes in emissions, says Jenny Stavrakou, a climate scientist at the Royal Belgian Institute for Space Aeronomy in Brussels who wasn’t involved in the study. Since those data are continuously updated, they can reveal daily changes in transportation emissions caused by lockdowns, she says. “It’s an innovative approach.”
Google’s mobility data revealed that 4 billion people reduced their travel by more than 50 percent in April alone. By adding more traditional emissions estimates to fill in gaps (SN: 5/19/20), the researchers analyzed emissions trends across 123 countries from February to June. The researchers found that the peak drop occurred in April, when globally averaged CO2 emissions and nitrogen oxides fell by roughly 30 percent from baseline, mostly due to reduced driving.
Fewer greenhouse gases should result in some cooling of the atmosphere, but the researchers found that effect will be largely offset by the roughly 20 percent fall in sulfur aerosols in April. These industrial emissions reflect sunlight and thus have a cooling effect. With fewer shading aerosols, more of the sun’s energy can heat the atmosphere, causing warming. On the whole, the stark drop in emissions in April alone will cool the globe a mere 0.01 degrees Celsius over the next five years, the study finds.
In the long-term, the massive, but temporary, shifts in behavior caused by COVID-19 won’t change our current warming trajectory. But large-scale economic recovery plans offer an opportunity to enact climate-friendly policies, such as invest in low-carbon technologies, that could avert the worst warming (SN: 11/26/19). That could help reach a goal of cutting total global greenhouse gas emissions by 52 percent by 2050, limiting warming to 1.5 degrees Celsius above preindustrial levels through 2050, the researchers say.

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The intense heat wave that gripped Siberia during the first half of 2020 would have been impossible without human-caused climate change, a new study finds. Researchers with the World Weather Attribution Network report that climate change made the prolonged heat in the region at least 600 times more likely — and possibly as much as 99,000 times more likely.
“We wouldn’t expect the natural world to generate [such a heat wave] in anything less than 800,000 years or so,” climate scientist Andrew Ciavarella of the U.K. Met Office in Exeter, England, said July 14 in a news conference. It’s “effectively impossible without human influence.”
The new study, posted online July 15, examined two aspects of the heat wave: the persistence and intensity of average temperatures across Siberia from January to June 2020; and daily maximum temperatures during June 2020 in the remote Siberian town of Verkhoyansk.

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Tiny Verkhoyansk made international headlines when it logged a record high temperature of 38° Celsius (100.4° Fahrenheit) on June 20 (SN: 6/23/20). The record was just one extreme amid a larger and longer event in the region that has led to a series of human and natural disasters (SN: 7/1/20). Those include wildfires across Siberia, the collapse of a fuel tank in the mining city of Norilsk due to sagging permafrost, and heat health effects (SN: 4/3/18).
Using observational data from Verkhoyansk and other Siberian weather stations, the researchers first assessed the rarity of the observed temperatures and determined temperature trends. Then they compared these observations with hundreds of climate simulations using different greenhouse gas warming scenarios. 

Had such a hot spell occurred in 1900 instead of 2020, it would have been at least 2 degrees cooler on average, the researchers found. In Verkhoyansk, climate change amped up June temperatures by at least 1 degree relative to 1900. And such heat waves are likely to become more common in the near future, the scientists found: By 2050, temperatures in Siberia could increase by between 2.5 degrees to as much as 7 degrees compared to the year 1900, the report finds. More

On June 20, a remote Siberian town called Verkhoyansk logged a temperature of 38° Celsius (100.4° Fahrenheit), likely setting a new high-temperature record for the Arctic Circle (SN: 6/23/20). But that new record didn’t occur in a vacuum: It’s part of a long-term trend of historically hot temperatures in Siberia linked to climate change, and […] More