Scientists at Stanford University have discovered a surprising shift in the Arctic Ocean. Exploding blooms of phytoplankton, the tiny algae at the base of a food web topped by whales and polar bears, have drastically altered the Arctic’s ability to transform atmospheric carbon into living matter. Over the past decade, the surge has replaced sea ice loss as the biggest driver of changes in uptake of carbon dioxide by phytoplankton.
The research appears July 10 in Science. Senior author Kevin Arrigo, a professor in Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth), said the growing influence of phytoplankton biomass may represent a “significant regime shift” for the Arctic, a region that is warming faster than anywhere else on Earth.
The study centers on net primary production (NPP), a measure of how quickly plants and algae convert sunlight and carbon dioxide into sugars that other creatures can eat. “The rates are really important in terms of how much food there is for the rest of the ecosystem,” Arrigo said. “It’s also important because this is one of the main ways that CO2 is pulled out of the atmosphere and into the ocean.”
A thickening soup
Arrigo and colleagues found that NPP in the Arctic increased 57 percent between 1998 and 2018. That’s an unprecedented jump in productivity for an entire ocean basin. More surprising is the discovery that while NPP increases were initially linked to retreating sea ice, productivity continued to climb even after melting slowed down around 2009. “The increase in NPP over the past decade is due almost exclusively to a recent increase in phytoplankton biomass,” Arrigo said.
Put another way, these microscopic algae were once metabolizing more carbon across the Arctic simply because they were gaining more open water over longer growing seasons, thanks to climate-driven changes in ice cover. Now, they are growing more concentrated, like a thickening algae soup.
“In a given volume of water, more phytoplankton were able to grow each year,” said lead study author Kate Lewis, who worked on the research as a Ph.D. student in Stanford’s Department of Earth System Science. “This is the first time this has been reported in the Arctic Ocean.”
New food supplies
Phytoplankton require light and nutrients to grow. But the availability and intermingling of these ingredients throughout the water column depend on complex factors. As a result, although Arctic researchers have observed phytoplankton blooms going into overdrive in recent decades, they have debated how long the boom might last and how high it may climb.
By assembling a massive new collection of ocean color measurements for the Arctic Ocean and building new algorithms to estimate phytoplankton concentrations from them, the Stanford team uncovered evidence that continued increases in production may no longer be as limited by scarce nutrients as once suspected. “It’s still early days, but it looks like now there is a shift to greater nutrient supply,” said Arrigo, the Donald and Donald M. Steel Professor in Earth Sciences.
The researchers hypothesize that a new influx of nutrients is flowing in from other oceans and sweeping up from the Arctic’s depths. “We knew the Arctic had increased production in the last few years, but it seemed possible the system was just recycling the same store of nutrients,” Lewis said. “Our study shows that’s not the case. Phytoplankton are absorbing more carbon year after year as new nutrients come into this ocean. That was unexpected, and it has big ecological impacts.”
Decoding the Arctic
The researchers were able to extract these insights from measures of the green plant pigment chlorophyll taken by satellite sensors and research cruises. But because of the unusual interplay of light, color and life in the Arctic, the work required new algorithms. “The Arctic Ocean is the most difficult place in the world to do satellite remote sensing,” Arrigo explained. “Algorithms that work everywhere else in the world—that look at the color of the ocean to judge how much phytoplankton are there—do not work in the Arctic at all.”
The difficulty stems in part from a huge volume of incoming tea-colored river water, which carries dissolved organic matter that remote sensors mistake for chlorophyll. Additional complexity comes from the unusual ways in which phytoplankton have adapted to the Arctic’s extremely low light. “When you use global satellite remote sensing algorithms in the Arctic Ocean, you end up with serious errors in your estimates,” said Lewis.
Yet these remote-sensing data are essential for understanding long-term trends across an ocean basin in one of the world’s most extreme environments, where a single direct measurement of NPP may require 24 hours of round-the-clock work by a team of scientists aboard an icebreaker, Lewis said. She painstakingly curated sets of ocean color and NPP measurements, then used the compiled database to build algorithms tuned to the Arctic’s unique conditions. Both the database and the algorithms are now available for public use.
The work helps to illuminate how climate change will shape the Arctic Ocean’s future productivity, food supply and capacity to absorb carbon. “There’s going to be winners and losers,” Arrigo said. “A more productive Arctic means more food for lots of animals. But many animals that have adapted to live in a polar environment are finding life more difficult as the ice retreats.”
Phytoplankton growth may also peak out of sync with the rest of the food web because ice is melting earlier in the year. Add to that the likelihood of more shipping traffic as Arctic waters open up, and the fact that the Arctic is simply too small to take much of a bite out of the world’s greenhouse gas emissions. “It’s taking in a lot more carbon than it used to take in,” Arrigo said, “but it’s not something we’re going to be able to rely on to help us out of our climate problem.”
Earth’s oceans are simmering with the heat trapped by increasing amounts of greenhouse gases. But one patch of water in the North Atlantic is stubbornly resisting the trend, and actually dropping in temperature.
A new study adds detail to the phenomenon, revealing there’s more than one cause at work.
A team of researchers from the Max Planck Institute for Meteorology in Germany applied long-term climate modelling to simulate various configurations to find which match the observed plunge in temperature.
One of the factors they identified comes as no real surprise, backing up previous studies that show a current of water called the Atlantic meridional overturning circulation (AMOC) has weakened significantly since the mid-20th century.
When running at full steam, the circulation takes warm, salty surface waters from the tropics near the Gulf of Mexico north towards the European coast, exchanging it for cold, fresh water supplied by melting ice.
Exactly what might be causing this highway of tropical water to slow down isn’t all that clear, though some models suggest more meltwater from Greenland coupled with rising global temperatures would fit what we’re seeing.
With warmer temperatures making the ocean water more buoyant, it’s less likely to drop as quickly, slowing the spiral. Meanwhile, a good dose of fresh water trickling in from melting Arctic ice and higher rainfall would also impede the circulating currents by forming a layer of less salty water on the surface.
Still, data on the AMOC aren’t the highest quality prior to 2004, leaving open the small possibility that the slow-down could be a return to business as usual rather than something triggered by a warming planet.
To tease out connections between Earth’s climate and the cold blob, the researchers behind this latest study used a detailed planetary climate model to couple variations in energy, carbon dioxide, and water across the ocean, land, and atmosphere.
Simulations run through this model allowed them to see what might happen if they forced the AMOC to churn away at full speed, leaving the atmosphere to act as a major influencing factor all on its own.
Sure enough, there was a small but noticeable effect. As the incoming warm waters cooled down, they produced low-lying clouds that would reflect incoming radiation, in turn cooling the surface even further.
Next, the team ran another scenario that looked only at the AMOC’s transport of heat, finding it wasn’t just carrying less energy, but was dumping more of it into the Arctic’s circulating water currents.
For complicated reasons, these subpolar circulations are picking up speed, drawing heat from the AMOC and leaving the cold blob even colder.
There’s still plenty of work to be done on building up these explanations and determining how much of an impact our insatiable desire to burn fossil fuels has had on what would otherwise be a natural cycle.
But the study goes a long way in showing how important it is that we take into account diverse factors in assessing local and global changes to the climate.
No doubt researchers will be paying even closer attention to the AMOC’s strength in coming years. But knowing exactly how this cold blob operates in a changing climate will help us better understand what to expect in a future that’s likely to be several degrees warmer.
A small glacier in the Arctic region of Norwegian archipelago Svalbard, as photographed by NASA’s Airborne Tropical Tropopause Experiment (ATTREX). This is one of the seven regions where ice loss is accelerating, causing the depletion of freshwater resources. Credit: NASA/John Sonntag
Continuous monitoring of glaciers and ice caps has provided unprecedented insights to global ice loss that could have serious socioeconomic impacts on some regions.
Seven of the regions that dominate global ice mass losses are melting at an accelerated rate, a new study shows, and the quickened melt rate is depleting freshwater resources that millions of people depend on.
The impact of melting ice in Greenland and Antarctica on the world’s oceans is well documented. But the largest contributors to sea level rise in the 20th century were melting ice caps and glaciers located in seven other regions: Alaska, the Canadian Arctic Archipelago, the Southern Andes, High Mountain Asia, the Russian Arctic, Iceland and the Norwegian archipelago Svalbard. The five Arctic regions accounted for the greatest share of ice loss.
And this ice melt is accelerating, potentially affecting not just coastlines but agriculture and drinking water supplies in communities around the world, according to the study by scientists at NASA’s Jet Propulsion Laboratory; the University of California, Irvine; and the National Center for Atmospheric Research in Boulder, Colorado. The study was led by Enrico Ciraci, a UCI graduate student researcher in Earth system science.
“In the Andes Mountains in South America and in High Mountain Asia, glacier melt is a major source of drinking water and irrigation for several hundred million people,” said study coauthor Isabella Velicogna, a senior scientist at JPL and professor of Earth system science at UCI. “Stress on this resource could have far-reaching effects on economic activity and political stability.”
The researchers based their work on data from the recently decommissioned U.S.-German Gravity Recovery and Climate Experiment (GRACE) pair of satellites that operated from 2002 to 2017, and their successor pair, GRACE Follow On (launched in 2018). The researchers calculated that, on average, these seven regions lost more than 280 billion tons of ice per year.
This ice loss contributed a total of 13 millimeters (0.5 inches) in global sea level rise between 2002 and 2019, and the rate has increased from 0.7 millimeters (0.028 inches) per year in 2002 to 0.9 millimeters (0.035 inches) per year in 2019.
As with GRACE, the GRACE-FO satellites continuously measure very slight changes in Earth’s gravitational pull as they orbit the Earth. Over time, shifts in the distribution of water are the largest source of gravity changes on the planet, so scientists can use the measurements of gravity change to track variations in the mass of water as it cycles from the ice caps and glaciers to the oceans.
GRACE was a joint mission of NASA and the German Aerospace Center, in partnership with the University of Texas at Austin. GRACE-FO is a partnership between NASA and the German Research Centre for Geosciences. When it launched in May 2018, 11 months had passed since GRACE made its last measurements.
Velicogna and her coauthors closed the resulting data gap between the end of GRACE and the initiation of GRACE-FO by using a state-of-the-art modeling tool called Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2) from NASA’s Global Modeling and Assimilation Office. MERRA-2 utilizes a host of independent observational datasets to boost the precision of its estimates. For this study, the researchers noted how well the MERRA-2 results lined up with the GRACE and GRACE-FO data, giving them a high degree of confidence of what these satellites would have observed if one or both were operating in the period of the data gap.
Having a record based on the long-term, precision measurements of hundreds of thousands of the world’s glaciers for over 18 years, Velicogna said, significantly enhances our understanding of their evolution.
“This paper demonstrates that GRACE-FO, in addition to GRACE, is providing precise, reliable, worldwide observations of the fate of mountain glaciers, which are not only important for understanding sea level change, but also for managing our freshwater resources,” she said.
The study, titled “Continuity of the Mass Loss of the World’s Glaciers and Ice Caps From the GRACE and GRACE Follow-On Missions,” was published April 30 in Geophysical Research Letters.
JPL managed the GRACE mission and manages the GRACE-FO mission for NASA’s Earth Science Division in the Science Mission Directorate at NASA Headquarters in Washington. The California Institute of Technology (Caltech) in Pasadena, California, manages JPL for NASA.
This map shows the amount of ice gained or lost by Antarctica between 2003 and 2019. Dark reds and purples show large average rates of ice loss near the coasts, while blues show smaller rates of ice gain in the interior.
Two new satellite images remind us that Earth’s ice sheets are losing so much mass it’s becoming obvious from space.
In the vivid new maps published as part of an April 30 study in the journalScience, researchers illustrated 16 years of ice loss inGreenland andAntarctica as seen by a laser-emitting NASA satellite. The images paint a picture of rapid melt around the coasts of both regions (shown in red and purple in the maps), far outweighing modest ice-mass gains (shown in light blue) farther inland.
Greenland’s ice sheet lost an average of 200 gigatons of ice per year, while Antarctica’s ice sheet lost an average of 118 gigatons per year; for reference, a single gigaton of ice is enough to fill 400,000 Olympic-sized swimming pools, the researcherssaid in a statement.
All that melting ice was responsible for a total 0.55 inches (14 millimeters) of sea-level rise between 2003 and 2019, the researchers found. That rise puts Earthon track for the worst-case climate warming scenario laid out in the Intergovernmental Panel on Climate Change’s (IPCC) latest report, previous research found. That scenario would put hundreds of millions of people living in coastal communities at risk of losing their homes — or their lives — to flooding.
For the new study, the researchers used the newest data from NASA’s ICESat-2 satellite, which launched in 2018 to monitor elevation changes on land (and ice) around the world by bathing the planet in laser beams. The team compared 2019 elevation levels with data recorded by the satellite’s predecessor — named simply ICESat — between 2003 and 2009. At thousands of locations where the two datasets overlapped, the team could see precisely how much ice had vanished from Greenland and Antarctica between 2003 and 2019.
Ice shelves — enormous ledges of ice floating over the ocean at the edges of Greenland and Antarctica — lost the most mass by far in both regions, the researchers said. While ice shelves are already partially submerged in water and therefore do not actively raise sea levels when they melt, they provide a structural integrity to glaciers that prevents ice farther inland from gushing into the sea.
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“It’s like an architectural buttress that holds up a cathedral,” study co-author Helen Amanda Fricker, a glaciologist at Scripps Institution of Oceanography at the University of California, San Diego, said in the statement. “The ice shelves hold the ice sheet up. If you take away the ice shelves, or even if you thin them, you’re reducing that buttressing force, so the grounded ice can flow faster.”
Predictably, the new research shows, as the ice shelves surrounding Antarctica and Greenland have thinned and melted over the last two decades, grounded ice farther inland has thinned and melted too.
The new analysis reveals, with unprecedented detail, the response of these ice sheets to changes in climate, “revealing clues as to why and how the ice sheets are reacting the way they are,” study co-author Alex Gardner, a glaciologist at NASA’s Jet Propulsion Laboratory in Pasadena, California, said in the statement.
Sea ice near the shore is crucial for dogsled travel for Arctic communities. But a new study shows that when temperatures crank up, it disappears faster—and it’s set to grow a lot worse by the end of the century.
Published in Nature Climate Change on Monday, the paper uses satellite imagery from 2000 to 2018 over 28 communities in northern Canada and western Greenland to track changes in so-called shorefast sea ice during the spring. That ice is a key connector for these remote, largely indigenous communities. During the winter and spring, it acts as a bridge to bring communities together and allows them to continue historic cultural practices that are essential to their way of life, such as seal hunting and dogsledding. But as the Arctic warms, it’s likely to breakup earlier and put those activities in jeopardy.
“Broadly speaking, the Arctic region faces some of the most dramatic impacts of climate change, and it is important to understand not only how these changes may feedback into global climate but also how they may affect local Arctic communities,” author Sarah Cooley, a PhD candidate at Brown University, told Earther in an email. “This result highlights the fact that climate change will not affect all places equally even within the Arctic and emphasizes the importance of taking localized, community-relevant approaches to studying climate change.”
The researchers looked at surface air temperatures to assess whether there was a connection and found that ice breakup is associated with spikes in temperature in 25 of the 28 communities they looked at. Though there’s no clear temperature trend over the relatively short satellite record they used for the study, the observing the correlation between shorefast ice loss and temperature allowed them to model what the future would look like under different climate scenarios.
They used three climate scenarios ranging from one where we draw down emissions rapidly to one where they rise. In all of the scenarios, there is a loss in the number of days the regions see this ice cover. Unchecked climate change would lead ice to break up 20 days earlier on average by century’s end while cutting emissions rapidly would only lead shorefast ice breakups happening an average of 7 days earlier. The findings also show that the colder it is, the more ice it’s projected to lose come spring. Regardless, the future of ice and the communities that depend on it will depend on how quickly leaders take action to avoid the worst.
The study only looked at temperature and ice. The team expects, however, that other weather patterns such as wind and waves also impact ice’s breakup date. There’s growing evidence that the Arctic could see more intense storms so the study may well be an underestimate of what the future holds for shorefast ice.
The Arctic has been transforming much sooner than scientists had expected. For Inuit communities, the climate crisis is already erasing criticalpieces of their lives and history. Taking a closer look at ice thickness and ice stability will give scientists an even-fuller picture of the consequences of loss.
There was a dramatic melting of Greenland’s ice sheet in the summer of 2019, researchers have confirmed, in a study that reveals the loss was largely down to a persistent zone of high pressure over the region.
The ice sheet melted at a near record rate in 2019, and much faster than the average of previous decades. Figures have suggested that in July alone surface ice declined by 197 gigatonnes – equivalent to about 80 million Olympic swimming pools.
Now experts have examined the level of melting in more detail, revealing what drove it. Crucially, the team note, the high pressure conditions lasted for 63 of the 92 summer days in 2019, compared with an average of just 28 days between 1981 and 2010. A similar situation was seen in 2012, a record bad year for melting of the ice sheet.
The team say the climate models of the Intergovernmental Panel on Climate Change (IPCC) have not taken into account such unusual conditions. If such high pressure zones become a regular annual feature, future melting could be twice as high as currently predicted, a result that could have serious consequences for sea level rise.
“This melt event is a good alarm signal that we urgently need to change our way of living to hold [back] global warming because it is likely that the IPCC projections could be too optimistic for [the] Arctic,” said Dr Xavier Fettweis, co-author of the research from the University of Liege, adding that the atmospheric conditions were unlikely to be down to natural climatic variability and could be driven by global heating.
Writing in the journal the Cryosphere, Fettweis and his co-author Marco Tedesco from the Lamont-Doherty Earth Observatory at Columbia University report how they used satellite data, climate models and global weather patterns to explore the melting of the surface of the ice sheet last year.
Among their findings the team report that almost 96% of the ice sheet underwent melting at some time in 2019, compared with an average of just over 64% between 1981 and 2010.
Using models, the pair also found that about 560Gt of meltwater runoff was generated in the summer of 2019. The surface mass balance, the amount of ice the sheet gained from rain and snowfall minus the amount lost through meltwater run off and evaporation, was just 54Gt a year– about 320Gt a year lower than the average across the earlier decades, and the greatest such drop on record.
Further analysis showed the level and distribution of melting to be closely tied to a number of factors, including levels of snowfall and reflection of sunlight – known as albedo – as well as cloudiness and absorption of sunlight. All of these, they note, were influenced by the persistent high pressure zone over the ice sheet last summer.
Dr Poul Christoffersen, a glaciologist at the Scott Polar Research Institute at the University of Cambridge, who was not involved in the study, welcomed the research, noting only 2012 had a higher meltwater runoff in recent years.
“Clearly, this shows that extreme melt events are becoming a lot more frequent,” he said, adding that the new study showed that persistent atmospheric high pressure was an important factor, resulting in clear skies and a lack of snowfall in the south and warm, moist air being brought to northern parts of the ice sheet. “In that sense, the extreme melt years can be seen as natural events exacerbated by climate change,” said Christoffersen.
Prof Andy Shepherd from the University of Leeds said a fall in surface mass balance was concerning. “If that drops below zero, then the ice sheet is no longer viable because in every year it is losing more ice than it gains,” he said, adding that that was not even counting the loss of icebergs. “Even if the glaciers stopped flowing, which is not going to happen, it would mean that the ice sheet still can’t survive,” he said.
The polar ice caps are melting six times faster than in the 1990s, according to the most complete analysis to date.
The ice loss from Greenland and Antarctica is tracking the worst-case climate warming scenario set out by the Intergovernmental Panel on Climate Change (IPCC), scientists say. Without rapid cuts to carbon emissions the analysis indicates there could be a rise in sea levels that would leave 400 million people exposed to coastal flooding each year by the end of the century.
Rising sea levels are the one of the most damaging long-term impacts of the climate crisis, and the contribution of Greenland and Antarctica is accelerating. The new analysis updates and combines recent studies of the ice masses and predicts that 2019 will prove to have been a record-breaking year when the most recent data is processed.
The previous peak year for Greenland and Antarctic ice melting was 2010, after a natural climate cycle led to a run of very hot summers. But the Arctic heatwave of 2019 means it is nearly certain that more ice was lost last year.
The average annual loss of ice from Greenland and Antarctica in the 2010s was 475bn tonnes – six times greater than the 81bn tonnes a year lost in the 1990s. In total the two ice caps lost 6.4tn tonnes of ice from 1992 to 2017, with melting in Greenland responsible for 60% of that figure.
The IPCC’s most recent mid-range prediction for global sea level rise in 2100 is 53cm. But the new analysis suggests that if current trends continue the oceans will rise by an additional 17cm.
“Every centimetre of sea level rise leads to coastal flooding and coastal erosion, disrupting people’s lives around the planet,” said Prof Andrew Shepherd, of the University of Leeds. He said the extra 17cm would mean the number of exposed to coastal flooding each year rising from 360 million to 400 million. “These are not unlikely events with small impacts,” he said. “They are already under way and will be devastating for coastal communities.”
Erik Ivins, of Nasa’s Jet Propulsion Laboratory, in California, who led the assessment with Shepherd, said the lost ice was a clear sign of global heating. “The satellite measurements provide prima facie, rather irrefutable, evidence,” he said.
Almost all the ice loss from Antarctica and half of that from Greenland arose from warming oceans melting the glaciers that flow from the ice caps. This causes glacial flow to speed up, dumping more icebergs into the ocean. The remainder of Greenland’s ice losses are caused by hotter air temperatures that melt the surface of the ice sheet.
The combined analysis was carried out by a team of 89 scientists from 50 international organisations, who combined the findings of 26 ice surveys. It included data from 11 satellite missions that tracked the ice sheets’ changing volume, speed of flow and mass.
About a third of the total sea level rise now comes from Greenland and Antarctic ice loss. Just under half comes from the thermal expansion of warming ocean water and a fifth from other smaller glaciers. But the latter sources are not accelerating, unlike in Greenland and Antarctica.
Shepherd said the ice caps had been slow to respond to human-caused global heating. Greenland and especially Antarctica were quite stable at the start of the 1990s despite decades of a warming climate.
Shepherd said it took about 30 years for the ice caps to react. Now that they had a further 30 years of melting was inevitable, even if emissions were halted today. Nonetheless, he said, urgent carbon emissions cuts were vital. “We can offset some of that [sea level rise] if we stop heating the planet.”
The IPCC is in the process of producing a new global climate report and its lead author, Prof Guðfinna Aðalgeirsdóttir, of the University of Iceland, said: “The reconciled estimate of Greenland and Antarctic ice loss is timely.”
She said she also saw increased losses from Iceland’s ice caps last year. “Summer 2019 was very warm in this region.”
It was already known that polar bears have cannibalistic trades however they never acted on Earth due to abundance of food and resources.
The increasing temperature, as well as human involvement in the Arctic, has resulted in the ice plains to melt. this has completely destroyed the habitats through which a polar bear would normally hunt for food.
According to polar bear expert, Ilya Mordvintsev at Moscow’s Severtsov Institute of Problems of Ecology and Evolution, “Cases of cannibalism among polar bears are a long-established fact, but such cases used to be found rarely while now they are recorded quite often. We state that cannibalism in polar bears is increasing.”
According to the researchers, the reason for this behaviour is the shortage of food supply in an already delicate habitat. Large male polar bears are attacking female and cubs — since they’re an easy target.
Normally they hunt on sea ice, feasting on seals. But with ice melting away, the bears are forced to be near the shore where they cannot hunt the usual way.
Human intervention has also severely impacted the habitat. The Gulf of Ob is now being used to extract Arctic Liquid Natural Gas with ships commonly passing through the route.
Another Russian scientist Vladimir Sokolov who has led numerous expeditions in the past stated that Polar Bears this year are also experiencing weather hotter than they’re used to — specifically towards the Spitsbergen Island to the north in Norway.
Many bears have also started to hoard bodies that they’ve killed by burying them in the snow to be consumed later. This phenomenon is called caching and is common in brown bears, but not quite in polar bears.
A scientific paper published in 1985 was the first to report a burgeoning hole in Earth’s stratospheric ozone over Antarctica. Scientists determined the cause to be ozone-depleting substances—long-lived artificial halogen compounds. Although the ozone-destroying effects of these substances are now widely understood, there has been little research into their broader climate impacts.
A study published today in Nature Climate Change by researchers at Columbia University examines the greenhouse warming effects of ozone-depleting substances and finds that they caused about a third of all global warming from 1955 to 2005, and half of Arctic warming and sea ice loss during that period. They thus acted as a strong supplement to carbon dioxide, the most pervasive greenhouse gas; their effects have since started to fade, as they are no longer produced and slowly dissolve.
Ozone-depleting substances, or ODS, were developed in the 1920s and ’30s and became popularly used as refrigerants, solvents and propellants. They are entirely manmade, and so did not exist in the atmosphere before this time. In the 1980s a hole in Earth’s stratospheric ozone layer, which filters much of the harmful ultraviolet radiation from the sun, was discovered over Antarctica. Scientists quickly attributed it to ODS.
The world sprang into action, finalizing a global agreement to phase out ODS. The Montreal Protocol, as it is called, was signed in 1987 and entered into force in 1989. Due to the swift international reaction, atmospheric concentrations of most ODS peaked in the late 20th century and have been declining since. However, for at least 50 years, the climate impacts of ODS were extensive, as the new study reveals.
Scientists at Columbia’s School of Engineering and Applied Science and the Lamont-Doherty Earth Observatory used climate models to understand the effects of ODS on Arctic climate. “We showed that ODS have affected the Arctic climate in a substantial way,” said Lamont-Doherty researcher Michael Previdi. The scientists reached their conclusion using two very different climate models that are widely employed by the scientific community, both developed at the U.S. National Center for Atmospheric Research.
The results highlight the importance of the Montreal Protocol, which has been signed by nearly 200 countries, say the authors. “Climate mitigation is in action as we speak because these substances are decreasing in the atmosphere, thanks to the Montreal Protocol,” said Lorenzo Polvani, lead author of the study and a professor in Columbia’s Department of Applied Physics and Applied Mathematics. “In the coming decades, they will contribute less and less to global warming. It’s a good-news story.”
New research warns that the earth may be approaching key tipping points, including the runaway loss of ice sheets, that could fundamentally disrupt the global climate system. A growing concern is a change in ocean circulation, which could alter climate patterns in a profound way.
Some of the most alarming science surrounding climate change is the discovery that it may not happen incrementally — as a steadily rising line on a graph — but in a series of lurches as various “tipping points” are passed. And now comes a new concern: These tipping points can form a cascade, with each one triggering others, creating an irreversible shift to a hotter world. A new study suggests that changes to ocean circulation could be the driver of such a cascade.
A group of researchers, led by Tim Lenton at Exeter University, England, first warned in a landmark paper 11 years ago about the risk of climate tipping points. Back then, they thought the dangers would only arise when global warming exceeded 5 degrees Celsius (9 degrees Fahrenheit) above pre-industrial levels. But last week, Lenton and six co-authors argued in the journal Nature that the risks are now much more likely and much more imminent. Some tipping points, they said, may already have been breached at the current 1 degree C of warming.
The new warning is much starker than the forecasts of the Intergovernmental Panel on Climate Change, which critics say has until now played down the risks of exceeding climate tipping points, in part because they are difficult to quantify.
The potential tipping points come in three forms: runaway loss of ice sheets that accelerate sea level rise; forests and other natural carbon stores such as permafrost releasing those stores into the atmosphere as carbon dioxide (CO2), accelerating warming; and the disabling of the ocean circulation system.
Researchers’ biggest fear is for the future of the ocean circulation system, which moves heat around the world and may dictate global climate.
The researchers once considered these tipping points to be largely independent of each other. Now they warn that the world faces a “cascade” of abrupt shifts in the planet’s climate system, as global warming takes hold. “We might already have crossed the threshold for a cascade of inter-related tipping points,” they wrote in Nature. This “could trigger a shift in the state of the Earth system as a whole,” one of the authors, Will Steffen of the Australian National University in Canberra, told Yale Environment 360.
Their biggest fear is for the future of the global ocean circulation system, which moves heat around the world and may dictate global climate. They say melting Greenland ice in a warmer Arctic has driven a key component of ocean circulation to a thousand-year low. Further decline, which would lead to a shift in heat distribution around the planet, could trigger forest collapse in the Amazon; cause near-permanent drought in Africa’s Sahel region; disrupt Asian monsoons; rapidly warm the Southern Ocean, which would cause a surge in global sea levels as the West Antarctic Ice Sheet disintegrates; and potentially shift the planet to a new climate regime they call “hothouse Earth.”
One climate scientist, Mike Hulme of the University of Cambridge, dismissed the new analysis as “a speculative opinion from a small group of self-selecting scientists.” He added that “there are no new research findings presented here” and that “many earth systems scientists would challenge the view” that the earth is close to crossing major tipping points. Lenton and his co-authors accept there is speculation involved, but argue that “given its huge impact and irreversible nature… to err on the side of danger is not a responsible option.”
The “climate emergency” is not just political rhetoric, they argue. It is now an identifiable scientific fact. Their message to the latest UN climate negotiations, under way in Madrid this week, is that the world may be almost out of time to prevent what they call an “existential threat to civilization.” Their study was released as a new report said that greenhouse gas emissions have hit a record high, with 40.6 billion tons of CO2 being pumped into the atmosphere in 2019.
The term “climate tipping points” was first coined 15 years ago by Hans Joachim Schellnhuber, former director of the Potsdam Institute for Climate Impact Research in Germany and a co-author of the new analysis, to describe how, under pressure from global warming, parts of the climate system could suddenly collapse or run out of control.
In their new analysis, the researchers conclude that of the 15 potential tipping points they identified in 2008, seven now show signs of being “active,” along with two others they have added to their list.“ That doesn’t mean a tipping point has necessarily been reached,” says Lenton. “But it means the system in question is showing evidence of change, of heading in the wrong direction.”
Four of these nine active tipping points involve thawing ice. Arctic sea ice is rapidly disappearing, and ice loss is accelerating on all three of the planet’s large, land-based ice sheets: Greenland, West Antarctica, and the Wilkes Basin in East Antarctica. Lenton says two of these, the West Antarctic Ice Sheet and Wilkes Basin, “are showing evidence consistent with having passed a tipping point,” meaning further ice loss may be unstoppable.
Greenland may not be far behind.“ Models suggest that the Greenland Ice Sheet could be doomed at 1.5 degrees C [2.7 degrees F] of warming, which could happen as soon as 2030,” the researchers report. Exceeding the three ice sheet tipping points could eventually cause an irreversible rise in sea levels of about 13 meters (43 feet), says Lenton.
Unlike the slowly deteriorating ice sheets, passing biospheric tipping points will produce abrupt, immediate, and obvious changes.
This may take centuries or millennia to play out, as the ice sheets slowly disappear into the ocean. But it will be virtually unstoppable, because once a thaw sets in, the surface of the ice sheet is lowered, exposing it to ever warmer air at lower altitudes.
Four more of the already-active tipping points involve the biosphere and its stores of carbon. The Amazon is suffering recurring droughts and forest dieback. In the boreal forests of the far north, rising temperatures are triggering epidemics of forest fires and pests. Meanwhile, permafrost is thawing and releasing methane, a greenhouse gas; and in the tropics, coral reefs are suffering massive die-offs, threatening wider ocean ecosystems.
Unlike the slowly deteriorating ice sheets, passing biospheric tipping points will often produce abrupt, immediate, and obvious changes, say the researchers. These may also be imminent. For instance, deforestation in the Amazon is already reducing rainfall and lengthening the dry season to a point where the rest of the trees die or are consumed by fires.
Carlos Nobre of the University of Sao Paulo, who was not involved in the present analysis, says that “when the dry season becomes longer than four months, tropical forest turns to savanna.” He puts the Amazon tipping point at 40-percent tree loss, a figure that changing global climate could reduce to between 20 and 25 percent by 2050. That is disturbingly close to the current total loss, reckoned to be approaching 20 percent.
Lenton says abrupt releases of CO2 from these natural carbon stores would drastically reduce the leeway the world has for avoiding global warming above 1.5 degrees, the preferred target set by the 2015 Paris Agreement. That probably requires limiting future CO2 emissions to about 500 billion tons — roughly 12 years’ emissions at current rates. But abrupt forest dieback in the Amazon and boreal forests, coupled with methane emissions from thawing permafrost, could use up 300 billion tons of that emissions budget, Lenton says.
The basic mechanisms behind these tipping points have been well-known for some years, though the predictions of the time it will take before they are activated have become much shorter. But the real new concern, says Lenton, is the identification of the potential for tipping point “cascades,” in which breaching one tipping point triggers breaches of others, leading to a rapid escalation of damage.
Lenton, Steffen, and others argued last year that 2 degrees C of warming “could activate… a domino-like cascade that could take the Earth system to even higher temperatures.” Such a change to what they called “hothouse Earth” would be irreversible, they said, even if greenhouse gas emissions were brought to zero.
The lynchpin of one such cascade, they say, is the ninth tipping point that they have identified to be active — a critical feature of the global ocean circulation system, centered in the North Atlantic and known as the Atlantic Meridional Overturning Circulation (AMOC).
“In our view, the evidence from tipping points alone suggests that we are in a state of planetary emergency,” the scientists wrote.
The AMOC is currently initiated by evaporation of warm water moving north, which leaves behind saltier, denser water that sinks to the sea bed. It is responsible for driving the ocean circulation, distributes heat around the globe, and may be the prime regulator of the climate.
Stefan Rahmstorf, an oceanographer at the University of Potsdam and a co-author of the new analysis, told e360: “The AMOC stands at the center of tipping-point cascades because of its large-scale heat transport.” It is, he says, the main reason why the Northern Hemisphere is warmer than the Southern Hemisphere. But it is being disrupted.
“Arctic warming and Greenland melting are driving an influx of fresh water into the North Atlantic,” he says. The fresher water is less dense and sinks less. Rahmstorf calculates that, as a result, the AMOC has weakened by about 15 percent since global warming took hold in 1975. “It is now at its weakest in the past millennium, or even longer,” he says.
This decline of the ocean circulation threatens to trigger other tipping points elsewhere. “A slowdown of the AMOC reduces rainfall over the Amazon basin, increasing the probability of crossing a tipping point there,” says Steffen. It could also mess with monsoon systems in Asia and West Africa, triggering drought in the Sahel. And by bringing warm waters into the Southern Ocean, it would further destabilize ice in Antarctica, unleashing an acceleration in global sea level rise.
Most climate models predict a continued weakening of the AMOC through the 21st century. It remains unclear how close it might be to a tipping point, the researchers admit. But Lenton says that historically the AMOC appears to jump between different stable states. “The question,” says co-author Johan Rockstrom, who is director of the Potsdam Institute for Climate Impact Research, “is, what are the pressure points where we might cross a threshold and trigger a state change?”
In the face of this threat, the researchers wade into the political debate about whether — as the European Parliament voted last month — the world should declare a climate emergency. “In our view, the evidence from tipping points alone suggests that we are in a state of planetary emergency,” their Nature paper concludes.
They justify this claim by attempting to define a climate emergency in mathematical terms, as a product of the extent of the threat, the probability of it happening, and the urgency, defined as how much time we have left to act. They argue that the current climate crisis fits that definition, with huge risks, increasing likelihood, and time fast running out.
This claim has drawn fire from some scientists. Hulme says such a calculation is “deeply misleading and dangerous… It is a bid by these scientists to place themselves as arbiters of whether or not we are in a climate emergency.” It is for society as a whole to decide what an emergency is, not scientists, he said.
“I am definitely not bidding to be an arbiter of climate emergency,” insists Lenton. “I am just trying to offer some scientific support for the already loud societal claims for climate emergency.” Referring to ongoing global youth protests demanding action to reduce greenhouse gas emissions, Lenton added, “The schoolkids are right.”
“We need to reach a social tipping point,” of low-carbon living, says an expert, “before we reach a planetary one.”
Hulme is also concerned about unintended consequences, such as encouraging politicians to embark on geo-engineering projects like deploying devices to shade us from solar radiation. “Calling a planet-wide emergency,” says Hulme, “can only accelerate the day when solar climate engineering is actively pursued” — something he opposes. Hulme and Lenton signed a joint statement, published in Nature Climate Change in 2015, warning of just such an eventuality.
Lenton says he remains opposed to geo-engineering, which he calls “as risky as the risks we are trying to avoid.” He thinks the threats the world faces are too great for scientists to stand on the political sidelines, especially given the world’s current failure to act to head off climate disaster.
“The current approach of the UN Climate Change Convention is a failure,” says Steffen. But he is not without hope. He believes declining fertility, innovation towards low-carbon energy, and growing movements for “greener” consumption all suggest that human society may be reaching its own tipping point in responding to the crisis. The bottom line, he says, is that “we need to reach a social tipping point, before we reach a planetary one.”