Ice caps
Once melting glaciers shut down the Gulf Stream, we would see extreme climate change within decades, study shows
Authors: René van Westen, Utrecht University; Henk A. Dijkstra, Utrecht University, and Michael Kliphuis, Utrecht University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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Superstorms, abrupt climate shifts and New York City frozen in ice. That’s how the blockbuster Hollywood movie “The Day After Tomorrow” depicted an abrupt shutdown of the Atlantic Ocean’s circulation and the catastrophic consequences.
While Hollywood’s vision was over the top, the 2004 movie raised a serious question: If global warming shuts down the Atlantic Meridional Overturning Circulation, which is crucial for carrying heat from the tropics to the northern latitudes, how abrupt and severe would the climate changes be?
Twenty years after the movie’s release, we know a lot more about the Atlantic Ocean’s circulation. Instruments deployed in the ocean starting in 2004 show that the Atlantic Ocean circulation has observably slowed over the past two decades, possibly to its weakest state in almost a millennium. Studies also suggest that the circulation has reached a dangerous tipping point in the past that sent it into a precipitous, unstoppable decline, and that it could hit that tipping point again as the planet warms and glaciers and ice sheets melt.
In a new study using the latest generation of Earth’s climate models, we simulated the flow of fresh water until the ocean circulation reached that tipping point.
The results showed that the circulation could fully shut down within a century of hitting the tipping point, and that it’s headed in that direction. If that happened, average temperatures would drop by several degrees in North America, parts of Asia and Europe, and people would see severe and cascading consequences around the world.
We also discovered a physics-based early warning signal that can alert the world when the Atlantic Ocean circulation is nearing its tipping point.
The ocean’s conveyor belt
Ocean currents are driven by winds, tides and water density differences.
In the Atlantic Ocean circulation, the relatively warm and salty surface water near the equator flows toward Greenland. During its journey it crosses the Caribbean Sea, loops up into the Gulf of Mexico, and then flows along the U.S. East Coast before crossing the Atlantic.
This current, also known as the Gulf Stream, brings heat to Europe. As it flows northward and cools, the water mass becomes heavier. By the time it reaches Greenland, it starts to sink and flow southward. The sinking of water near Greenland pulls water from elsewhere in the Atlantic Ocean and the cycle repeats, like a conveyor belt.
Too much fresh water from melting glaciers and the Greenland ice sheet can dilute the saltiness of the water, preventing it from sinking, and weaken this ocean conveyor belt. A weaker conveyor belt transports less heat northward and also enables less heavy water to reach Greenland, which further weakens the conveyor belt’s strength. Once it reaches the tipping point, it shuts down quickly.
What happens to the climate at the tipping point?
The existence of a tipping point was first noticed in an overly simplified model of the Atlantic Ocean circulation in the early 1960s. Today’s more detailed climate models indicate a continued slowing of the conveyor belt’s strength under climate change. However, an abrupt shutdown of the Atlantic Ocean circulation appeared to be absent in these climate models.
This is where our study comes in. We performed an experiment with a detailed climate model to find the tipping point for an abrupt shutdown by slowly increasing the input of fresh water.
We found that once it reaches the tipping point, the conveyor belt shuts down within 100 years. The heat transport toward the north is strongly reduced, leading to abrupt climate shifts.
The result: Dangerous cold in the North
Regions that are influenced by the Gulf Stream receive substantially less heat when the circulation stops. This cools the North American and European continents by a few degrees.
The European climate is much more influenced by the Gulf Stream than other regions. In our experiment, that meant parts of the continent changed at more than 5 degrees Fahrenheit (3 degrees Celsius) per decade – far faster than today’s global warming of about 0.36 F (0.2 C) per decade. We found that parts of Norway would experience temperature drops of more than 36 F (20 C). On the other hand, regions in the Southern Hemisphere would warm by a few degrees.
These temperature changes develop over about 100 years. That might seem like a long time, but on typical climate time scales, it is abrupt.
The conveyor belt shutting down would also affect sea level and precipitation patterns, which can push other ecosystems closer to their tipping points. For example, the Amazon rainforest is vulnerable to declining precipitation. If its forest ecosystem turned to grassland, the transition would release carbon to the atmosphere and result in the loss of a valuable carbon sink, further accelerating climate change.
The Atlantic circulation has slowed significantly in the distant past. During glacial periods when ice sheets that covered large parts of the planet were melting, the influx of fresh water slowed the Atlantic circulation, triggering huge climate fluctuations.
So, when will we see this tipping point?
The big question – when will the Atlantic circulation reach a tipping point – remains unanswered. Observations don’t go back far enough to provide a clear result. While a recent study suggested that the conveyor belt is rapidly approaching its tipping point, possibly within a few years, these statistical analyses made several assumptions that give rise to uncertainty.
Instead, we were able to develop a physics-based and observable early warning signal involving the salinity transport at the southern boundary of the Atlantic Ocean. Once a threshold is reached, the tipping point is likely to follow in one to four decades.
The climate impacts from our study underline the severity of such an abrupt conveyor belt collapse. The temperature, sea level and precipitation changes will severely affect society, and the climate shifts are unstoppable on human time scales.
It might seem counterintuitive to worry about extreme cold as the planet warms, but if the main Atlantic Ocean circulation shuts down from too much meltwater pouring in, that’s the risk ahead.
This article was updated on Feb. 11, 2024, to fix a typo: The experiment found temperatures in parts of Europe changed by more than 5 F per decade.
Originally published under Creative Commons by 360info™ .
360Info is an open access global information agency of journalists working with academics to address the world’s biggest challenges and offer practical solutions. More by 360info.
Throughout 2023, the area of ocean around Antarctica covered by sea ice was so far below the norm that scientists have struggled to communicate their shock.
This month, as the sea ice shrinks to its smallest point of the year, it is once again tracking well below its previous levels.
Research released in September 2023 shows that ocean warming was a key contributor to the dramatic change in sea ice.
The question is where the heat comes from.
A new satellite launched recently may provide the key to understanding how the ocean transports heat to Antarctica’s margins where it has a devastating impact on sea ice and ice shelves.
Sea ice insulates the ocean, reflects heat, drives currents, supports ecosystems and protects ice shelves.
Parts of the ocean 100–200m below the surface began to warm in 2015, and those same regions lost substantial sea ice in 2016. Since then, the warm subsurface ocean seems to have maintained the low sea-ice coverage.
The record-breaking low sea ice of 2023 may be the new abnormal, the beginning of the inevitable decline in Antarctic sea ice, long projected by climate models.
For millions of years, the icy continent has been ring-fenced by the Antarctic Circumpolar Current, separating the warm northern waters from the cold polar ocean.
Flowing clockwise around Antarctica and driven by westerly winds, the current is the world’s strongest, with a flow 100 times stronger than all rivers combined.
The Antarctic Circumpolar Current flows around Antarctica, keeping warm water out — but eddies can let heat through.
The current ‘feels’ the seafloor and the mountains in its path. Where it encounters barriers like ridges or seamounts, ‘wiggles’ are created in the water flow that form eddies.
Ocean eddies are the weather systems of the seas, and they play a key role in transporting heat through the circumpolar current to the ocean around Antarctica. But they’re small and hard for satellites to see.
Broad-scale ocean mapping identifies at least five major ‘heat flux gates’ or eddy hotspots in the circumpolar current.
One is south of Australia, about halfway between Tasmania and Antarctica.
To understand the ocean dynamics happening now and how these may change in the future, we need much higher-resolution data to see smaller-scale features like the eddy hotspots.
Enter the Surface Water and Ocean Topography (Swot) satellite. Jointly developed by Nasa and French space agency Centre National d’Études Spatiales (CNES), the Swot satellite measures differences in the height of the ocean within a few centimetres from an orbit of more than 890km above the surface.
The advanced radar altimeters on the two-tonne satellite detect surface water features with 10 times better resolution than previous technologies.
Oceanographers say it’s like a short-sighted person looking at a tree in the distance, and then putting on glasses to reveal all the leaves.
As Swot passes over the Southern Ocean, the high-resolution topography it records of the shape of the ocean surface shows the fine streams of current to capture the eddy hotspots spinning off the Antarctic Circumpolar Current.
This means scientists can monitor these smaller-scale circulation features thought to be responsible for transporting most of the heat and carbon from the upper ocean to deeper layers – a critical buffer against global warming.
For the first time we can see them on the surface in detail – but we still need to work out what’s happening beneath the waves.
In November 2023, scientists were able to validate the Swot satellite data from an eddy hotspot in the Southern Ocean in an ambitious voyage on CSIRO research vessel investigator.
The five-week FOCUS voyage travelled 850 nautical miles south of Hobart to the Macquarie meander, one of the five eddy hotspots.
A meander may sound gentle and slow, but in fact it’s where the world’s strongest current races through a series of hairpin bends, steered by mountains on the seafloor.
As the satellite passed overhead, the team led by CSIRO and the Australian Antarctic Program Partnership deployed a variety of high-tech observational equipment.
Researchers and crew anchored a tall mooring 3.6km high at the centre of the survey area, carrying over 54 instruments on a cable stretching from the seafloor to near the surface.
They also released free-floating autonomous instruments like floats, drifters and gliders into the eddies, while more than a hundred CTDs – conductivity, temperature, and depth sensors – plumbed the depths and a Triaxus was towed behind the ship through the satellite’s path.
Researchers use a variety of instruments to understand the ocean. Some float along the surface, some dive deep in the water, and some follow directed paths using motors.
The wealth of information gathered by all these instruments ‘ground-truths’ and validates the satellite data from the surface.
The Antarctic is rapidly changing, and with further disruptions to the sea-ice cycle on the cards, there’s a race to understand why.
Strong winds over the Southern Ocean have been increasing for decades and are likely to continue. It’s expected this will send more heat southward through leaky meanders, accelerating ice shelf melting in Antarctica and sea level rise.
Ultimately, this research aims to turn daily maps of ocean sea surface height from satellites into daily maps of the movement of heat in the Southern Ocean toward Antarctica.
This is vital information in a climate crisis. It will help governments plan how to respond to ocean warming and rising sea levels and how quickly action is needed.
At the same time, as the transition to a net-zero world gathers momentum and carbon levels in the atmosphere start to level out, we need to be able to track the response of the Southern Ocean and the global climate system.
Ariaan Purich is a lecturer in the School of Earth, Atmosphere and Environment at Monash University, and a Chief Investigator with the Australian Research Council Special Research Initiative Securing Antarctica’s Environmental Future. She has worked previously with CSIRO.
Edward Doddridge is a physical oceanographer working in the Australian Antarctic Program Partnership (AAPP) based at the Institute for Marine and Antarctic Studies (IMAS) in Hobart, Tasmania.
Benoit Legresy works with CSIRO as climate scientist leading the Oceans group and is a co-leader of the oceanography project with the Australian Antarctic Program Partnership.
Edward Doddridge (University of Tasmania) and Ariaan Purich (Monash University) both receive funding from the Australian Research Council.
The research of the FOCUS voyage is supported by a grant of sea time on RV Investigator from the CSIRO Marine National Facility which is supported by the Australian Government’s National Collaborative Research Infrastructure Strategy (NCRIS).
The Australian Antarctic Program Partnership is funded by the Australian Government Department of Climate Change, Energy, the Environment and Water through the Antarctic Science Collaboration Initiative.
Authors:
Ariaan Purich is a lecturer in the School of Earth, Atmosphere and Environment at Monash University, and a Chief Investigator with the Australian Research Council Special Research Initiative Securing Antarctica’s Environmental Future. She has worked previously with CSIRO.
Edward Doddridge is a physical oceanographer working in the Australian Antarctic Program Partnership (AAPP) based at the Institute for Marine and Antarctic Studies (IMAS) in Hobart, Tasmania.
Benoit Legresy works with CSIRO as climate scientist leading the Oceans group and is a co-leader of the oceanography project with the Australian Antarctic Program Partnership.
Edward Doddridge (University of Tasmania) and Ariaan Purich (Monash University) both receive funding from the Australian Research Council.
The research of the FOCUS voyage is supported by a grant of sea time on RV Investigator from the CSIRO Marine National Facility which is supported by the Australian Government’s National Collaborative Research Infrastructure Strategy (NCRIS).
The Australian Antarctic Program Partnership is funded by the Australian Government Department of Climate Change, Energy, the Environment and Water through the Antarctic Science Collaboration Initiative.
Authors:
YanaBu, Shutterstock
Katrin Meissner, UNSW Sydney; Deepashree Dutta, University of Cambridge, and Martin Jucker, UNSW Sydney
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Our planet has warmed by about 1.2°C since 1850. But this warming is not uniform. Warming at the poles, especially the Arctic, has been three to four times faster than the rest of the globe. It’s a phenomenon known as “polar amplification”.
Climate models simulate this effect, but when tested against the past 40 years of warming, these models fall short. The situation is even worse when it comes to modelling past climates with very high levels of greenhouse gases.
This is a problem because these are the same models used to project into the future and forecast how the climate will change. They are likely to underestimate what will happen later this century, including risks such as ice sheet melting or permafrost thawing.
In our new research published today in Nature Geoscience we used a high-resolution model of the atmosphere that includes the stratosphere. We found a special type of cloud appears over polar regions when greenhouse gas concentrations are very high. The role of this type of cloud has been overlooked so far. This is one of the reasons why our models are too cold at the poles.
Polar Stratospheric Clouds over Norway (Night Lights Films – Adrien Mauduit)
Back to the future
Looking into past climates can give us glimpses of possible futures for a range of extreme conditions. For us, this means we can use Earth’s history to find out how well our climate models perform. We can test our models by simulating episodes in the past when Earth was much warmer. The advantage of this is that we have temperature reconstructions for these episodes to evaluate the models, as opposed to the future, for which measurements are not available.
If we go back 50 million years or so, our planet was very hot. Carbon dioxide (CO₂) concentrations ranged between 900 and 1,900 parts per million (ppm), compared with 415 ppm today. Methane (CH₄) concentrations were likely also much higher.
Canada’s arctic archipelago was covered in lush rainforests inhabited by alligators, turtles, lizards and mammals.
For these plants and animals to survive, conditions must have been warm and ice-free year-round. Indeed, surface ocean temperatures exceeded 20°C near the north pole (at about 87°N) and 25°C in the Southern Ocean (at about 67°S).
This period called the early Eocene is a perfect test bed for our models, because it was globally very warm, and the poles were even warmer, meaning it was a climate with extreme polar amplification. In addition, the Eocene is recent enough for temperature reconstructions to be available.
But as it turns out, the models fail again. They are much too cold at high latitudes. What are our models missing?
Polar stratospheric clouds
In 1992 American paleoclimatologist Lisa Sloan suggested polar stratospheric clouds might have caused extreme warming at high latitudes in the past.
These clouds are a rare and beautiful sight today. They are also called nacreous or mother-of-pearl clouds for their vivid and sometimes luminous colours.
They form at very high altitudes (in the stratosphere) and at very low temperatures (over the poles). In the present day climate, they appear mainly over Antarctica, but have also been observed during winter months over Scotland, Scandinavia and Alaska, at times when the stratosphere was particularly cold.
Just like greenhouse gases, they absorb infrared radiation emitted by the Earth’s surface and re-emit a portion of this energy back to the surface. This suggests polar stratospheric clouds could be one of the missing puzzle pieces.
They warm the surface. And their effect could be significant, especially in winter, when the sun does not rise. But they are difficult to simulate in a climate model, so most models ignore them. This omission could explain why climate models miss some of the polar warming, because they miss a process that warms the poles.
Three decades after Sloan’s paper, a few atmosphere models are finally complex enough to allow us to test her hypothesis. In our research we use one of them and find that under certain conditions, the additional warming due to these polar stratospheric clouds exceeds 7°C during the winter months. This significantly reduces the gap between climate models and temperature evidence from the early Eocene. Sloan was right.
Implications for future projections
Our research explains why climate models don’t work so well for past climates when greenhouse gas levels were much higher than they are today. But what about the future? Should we be concerned?
There is some good news. While polar stratospheric clouds do warm the poles, they won’t be as common in the future as they were in the distant past, even if both CO₂ and CH₄ reach very high levels.
This is due to another difference between the Eocene and today: the position of continents and mountains, which were different back then and which also influence the formation of polar stratospheric clouds. So even if we hit early Eocene levels of CH₄ and CO₂ in the future, we would expect less polar stratospheric cloud to be formed. This suggests the standard climate models are better at predicting the future than the past.
It’s therefore unlikely the Arctic and Antarctica will be covered by these beautiful clouds anytime soon. But our research shows evidence from past climates can reveal processes that only become important when greenhouse gas concentrations are high. Some of these processes are not included in our models because models are tested against present day observations and other processes simply seemed more important to include. Looking into the past is a way of broadening our horizon and learning for the future.
Authors: Kaitlin Naughten, Ocean-Ice Modeller, British Antarctic Survey; Jan De Rydt, Associate Professor of Polar Glaciology and Oceanography, Northumbria University, Newcastle, and Paul Holland, Ocean and Ice Scientist, British Antarctic Survey
This article is republished from The Conversation under a Creative Commons license. Read the original article.
The rate at which the warming Southern Ocean melts the West Antarctic ice sheet will speed up rapidly over the course of this century, regardless of how much emissions fall in coming decades, our new research suggests. This ocean-driven melting is expected to increase sea-level rise, with consequences for coastal communities around the world.
The Antarctic ice sheet, the world’s largest volume of land-based ice, is a system of interconnected glaciers comprised of snowfall that remains year-round. Coastal ice shelves are the floating edges of this ice sheet which stabilise the glaciers behind them. The ocean melts these ice shelves from below, and if melting increases and an ice shelf thins, the speed at which these glaciers discharge fresh water into the ocean increases too and sea levels rise.
In West Antarctica, particularly the Amundsen Sea, this process has been underway for decades. Ice shelves are thinning, glaciers are flowing faster towards the ocean and the ice sheet is shrinking. While ocean temperature measurements in this region are limited, modelling suggests it may have warmed as a result of climate change.
We chose to model the Amundsen Sea because it is the most vulnerable sector of the ice sheet. We used a regional ocean model to find out how ice-shelf melting will change here between now and 2100. How much melting can be prevented by reducing carbon emissions and slowing the rate of climate change – and how much is now unavoidable, no matter what we do?
Rapid change is locked in
We used the UK’s national supercomputer ARCHER2 to run many different simulations of the 21st century, totalling over 4,000 years of ocean warming and ice-shelf melting in the Amundsen Sea.
We considered different trajectories for fossil fuel burning, from the best-case scenario where global warming is limited to 1.5°C in line with the Paris Agreement, to the worst, in which coal, oil and gas use is uncontrolled. We also considered the influence of natural variations in the climate, such as the timing of events such as El Niño.
The results are worrying. In all simulations there is a rapid increase over the course of this century in the rate of ocean warming and ice-shelf melting. Even the best-case scenario in which warming halts at 1.5°C, something that is considered ambitious by many experts, entails a threefold increase in the historical rate of warming and melting.
What’s more, there is little to no difference between the scenarios up to 2045. Ocean warming and ice-shelf melting in the 1.5°C scenario is statistically the same as in a mid-range scenario, which is closer to what existing pledges to reduce fossil fuel use over the coming decades would produce.
The worst-case scenario shows more melting than the others, but only from around mid-century onwards, and many experts think this amount of future fossil fuel burning is unrealistic anyway.
The results imply that we are now committed to rapid ocean warming in the Amundsen Sea until at least 2100, regardless of international policies on fossil fuels.
The increases in warming and melting are the result of ocean currents strengthening and driving more warm water from the deep ocean towards the shallower ice shelves along the coast. Other studies have suggested this process is behind the ice shelf thinning measured by satellites.
How much will the sea level rise?
Melting ice shelves are a major cause of sea-level rise, but not the whole story. We can’t put a number on how much sea levels will rise without also simulating the flow of Antarctic glaciers and the rate of snow accumulating on the ice sheet, which our model didn’t include.
But we have every reason to believe that increased ice-shelf melting in this region will cause the rate at which sea levels are rising to speed up.
The West Antarctic ice sheet is already contributing substantially to global sea-level rise and is losing about 80 billion tonnes of ice a year. It contains enough ice to cause up to 5 metres of sea-level rise, but we don’t know how much of it will melt, and how quickly. Our colleagues around the world are working hard to answer this question.
Courage and hope
There are some consequences of climate change that can no longer be avoided, no matter how much fossil fuel use falls. Substantial melting of West Antarctica up to 2100 may now be one of them.
How do you tell a bad news story? The conventional wisdom is that you’re supposed to give people hope: to say that there’s a disaster behind one door, but we can avoid it if only we choose a different one. What do you do when your science tells you that all doors lead to the same disaster?
Kate Marvel, an atmospheric scientist, said that when it comes to climate change, “we need courage, not hope … Courage is the resolve to do well without the assurance of a happy ending”. In this case, courage means shifting our attention to the longer term.
The future will not end in 2100, even if most people reading this will no longer be around. Our simulations of the 1.5°C scenario show ice-shelf melting starting to plateau by the end of the century, suggesting that further changes in the 22nd century and beyond may still be preventable. Reducing sea-level rise after 2100, or even slowing it down, could save many coastal cities.
Courage means accepting the need to adapt, protecting coastal communities where it’s possible to do so, and rebuilding or abandoning them where it’s not. By predicting future sea-level rise in advance, we’ll have time to plan for it – rather than wait until the ocean is on our doorstep.
Kaitlin Naughten, Ocean-Ice Modeller, British Antarctic Survey; Jan De Rydt, Associate Professor of Polar Glaciology and Oceanography, Northumbria University, Newcastle, and Paul Holland, Ocean and Ice Scientist, British Antarctic Survey
This article is republished from The Conversation under a Creative Commons license. Read the original article.
The situation in Antarctica, both what’s currently being observed and the latest research, and the consequences are succinctly explained by Prof. Nerilie Abrahm from the Australian National University. Further consequences are discussed in the second half of the video.