Ice sheet melting: it’s not just about sea level rise

Originally published at The Science Breaker

Climate change is causing the Greenland and Antarctic Ice Sheets to melt, which releases cold, fresh meltwater into the nearby ocean. This meltwater causes sea level rise, but a lesser-known side effect is the disruption of deep ocean currents and climate patterns worldwide. Our modelling study investigated these processes.

You’ve probably heard that climate change is melting the polar ice caps – but what does this actually mean? It refers to the Greenland and Antarctic Ice Sheets, which are large systems of interconnected glaciers, kilometres thick. They are formed by snow falling on land, which compacts into ice and slowly flows downhill towards the ocean. When the ice sheets come into contact with a warming atmosphere or ocean, they begin to melt faster than new ice can form. This releases cold, fresh meltwater into the surrounding ocean. The most well-known consequence of this process is sea-level rise, as the volume of the ocean increases. Unfortunately, there are other side effects beyond sea level rise.

The oceans around Greenland and Antarctica are unusual because they are the only regions of the world’s oceans with significant vertical mixing. Everywhere else, the ocean is stratified, forming layers of water organised by density, with the lightest water at the surface and the heaviest water at the seafloor. The layers don’t interact with each other very much. But in a few locations around the coast of Antarctica, as well as in the North Atlantic Ocean near Greenland, surface water becomes cold and salty enough to sink into the deep ocean. Then it slowly travels around the world for about a thousand years, like a deep-ocean conveyor belt, before resurfacing. This process of “deep water formation”, occurring in just a few regions, affects deep ocean currents which transport heat around the world and influence climate patterns worldwide. But what happens when ice sheet meltwater is released into these deep water formation regions? How are the ocean currents and climate patterns affected?

Our study addressed this question using two different models: an ice sheet model, which simulates the flow of the glaciers making up the Greenland and Antarctic Ice Sheets, and a climate model, which simulates the global atmosphere, ocean, sea ice, and vegetation. Both models run on supercomputers and solve huge numbers of physics equations. Our study was novel because the ice sheet model and the climate model were able to communicate with each other, exchanging information regularly throughout the simulations.

We ran a number of different simulations over the 21st century, using several different scenarios for fossil fuel emissions, which might decline in the future but might continue to grow. In some experiments, the climate model considered the effect of ice sheet meltwater in its calculations; in other experiments, it ignored the meltwater. This allowed us to isolate the impact of ice sheet meltwater on the climate system.

In our simulations, ice sheet melting slowed down the rate of nearby deep water formation. The fresh meltwater reduced the density of the surface ocean, making it more difficult for surface waters to sink. In the North Atlantic, this reduction in deep water formation altered the pathways of nearby ocean currents. The Gulf Stream, which travels up the east coast of North America, and its extension the North Atlantic Drift, which cuts across the Atlantic towards Europe, were redirected such that less heat was transported from North America to Europe. While both locations still warmed (due to climate change), eastern North America experienced a bit of extra warming, while in Europe some of the warmings were canceled out. Furthermore, temperatures became more variable in many regions, indicating a greater prevalence of extreme weather.

Around Antarctica, deep water formation connects the cold atmosphere to the warmer subsurface ocean. This allows the ocean to release heat, warming the atmosphere while cooling the deep ocean. In our simulations, a reduction in deep water formation suppressed this effect, trapping heat beneath the ocean surface. Ice sheet melting, therefore, caused the atmosphere around Antarctica to warm less than it otherwise would have, while the subsurface ocean warmed more dramatically. This result is particularly troubling because the Antarctic Ice Sheet is in contact with these regions of the ocean. It suggests a vicious cycle whereby ice sheet melting causes subsurface ocean warming, which causes more ice sheet melting, and so on.

It is now clear that the effects of ice sheet melting are not just limited to sea-level rise. We can expect ice sheet meltwater to have many more side effects, on ocean currents and weather patterns. But how much will the ice sheets actually melt? That depends on fossil fuel emissions, and how they change in the future. Our study used models, but the same experiment is currently being run in the real world, in real-time.

Original Article:
N. R. Golledge et al., Global environmental consequences of twenty-first-century ice-sheet melt. Nature 566, 65-72 (2019)

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How I became a scientist

For the first fourteen years of my life, I thought science was boring. As far as I could tell, science was a process of memorising facts: the order of the planets, the names of clouds, the parts of a cell. Sometimes science meant building contraptions out of paper and tape to allow an egg to survive a two-metre fall, and I was really terrible at that sort of thing. So instead I spent all my spare time reading and writing, and decided to be a novelist when I grew up.

This all changed in my first year of secondary school, when I met the periodic table. I don’t know what it says about me that my first spark of interest in science boiled down to “This chart is very nicely organised”. (As anyone who has seen my home library will attest, I really like organising things.) The periodic table quickly acted as a gateway drug to basic physics and chemistry. Science still meant memorising facts, but suddenly they were much more interesting facts.

In the next year of secondary school, maths also became interesting. Until then, maths had been easy to the point of tedium. Most of my time in maths had been spent triple-checking my answers. But now maths was streamed into three different courses, and I chose the most difficult one, and it was wonderful. There is nothing quite as exhilarating as being challenged for the first time.

So now I had a dilemma. I wasn’t so interested in being a novelist any more, and I really liked maths and science. But at my school, all the best maths and science students went on to be doctors. Whereas I was so squeamish about medical things that I would hide from the television whenever my older sisters watched ER. I was also something of a hypochondriac. These are not qualities which are prized by the medical profession.

It was very important for a teenager in the early 2000s to know exactly what they wanted to be when they grew up, so I worried about this a lot. For a while I tried to convince myself to be a doctor anyway. I had no interest in dentistry or pharmacy, which were the other options presented to me. I seriously considered becoming an optometrist, but the faint possibility that I might have to deal with an eyeball that had popped out of someone’s head was enough to turn me against the idea. Some of the strong maths and science students at my school had gone on to become engineers, but I thought that probably involved the same sorts of skills as building egg-protecting devices.

At the same time as this inner turmoil, something else was going on: I was becoming interested in the environment. This was mostly a result of peer pressure. There was a very cool group of students, most in the year above mine, who had started an environmental club. Once a week, I came to school extra early in the morning to hang out with them at club meetings. And we had long and fascinating discussions, ranging from the best way to save water in the school’s bathrooms to environmental policy in the Canadian government.

I started to wonder if there was a career path which connected the environment with maths and science. I went on my school’s career-matching website to find out, and filled in the questionnaire. The website recommended I become a chemist who tested water samples from industrial plants to make sure they weren’t polluting the local environment. I wasn’t particularly inspired by this idea. I remember reading over all the other careers on the website, but I don’t remember seeing anything about academia or scientific research. And, I mean, fair enough. Given the massive oversupply of PhDs in the modern world, I understand why schools wouldn’t want to funnel students in that direction.

Meanwhile, back in the environmental club, names were being drawn out of a hat. One of the local universities was holding a climate change conference for secondary school students, and my school had been allocated three places. I was one of the lucky ones, and a few weeks later I rode the bus to the city centre for the conference.

The first presentation was called “The Science of Climate Change” and it was delivered by Danny Blair, a climatologist at the university. He talked about many different things and all of them were fascinating and I scrawled tiny notes in a tiny notebook as quickly as I could. But I particularly remember him explaining how scientists can use ice cores to figure out the temperature from hundreds of thousands of years ago. In short, oxygen has different isotopes, some of which are heavier than others. When the oxygen atoms join H2O molecules, they form “heavy water” and “light water”. Heavy water needs more energy, and therefore a higher temperature, to evaporate from the ocean and eventually fall as precipitation somewhere else. So by measuring the ratio of heavy water to light water in the ice cores, you can figure out what the global temperature was when each layer of snow fell.

Sitting there with my tiny notebook, I thought this was just the most fascinating thing I’d ever heard. This was the very first time I’d seen a practical application of the periodic table which brought me joy and excitement, rather than despair that I might end up testing water samples for the rest of my life. And it slowly dawned on me that this job called “scientist” basically meant you could study whatever you found interesting, and get paid to do so. “Right then,” I thought, “I’ll be a scientist.”

It’s eleven years later and I still haven’t changed my mind. I didn’t become an ice core scientist, but I did end up studying a different part of the climate system which I found even more interesting. Academia is not perfect, but there is no other way I’d rather spend my working life. Far from memorising an endless stream of facts, it turns out that science is full of creativity and solving mysteries. My work is always changing and growing, and I never get bored.

How does the Weddell Polynya affect Antarctic ice shelves?

The Weddell Polynya is a large hole in the sea ice of the Weddell Sea, near Antarctica. It occurs only very rarely in observations, but is extremely common in ocean models, many of which simulate a near-permanent polynya. My new paper published today in Journal of Climate finds that the Weddell Polynya increases melting beneath the nearby Filchner-Ronne Ice Shelf. This means it’s important to fix the polynya problems in ocean models, if we want to use them to study ice shelves.

The Southern Ocean surrounding Antarctica is cold at the surface – often so cold that it freezes to form sea ice – but warmer below. The deep ocean is about 1°C, which might not sound warm to you, but to Antarctic oceanographers this is positively balmy. If regions of the Southern Ocean start to convect, with strong top-to-bottom mixing, the warm deep water will come to the surface and melt the sea ice.

In observations, this doesn’t happen very often, and it only seems to happen in one region: the Weddell Sea, in the Atlantic sector of the Southern Ocean. Satellites spotted a large polynya (about the size of the UK) for three winters in a row, from 1974-1976. But then the Weddell Polynya disappeared until 2017, when a much smaller polynya (about a tenth of the size) showed up for a few months in the spring. We haven’t seen it since.

holland_polynya_1975

The Weddell Polynya in the winter of 1975. (Holland et al., 2001)

nsidc_polynya_2017

The Weddell Polynya in the spring of 2017. (NSIDC)

By contrast, models of the Southern Ocean simulate Weddell Polynyas very enthusiastically. In many ocean models, it’s a near-permanent feature of the Weddell Sea, and is often much larger than the observed polynya from the 1970s. This can happen very easily if the model’s surface waters are slightly too salty, which makes them dense enough to sink, triggering top-to-bottom convection. We also think it might have something to do with imperfect vertical mixing schemes.

It’s a rite of passage for Southern Ocean modellers that sooner or later you will work with a model that forms massive polynyas, all the time, and you can’t make them go away. I spent months and months on this during my PhD, and eventually I gave up and did “surface salinity restoring” to prevent the salty bias from forming. Basically, I killed it with freshwater. If you throw enough freshwater at this problem, the problem will go away.

So when the little Weddell Polynya of 2017 showed up, I was paying attention. And when the worldwide oceanography community jumped on the idea and started publishing lots of papers about the Weddell Polynya, I was paying attention. But soon I noticed that there was an important question nobody was trying to answer: what does the Weddell Polynya mean for Antarctic ice shelves?

Ice shelves are the floating edges of the Antarctic Ice Sheet. They’re in direct contact with the ocean, and they slow down the flow of the glaciers behind them. Ice shelves are what stand between us and massive sea level rise, so we should give them our respect. But ocean modellers have largely neglected them until now, because ice shelf cavities – the pockets of ocean between the ice shelf and the seafloor – are quite difficult to model. This is changing as supercomputers improve and high resolution becomes more affordable. More and more ocean models are adding ice shelf cavities to their simulations, and calculating melt rates at the ice-ocean interface. So if it turns out that the Weddell Polynya contaminates these ice shelf cavities, it would be even more important to fix the models’ polynya biases. It would also be interesting from an observational perspective, especially if the polynya shows up again soon.

At the time I started wondering about the Weddell Polynya and ice shelves, I was conveniently already setting up a new model of the Weddell Sea, which includes ice shelves. This model doesn’t produce Weddell Polynyas spontaneously, and for that I am eternally grateful. But I found a way to create “idealised” polynyas in the model, by choosing particular regions and forcing the model to convect there, whether or not it wanted to. This way I had control over where the polynyas occurred, how large they were, and how long they stayed open. I could run simulations with polynyas, compare them to a simulation with no polynyas, and see how the ice shelf cavities were affected.

I found that Weddell Polynyas do increase melt rates beneath nearby ice shelves. This happens because the polynyas cause density changes in the ocean, which allows more warm, salty deep water to flow onto the Antarctic continental shelf. The melt rates increase the larger the polynya gets, and the longer it stays open. This is bad news for Southern Ocean models with massive, permanent polynyas.

First I looked at the Filchner-Ronne Ice Shelf (FRIS), the second-largest ice shelf in Antarctica, and the focus of my Weddell Sea research these days. On the continental shelf in front of FRIS, the sea ice formation is so strong that the warm signal from the Weddell Polynya gets wiped out. The water ends up at the surface freezing point anyway, and the extra heat is lost to the atmosphere. But the salty signal is still there, and these salinity changes cause the ocean currents beneath FRIS to speed up. Stronger circulation means stronger ice shelf melting, in this case by up to 30% for the largest Weddell Polynyas.

For smaller ice shelves in the Eastern Weddell Sea, the nearby sea ice formation is weaker. So both the warm signal and the salty signal from the Weddell Polynya are preserved, and the ice shelf cavities are flooded with warmer, saltier water. Melting beneath these ice shelves increases by up to 80%.

The modelled changes are smaller for Weddell Polynyas which match observations, in terms of size as well as duration. So if the Weddell Polynya of the 1970s affected the FRIS cavity, it probably wasn’t by very much. And the effect of the little 2017 polynya was probably so small that we’ll never detect it.

However, these results should send a message to Southern Ocean modellers: you really need to fix your polynya problem if you want to model ice shelf cavities. I’m sorry.

Climate change and compassion fatigue

I’m a climate scientist, and I don’t worry about climate change very much. I think about it every day, but I don’t let it in. To me climate change is a fascinating math problem, a symphony unfolding both slowly and quickly before our very eyes. The consequences of this math problem, for myself and my family and our future, I keep locked in a tiny box in my brain. The box rarely gets opened.

The latest IPCC special report tells the world what I and all of my colleagues have known for years: we’re seriously running out of time. In order to keep climate change in the category of “expensive inconvenience” rather than “civilisation destroyer”, we’re going to have to decarbonise the global economy in less time than many of the people reading this have been alive. But given the priorities of most of the world’s governments, it seems uncomfortably plausible that we’ll be facing the sort of post-apocalyptic wasteland I’ve only ever seen in movies. Will the rich and privileged countries be able to buy their way out of this crisis? Maybe. But maybe not.

I know all this. I’ve known it for years and it’s why I chose the career that I did. It’s the backdrop to my every working day. But I can’t seem to imagine my future intersecting with this future. I can’t picture myself or my family as part of the movie, only as part of the audience. It feels deeply intangible, like my own death.

Instead I surround myself with the comforting minutia of academic life. I worry about small things, like how I’m going to fix the latest problem with my model, and slightly larger things, like what I’m going to do when my contract runs out and whether I will ever get a permanent job. But mostly I just really enjoy studying the disaster. An ice sheet which is falling apart is far more interesting than a stable ice sheet, and I feel privileged to have access to such a good math problem. So I work until my brain feels like it might turn into liquid and slide out of my ears, then I cycle home in the mist and eat Cornish pasties on the couch with my husband while watching the BBC. In so many ways, I love this life. And I don’t worry about climate change, I don’t open that box, for months at a time.

“Compassion fatigue” is a term used to describe healthcare professionals who become desensitised to tragedy and suffering, and lose the ability to empathise with their patients. It begins as a coping strategy, because fully absorbing the emotional impact of such harrowing work would eventually make it impossible to get up in the morning. I think I have compassion fatigue with climate change. The more I study it, the less I actually think about it. The scarier it gets, the less I seem to care.

And maybe this is okay. Maybe compartmentalisation is the healthiest response for those of us close to the issue. Accept the problem, fully let it in, and decide what you’re going to do to help. Then lock up that box in your brain and get on with your piece of the fight. Find joy in this wherever you can. Open up the box once in a while, to remind yourself of your motivation. But for the most part ignore the big picture and keep yourself healthy and happy so that you can keep going. Even if this, in and of itself, is a form of denial.

Future projections of Antarctic ice shelf melting

Climate change will increase ice shelf melt rates around Antarctica. That’s the not-very-surprising conclusion of my latest modelling study, done in collaboration with both Australian and German researchers, which was just published in Journal of Climate. Here’s the less intuitive result: much of the projected increase in melt rates is actually linked to a decrease in sea ice formation.

That’s a lot of different kinds of ice, so let’s back up a bit. Sea ice is just frozen seawater. But ice shelves (as well as ice sheets and icebergs) are originally formed of snow. Snow falls on the Antarctic continent, and over many years compacts into a system of interconnected glaciers that we call an ice sheet. These glaciers flow downhill towards the coast. If they hit the coast and keep going, floating on the ocean surface, the floating bits are called ice shelves. Sometimes the edges of ice shelves will break off and form icebergs, but they don’t really come into this story.

Climate models don’t typically include ice sheets, or ice shelves, or icebergs. This is one reason why projections of sea level rise are so uncertain. But some standalone ocean models do include ice shelves. At least, they include the little pockets of ocean beneath the ice shelves – we call them ice shelf cavities – and can simulate the melting and refreezing that happens on the ice shelf base.

We took one of these ocean/ice-shelf models and forced it with the atmospheric output of regular climate models, which periodically make projections of climate change from now until the end of the century. We completed four different simulations, consisting of two different greenhouse gas emissions scenarios (“Representative Concentration Pathways” or RCPs) and two different choices of climate model (“ACCESS 1.0”, or “MMM” for the multi-model mean). Each simulation required 896 processors on the supercomputer in Canberra. By comparison, your laptop or desktop computer probably has about 4 processors. These are pretty sizable models!

In every simulation, and in every region of Antarctica, ice shelf melting increased over the 21st century. The total increase ranged from 41% to 129% depending on the scenario. The largest increases occurred in the Amundsen Sea region, marked with red circles in the maps below, which happens to be the region exhibiting the most severe melting in recent observations. In the most extreme scenario, ice shelf melting in this region nearly quadrupled.

Percent change in ice shelf melting, caused by the ocean, during the four future projections. The values are shown for all of Antarctica (written on the centre of the continent) as well as split up into eight sectors (colour-coded, written inside the circles). Figure 3 of Naughten et al., 2018, © American Meteorological Society.

So what processes were causing this melting? This is where the sea ice comes in. When sea ice forms, it spits out most of the salt from the seawater (brine rejection), leaving the remaining water saltier than before. Salty water is denser than fresh water, so it sinks. This drives a lot of vertical mixing, and the heat from warmer, deeper water is lost to the atmosphere. The ocean surrounding Antarctica is unusual in that the deep water is generally warmer than the surface water. We call this warm, deep water Circumpolar Deep Water, and it’s currently the biggest threat to the Antarctic Ice Sheet. (I say “warm” – it’s only about 1°C, so you wouldn’t want to go swimming in it, but it’s plenty warm enough to melt ice.)

In our simulations, warming winters caused a decrease in sea ice formation. So there was less brine rejection, causing fresher surface waters, causing less vertical mixing, and the warmth of Circumpolar Deep Water was no longer lost to the atmosphere. As a result, ocean temperatures near the bottom of the Amundsen Sea increased. This better-preserved Circumpolar Deep Water found its way into ice shelf cavities, causing large increases in melting.

Slices through the Amundsen Sea – you’re looking at the ocean sideways, like a slice of birthday cake, so you can see the vertical structure. Temperature is shown on the top row (blue is cold, red is warm); salinity is shown on the bottom row (blue is fresh, red is salty). Conditions at the beginning of the simulation are shown in the left 2 panels, and conditions at the end of the simulation are shown in the right 2 panels. At the beginning of the simulation, notice how the warm, salty Circumpolar Deep Water rises onto the continental shelf from the north (right side of each panel), but it gets cooler and fresher as it travels south (towards the left) due to vertical mixing. At the end of the simulation, the surface water has freshened and the vertical mixing has weakened, so the warmth of the Circumpolar Deep Water is preserved. Figure 8 of Naughten et al., 2018, © American Meteorological Society.

This link between weakened sea ice formation and increased ice shelf melting has troubling implications for sea level rise. The next step is to simulate the sea level rise itself, which requires some model development. Ocean models like the one we used for this study have to assume that ice shelf geometry stays constant, so no matter how much ice shelf melting the model simulates, the ice shelves aren’t allowed to thin or collapse. Basically, this design assumes that any ocean-driven melting is exactly compensated by the flow of the upstream glacier such that ice shelf geometry remains constant.

Of course this is not a good assumption, because we’re observing ice shelves thinning all over the place, and a few have even collapsed. But removing this assumption would necessitate coupling with an ice sheet model, which presents major engineering challenges. We’re working on it – at least ten different research groups around the world – and over the next few years, fully coupled ice-sheet/ocean models should be ready to use for the most reliable sea level rise projections yet.

A modified version of this post appeared on the EGU Cryospheric Sciences Blog.

Life after PhD

To continue my tradition of trying out all the Commonwealth countries, since my last post I have moved to the UK and begun a postdoc at the British Antarctic Survey in Cambridge. The UK is far nicer than Australians will lead you to believe – there are indeed sunny days, and gorgeous coastline, and great wildlife. None of these things are quite at Australian levels, but there are other things that at least partially make up for it. Like central heating, and the absence of huntsman spiders.

My PhD is now completely wrapped up, and I can officially use the title Dr., so I get very excited about filling in forms. For my postdoc I’m continuing to study interactions between Antarctic ice shelves and the ocean, but using a different ocean model (MITgcm), and focusing on a specific region (the Filchner-Ronne Ice Shelf in the Weddell Sea). This project includes some ice-sheet/ocean coupling, which I’m enormously, ridiculously excited about.

A postdoc is far more relaxing than a PhD, and far less existential. I know I’m only a few months in, but many of my colleagues hold a similar opinion. At last, there is no monolithic Thesis that everything is building up to, no pressure for all your research threads to converge into a coherent narrative before your scholarship runs out, no need to justify your continued existence (“how long have you been here, again?”) There is just a period of time for which your postdoc is funded, and you do as much science as you can during that time. You have more confidence in your own abilities, since you’ve done vaguely similar things before, and everyone else seems to take you more seriously too.

Much has been written on the mental health risks of doing a PhD, both in the scientific literature and in the media. I won’t pretend to be an alarming example of this, because many students have a much, much harder time than I did. But I did operate under elevated stress during the last year and a half of my PhD, and I noticed the effect this had on my life. Regular exercise was very effective in keeping my spirits up, but it didn’t really help the insomnia.

Here’s the pleasantly surprising bit: these effects appear to go away when you finish your PhD. I don’t know what else I expected – that I would be scarred for life? All I know is that I’ve slept well nearly every night since the day I submitted my thesis. And when I look at my giant list of things to do with my model, I don’t feel overwhelmed. I just feel excited.