Glaciers are disappearing around the planet, and the questions of how scientists monitor glaciers and how these glaciers affect global sea level rise are very important for the future.
For The Big Questions, IFLScience’s podcast, we spoke to Dr Peter Davis, a physical oceanographer from the British Antarctic Survey to discuss these questions and his research on Thwaites Glacier in Antarctica.
How do you monitor the changes in ice shelves and glaciers?
Peter Davis (PD): There are two or three different ways. The first and most indirect way is through satellite automations. Whizzing around the planet way up in space there are lots of satellites that we use to observe the ice shelves. We can look at their thickness, we can look at simple imagery of them, and see how they’re responding in time.
Then there’s what I do – in-situ observation a.k.a. deep fieldwork. We go to the ice shelves and the glaciers and we either observe them from the surface, or we take observations of snowfall and weather. We use seismic, using sound essentially to understand the makeup and how thick they are.
We also observe the ocean underneath. Using techniques known as hot water drilling to drill through the ice shelf, we deploy instruments in the ocean cavities underneath the floating ice shelves. These instruments let us monitor how the ocean is changing, how it’s circulating, its temperature, and how the ice shelf is melting from underneath. We use all that information to build a picture of how the ice shelves and glaciers are evolving over time.
Why is it important to understand how the glacier or ice shelf is melting from beneath?
(PD): Essentially, the key idea is that glaciers flow in off the continent of Antarctica, like massive rivers of ice. They’re very slow moving but they drift towards the ocean and when the ice that’s on the ground gets into the ocean, it causes sea levels to rise.
Ice shelves, which are floating extensions of these glaciers, act as a cork that holds back the ice that’s on the land. They prevent the ice from getting into the ocean and therefore they control sea level rise. The problem is when you melt an ice shelf from beneath, it weakens its ability to hold the ice back on the land and it allows the sea level rise to occur more quickly.
When we observe an ice shelf or glacier from the surface, what we are essentially looking at is how the surface is changing. It’s basically changing its height, but that integrates a lot of different processes, of which melt is just one part. We have to make a lot of assumptions about the other processes in order to determine how much of it is melting.
It’s a much better, although certainly harder and more logistically intense, operation to go to the ice shelf and observe its melt rate directly, either using radars we put on the surface or drilling through and putting instruments looking up at it from beneath to measure the melt rate directly.
What are the changes we are seeing in Antarctica that are affecting global sea level rise?
(PD): We are seeing that warm water that is offshore of the Antarctic continent is being forced onto the continental shelves and underneath the ice shelves more rapidly. When ice shelves are in balance, the amount of ice or grounded ice that crosses over the grounding line – that’s the point where the ice shelf first starts floating – matches the amount of ice that’s lost through melting and calving of icebergs.
The problem is that we are getting more warm water that’s coming onto the shelves; it’s driving more melting from beneath and it’s knocking ice shelves or glaciers out of balance. This means that we’re getting more ice from the land into the ocean, causing sea levels to rise.
Are there specific areas in Antarctica that are more affected than others?
(PD): Yes, West Antarctica. Antarctica is generally split into two broad areas, East Antarctica and West Antarctica. East Antarctica is much larger, but West Antarctica is the area we are most concerned about, and this is because the warm water that’s driving the ice shelf melting gets much closer to the continental shelf in West Antarctica than it does in East Antarctica.
In East Antarctica, for a variety of different reasons, there’s a lot of cold water on these continental shelves that essentially protects the ice shelves from the warm water that’s flowing around the continent. Currently, our best observations show that warm water isn’t readily accessing East Antarctica ice shelves. Whether that continues into the future is an open question but currently, West Antarctica is the area of most concern.
How much have things changed in West Antarctica?
(PD): In terms of big, obvious changes, we’ve seen a number of ice shelves in the Antarctic peninsula – which is kind of the boundary between East and West – collapse entirely. The Larsen A ice shelf has gone, and the Larsen B ice shelf has gone. They’ve been collapsing from the North to the South as atmospheric temperatures warm, and then further around in the Amundsen Sea, we’re seeing warm water melting feedback. We’ve seen grounding lines retreat very rapidly, 2 kilometers (1.2 miles) a year. We’ve seen big calving events from the front of these glaciers – ice fronts have retreated inland.
Over what time period have we seen these changes?
(PD): Probably over the last 10 to 20 years. The problem is that we see a lot of individual events and then attribution [to a particular time] is actually quite difficult, but certainly it’s been a slowly evolving process. It was perhaps around the 1970s or 1980s when we first began to sense that this warm water was coming onto the shelf.
We’ve seen these successive events and excessive record retreats, but I don’t think you can really point to one particular period of time when something changed or happened. It’s an ongoing process.
What complicates matters is that we know the region has these natural long-term cycles that take 10 years to go from one state to the next, and then back to the original state. When we’ve only been observing for 20 years, particularly not in situ, it’s quite difficult to pick out from the observations exactly what started when.
Of course, we have models that give us a much bigger picture and tell us why it’s changing and how it’s going to change, but picking out the attribution of those changes is more difficult.
Are you using the model to try and work out the changes that are natural to Antarctica and its glaciers, and those that might be attributable to anthropogenic climate warming?
(PD): Definitely – there are already people working on that exact question. The great thing about computer models is you can run many different simulations with many different initial conditions and the way that we force the model.
One of the things that researchers can do is run the models using historical climate forcing without greenhouse gas emissions several times. That tells us something about the natural variability in the system that we see and the benefit of that is knowing that what we see in the real world is only one instance of all possible cycles. It’s a chaotic system.
Then, they can redo all the simulations with greenhouse gas forcing and begin to look at the differences between all the different simulations. As they haven’t just done it once – they’ve done it hundreds of times – they can begin to pick out what’s natural and what’s not natural and start attributing change to greenhouse gas emissions, natural variation, and natural forcing.
You work specifically on Thwaites Glacier. What kind of changes have you seen there in the last decade or so?
(PD): Thwaites is split into two separate dynamic regions. We have the Thwaites main trunk and we have the Thwaites Eastern Ice Shelf. The main trunk has disintegrated quite rapidly into more of a loose mélange of these blocks of ice, whereas the Eastern Ice Shelf has retained more of an ice shelf structure. It has a more classical floating tongue that’s out over the ocean, but even that is now beginning to break up. We’re seeing big rifts and cracks in the ice, across the ice shelf’s surface and all the evidence is pointing to a collapse of that ice shelf in the next 10 to 20 years.
Do you know why there is a difference between these two parts?
(PD): The answer has to do with the seabed topography. The ice shelves, once they float off the continent, can encounter what is known as “pinning points”. This is where you get shallow seabed topography that intersects with the ice base, and it creates a place for the ice to grip onto.
In front of the Eastern Ice Shelf, there remains a pinning point. The ice shelf is still thick enough that it can hold onto that seabed, and it can retain its shape, whereas on the main trunk, there is no longer a pinning point. Essentially, what was the ice shelf has become unconstrained and it’s just flown out into these big blocks.
If the collapse of this glacier happens in the next 15 or 20 years, what are we looking at in terms of short-term and longer-term consequences, for both the glacier in West Antarctica and global sea levels?
(PD): To clarify, we’re talking about the collapse of the ice shelf – the floating bit – in 15 to 20 years. The collapse of the glacier – the grounded bit – whilst possible, is certainly not something we are expecting to happen within 100, 200, or even 300 years. It’s a multi-century to millennial timescale.
However, if the ice shelf collapsed, we would immediately see a greater flux of ice from the ground into the ocean. That would, in turn, immediately cause the rate of sea level rise to increase. If, in the worst-case scenario, that triggered some sort of unstable collapse of the glacier that played itself out over many centuries, the sea level rise could be on the order of feet, as the glacier continued to flow into the ocean.
There are suggestions that if the Thwaites Glacier was lost, that may destabilize wider parts of West Antarctica, the whole Amundsen Bay and Sea may become unstable, and then you’re looking at really catastrophic rates of sea level rise. But, to stress again, this is not a decade-long process – this is centuries to thousands of years.
This interview was part of IFLScience’s The Big Questions and has been edited for length and clarity. Subscribe to our newsletter so you don’t miss out on the biggest stories each week.
Source Link: How Is Climate Change Affecting Glaciers And What Are The Consequences?
