This post was co-authored with Natalya Gomez, Associate Professor, Canada Research Chair in Geodynamics of Ice sheet – Sea level interactions at McGill University.
The Antarctic Ice Sheet faces an uncertain future under climate change. As the Earth’s air and oceans warm, the ice sheet is starting to melt at an ever-faster rate. As it melts it contributes to sea level rise, causing harm to coastal and island communities around the world.
To more accurately project how sea levels will rise in the future, scientists also need to consider the structure of the solid Earth that the ice sheet rests upon. This often ignored variable plays an important role in how the ice sheet responds to warming—and how much, and how quickly, sea levels rise in the years ahead. Our new research, published in Science Advances, investigates ice sheet and solid Earth interactions, and underscores that making steep cuts in heat-trapping emissions now is crucial to prevent high sea level rise later.
Connecting the ice and the Earth below
You may remember from taking science classes that the interior of the Earth is not completely solid. There is a viscous (meaning flowy or squishy) mantle under the Earth’s crust. In some places, the mantle is squishier than in others, and this is true for several key areas under the Antarctic Ice Sheet. That squishiness, it turns out, is a significant factor in determining the rate of ice sheet loss under different scenarios.
Our research, led by Dr. Natalya Gomez at McGill University, uses computer models to simulate interactions between the ice sheet and solid Earth, and the resulting sea level responses. We compared simulations which use different models of the Earth’s interior—one which includes new observational evidence of the Earth’s structure, allowing us to consider where it is more or less squishy, and some where we neglect squishiness all together or simply assume the Earth’s interior structure is the same everywhere. We then test each version under different heat-trapping emissions scenarios to understand possible future sea level responses.
The physical dynamics of ice sheet loss
Our simulations show that there are two main mechanisms— the sea level feedback and water expulsion effect—at play as the solid Earth and ice interact in Antarctica. These mechanisms influence how much sea levels rise, on average, along global coastlines away from Antarctica.
To understand how the first mechanism, the sea level feedback, works, we need to know a little more about ice sheets. The Antarctic Ice Sheet has grounded ice, which is ice that sits on the solid Earth below it and ice shelves, which extend out from the grounded portion and float on the ocean surface. The floating ice shelves stabilize the grounded ice behind them. If ice shelves melt away, the grounded ice behind them flows more quickly out to sea, contributing to sea level rise.
The Antarctic Ice Sheet is a marine-based ice sheet, meaning there are places where the grounded ice is meeting the solid Earth below the ocean surface. When the ocean warms and the ice sheet starts to melt, the depth of this water determines how quickly the ice sheet ‘retreats’ into the grounded sections. When we run our models and simulate how the ice sheet responds in a warming world, if we neglect the Earth’s interior structure and the solid Earth beneath the ice sheet stays rigid in the model, then the depth where the ice sheet is grounded doesn’t change. We can see this in the top left panel of Figure 1.
But if we use a realistic model of the Earth’s interior informed by observations, then in places where the Earth is squishier under the ice sheet, the Earth’s crust can rebound as the ice sheet’s enormous mass melts off it and stops providing a force pushing the Earth’s crust down. We can see this in the bottom left panel of Figure 1. The rate of ice sheet melting, and flow of grounded ice, depends on how deep the water is where the ice meets the Earth below it. Ice sheets grounded in deeper water flow faster. If the ice sheet melts back and the Earth that used to be beneath it springs up in response, then the ice sheet in that region suddenly finds itself in shallow water, which slows the flow of grounded ice out to sea. This is what is called the sea level feedback. The name comes from how far below the sea surface the grounded portion of the ice is, and how much that changes as the ice sheet melts and the solid Earth responds by rebounding upwards.
The second mechanism is called the water expulsion effect. We have discussed how Antarctica is a marine-based ice sheet where parts of the ice sheet meet the solid Earth below sea level. This means when the ice sheet melts away, those places are now ocean. These are called marine basins. But recall that, as the ice sheet melts away, the Earth below it that was previously pressed down by its weight now springs up when it is removed. As the Earth rises, the water in exposed marine basins needs to go somewhere. It gets expelled out into the global oceans causing sea levels to rise farther from the ice sheet, as waters get shallower close by the ice sheet.
How do heat-trapping emissions factor in?
When we run our models with low emissions scenarios, the planet doesn’t warm as much or as fast and the ice sheet melts relatively slowly. In these scenarios, the solid Earth starts rising as the ice sheet melts, which lifts the ice sheet up, reducing contact with the warming waters and slowing the melt and the seaward flow of the grounded ice. The sea level feedback is the dominant factor here and it helps protect the ice sheet. We find that global average sea level rise is up to 40% lower in this scenario. That makes a huge difference to coastal communities around the world impacted by flooding, storm surge, and saltwater contamination of fresh water sources.
In high emissions scenarios, the Earth warms quickly and substantially, and the ice sheet melts quickly, moving into grounded sections and rapidly raising sea levels. In this scenario, the ice sheet melts too quickly for the sea level feedback to provide much protection. Instead, the water expulsion effect dominates, and sea levels rise substantially. This is a terrible scenario for communities around the world because not only does sea level rise much higher, but it rises so quickly that it becomes much harder to implement adaptation strategies.
Mapping where sea level rise is highest
The Earth’s interior, the ice sheet, and the global oceans all interact in complex ways, and interactions between them have a large influence on sea level responses. While so far, we have been discussing the impact of ice sheet and solid Earth interactions on average sea level rise, sea levels experienced along the global coastlines will be spatially variable. Regionally, sea levels differ from the average value due to a variety of effects. As an ice sheet loses mass, the solid Earth beneath rebounds in response, the Earth’s gravity is altered (massive ice sheets pull water towards them), and the Earth’s rotation axis shifts position redistributing water around the globe. Our models simulate these effects.
Our study also looks at where sea level rise impacts from the Antarctic Ice Sheet effects are projected to be highest. Two sea level maps from the paper are in Figure 2. These show projected sea level rise at the year 2150. The map on the left shows the low emissions scenarios, where the sea level feedback dominates, and sea level rise is minimized. The darker blue is higher sea level rise. Looking at the two darkest shades of blue, we see the highest projected sea level rise (0.33-0.35 m or 1.1-1.5 ft above levels at the year 2000) is in the Indian Ocean basin, southwest and northeast Pacific, and the North Atlantic and Caribbean. The map on the right is the high emissions scenario where the water expulsion effect dominates, and sea level rises substantially more. Here, the highest projected impacts occur across almost the entirety of ocean basins above the equator, with particularly high impacts in the Pacific and North Atlantic. Note that the darkest blue is 3.6 m of sea level rise, or 11.8 ft! That would be a devastating situation for coastal communities to deal with, especially for low lying and atoll nations who have been sounding the alarm on climate change and sea level rise for decades, and are not responsible for rising emissions. This is a clear case of climate injustice.
It’s not too late to slow sea level rise
Last year was the hottest year in recorded history and this year is again looking like it will break that record. We just experienced the hottest days ever recorded, though those records may also break yet again as we head into August. The record high temperatures are impacting Antarctica as well. Since 1992, ongoing international negotiations have led to global agreements to address climate change and reduce the heat-trapping emissions that have been rising over the past few centuries. Unfortunately, nations have failed to live up to their commitments under international climate agreements, and heat trapping emissions have continued to rise over the past 30 years.
Sea level rise is just one of the devastating impacts of climate change, but it is one that will haunt us and future generations far into the future. The policy choices that are made now will determine how ice sheets respond for centuries to come. We need political leaders to make the right choices and reduce heat trapping emissions now. We need a fast, fair phaseout of fossil fuels, and steep reductions in methane emissions—including from agriculture and oil and gas operations— to keep the Paris Agreement and Global Methane Pledge on track. The time for action is now.