This post was co-authored by Natalya Gomez, an associate professor in the Canadian Research Faculty of 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 melting at an accelerating rate. As it melts, it contributes to sea level rise, harming coastal and island communities around the world.

To more accurately predict how sea levels will rise in the future, scientists must also consider the structure of the solid Earth on which the ice sheet rests. This often-ignored variable plays a major role in how the ice sheet responds to warming, and how much and how fast sea levels will rise in the coming years. Our new research, published in Scientific progressexamines the interactions between the ice sheet and the solid Earth and emphasizes that implementing large reductions in emissions of heat-trapping substances now is crucial to prevent sea levels from rising too high in the future.

Connecting the ice and the earth beneath

You may remember from science class that the Earth’s interior isn’t completely solid. There’s a viscous (meaning fluid or squishy) mantle beneath the crust. In some places, the mantle is squishier than in others, and that’s true of several key areas beneath the Antarctic ice sheet. That squishiness turns out to be a major factor in determining the rate of ice sheet loss under different scenarios.

Our research, led by Dr. Natalya Gomez of McGill University, uses computer models to simulate interactions between the ice sheet and the solid Earth, and the resulting sea-level responses. We compared simulations that use different models of the Earth’s interior: one that incorporates new observational evidence of the Earth’s structure, allowing us to consider where it is more or less squishy, ​​and another where we neglect squishiness altogether or simply assume that the Earth’s internal structure is the same everywhere. We then test each version under different scenarios for heat-trapping emissions 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 the water expulsion effect, which are at play when the solid Earth and the ice in Antarctica interact. These mechanisms influence how much sea level rises on average along the global coastlines far from Antarctica.

To understand how the first mechanism works, the feedback of the sea levelwork, we need to know a little more about ice sheets. The Antarctic ice sheet has grounded ice, which is ice that lies on the solid earth below it, and ice shelves, which extend from the grounded portion and float on the ocean surface. The floating ice shelves stabilize the grounded ice behind it. As ice shelves melt, the grounded ice behind them flows more quickly into the sea, contributing to sea level rise.

The Antarctic ice sheet is a sea-based ice sheet, meaning that there are places where the grounded ice meets the solid Earth beneath the ocean surface. As the ocean warms and the ice sheet begins to melt, the depth of this water determines how quickly the ice sheet “retreats” into the grounded parts. When we run our models and simulate how the ice sheet responds in a warming world, if we neglect the internal structure of the Earth and the solid Earth beneath the ice sheet remains rigid in the model, then the depth at which the ice sheet is grounded does not 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 softer beneath the ice sheet, the crust can bounce back as the huge mass of the ice sheet melts and no longer exerts a force on the crust. We can see this in the lower left panel of Figure 1. The rate at which the ice sheet melts and the flow of the grounded ice depends on how deep the water is where the ice meets the Earth below. Ice sheets that are grounded in deeper water flow faster. When the ice sheet melts again and the Earth that was beneath it bounces up in response, the ice sheet in that area is suddenly in shallow water, which slows the flow of the grounded ice out to sea. This is what is called sea-level feedback. The name comes from how far below the sea surface the grounded part of the ice is and how much that changes as the ice sheet melts and the solid Earth responds by bouncing up.

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 that when the ice sheet melts, those areas are now ocean. These are called marine basins. But remember that as the ice sheet melts, the earth beneath it that was previously being pushed down by its weight now springs upward as it is removed. As the earth rises, the water in exposed marine basins has to go somewhere. It is expelled into the global oceans, causing sea level to rise further away from the ice sheet because waters become shallower near the ice sheet.

What role do heat-trapping emissions play in this?

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 begins to rise as the ice sheet melts, lifting the ice sheet upward, reducing its contact with the warming waters and slowing the melting and seaward flow of the grounded ice. The sea level feedback is the dominant factor here, helping to protect the ice sheet. We see global average sea level rise being up to 40% lower under this scenario. That makes a huge difference to coastal communities around the world that are affected by flooding, storm surges, and saltwater pollution of freshwater sources.

In high-emission scenarios, the Earth warms rapidly and substantially and the ice sheet melts rapidly, causing it to move toward grounded areas and causing sea level to rise rapidly. 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 level rises substantially. This is a terrible scenario for communities around the world, because sea level not only rises much higher, but also 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 oceans all interact in complex ways, and interactions between them have a major impact on sea level responses. Although we have discussed the impact of ice sheet–solid Earth interactions on mean sea level rise so far, sea level along global coastlines will be spatially variable. Regionally, sea levels will differ from the mean value due to several effects. As an ice sheet loses mass, the solid Earth beneath it rebounds in response, Earth’s gravity changes (massive ice sheets pull water toward them), and Earth’s rotational axis shifts, redistributing water around the globe. Our models simulate these effects.

Our study also looks at where the sea level rise effects of the Antarctic ice sheet are expected to be highest. Two sea level maps from the paper are shown in Figure 2. These show the expected sea level rise in 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 that the highest expected sea level rise (0.33–0.35 m or 1.1–1.5 ft above year 2000 levels) is in the Indian Ocean Basin, the southwestern and northeastern Pacific Oceans, 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 rise is significantly more. Here, the highest expected impacts occur in almost all 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 countries that have been sounding the alarm about climate change and sea level rise for decades, and are not responsible for the rising emissions. This is a clear case of climate injustice.

It is not too late to slow sea level rise

Last year was the warmest year in recorded history and this year looks set to break that record again. We just experienced the warmest days on record, although those records could be broken again as we head into August. The record high temperatures are also affecting Antarctica. Since 1992, ongoing international negotiations have led to global agreements to address climate change and reduce heat-trapping emissions that have been increasing over the past few centuries. Unfortunately, countries have failed to meet 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 consequences of climate change, but it is one that will haunt us and future generations well into the future. The policy choices 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 rapid, fair phase-out of fossil fuels and deep reductions in methane emissions, including from agriculture and oil and gas, to keep the Paris Agreement and the Global Methane Pledge on track. The time for action is now.