NASA scientists traverse the icy landscape of Antarctica, carrying scientific instruments and equipment for cold weather with them. The team was tasked with collecting ground data to verify the accuracy of the measurements made by the IceSat-2 satellite. Photo credit: NASA / Kelly Brunt
When an ice cube is exposed to a heat source such as warm water or air, it melts. So it’s no wonder that our glaciers and ice sheets are melting due to the warming climate. However, it is nowhere near as easy to predict how much glaciers and ice sheets will melt and how fast – key components of sea level rise -.
Glaciers and ice sheets are far more complex structures than ice cubes. They arise when snow accumulates and is compressed into ice over many years by fresh snow. As they grow, they begin to move slowly under the pressure of their own weight, dragging smaller stones and debris with them across the land. Glacier ice, which extends over large land masses as in Antarctica and Greenland, is considered an ice sheet.
Greenland and Antarctica are home to most of the world’s glacier ice – including just two ice sheets – which makes them of particular interest to scientists. Together, the two regions also contain enough ice to raise sea levels by nearly 65 meters if it were to melt all at once. Adaptability and our long-term survival in a changing world. Recognition: NASA
The processes in which glaciers and ice sheets lose mass are also more complex. The surface of an ice cube melts when exposed to ambient air (warm). And while warm air certainly melts the surface of glaciers and ice sheets, they are also significantly affected by other factors, including the sea water surrounding them, the terrain (both land and sea) over which they move, and even your own Meltwater.
Greenland and Antarctica are home to most of the world’s glacier ice, including just two ice sheets. These thick ice sheets – about 3,000 meters (10,000 feet) and 4,500 meters (15,000 feet) thick, respectively – contain most of the freshwater stored on Earth, which makes them of particular interest to scientists. Together, the two regions also contain enough ice that if it melted all at once, it would raise sea levels nearly 65 meters – which makes studying and understanding them not only interesting but also crucial to our proximity. Adaptability and our long-term survival in a changing world.
Ice loss in Greenland
A glacier is considered “in equilibrium” when the amount of snow that falls and accumulates on its surface (the accumulation zone) is equal to the amount of ice lost through melting, evaporation, calving, and other processes.
However, since the annual air temperatures in the Arctic rise faster than anywhere else in the world, this equilibrium can no longer be achieved in Greenland. The warmer sea water around the island’s tidal glaciers is also problematic.
“It’s basically like pointing a hair dryer at an ice cube while the ice cube is also sitting in a warm water pot,” said Josh Willis, lead researcher at NASA’s Oceans Melting Greenland (OMG), a project studying the effects of Sea water temperature on melting ice in the region. “The glaciers are melted simultaneously by heat from above and below.”
Although the warm air and the warm water contribute to individual melting, the interaction between the melt water from the glacier and the warm sea water also plays an important role.
When warm summer air melts the surface of a glacier, the meltwater drills holes through the ice. It makes its way to the bottom of the glacier, where it runs between the ice and the glacier bed, and eventually shoots into the surrounding ocean in a cloud on the glacier floor. The meltwater plume is lighter than the surrounding seawater because it does not contain salt. So it rises to the surface and mixes the warm sea water upwards. The warm water then rubs against the bottom of the glacier and melts even more glaciers. This often leads to calving – ice cracks and breaking into large chunks of ice (icebergs) – at the front end or at the end of the glacier. Photo credit: NASA
When warm summer air melts the surface of a glacier, the meltwater drills holes through the ice. It makes its way to the bottom of the glacier, where it runs between the ice and the glacier bed, and eventually shoots into the surrounding ocean in a cloud on the glacier floor.
The meltwater plume is lighter than the surrounding seawater because it does not contain salt. So it rises to the surface and mixes the warm sea water upwards. The warm water then rubs against the bottom of the glacier and melts even more glaciers. This often leads to calving – ice cracks and breaking into large chunks of ice (icebergs) – at the front end or end point of the glacier.
This image shows a region of the ocean floor off the coast of northwest Greenland that was mapped as part of NASA’s Oceans Melting Greenland (OMG) mission. This five-year suborbital mission from Earth Ventures will test the link between ocean warming and ice loss in Greenland. The data shown here is used to understand the ways warm water can reach glacier edges. The color overlay on the water shows the depth of the ocean floor, with deep blue colors representing depths of more than 1,000 meters. A deep trench extends south and west of Cornell Glacier, shown in the upper right corner. Photo credit: NASA / JPL-Caltech
The intricate shape of the seabed around Greenland affects how easily this warm water melt can occur. In some areas it acts as a barrier, preventing the deep, warmer waters of the Atlantic Ocean from reaching the glacier fronts. However, similar to the terrain above water, the underwater terrain contains other features such as deep canyons. The gorges cut into the continental shelf and let in the Atlantic water. Glaciers in these waters melt faster than those in which the warm water is blocked by underwater ridges or sills.
Ice loss in the Antarctic
In Antarctica, where similar surface and ocean melting processes occur, the topography and bedrock on which the ice sheet resides significantly affects the stability of the ice sheet and its contribution to sea level rise.
The researchers divide Antarctica into two regions based on the relationship between the ice and the bedrock below. East Antarctica, the area east of the Transantarctic Mountains, is extremely high and has the thickest ice in the world. The bedrock under the ice sheet is also mostly above sea level. These features help keep the east side relatively stable. The West Antarctic, on the other hand, is lower and most of the ice sheet there is thinner. In contrast to the east, the ice sheet in West Antarctica is located on bedrock below sea level.
Image credit: NOAA
“In West Antarctica these glaciers rest on underwater bedrock. As in Greenland, there is a layer of warmer seawater under the cold surface layer. This warm water can flow onto the continental shelf and then under the ice shelves – the floating ice that stretches from the glaciers and the ice sheet, ”said Helene Seroussi, scientist at the NASA Jet Propulsion Laboratory. “The water melts the ice shelves from below, which can cause them to thin and break off.”
This is important because the ice shelves act like corks. They hold back the ice that flows upstream and slow its approach to the ocean, where it raises sea levels. When the ice shelves calve, the cork is essentially removed allowing more inland ice to flow freely into the ocean. In addition, this leads to the retreat of the grounding zone – the area where the ice separates from the bedrock and begins to float.
The visualization shows how ocean currents flow around and under Pine Island Glacier in Antarctica. When the water gets under the ice shelf, it erodes the ice shelf from the bottom and becomes thinner. The visualization was carried out using the V3 ocean circulation model “Estimation of the circulation and climate of the ocean” (ECCO), the 100 meter surface height model “Reference height model of the Antarctic” (REMA) and the 450 m bed topography and the ice thickness of the BedMachine Antarctica creates V1 data sets. The surface is mapped with scenes from NASA’s LandSat 8 satellite. To clarify, exaggeration factors of 4 and 15 – above and below sea level – were used. Photo credit: NASA / Cindy Starr
“The grounding zone describes floating ice that is already on the sea level budget from grounded ice that is not on the budget,” said ICESat-2 scientist Kelly Brunt of NASA’s Goddard Space Flight Center and the University of Maryland. “Floating ice is like an ice cube floating in a glass. It doesn’t run over the glass when it melts. However, when non-floating ice is added to the ocean, more ice cubes are added to the glass, causing the water level to rise. ”
The bedrock in West Antarctica is also sloping backwards – meaning it is higher on the edges and gradually deepens inland. Every time the grounding zone retreats inland, thicker ice is exposed to seawater and the glacier or ice sheet is grounded in deeper water. This allows even more ice to flow upstream into the ocean.
“It’s worrying in West Antarctica because if we push back the grounding zones, the downward, backward slope means there really is no backstop, nothing to break that cycle of melting and retreat,” Brunt said. “Our maps of the bedrock under the ice sheet are not as comprehensive as in Greenland, also because Antarctica is far less accessible. Because of this, we really don’t know if there are any small bumps or peaks down there that could slow down the retreat. ”
West Antarctic glaciers like Thwaites and Pine Island are already retreating faster than in the past. This is problematic as they provide a primary route for ice from the West Antarctic Ice Sheet to enter the Amundsen Sea and raise sea levels.
Overall, melting and ice loss have accelerated at both poles in recent years. The more we learn about the processes and interactions that cause it, some of which have been discussed here, the better we can accurately and precisely predict sea level rise far into the future.
