Antarctic ice sheet

The Antarctic ice sheet is a massive continental glacier. It covers about 98% of the Antarctic continent. This ice sheet is the largest of Earth's two current ice sheets. It holds a staggering amount of ice. This volume is equivalent to 61% of all fresh water on Earth. The ice surface is nearly continuous across the land. Only a few areas remain ice-free. These include the dry valleys, some mountain tops called nunataks, and sparse coastal bedrock.

Scientists divide the ice into three distinct regions. The first is the East Antarctic Ice Sheet, or EAIS. It is over 10 times larger than the West Antarctic Ice Sheet. The second is the West Antarctic Ice Sheet, known as the WAIS. Some glaciologists also include the Antarctic Peninsula as a third region. The third region is the Antarctic Peninsula (AP). These areas differ in their topography and how the ice flows. They also differ in their glacier mass balance. Mass balance refers to the difference between ice gained and ice lost.

The EAIS and the WAIS behave very differently. The EAIS sits at a higher elevation on a major land mass. This makes it less vulnerable to climate change. In the 20th century, the EAIS even showed limited cooling. Meanwhile, the WAIS warmed by over 0.1 °C per decade between the 1950s and 2000. The entire continent has warmed by more than 0.05 °C per decade since 1957. As of the early 2020s, the EAIS is actually gaining mass. This happens because more precipitation is freezing on top of the ice. However, the ice loss from WAIS glaciers is much greater. Two important glaciers are Thwaites and Pine Island Glacier.

The WAIS is classified as a marine-based ice sheet. This means its bed lies below sea level in many places. If the ice were gone, this area would be seabed. This location makes the WAIS vulnerable to marine ice sheet instability. This instability is difficult for scientists to simulate in models. If this instability is triggered before 2100, sea levels could rise by tens of centimeters more. This would happen especially with high overall warming. The WAIS is bounded by the Ross and Filchner-Ronne ice shelves. It also includes outlet glaciers that drain into the Amundsen Sea.

Ice loss has a massive impact on the Southern Ocean. Antarctica generates 1,100 to 1,500 billion tons of fresh meltwater every year. This meltwater flows into the ocean and dilutes the salty Antarctic bottom water. This process is called freshening. It increases stratification, which means the water layers become more stable. This weakens the lower cell of the Southern Ocean overturning circulation. Since the 1970s, the upper cell circulation has strengthened by 3 to 4 sverdrup. A sverdrup is a flow of one million cubic meters per second. This represents a 50% to 60% increase in flow. Conversely, the lower cell has weakened by 10% to 20%.

A total collapse of the circulation could be a tipping point. A tipping point is a change that is difficult to reverse. Some studies suggest the circulation could lose half its strength by 2050. A complete collapse might not happen until near the year 2300. This would change precipitation patterns in the Southern Hemisphere. It could also impact marine ecosystems and fisheries. The loss of the ice sheet would take a long time. It could take between 2,000 and 13,000 years. However, high emissions might shorten this to only 500 years.

History shows that Antarctica has changed many times. The icing of Antarctica began between 45.5 and 60 million years ago. This happened during the Late Cenozoic. Declining carbon dioxide levels were the primary cause. There was a major event 34 million years ago. This was the Eocene–Oligocene extinction event. During this time, CO2 levels were about 760 parts per million. The ice sheet also declined during the warm Pliocene epoch. This was between five and three million years ago. Scientists use paleoclimate research to understand these ancient changes.