Antarctica’s heaving oceans are covered each winter by a dynamic cap of sea ice that extends from the continent and spans up to twenty million square kilometres. As the oceans and atmosphere warm in summer, the sea ice melts and it shrinks to about three million square kilometres. This dramatic seasonal cycle is one of the world’s largest annual geophysical changes, and has a substantial effect on ocean circulation, regional and global climate.

The vast white expanse might at first appear to be a barren and inhospitable environment. In reality, it supports a thriving ecosystem of unique marine life from microorganisms to zooplankton to iconic penguins, seals and whales. While much uncertainty still exists, scientific modelling suggests that sea ice extent in Antarctica may gradually decline as the world’s climate warms. New Zealand researchers are working, with their international colleagues and the support of Antarctica New Zealand, to unravel the complex systems that interact to drive sea ice change.

Antarctic sea ice drives circulation in both the atmosphere and the ocean, dominating the southern latitudes, and ultimately influencing the whole of Earth’s climate system. It forms when ocean water is cooled to just below its freezing temperature, first creating tiny crystals called frazil ice, and gradually developing into more consolidated structures. As the sea ice forms, salt is expelled which increases the salinity of water below. These salty waters then sink to the ocean floor and flow towards the equator, driving global ocean circulation. The white surface of the sea ice reflects considerably more sunlight than open water, reducing the heat that enters the ocean.

The annual freeze and melt of sea ice does not directly affect sea level. However, the presence of sea ice at the ocean’s surface lessens the action of waves which can help to protect Antarctica’s ice shelves. These massive slabs of glacial ice, hundreds of kilometres wide and hundreds of metres thick, are the floating extensions of the land-based ice sheet. In turn, the ice shelves stem the flow of the land-based ice sheet, reducing the rate at which it is discharged into the ocean and therefore limiting rates of global sea level rise. Ice shelves are eroded from below by contact with the ocean and could become destabilised when that water warms. Very cold meltwater released in this process can produce ice crystals which can become incorporated into the sea ice. Studying the sea ice thus provides insight into the health and stability of ice shelves and the ocean that fills the cavities beneath them.

Antarctic sea ice supports an incredible diversity of life. Specialised sea ice algae grow throughout the sea ice, within the brine channels that course through its structure. At the base of the sea ice, the algae form dense mats that are grazed by many small marine organisms, including krill, which in turn are a vital food source for other animals including fish, penguins and whales. Life is abundant at the edges of the sea ice where nutrients are released to the ocean, stimulating phytoplankton production and fuelling the food chain. In addition, the sea ice platform is critical for the reproductive success of many Antarctic species. The furrows and ripples under the sea ice provide a protective space for the eggs of Antarctic fish. Emperor penguins congregate on the sea ice during the darkness of winter, raising chicks that are ready to fledge when the sea ice breaks up. Sea ice also provides a sanctuary for Weddell seals to haul out, rest and moult, and for females to give birth.

Given the great importance of sea ice to both Antarctica and the rest of the world,New Zealand scientists are working hard to understand how a changing climate will affect the annual cycle of sea ice growth and melt. But the puzzle is complex. Unlike the Arctic, where sea ice has shown a significant decline since satellite records began in the 1970s, Antarctic sea ice is not yet demonstrating a clear signal. For many years, the total extent of Antarctic sea ice was increasing, although there was large regional variability in this trend. However, in 2014 the sea ice extent started to shrink rapidly and unexpectedly, to a record low in 2017. The sea ice seemed to be rebounding from this low, until another record low was measured early in 2022. The variability of these patterns makes future predictions of sea ice extent in a warming climate very difficult, and highlights the complexity of the atmospheric and oceanic processes that are involved in determining the rate of sea ice formation and decline.

New Zealand researchers untangle these complexities with the support of Antarctica New Zealand. Teams use a suite of technologies to study the sea ice from space, from the air, from its surface and even from underneath.Satellites have been providing information on Antarctic sea ice extent since the 1970s. These increasingly precise images provide robust information on sea ice area, but are less well equipped to estimate sea ice thickness.New Zealand researchers are part of an international collaboration using the ‘EM-Bird’ (an electromagnetic induction device, flown beneath an aircraft) to complement the satellite imagery and provide information on sea ice thickness. These airborne studies are strengthened by fieldwork on the sea ice of McMurdo Sound, in the Ross Sea region.

The timeframe for safe operation on the sea ice each year is short, so October and November are abuzz with scientists taking their opportunity to sample, measure and observe the ice and ocean below. Recent innovations have combined cutting-edge technology with kiwi ingenuity to study the physical properties of sea ice and its role as habitat.The use of remotely operated vehicles that can ‘swim’ beneath the sea ice has provided new opportunities to study supercooled water, which contributes to sea ice formation, as well as the diversity and function of the sea ice algae community. These new technologies are complemented by a more than twenty-year long record of sea ice growth and decline in McMurdo Sound, which is facilitated each year by the deployment of a ‘sea ice mass balance station’ during the sea ice growth period. Ultimately, these studies, in combination with international efforts, seek to understand the drivers of sea ice change.

The answers are complex, but New Zealand research is contributing to improved clarity about the future of this uniquely important Antarctic phenomenon.