Two paths to a shared fate

A combination picture shows a segment of the largest ice shelf in the Arctic breaking away. PHOTO: ESA/REUTERS

Last month, in the late northern hemisphere summer, Arctic sea ice shrunk to its second-lowest extent since satellite records began in the late 1970s. The record lowest extent occurred in 2012, when a storm broke up already weakened sea ice.

Sea ice is frozen sea water, so it is particularly susceptible to storms, warming ocean waters and a warming atmosphere. Arctic sea ice has shrunk in both extent and thickness over the past 20 years. Up until the late 1990s, large areas of Arctic sea ice never completely melted in the summer and only relatively small amounts exited the Arctic Ocean basin. The result was that the northern oceans used to be mainly covered with sea ice that lasted for more than one year. Since the late 1990s, this has changed: increasing temperatures and stronger storms have resulted in weaker ice that melts and breaks up more easily, and is more likely to be forced out through Fram Strait into warmer seas. Now Arctic sea ice that is only a few months old is the new normal in some areas. The result is thinner sea ice, and this has had profound impacts on the animals and people that live in the Arctic.

Late September 2020 marked the end of winter for Antarctic sea ice, with a greater overall extent than for 2019. February 2021 will be the end of the Antarctic summer, and it will be interesting to see what the sea ice does this summer. Antarctic sea ice has not declined rapidly like Arctic sea ice, and in some regions Antarctic sea ice has even increased the area it covers. Scientists do not yet fully understand the reasons for this different sea ice response to climate change between the Arctic and Antarctic, and between different regions of the Antarctic. From a New Zealand perspective, it is vital to understand what will happen to Antarctic sea ice in the future because changes in Antarctic sea ice extent can lead to changes in weather patterns over New Zealand. In winter, Antarctic sea ice is a closer neighbour to New Zealand than Tasmania.

So why has Arctic sea ice shrunk more in our warming world than Antarctic sea ice? Some geographic differences are important: the Arctic Ocean is an ocean mostly surrounded by land and is relatively shallow, whereas the Antarctic is surrounded by the deep Southern Ocean. Global warming due to increases in greenhouse gases has greater effects in polar regions, a phenomenon called ‘‘polar amplification’’. In the Arctic, the loss of highly reflective (which scientists call ‘‘high albedo’’) snow-covered sea ice exposes the ocean to direct warming, which then leads to further ice melt, a vicious cycle known as ice-albedo feedback. In contrast, the vast Southern Ocean has absorbed large amounts of heat from human-caused climate change, and is therefore acting as a temporary buffer for sea ice in some areas. Meltwater from the fresh land-based ice sheet and floating ice shelves might be temporarily protecting the sea ice by reducing the heat moving upwards from the ocean to melt sea ice. The strong winds of the Antarctic may also play a role in compensating for climate impacts on sea ice.

In the long term, increasing the temperature of Earth’s atmosphere and oceans will reduce the area and thickness of Antarctic sea ice. Urgent, worldwide reductions of greenhouse gas emissions are the only way to prevent Antarctic sea ice shrinking dramatically in the future.

 - Dr Inga Smith is the co-director of He Kaupapa Hononga: Otago’s Climate Change Research Network and is based in the University of Otago department of physics. Each week in this column, one of a panel of writers addresses issues of sustainability.