Scientists have long been searching for an explanation for the contrasting fortunes of sea ice at the Earth’s poles. While the Arctic has seen a rapid decline in sea ice extent in recent decades, the seas around Antarctica have slowly gained ice.
July 7, 2016 — Now, a new study suggests a natural shift from one state to another in the Pacific Ocean is responsible for the increase in Antarctic sea ice.
However, as the Pacific Ocean state shifts once again, growth is now likely to slow and ice extent may even start to shrink, the lead author tells Carbon Brief.
From the beginning of the satellite record in 1979 to the end of the 20th century, the area around Antarctica covered by sea ice grew by a modest 0.12m square kilometres (sq km) per decade. But between 2000 and 2014, a growth spurt saw sea ice increase by 0.57m sq km per decade.
You can see how Antarctic sea ice has changed since 1979 in the lower chart below, from Carbon Brief’s climate dashboard. For comparison, the upper chart shows the losses of Arctic sea ice over the same period.
The increase in Antarctic sea ice is seemingly at odds with the continued warming of the climate in response to rising atmospheric greenhouse gases. This apparent contradiction has prompted a lot of speculation in the media and by scientists in recent years.
The new study, just published in Nature Geoscience, says it has identified the primary cause behind the boost in sea ice growth – a natural cycle in the Pacific Ocean.
This cycle goes by two names: the Interdecadal Pacific Oscillation (IPO) and the Pacific Decadal Oscillation (PDO). Strictly speaking, the IPO refers to the entire Pacific Ocean, while the PDO refers to just the North Pacific, but the terms are often used interchangeably.
The IPO has two opposite phases: positive (also known as the “warm” phase) and negative (“cool”). The phases affect the strength of the trade winds that blow east-to-west across the tropical Pacific Ocean. These winds are themselves driven by warm air rising along the equator and the rotation of the Earth.
During a negative IPO phase, the trade winds strengthen, driving heat into the deep Pacific Ocean, which brings cooler water to the surface.
Since 2000, the IPO has been in an extended negative phase. Its cooling effect is thought to have a role in the recent slowdown in global surface temperature rise.
Using a set of 260 global climate model simulations, the researchers identified those that most accurately replicated this negative IPO phase. When they looked specifically at these results, they all showed an increase in Antarctic sea ice as well.
Their analysis identifies a chain reaction that explains how a negative IPO ultimately results in Antarctica gaining sea ice.
The cooler conditions in the Pacific during a negative phase mean less moisture evaporates from the ocean, causing lower than average rainfall in the eastern Pacific. The reduced rainfall, in turn, affects a low pressure weather system called the Amundsen Sea Low (ASL), which tends to sit off the coast of West Antarctica.
Lead author Prof Jerry Meehl, from the National Centre for Atmospheric Research (NCAR) in Colorado, explains to Carbon Brief:
As the sea ice is blown away from the land, it exposes open water, which then freezes, increasing the extent of sea ice.
Natural variations the tropical southwest Pacific Ocean and in the tropical Atlantic Ocean also affect the ASL and Antarctic sea ice, Meehl says, but they play a secondary role compared to the IPO.
The findings help explain how sea ice around the Antarctic has increased while the climate system has continued to warm, Meehl tells Carbon Brief:
As for the future, evidence suggests that the IPO shifted to a positive phase in 2014, says Meehl:
The findings show how tricky it can be to separate natural and human-caused impacts on the climate, says Prof John Turner, a climatologist at the British Antarctic Survey, who wasn’t involved in the study. He tells Carbon Brief:
In fact, the extended negative phase of the IPO since 2000 was itself likely influenced by both human and natural factors, Meehl says.
While the phase occurred as part of its natural cycle between warm and cool phases in the Pacific Ocean, it might well have been enhanced by the combination of a series of moderate volcanic eruptions, an increase in air pollution over Asia, and reduced output from the sun, he adds.
by Robert McSweeney | Carbon Brief