WHERE OCEAN MAKES ICE
THE FROZEN OCEAN AT THE BOTTOM OF THE WORLD.
Every austral autumn; in the seas around the Antarctic, the Sun dips below the horizon and won’t be seen again for months. Its disappearance ushers in the start of the Antarctic winter. The air temperature plummets, the ocean follows suit. The polar seas drops to -1.8°C. During this long and cold polar night, an ocean area one and a half times the size of the United States freezes. This change effectively doubles the ice covered area of the continental Antarctic to the south, and is one of the most dramatic changes to the surface of our planet each year.
Unlike continental ice sheets also found in the polar regions, changes in the sea ice cover have no direct influence on sea level rise. But the colossal change between an ice covered sea and an open ocean has implications of its own. The waxing and waning of the Antarctic sea ice cover is a primary driver of global ocean circulation, which is responsible for the movement of water masses all around the globe. In addition, in the southern spring, sea ice acts like a giant mirror, covering the otherwise dark ocean, reflecting the Sun’s energy back to space and changing the energy balance of the entire planet. In turn, it is a fundamental part of the global climate system, acting like a giant washing machine circulating water around the planet, while also working like a continent sized air conditioning unit.
But even now, in 2017, we don’t really know how much of it there is.
The planet, and indeed the Southern Ocean is warming, so it might come as a surprise, that unlike it’s Arctic counterpart, the Antarctic sea ice cover has actually grown in size since we started measuring its extent with satellites.
Since 1979 the Antarctic sea ice cover has increased by 3% per decade, a small amount, especially when compared to the 13% per decade decline measured in the Arctic. But it begs the question, why in a warming world is the Antarctic sea ice cover expanding? And perhaps more importantly, is this trend expected to reverse? Although we have been able to measure sea ice extent, adequate datasets on its thickness still elude us. This means we don’t know if its volume is changing. Volume is a more holistic measure of the system and informs us whether it is growing or shrinking in its entirety. As part of a global scientific effort, in 2011 and 2013 we went to the Antarctic to help establish why.
The only way to measure sea ice thickness at the required hemispheric scales is using satellites. These special type of satellites, called altimeters have been in operation for over 20 years. Advancing technology has permitted these instruments, carried into orbit on these satellites to continuously improve. From 700 km above the Earth’s surface, travelling at 7 kms per second, these instruments can discern changes in the distance between the waterline and the top of the ice, a distance typically only in the order of 20 cm.
This distance between the sea surface and the ice surface is termed the freeboard. Freeboard is the above water segment of the floating ice which can then be used to estimate the total thickness of the sea ice.
There are many uncertainties with these measurements, and that is why it is necessary to travel to the Antarctic to ‘ground truth’ the satellite data. With confidence in the measurements and methods, proven by ground truthing, we can then attempt to map the entire hemisphere’s sea ice cover.
Accurately measuring Antarctic sea ice thickness is an ongoing effort. The dataset will help develop an understanding of why the Antarctic sea ice cover has been increasing. We can then improve our ability to forecast changes in the Southern Ocean, that ability being of great importance to help us make the right decisions for society as our climate changes.