Every year, the vast ocean around Antarctica freezes and thaws. Hi, I'm Dave Thurlow and this is the Weather Notebook. As Allan Coukell reports, scientists in that remote region are studying how the ice breaks up to learn clues about climate.
Inga Smith: "(sound of ice auger scraping). The ice is getting quite wet and quite soft, which means we are very close to breaking through the bottom."
AC: Standing on the frozen Ross Sea, a few kilometres off the coast of Western Antarctica, sea ice researchers are using an auger to extract a column of ice.
Inga Smith: (Clink) That's it! Oh! (glug) We've just broken through. ooooh!"
AC: Sea ice is an important part of the global climate system. It keeps the air cool by reflecting incoming sunlight back into space and the yearly cycle of freezing and thawing also helps to drive ocean currents.
And while the scientists will be taking this ice core back to the lab for further study, the two metres of ice we're standing on will soon be gone.
Inga Smith is from the University of Otago, in New Zealand:
Inga Smith: The sea ice doesn't melt like normal ice. When the sea ice starts to freeze, it captures pockets of salt water, so you've essentially got little bits of freshwater ice with lots of concentrated salt. As it warms up, that salt water starts to eat its way through; it will actually start to disintegrate internally -- it'll get weaker, so you won't just see sea ice melting away. What you'll see is you'll be standing on metre-thick ice. Waves will come in and it will break off huge sections of it. They'll drift north to where it's warmer and then they'll start to melt.
AC: At its annual maximum - usually about September - the sea ice around Antarctica covers about 19 million square kilometres - an area over twice the size of the United States.
By February, it will contract to less than a fifth of that size.
That makes it one of the largest seasonal surface changes on the planet.
Thanks today to Allan Coukell of New Zealand. The Weather Notebook is a production of the Mount Washington Observatory. You can learn more by logging on to our website at weathernotebook.org.