For Antarctica, the recent accelerated warming is estimated to be about two-thirds the result of natural variability with the role of rising greenhouse gases contributing about one-third, said Kyle Clem, a post-doctoral research fellow at New Zealand’s Victoria University.
The rapid warming “lies within the upper bounds of natural variability”, Dr Clem said. “It’s extremely rare and it appears very likely that humans played a role.”
The research shows “there’s no place on earth that’s immune to global warming”, he said. “There’s nowhere to hide – not even up on the Antarctic Plateau.”
Sitting at 2835 metres above sea level – or 600 metres higher than Mt Kosciuszko – on a rocky continent, the South Pole is exposed to different weather processes than its polar opposite. By contrast, the North Pole rests on shifting sea ice with the seabed more than four kilometres below.
Dr Clem, along with other researchers from the US and the UK, found changing circulation patterns in the Pacific and Southern Ocean determine which parts of Antarctica warm or cool.
For instance, the western tropical Pacific has periods when is warmer or cooler than usual.
The warmer period – known as the negative phase of the so-called Interdecadal Pacific Oscillation – set in about 2000. During this phase, there is more storm activity in the tropics which in turn spawns more high- and low-pressure systems that send heat far into the high latitudes.
The circumpolar westerly winds, which have been strengthening and contracting polewards under climate change – also play a role in amplifying the transfer of warmth into Antarctica.
When those two patterns align, as they have in recent decades, the South Pole warms but some parts, such as western Antarctica warm at a slow pace or even cool, as the frigid air shifts around.
Michael Mann, Director of the Earth System Science Centre at the University of Pennsylvania, said the study provided “a very detailed and useful analysis” of the forces at play in the far south.
If anything, though, the researchers’ use of model simulations to reach conclusions about regional trends probably understates the role of human-caused climate change.
“In short, what the authors attribute to natural internal cycles might just be a shift in atmospheric circulation that is actually due to human-caused climate change but isn’t accurately captured in the average over models,” Professor Mann said, in an email that included those italics.
The recent polar extremes – including eastern Siberian temperatures above 40 degrees – were important because “what happens in the poles doesn’t stay in the poles”, the prominent climate scientist said.
Changes at the South Pole itself were not as critical as the warming of the Southern Ocean, which is leading to the collapse of the West Antarctic ice shelves and the destabilisation of the interior ice sheet.
“This was not well predicted by climate models, meaning we are further along when it comes to the destabilisation of ice sheets and the commitment to rising sea levels than we expected to be at this point,” Professor Mann said.
Peter Hannam writes on environment issues for The Sydney Morning Herald and The Age.