Kiwi scientists have revealed the South Pole has been heating up at three times the global rate - a finding contrary to long-held views that it's been safe from warming.
But the researchers say that not all of the warming – an average temperature rise of 1.8C over the past three decades – can be put down to climate change.
Scientists have generally considered the pole to be buffered from global warming, due to its entrenched extreme cold and its remoteness from climate change's drivers.
But the new study, led by Victoria University researchers and just published in Nature Climate Change, finds the temperature rise appears to be linked to what's going on in the tropics.
Specifically, an increased upward motion in the tropics appeared to have strengthened cyclones in the South Atlantic Ocean – encouraging warm, moist air to travel further into the continental interior of Antarctica.
While teasing out how much of that warming could be pinned to increasing greenhouse gas emissions was tough, the researchers estimated it could account for 1C across the three decades.
"That's compared to the 1.8C of warming observed and the dramatic cooling of more than 1C during the 1980s and 1990s," said the paper's lead author, postdoctoral research fellow Dr Kyle Clem.
Therefore, decadal temperature fluctuations exceeded the anthropogenic signal by a factor of three.
"We also found climate models that did not have anthropogenic greenhouse gases were able to produce 30-year warming rates greater than the observed 1.8C, further demonstrating how extreme 'natural' temperature fluctuations are at the South Pole."
Clem said interest in South Pole temperatures was piqued by work on an annual state of the climate report for the US-based National Oceanic and Atmospheric Administration.
He began looking at data from weather stations across Antarctica, including one at the South Pole.
"That is a year-round base and they have been taking routine observations since 1957," he said.
"So it is one of the longest-running stations in Antarctica with continuous records. Their monthly temperature record is 100 per cent complete since 1957, which is incredible.
"I was looking at the anomalies at the different stations around Antarctica and saw that the South Pole was very warm in 2018 and it was 2.4C warmer than the 1981-2010 climatology, which we use as a standard base period.
"I looked a bit closer and that was the warmest year ever recorded at the South Pole. And its neighbour in Antarctic terms — the Russian station Vostok up on the East Antarctic Plateau — was also 2.4C above its climatology and also had its warmest year on record dating back to 1958."
Ultimately, he and fellow researchers found the 0.61C plus or minus 0.34C per decade polar warming was the end result of a succession of events in the tropics.
"The warming resulted from a strong cyclonic anomaly in the Weddell Sea, caused by increasing sea-surface temperatures in the western tropical Pacific Ocean, which transferred warm and moist air from the South Atlantic into the Antarctic interior," he explained.
"This shows how intimately linked the climate of Antarctica is to tropical variability. Our study also shows how atmospheric internal variability can induce extreme regional climate change throughout the Antarctic interior, which has masked any anthropogenic warming signal there during the twenty first century.
"In fact the South Pole over the past 30 years has warmed more than three times faster than the global average warming, while over the same period the warming on the Antarctic Peninsula and across West Antarctica stopped and even reversed."
While the observed warming is in large part linked to natural variability, humans have clearly also played their part, he said.
"Over the full range of all possible 30-year trends in climate models without anthropogenic greenhouse gases, the observed warming lies in the upper 0.1 per cent, meaning it is extremely rare and that the recent warming was probably pushed to such an extreme level by anthropogenic forcing."
How Earth's largest ice shelf is melting
Meanwhile, Kiwi scientists have gained a deeper understanding of how the world's largest ice shelf is being melted from below.
Antarctica's Ross Ice Shelf is a vast frozen block that measures many hundred metres thick, and sprawls across some 480,000 sq km – equivalent to the size of France.
Scientists have been urgently trying to understand how it's responding to climate change – and one Niwa-led study last year found part of the shelf was warming at a rate 10 times faster than the rest of it.
Now, another just-published Niwa paper has taken scientists closer to understanding the complex processes driving melting on the continent, which is critical to forecasting future sea level rise.
New measurements in the paper were just the second set that had been taken in the area – the first having been recorded in 1977, over a much shorter timeframe.
Niwa marine physicist Dr Craig Stevens carried out the latest measurements during a 2017 expedition, in which a hot water drill bored 350m metres through the ice to the ocean below.
Instruments were then lowered through the hole and left for two years to record ocean currents and water temperature with data transmitted via satellite.
"Future climate projections largely depend on the longevity of the ice shelves so it is important to see what has changed," Stevens said.
"Bringing together observed and modelled perspectives is critical for improving our predictions of ice sheet evolution and how that will impact the planet."
He said the data collected, and comparisons with the 1977 data, provide valuable insight into the how the ocean circulates in the cavity under the ice shelf and the complexity of the processes involved.
Some key differences were observed between the measurements and those taken 40 years ago including salinity which reflects continent-wide changes.
His research team also observed short-term refreezing on the underside of the ice shelf, which had implications for the melt rate and the dynamics of the upper ocean.
"The upper part of the ocean cavity seawater is the water that does all the damage in terms of ice melting," Stevens explained.
"It's only a few hundreds of a degree Celsius warmer than the 1970s data. It is however much saltier, by as much as the top to bottom difference in salt content, which has much more of an effect on how the cavity water moves and circulates.
"This is important because one of the big issues in the region is the ocean freshening over the last decades appears to be reversing."
"While it would be nice to identify change, this work is still at the discovery stage where we are measuring how these systems actually operate and the ways that this behaviour differs from the open ocean where we have built so much of our present understanding."
In any case, the new insight emphasised the importance of making these observations in the environment where they are taking place, he said.
"These virtually unexplored areas are hidden from the sun and wind by hundreds of metres of ice and are subject to processes that we are not used to seeing in the open ocean."
The next phase will explore how to integrate these new measurements into the climate models.
"We have dramatically lifted the amount of data available and provided a basis for better understanding the complex processes that drive melting."
The research, just published in the journal PNAS: Physical Sciences, was facilitated by the Aotearoa New Zealand Ross Ice Shelf Programme, the Victoria University of Wellington Hot Water Drilling initiative and Antarctica New Zealand.