Scientists find ancient Antarctic ice melt could happen again, raising sea levels by three metres.
During my first trip to Australia back in 1990, in the days when we had no mobiles and travellers had to queue up outside a telephone box, a breath of chilly air (by Australian winter standards) prompted a local next to me in the line to comment “that’s a chilly wind coming up off the pole”. To my Scottish ears, it was a weird idea that the cold should come “up off the pole” rather than down from the Arctic. I chuckled to myself as I understood I was definitely at the other end of the world.
Penguins in the hothouse?
This week that conversation came to my mind when I heard about the record warm temperature recorded in Antarctica. The Financial Times had a quirky story of a researcher “weighing baby penguins when she noticed the unusually warm wind sweeping over the colony that lives on a rocky outcrop at the tip of the Antarctic Peninsula”. On Thursday February 6 2020, a temperature of 18.3C was recorded at the Argentinian base Esperanza, on the Antarctic peninsula, the region which is heating up much faster than the rest of the continent at the bottom of the world. The temperature, the highest ever recorded in Antarctica, was a staggering 0.8C higher than the previous record from five years ago, and way off the normal range for this time of year, which is somewhere between -5C and +5C.
As I write this, reports have come in of more temperature records at the bottom end of the world. The Antarctic has registered a temperature of more than 20C (68F) for the first time in history, prompting fears of climate instability in the world’s greatest repository of ice, writes Jonathan Watts in the Guardian. “The 20.75C logged by Brazilian scientists at Seymour Island on 9 February was almost a full degree higher than the previous record of 19.8C, taken on Signy Island in January 1982,” Watts goes on.
We have come a long way from the time when Antarctica, the ice continent in the far south of our planet, the coldest place on earth, was considered immune to climate warming, unlike the Arctic region up north, which has been known for much longer to be warming more than twice as fast as the global average. And we have made that huge leap in a tiny period of time.
Warming from the bottom up
The record temperatures at Esperanza base may well have been influenced by the Föhn effect, when strong winds blowing across mountain ranges cause warm air to form on the downwind side of the mountain, those on Seymour Island close by influenced by shifts in ocean currents and El Niño events. But that does not detract from the overall conviction of the majority of scientists that Antarctica, especially West Antarctica, is being affected by the overall global warming trend, and that those changes could, in turn, trigger feedback mechanisms which will warm the rest of the planet even more.
It’s not just a matter of air temperature. Warming seas are also melting the Antarctic ice from below. The IPCC tells us that since 1955, over 90% of the excess heat trapped by greenhouse gases has been stored in the oceans. A study published in January 2020, in Advances in Atmospheric Sciences says the world’s oceans in 2019 (especially the upper 2000 metres) were “the warmest in recorded human history”. And “it is well established that the Southern Ocean has taken up most of the global warming heat since 1970”, the paper explains.
Scientific instrument records go back just 150 years – an impossibly short time, a blink in the evolution of the planet. But we are living in the Anthropocene and the planet has never before been subjected to such rapid and wide-ranging interference. And we don’t have to rely solely on that modern data.
The ice-age detectives
“The future is heading far beyond the range of anything we’ve seen observed in the scientific instrumental record of the last 150 years,” says Chris Turney of UNSW, Australia, lead author of a new international study of the Antarctic ice sheet published on February 12 in Proceedings of the National Academy of Sciences (PNAS).
“We have to look further into the past if we’re going to manage future changes,” Turney explains.
“The better we understand the climate of the past, the better we can deduce what this means for the future”, adds co-author Stefan Rahmstorf from Germany’s Potsdam Institute for Climate Impact Research (PIK).
And the findings are not reassuring.
The team focussed on a period known as the Last Interglacial, 129,000 – 116,000 years ago.This was the most recent warm period to our current interglacial period. As PIK puts it, “the last phase of climate history with global temperatures similar to those that the world is heading towards due to manmade global warming in the coming decades.”
During the period studied, polar ocean temperatures were probably less than 2°C warmer than today, which Turney says make the period useful when it comes to considering how future global warming might affect ice dynamics and sea levels.
I asked Professor Turney to clarify the relation between 2°C of ocean warming and 2°C of overall warming. He told me in an email:
“When we’re talking about oceans we’re referring to sea surface temperatures. And with regards the 2˚C rise relative to pre-industrial, the ocean temperatures are used as an average estimate for the globe (because of dominance of ocean area on the planet). In the model runs we used a range of ocean and atmospheric temperatures to force the ice sheets. This 2˚C warming of the ocean does not include any atmospheric increase. If you include atmospheric warming the ice loss from Antarctica (and sea level rise) would be slightly higher.”
During the Last Interglacial global mean sea levels were between 6 and 9 metres higher than today, with some scientists saying they could even have been 11 metres higher. Melting of the Greenland Ice Sheet is said to account for two metres of that. Ocean expansion from warmer temperatures and melting mountain glaciers are thought to have caused less than one metre of increase.
“We now have some of the first major evidence that West Antarctica melted and drove a large part of this sea level rise”, says Turney.
Why we need to look at old ice
So what can we deduce from that about our current situation?
“Here, the analysis of climate history reveals that we would probably completely lose the West Antarctic Ice Sheet in a two-degree Celsius warmer world”, says co-author Stefan Rahmstorf.
The extreme ice loss caused “a multi-metre rise in global mean sea levels – and it took less than 2°C of ocean warming for it to occur”, his colleague Turney explains. “It could happen again”.
The data indicate that “Antarctica is highly vulnerable to projected increases in ocean temperatures and may drive ice-climate feedbacks that further amplify warming.”
The team conducted their research on what is known as a blue ice area, which they say is the “perfect laboratory”. These areas are created by fierce winds blowing over mountains, removing the top layer of snow and eroding the exposed ice. As the ice is removed, ancient ice flows up to the surface, giving an insight into the history of the ice sheet. “Instead of drilling kilometres into the ice, we can simply walk across a blue ice area and travel back through millennia. By taking samples of ice from the surface we are able to reconstruct what happened to this precious environment in the past,” Turney explains.
When the ice tips
What makes the West Antarctic Ice Sheet so sensitive is that it rests on bedrock well below sealevel, fringed by large floating ice shelves. The water is getting warmer all the time. When it gets into cavities beneath the ice shelves, ice melts from below. The shelves get thinner, making the central ice sheet highly vulnerable to warming ocean temperatures.
Models using the data the team collected suggest a 3.8 metre sea level rise during the first thousand years of a 2°C warmer ocean. They also suggest that most of the sea level rise would occur after the loss of the ice shelves, which collapsed within the first two hundred years of higher temperatures. Zoe Thomas is a co-author at the report and Fellow at UNSW.
“The positive feedbacks between a warming ocean, ice shelf collapse, and ice sheet melt suggest that the West Antarctic may be vulnerable to passing a tipping point”, she says.
“As it reaches the tipping point, only a small increase in temperature could trigger abrupt ice sheet melt and a multi-metre rise in global sea level”.
The IPCC currently estimates that global sea level will rise between 40cm and 80cm over the next century. Antarctica would only contribute around 5 cm of this. But the researchers are worried that the contribution could actually be much higher.
“Recent projections suggest that the Antarctic contribution may be up to ten times higher than the IPCC forecast, which is deeply worrying,”, says Chritopher Fogwill, another of the authors and Director of the Institute for Sustainable Futures at the UK University of Keele.
“Our study highlights that the Antarctic Ice Sheet may lie close to a tipping point, which once passed may commit us to rapid sea level rise for millennia to come”.
‘The authors of the study Record-Setting Ocean Warmth Continued in 2019 stress that ocean warming will continue even if the global mean surface air temperature can be stabilized at or below 2°C (the key policy target of the Paris Agreement) in the 21st century (…) “due to the long-term commitment of ocean changes driven by GHGs.”
Ocean and large ice sheets are slow to respond and will continue to change even after global temperature is stabilized.
A new paper just published by the Potsdam Institute for Climate Impact Research The Antarctica Factor: model uncertainties reveal upcoming sea level risk finds that ice loss in Antarctica could become a major risk for coastal protection even in the near term. The findings of the international team coordinated by PIK are based on “an exceptionally comprehensive comparison of state-of-the-art computer models from around the world.”
“While we saw about 19 centimeter of sea-level rise in the past 100 years, Antarctic ice loss could lead to up to 58 centimeter within this century. Coastal planning cannot merely rely on the best guess. It requires a risk analysis. Our study provides exactly that: The sea level contribution of Antarctica is very likely not going to be more than 58 centimeters”, says lead-author Anders Levermann.
Still time to act
In the long term, the Antarctic ice sheet has the potential to raise sea level by tens of metres, the scientists conclude. That does not mean there is no point in reducing emissions and halting global warming, the experts insist.
“What we know for certain is that not stopping the burning of coal, oil and gas will drive up the risks for coastal metropolises from New York to Mumbai, Hamburg or Shanghai,” Levermann concludes.
His colleague at PIK, Stefan Rahmstorf stresses action on emissions is still essential, even if we have already triggered the collapse of the West Antarctic Ice Sheet :
“The speed of the ice loss can still be limited by limiting global warming. Once more, this underlines the importance of the Paris Climate Agreement with the aim to keep global warming to well below 2 degrees Celsius, or better still 1.5C.”
“Our findings show that we don’t want to get close to 2°C warming,” warns UNSW Professor Turney .
The 630 million people now estimated to live on land below projected annual flood levels for 2100, and the one billion people now estimated to occupy land less than 10 metres above current high tide lines are unlikely to disagree.