Over the last few decades there has been an overall decrease in the amount of ice at the North Pole, and an increase in the amount of ice at the South Pole. Should all the remaining ice melt at the North Pole it will have hardly any effect on global sea levels because it is sea ice not land ice. Should all the ice melt at the South Pole, well this could cause global sea levels to rise by quite a bit.
I wrote something along these lines during that Glasgow talkfest. I received various emails telling me that it was not the sea ice I should be concerned about, but rather the glaciers.
There are some very large glaciers at the South Pole. For a time there was concern about the Pine Island Glacier, but then it stabilised. Then there is the Thwaites Glacier. It is about the size of Britain, and melting.
I got chatting with my friend Arthur Day about this glacier, asking him whether we should be concerned, and he explained:
"By far the main glacier of concern is the Thwaites glacier and its ‘rapidly’ thinning ice shelf.
This enormous glacier, described in the mainstream media as ‘the world’s most terrifying glacier’, is about the size of Britain. It is one of the largest glaciers on Earth. It has recently gained notoriety because it is currently undergoing a phase of relatively rapid flow into the sea. There is a fear that the faster flow is the beginning of an ‘irreversible collapse’ that will eventually contribute a ‘devastating amount’ of meltwater to sea level rise. Fuelling this fear is the knowledge that the glacier feeds a marine ice sheet where almost all of the basement supporting it is well below sea level, potentially making it unstable. Currently it is believed this glacier alone could already be contributing about 4% to global sea level rise.
If its ice shelf is weakened from beneath due to melting by ‘warm’ sea water, then it could lead to destabilisation of the main glacier behind, making it flow into the sea much faster than otherwise. This in turn could contribute to a faster rate of sea-level rise. While there is a lot of concern about this, there is no actual evidence it is anything more than a natural process. The glacier flows into the Amundsen Sea along the West-facing coastline of the Antarctic Peninsula and, today, its movement can be easily measured. However, just because it has been possible to easily measure the flow of the glacier over the few decades since the satellite era began in 1979, it cannot be claimed that ‘fast’ glacier flow is a ‘new’ phenomenon and, therefore, ‘unprecedented’. Glacier flow and ice shelf melting needs to be assessed from a much longer-term historical perspective with one eye on what glaciology teaches us about the complex dynamics of ice flow. For example, glaciers can start and stop moving again for no apparent reason, as has recently been demonstrated by the sudden stabilisation of the Pine Island Glacier.
In January 2020, an expedition by the US-UK International Thwaites Glacier Collaboration melted a 30 cm wide and 600-metre-deep hole through the floating ice shelf in front of the Thwaites glacier. For the first time they were able to directly measure the temperature of the ‘warm’ sea water right at the point where the ice meets the sea. The water temperature was 2°C. But this is just one measurement. A single temperature snapshot in time and place does not constitute a trend!
The seawater beneath the ice shelves is linked to an upwelling of ‘warm’ circumpolar deep water carried on an offshoot of the Antarctic Circumpolar Current. This is the strongest and most important ocean current on Earth. It is the only current linking all the major oceans. It flows around the Antarctic continent from west to east, at least in part whipped along by the drag from the strong westerly winds that blow around the polar regions. It is estimated that this current transports somewhere between 100 and 150 million cubic metres of heat-carrying sea water per second. Without the Antarctic Circumpolar Current, and its impact on planetary heat redistribution, the global climate would be very different. One of the curiosities about the Southern Ocean is that the deep water circulating Antarctica, the ‘Circumpolar Deep Water’ and the ‘Antarctic Bottom Water’ circulations, are warmer than at the surface. This happens because the deep water is also more saline, making it denser despite its higher temperature.
The Southern Ocean is a critical component of the global climate system because it is a key region for the upwelling of deep ocean waters to the surface. These upwelling waters are very old and have not been to the surface of the ocean for centuries or even millennia. It means that this water has not interacted with the atmosphere since well before the industrial era, certainly not since any human impacts on the atmosphere due to increasing CO2 were even possible. These upwelling waters bring heat to the shallower seas of the Antarctic continental shelves. When an offshoot of this current emerges along the western coastline of the Antarctic Peninsula, it can find its way to shallower depths and interact with the ice shelves in front of marine-terminating glaciers such as the Thwaites Glacier. Assisted by the prevailing westerly winds, enough heat can be transported to locally warm the climate of West Antarctica, so this current has potential consequences for the marine-terminating glaciers and the stability of the West Antarctic Ice Sheet. If the current or the assisting winds change, then the local climate can change. In fact, careful analyses suggest the strength of the upwelling and the associated currents experience significant inter-decadal variability, perhaps driven by changes in the westerly wind patterns.
What really counts in terms of judging human impacts on this natural process is whether any mooted acceleration in melting at the ice shelves can be attributed to human activity, or not. Are any short-term changes, such as they are, simply a function of natural variability in the temperature, speed, and distance at which the ‘warm’ currents pass along the coastline, or are they ‘unprecedented’ changes that lie outside the range of natural variability? It remains entirely unclear how the circulation of circumpolar deep water might change in the future. If the cause of changes in circulation is unknown, then changes could simply be random and a function of entirely self-contained ‘internal’ natural variability in the flow of the current. Imagine for a moment the flow of water in a turbulent stream. Watching closely, does the water always follow exactly the same path past any point? Ocean currents are the same but they operate on vastly greater scales over much longer time frames. In climate science, natural internal climate variability is a well-established fact. It is most pronounced at the local scale within individual climatic regions. The volatile climate of polar regions like West Antarctica is a good example.
In any case, the combined amount of ice in the affected ice shelves is only a fraction of the total amount of shelf ice that fringes the rest of the Antarctic continent. The threat that ice shelf thinning poses to the stability of the glaciers behind them is the subject of extremely challenging computer modelling. Theoretical understanding is still incomplete and there are not enough measurements in the critical areas. While of concern and worthy of monitoring, because the Thwaites Glacier behind the ice shelf is ‘the size of Britain’, should a ‘collapse’ commence at some time in the coming centuries then it would still take many thousands of years to unfold. That is because a glacier this size cannot simply ‘collapse’. It is just too big for that to occur. Geological studies of past events over the last million years show that the transition between glacial, intermediate, and collapsed states takes one to several thousand years. This is an entirely natural process, but it hasn’t stopped the Thwaites being dubbed the ‘doomsday glacier’ in headline-grabbing news articles such as one by the BBC entitled ‘Antartctica melting: Climate change and the journey to the doomsday glacier’, which is an impression cemented into folklore by many similar articles.
There is an extensive body of scientific literature that documents the past geological and climate history of West Antarctica. It documents a multi-thousand-year record of entirely natural climate volatility. This record is stored in both the offshore sediments, beneath the ice shelves, and within the ice sheet itself. The geology shows there is no evidence that current-day natural swings in climate along the West Antarctic coastline are in any way unprecedented. The ice sheet has undergone multiple massive volume changes over just the last million years alone in response to the global ‘ice age’ glaciation cycles. At times, parts of the West Antarctic Ice Sheet have disappeared almost entirely, along with its associated massive Ross and Ronne-Filchner ice shelves which combined are hundreds of times more massive than the Thwaites ice shelf. But each time, the ice has returned and, ten thousand years ago, the ice sheet on the Marie Byrd Land coast in West Antarctica was actually more than 700 m thicker than today.
These ice sheet cycles have been repeating for millions of years and therefore any current thinning of the ice beneath the ice shelves cannot simply be attributed to human activity, just because we now have the technology to measure it.”
But Arthur, they are! By those who have no concept of our climate history, or of how much ice there is in Antarctica.
