by David Spratt
Third in a series. Read 1 | 2 | 3 | 4 | 5 | 6 | 7
 |
| Broken-up Arctic sea-ice |
Arctic warming is racing ahead of the worst-case estimates, now heating four times faster than the global average, and the region is undergoing abrupt climate change, understood as a transition of the climate system into a different mode on a time scale that is faster than the responsible forcing. In other words, it has passed a tipping point for rapid, system-level change.
Researchers say that the Arctic “is currently experiencing an abrupt climate change event … climate models underestimate the abruptness of the recent changes observed in the Arctic (and) climate models underestimate this ongoing warming”. [Models do not account well for warming due to sea-ice loss, but losing the reflective power of Arctic sea ice in the summer months would advance the 2ºC threshold by 25 years”.]
In winter, the Arctic Ocean and the surrounding seas extending to the edges of Russia, Alaska, Canada and Greenland are covered by a thin skin of more than 14 million square kilometres of floating sea-ice, which is reduced in extent each summer by warmer conditions. Summer sea ice now covers half the area it did in the 1980s, and because it is thinner, its volume has decreased 75%. There is almost no multi-year ice left.There is a close relationship between sea-ice changes and rapid contemporary Arctic and past Greenland warming, an ominous signal about Greenland’s future, and its impact on the AMOC.
There is a strong positive feedback: as reflective sea ice in summer is replaced by dark, heat-absorbing open water, more energy is absorbed and the region heats, more ice is lost and the remaining ice is thinner and more fragile for the next summer melt. Another feedback is at work, too: the polar jet stream creates an atmospheric boundary between the Arctic and mid-latitude conditions, but the stream’s destabilisation into a more meandering pattern caused by a hotter climate is now bringing unprecedented surges of heat into the Arctic. These have led to unprecedented heatwaves in Siberia and wildfires in Alaska.
The Arctic is inexorably heading towards the day, within the next decade or so, when there will be no summer sea-ice (defined as less than ten per cent of the area before the “big melt” commenced). This “blue water” event will lengthen to weeks and months if warming continues. There is uncertainty about the timing, “with some research suggesting that recent trends could lead to an ice-free Arctic as early as the 2020s and others suggesting 2030 or substantially later” depending on factors including future warming and natural variability. “The chance that there will be any permanent sea ice left in the Arctic after 2022 is essentially zero… Can we lose 75-80% of permanent ice and recover? The answer is no,” says Prof. James Anderson of Harvard University.
The consequences of a summer ice-free Arctic are profound. The region’s ecology would be unrecognisable, Greenland would be experiencing unstoppable melting with severe consequences for the global ocean circulation system. More permafrost stores would be mobilised and the Boreal forests would suffer more severe dieback. The temperature differential between the Arctic and the tropics would be so reduced that basic weather, atmospheric circulation and precipitation patterns would be transformed. A climate-induced ozone hole may form. Impacts will affect the whole globe, from the Amazon rainforest to the West African and Indian monsoons, and more severe El Ninos.
Arctic specialist Jason Box says that "If we don’t get atmospheric carbon down and cool the Arctic, the climate physics and recent observations tell me we will probably trigger the release of these vast carbon stores, dooming our kids’ to a hothouse Earth."
Greenland: “Beyond its viability threshold”
The Greenland Ice Sheet (GIS) contains enough ice to raise sea-levels by 7 metres. There is strong evidence that GIS has passed a tipping point, is losing mass at accelerated rates in the 21st century, melting seven times faster than it did in the 1990s, and that ice mass loss will continue to accelerate for the current climate conditions.
 |
| Tipping points and potential cascade effects |
This is in part due to rapid Arctic warming. But a number of feedback mechanisms are driving these events, including:
- Positive melt-elevation feedback, a non-linear mechanism in which melting reduces the ice sheet height, exposing the ice surface to warmer temperatures, which further accelerates melting. Researchers say this process is likely to be responsible for the observed destabilization of GIS and is an “early-warning signal for a forthcoming critical transition” with “substantially enhanced melting in the near future”.
- Algal blooms darkening ice, the change in reflectivity resulting in greater heat absorption and enhanced melting of the surface ice by as much as 20 percent. “Ice algae have started to colonize larger parts of Greenland. They’ve become an x-factor in the melting process,” says Prof. Jason Box.
- Enhanced rainfall: Rainfall makes GIS more prone to surface melt since it changes the surface reflectivity, and the melting reinforces itself: the rain melts snow, exposing the underlying darker ice, which absorbs more sunlight, increasing surface temperature, leading to more melting.
Box, professor in glaciology at the Geological Survey of Denmark and Greenland, says GIS has passed a tipping point/point of system viability:
“Technically, now [at 1.2°C] Greenland is beyond its viability threshold… 1.5°C [of warming would] mean the ‘beyond the threshold’ state is enhanced and the loss [of ice mass] becomes a complex, non-linear, amplified response guaranteeing the ice sheet remains beyond its viability threshold. [We are documenting] several physical processes and amplifiers that guarantee more rapid response of the ice than is currently encoded in climate models that project sea-level rise… we cannot yet rely on ice sheets models for credible sea level projections.”
[As an aside, many of the feedbacks and processes Box and others have described that are not accounted for in current Greenland models are also applicable for the Asian ice sheets: One-quarter of the Himalayan and Tien Shan ice sheets have already been lost.]
Ian Howat, professor of earth sciences and distinguished university scholar at Ohio State, agrees with Box: "Glacier retreat has knocked the dynamics of the whole ice sheet into a constant state of loss... Even if the climate was to stay the same or even get a little colder, the ice sheet would still be losing mass."
And a study published in 2020 found that GIS reached its tipping point 20 years ago, when widespread glacier retreat helped push the ice sheet from a balanced to an imbalanced state, and a “switch to a new dynamic state of sustained mass loss that would persist even under a decline in surface melt”, even if the oceans and atmosphere were to stop warming today.
Norwegian and US scientists who took a close look at the ice age history of Greenland found that all it takes to trigger GIS melting away is a mean sea surface temperature higher than seven degrees Celsius. And the present mean sea surface temperature is already 7.7°C.
 |
| Meltwater lake, Greenland |
Above-zero temperatures (and rainfall) are now being recorded at the summit of GIS, leading to melting and rainfall at the highest elevations. The esteemed glaciologist Konrad Steffen, who in 2020 lost his life in a GIS crevasse, said in 2018: “Once we reach the point that melting occurs up to the highest point, Greenland [ice sheet] will disappear. Eventually we will have a global sea-level rise of five metres. This will maybe happen in 50 to 100 years… There will be a migration of 300-500 million people away from the coast, and that will be quite a big unrest.”
There is a significant chance of a sea-ice free Arctic in the next decade or so. “Could the Greenland ice sheet survive if the Arctic were ice-free in summer and fall?”, asked researchers in 2007. Their answer:
“It has been argued that not only is ice sheet survival unlikely, but its disintegration would be a wet process that can proceed rapidly. Thus an ice-free Arctic Ocean, because it may hasten the melting of Greenland, may have implications for global sea level, as well as the regional environment, making Arctic climate change centrally relevant to the definition of dangerous human interference.”
