Ocean stratification. A condition characterized by the separation of layers of water of different temperatures and chemical make-up. A condition that has serious impacts to the geophysical nature of the worlds oceans, to the ability of oceans to support life, and to the stability of the vast glaciers of Antarctica — whose faces plunge as deep as hundreds of feet into the Southern Ocean.
In the Antarctic, today, what we see is a cold surface layer and a heating bottom layer. The cold surface layer is fed by an expanding pulse of chill, fresh water issuing from the melting glaciers of Antarctica. Over the years it has become more uniform, sequestering cold near the surface as warmth builds up in the depths below. The deeper hot layer is fed by warmer water issuing in from the tropics and heated to temperatures not seen for tens of thousands of years. This hot water bears a heavy burden of salt. So it is denser and it dives beneath the expanding fresh water layer. The insulating fresh, cold water layer prevents mixing between the bottom layer and the surface. Such mixing would cool the bottom layer. But instead the heat builds and builds and builds.
(Antarctica — visual difference in ice mass between now [right] and last glacial maximum [left]. By mid century, atmospheric greenhouse gas concentrations driven by humans could be high enough [550 ppm CO2e+] to melt all the remaining ice upon this now-frozen continent. Image source: NASA/Goddard Space Flight Center.)
Ocean currents bring the deep, hot water in contact with the base of Antarctica’s massive glaciers. And this intensely focused heat engine goes to work to rapidly melt the ice.
It is this condition of ongoing and intense melting of the ice sheet’s bases that terminate in faces of ice cliffs, hundreds of feet high and deeply submerged in the sea, that is driving the irreversible collapse of many glaciers in Antarctica. Already, due to this irreversible fall, the entire flank of West Antarctica is under collapse — locking in at least three feet of sea level rise from this region alone going forward.
But now, a new study finds that these conditions — the same conditions we observe today — led to the release of enough glacial ice from Antarctica alone at the end of the last ice age to raise sea levels by 3-4 meters (10-13 feet) in just 1-3 centuries.
From Nature Communications:
“The reason for the layering is that global warming in parts of Antarctica is causing land-based ice to melt, adding massive amounts of freshwater to the ocean surface,” said ARC Centre of Excellence for Climate System Science researcher Prof Matthew England an author of the paper.
“At the same time as the surface is cooling, the deeper ocean is warming, which has already accelerated the decline of glaciers on Pine Island and Totten. It appears global warming is replicating conditions that, in the past, triggered significant shifts in the stability of the Antarctic ice sheet.”
The last time this happened was 14,000 years ago as the Earth slowly warmed out of the end of the last ice age. But the result was anything but gradual:
“Our model simulations provide a new mechanism that reconciles geological evidence of past global sea level rise,” said researcher UNSW ARC Future Fellow Dr Chris Fogwill.
“The results demonstrate that while Antarctic ice sheets are remote, they may play a far bigger role in driving past and importantly future sea level rise than we previously suspected.”
“The big question is whether the ice sheet will react to these changing ocean conditions as rapidly as it did 14,000 years ago,” said lead author Dr Nick Golledge, a senior research fellow at Victoria’s Antarctic Research Centre.
These are critical questions. Ones that have serious impacts for the more than 700 million people now living within 10 meters of current sea level.
(Antarctic Ice Shelf thickness changes. Note the thinning of almost all the ice shelves along the margin of Antarctica. Ice shelves anchor interior ice, keeping it from rushing out through deep channels into the Southern Ocean. Rapidly thinning ice shelves is a precursor to glaciers rushing toward the sea. Image source: Nature Pritchard et al. 2012)
To this point it is worth noting that the pace of warming 14,000 years ago was on the order of 0.05 degrees Celsius each century. The current pace of human-driven warming over the past century was 20 times faster. This century, the warming is predicted to be as much as 500 times faster (3-5 C warming by 2100). So the question may we be — will Antarctica respond as ‘slowly’ as it did at the end of the last ice age? Slow as in ice outbursts that lead to sea levels rising by as much as 14 feet during one century.
(Hat Tip to Colorado Bob)