'Red alert' for Antarctic sea ice

... means a 'red alert' for much more too

Something weird is happening in Antarctica.

First it was the ozone hole - now mostly in the rear-view mirror (though maybe not entirely. Some scientists think that the ozone hole this year will be larger than usual because of increased water vapour from the Tongan volcanic eruption).

Now its climate change - bearing down on us at breakneck speed. It seems the fragile environment in Antarctic makes it the canary in the coalmine again.

My old Lauder boss, Gordon Keys, passed on an excellent ‘Red Alert’ article by another NIWA colleague - Natalie Robinson- that appeared in the June 2023 issue from the New Zealand Antarctic Society. I don’t know her personally, and the paper doesn’t seem to be freely available, so I’ll summarise the main points here and try to put them in context. Her scary but enlightening story reminds me of this, and this, among others of my recent posts here at uv.substack.com

Much of the change she’s talking about is illustrated at Zachary Wabe’s excellent website. The plots there (see samples below) show that in the last year or so, something seems to have changed in the waters surrounding Antarctica. Sea surface temperatures there have heated so much that amount of floating sea ice has suddenly started to decrease. She makes the point that at its annual peak in September this apron of ice effectively doubles the size of the continent. But that won’t be the case this year.

The most immediate consequence this reduction in sea ice from global warming lowers the overall surface reflectivity (albedo), leading to greater absorption of solar radiation in the ocean - and more warming. A dreaded positive feedback. But it doesn’t end there ..

The effects go much deeper (if you’ll pardon the pun), ultimately limiting the ability of oceans to continue absorb excess carbon dioxide.

Here’s why. The density of water is a function of both its temperature and its saltiness (salinity). You float noticeably higher in sea water because is density is about 3 percent greater than fresh water, giving a 3 percent increase in buoyancy. The salinity of seawater is reported in Practical Salinity Units (PSU, where 1 PSU = 1 gram of salt per kilogram of water). Its mean is about 35 PSU - as used in the plot below - but it can vary greatly, ranging from less than 15 PSU at the mouth of the rivers to more than 40 PSU in the Dead Sea.

The changes in Antarctic waters aren’t nearly as large, but they’re still important. In the process of freezing to form those ice sheets each winter, brine (or salty water) is rejected. This makes the floating ice less salty and the underlying water saltier.

That increased salinity of the remaining water makes it denser, and triggers a descent that’s the beginning of the large scale circulation of oceanic water. Conversely, in summer as the ice melts, the surface layer becomes less dense and floats on top.

About 40 percent of the deep water global oceanic circulation - the so-called Thermo-haline (thermo - from heat-carrying, haline from salinity-controlled) Conveyor Belt that transports heat throughout the globe is initiated by that pulsed descent of Antarctic water triggered by these changes in salinity. The Gulf Stream, which provides heat to northern Europe is just one component of this ‘Conveyor Belt’.

The trouble is that with less ice being formed, the surface waters are less salty, so less inclined to descend, leading to a slowing of the Conveyor Belt. And the changes are large. As the link shows (see below), the amount of sea ice formed this year is about 2.5 million square kilometres less than usual - 10 or 20 percent less (depending on the season) than in previous years. Uncharted territory again! This means potentially a huge reduction in the deep-water circulation.

She also makes the point that melting of sea ice around the Antarctic continent has a quite different effect from the melting of land-bound glaciers on the continent itself. Unlike floating sea-ice, the melting of those glaciers will inevitably lead to multi-metre rises in sea level, though the time scales are long - hopefully multi-century.

A more immediate effect of that ice-melt results from the discharge of fresh water to the ocean in summer, adding to the lower salinity water from melting sea ice (of course, because air temperatures are so low in Antarctica, this flow will occur only from those parts of the glacier that extend to the ocean). The fresh water is less dense than the underlying salty sea water, so tends to float on top, and so inhibiting downward motions in summer.

Finally, she makes the point that a couple of recent studies show that the effects of these changes in deep water circulation could be reduced by 40 percent or more this century if current emissions continue. That would have a drastic effect on the ability of oceans to absorb atmospheric carbon dioxide and excess heat energy. Without that capability, the surface temperature would rise much faster than in recent years. The trigger point for this runaway effect is for a temperature rise of about 2C underlining the urgent need to keep future increases below that level.

There’s plenty of evidence elsewhere that things are disturbingly different from business-as-usual. The anomalies this year not just confined to Antarctica. Bill McKibben talks of several others here. If you need more, see here. It looks more like a planetary red alert we’re heading into.

An interesting aside (which oceanographers have clearly thought of, but I hadn’t … ).
One component of sea level rise is the thermal expansion of the oceans. The rise in sea level from that source can be calculated from the decrease in density at higher temperatures. But at the same time, the increased temperatures will be accompanied by increased evaporation, which will lead to a counteracting increase in salinity and therefore an increase in density. As you can see from the above figure, effects of salinity changes could conceivably swamp those from thermal expansion. According to this plot, the effect of a change in salinity as small as 1 PSU can be enough to counteract the effect of a 5 degree change in temperature. Food for thought.