Cause and effect l Winter is here, but it's not enough

Closer to the south pole, however, these effects aren't as apparent.

And yet, the records there have left scientists baffled.

On July 31, the sea ice extent in the southern hemisphere reached its lowest level of 14.624 million square kilometres.

The level on the same date last year was 15.868 million square kilometres – the lowest till that time.

In 1979, when the winter ice was first observed, the Antarctic sea ice extent was 17.070 million square kilometres; so an overall drop of nearly 2.5 million square kilometres.

In that context, the drop might not seem like a lot. The ice extent still numbers in the millions of kilometres.

Yet, the decrease is right now alarming because, unlike the northern hemisphere, the south is in the dead of winter, when ice seasonally builds up. Antarctic sea ice shrinks to its lowest levels towards the end of February – the continent's summer.

This year’s low in the winter ice also came after the lowest extent on record for an annual minimum – 1.79 mn sq km on February 21.

The cycle of seasonal variability is the opposite of the Arctic, where the ice reaches its maximum in February to March, and its lowest in September.

For the Arctic, the July 31 value is 6.968 million square kilometres. The lowest was recorded in 2012, on September 17, 3.387 mn km sq.

Thus, the seasonal drops would appear common, even natural. And hence, the worry over the rate of decrease.

According to experts, it is once-in-every-7.5 million years-rare.

"To say unprecedented isn't strong enough," physical oceanographer Edward Doddridge told ABC News.

"For those of you who are interested in statistics, this is a five-sigma event. So it's five standard deviations beyond the mean. Which means that if nothing had changed, we'd expect to see a winter like this about once every 7.5 million years. It's gobsmacking," he said.

A five-sigma event indicates a statistical anomaly five standard deviations away from the mean, making it an exceptionally rare event with a very low probability of happening by chance.

But, the focus on the rarity of the event may actually be counterproductive.

Ted Scambos, a glaciologist at the University of Colorado Boulder, told CNN, “There’s no sense talking about the odds of it happening the way the system used to be, it’s clearly telling us that the system has changed.”

The question that then arises: Why would anyone care about melting ice in Antarctica?

And, if it is 7.5 million years rare, does it really have any far-reaching consequence?

The answers to both are simple and interlinked.

First, sea ice helps regulate the earth's temperature through a process called the albedo feedback: ice reflects the Sun's heat back into space, helping to regulate the temperature of the planet. If there's less ice to reflect that sunlight, the heat gets absorbed by the surface instead of being reflected out into space, in turn accelerating the warming of sea which then gets carried around the rest of the world.

Global sea surface temperatures were already at a record high for the time of the year both in May and June, with an El Nino having set in in May.

Second, the freeze and melt cycle of the ice drives global currents that transport nutrient-rich water to the rest of the ocean, feeding ecosystems.

As opposed to the Arctic – where the rate of decline has been more accurately measured at 12.6% a decade compared to the 1980-2010 average – the decline in the Antarctic has been so varied that no clear long-term trajectory has been observed.

The IPCC in 2019 said: “It is very likely that Antarctic sea ice cover exhibits no significant trend over the period of satellite observations (1979–2018). While the drivers of historical decadal variability are known with medium confidence, there is currently limited evidence and low agreement concerning the causes of the strong recent decrease (2016–2018), and low confidence in the ability of current-generation climate models to reproduce and explain the observations.”

The IPCC report warned with “high agreement” that the recent loss in the Antarctic Ice Sheet “could be irreversible over decades to millennia” and said that data are not yet sufficient to "determine whether these changes mark the beginning of irreversible retreat”.

The movement has been more steadily downward since 2016.

Because Antarctic sea ice extent has remained below climatological values since 2016, there is still potential for longer-term changes to emerge in the Antarctic, similar to the Arctic, the IPCC said.

Another question that remains: Is the decline due to anthropogenic warming?

"The consensus statement at the moment would be that this is largely anthropogenic forces that have caused the ocean to warm, for the atmosphere to be highly disturbed and to affect the sea ice," Australian Antarctic Program scientist and associate researcher at the University of Tasmania Dr Petra Heil told ABC News.

"We might end up in a new state," she said, expressing fears of the ice dropping to a level that it kicks over a tipping point, leading to a slowdown of deep ocean currents that carry heat, oxygen, carbon and nutrients for marine life across the globe.

The slowdown of the currents, incorrectly interpreted in the Hollywood movie The Day the Earth Stood Still, and more recently, and again incorrectly, confused with the collapse of the Gulf Stream, could lead to a breakdown of global climate, melting more ice and even changing rain patterns.