Declining Arctic sea ice slumps to 'uncharted territory' in worrying graph

Declining sea ice in the Arctic has plunged to "uncharted territory" for this time of year.

Normally sea ice extent fluctuates through the year. Some melts during the northern-hemisphere summer, before refreezing in the winter.

Ice that survives through multiple seasons becomes thicker and more durable.

But over the last few decades, more sea ice has been melting than refreezing. The ice that remains is 'younger' and more likely to melt the next season than the ice that has been around for several years.

Colorado meteorologist Bob Henson called it "truly uncharted territory", sharing a graph of this season's ice extent on social media.

It shows a noticeable plunge in sea ice extent in April, a sharper and lower drop than in previous years.

Arctic sea ice reached its lowest level on record in September 2012 at 3.387 million square kilometres.

It's almost half the extent of the lowest point in the 1981-2010 median, which dropped to 6.341 million square kilometres in the peak of the northern summer.

On the other side of the planet in the Antarctic, sea ice was thought to be resisting the issues plaguing the north as the levels remained relatively consistent.

However in 2016 the extent plummeted and has remained low since, leaving scientists fearing it has reached a "tipping point".

"This could be the moment that Antarctica is catching up with the Arctic," climate scientist Professor Gary Wilson told Newshub in 2017.

"In one sense it's exciting we're starting to see signs, in another sense it's daunting because when ice melts, it tends to melt rapidly."

2017 and 2018 echoed 2016's plunge, marking the lowest levels since 1997.

The average global temperature has risen 0.8degC since 1880, with the majority of the increase since 1975, according to NASA's Earth Observatory.

"A one-degree global change is significant because it takes a vast amount of heat to warm all the oceans, atmosphere, and land by that much," NASA explains on its site.

"In the past, a one- to two-degree drop was all it took to plunge the Earth into the Little Ice Age."