18,000 years ago, the Zeeland Switch destroyed the Earth’s glaciers

(ORDO NEWS) — Earlier this month, we completed our last annual snowline survey over New Zealand’s South Island (Te Waipunamu), providing a bird’s eye view of how the glaciers behaved last year.

This collection of aerial photographs adds to nearly half a century of perspective on the undeniable and dramatic impact of climate change on New Zealand’s frozen landscapes.

To put it bluntly, New Zealand’s glaciers look depleted. Another offshore heatwave from the Tasman Sea broke the record for the hottest year on record in late 2021, washing over the Southern Alps. This trend continued into the summer of 2022 in the Southern Hemisphere.

As New Zealand’s glaciers continue to feel the heat and shrink, bedrock that hasn’t seen daylight for centuries is exposed.

Pools filled with melt water begin to multiply throughout the landscape. In many cases, a ring of mud and rocks forms around some of New Zealand’s largest lakes, marking where the ice once reached.

Our current research focuses on these exposed rock ridges to trace New Zealand’s climate history.

The ridge-like mounds of rocks left behind by retreating glaciers are called moraines. Right in front of some of the largest glaciers in the Southern Alps, fresh moraines fringe turquoise lakes into which the ice flows. This scene leaves no room for denying the rapid retreat of the ice from the alpine landscape.

Downstream, more extensive moraines wind like ribbons around the massive lake basins along the edge of the Southern Alps. Some of these landforms stretch for miles, and they are evidence that the ice was much more extensive in the past.

18000 years ago the Zeeland Switch destroyed the Earths glaciers 2

We know that the processes that formed these moraines must have been similar to what we see today. But how old are they? What happened to the massive ice that was once there and why has it retreated?

A new mechanism to explain the rapid shift at the end of the last ice age – the so-called “Zealand switch” – is based on data from New Zealand moraines. This new hypothesis calls into question the long-held view of why glaciers have changed in the recent and distant past.

Although Zeeland Crossing focuses on the global retreat of ice in prehistoric times, we believe it could also explain what is happening to our glaciers now.

18000 years ago the Zeeland Switch destroyed the Earths glaciers 3Left: The map shows the moraines of Lake Ohau and Lake Pukaki with their ages in thousands of years before present. Right: Lake Pukaki (A) and Lake Ohau (B) moraines indicate that the ice retreated rapidly 18,000 years ago

Clues from a Nearly Sunken Continent

Geologists who study glaciers use rare chemical isotopes found in rocks to trace the history of the earth’s surface using a technique called cosmogenic surface dating.

This method measures how long rocks found on the surface today have been exposed to cosmic rays. Boulders that have been transported inside flowing ice have zero exposure history.

When they fall on the moraine and are exposed to cosmic rays from space, their “cosmic clock” starts, and rare isotopes begin to accumulate in the minerals of the rock.

Once the dates of irradiation of moraine boulders are established, they are tied to detailed maps that show the sequences of advance and retreat of ice.

On the large moraines around the lakes of the central Southern Alps – Pukaki, Tekapo and Ōhau – there are now hundreds of results indicating rapid changes that occurred about 18,000 years ago.

In the Tasman Sea, microfossils from sediment cores suggest that ocean currents and boundaries shifted at exactly the same time.

Climate modeling can explain the simultaneous changes on land and at sea as a major shift in the Southern Hemisphere westerly winds over the nearly submerged continent of Zeeland – hence the “Zealand switch” hypothesis.

When the Zealandia Switch turns on and spins south westerly winds, it promotes the export of water vapor from the tropics and an atmospheric circulation that stimulates warming in both hemispheres.

If the Zeeland switch hypothesis is confirmed, then the story of the origin of the Quaternary Ice Age and its impact on global climate, plant ecosystems and ancient fauna will have to be rewritten.

Zeeland change and loss of ice

Fast forward 18,000 years and the southerly winds of change are kicking in again. Subtropical waters enter the Tasman Sea, resulting in more frequent marine heatwaves. The temperature in New Zealand is rising sharply.

Atmospheric rivers filled with tropical moisture penetrate the Antarctic latitudes and bring with them record temperatures. The current situation shows signs of an increasing role for the Zeeland switch – but this time the Earth is in an interglacial, not an ice age.

Recent studies of the Southern Alps glaciers show that warm season temperatures in Australia and the trend towards higher snow cover levels are closely related. The rising trend of the snow line is also accelerating at an alarming rate.

18000 years ago the Zeeland Switch destroyed the Earths glaciers 4The New Zealand Summer Snowline (also known as the New Zealand Equilibrium Line Height) has continued to rise in recent years. It is expected to be at least 200 m above the 1981-2010 average by the next decade

The series of extremely hot years with exceptionally high snow lines that define this pattern has been linked to anthropogenic greenhouse gas emissions. Similar conclusions have been drawn for the recent acceleration in global ice loss.

18000 years ago the Zeeland Switch destroyed the Earths glaciers 5The rise of the snowline is accelerating in the Southern Alps. By 2035, many of the glaciers monitored by NIWA are expected to come close to extinction

These connections raise the possibility that human activity has pushed the Zeeland switch to the “ON” position and it may be stuck there for the foreseeable future.

If what happens is similar to how the Zeeland switch rolled up the ice age during the last ice age, we can expect large, rapid and global effects of climate reorganization.

The coming changes could also be the beginning of the end – the final cessation – for many glaciers to the north and south. The Conversation

Andrew Lorrie, Chief Scientist and Program Manager for the Southern Hemisphere Climate and Environment Program at the National Institute for Water and Atmospheric Research; Aaron Putnam, assistant professor at the University of Maine; David Burrell, geologist and geomorphologist GNS Science; George Denton, professor at the University of Maine, and Joellen Russell, professor at the University of Arizona.

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