Zealandia rules the wind

The sunken continent of Zealandia could play a key role in the Earth’s climate, researchers say.

Zealandia, which New Zealand is part of, sits at a critical crossroads between ocean currents and climate, according to an international team led by the University of Maine and including GNS Science.

The researchers believe ‘‘Zealandia switch’’ led to the world warming at the end of the last glaciation, with the southern hemisphere westerly wind belt – the strongest wind system on Earth – playing an important role.

‘‘The position of the westerlies in relation to Te Riu-a-Ma¯ui / Zealandia determines the way water moves between the warm Pacific Ocean and the cold Southern Ocean,’’ GNS Science geomorphologist David Barrell said.

‘‘Our modelling shows that changes in the southern westerly winds have farreaching consequences for global climate.’’

During the last glaciation, the westerlies were farther north and weaker, he said. ‘‘This lessened atmospheric convection in the tropical Pacific and reduced the production of moisture and transfer of heat into both hemispheres. It made the globe cooler.

‘‘At the end of the last glaciation, the westerlies shifted south and strengthened. This stirred up the ocean and generated more convection, moisture and warmer temperatures.’’

The Zealandia Switch mechanism related to the general position of the southern hemisphere westerly wind belt, and to the role of Zealandia and Australia in controlling ocean currents, the University of Maine (UMaine) said.

‘‘Zealandia presents a physical impediment to ocean current flow. When the westerly wind belt is farther north, the southward flow of warm ocean water from the tropical Pacific is directed north of the New Zealand landmass (glacial mode),’’ the university said. ‘‘With the wind belt farther south, warm ocean water extends to the south of New Zealand (interglacial mode).’’

Computer modelling showed global climate was affected by the latitude at which the westerlies were circulating. When they shifted south, the westerlies invigorated water circulation in the South Pacific and Southern oceans, and warmed surface ocean waters across much of the globe.

UMaine said the new hypothesis was fundamentally different from the long-held view that subtle changes in the Earth’s orbit influenced how the northern hemisphere continental ice sheets regulated ice age climates.

The new hypothesis was that the orbit changes affected the behaviour of southern hemisphere westerly winds, and it was that behaviour which lay at the heart of global ice age cycles.

A key part of the research was the development of timelines showing the extent of glaciers in New Zealand and South America during and since the last ice age. Those findings included a major recession in glaciers in the Southern Alps about 18,000 years ago due to significant climate warming.

The research team’s findings suggested ‘‘a big, fast and global end to the last ice age in which a southern-sourced warming episode linked the hemispheres’’.

UMaine said the southward shift of southern hemisphere westerlies at the end of the last ice age was accompanied by gradual but sustained release of carbon dioxide from the Southern Ocean. That may have helped lock the climate system into a warm interglacial mode.

The researchers suggested the introduction of fossil CO2 into the atmosphere may be reawakening the same dynamics that ended the last ice age, potentially propelling the climate into a new mode.

GNS Science’s Barrell said if the new hypothesis stood up to scrutiny, it would provide a better understanding of what drove prehistoric climate shifts.

Researchers believe ‘‘Zealandia switch’’ led to the world warming at the end of the last glaciation.