The Southern Hemisphere may be the missing link in answering longstanding questions about how ice ages wax and wane, according to a new study. There, say researchers, complex interactions among the westerly wind system, the Southern Ocean and the tropical Pacific can trigger rapid global changes in atmospheric temperature. The mechanism, dubbed the Zealandia Switch, relates to the position of the Southern Hemisphere westerly wind belt—the strongest wind system on Earth—and the continental masses of the southwest Pacific Ocean around New Zealand. The researchers suggest it could again play a role, though not necessarily a predictable one, as humans push the planet into a warmer state.
The research team, from the University of Maine, Columbia University’s Lamont-Doherty Earth Observatory and other institutions, published the study online in the journal Quaternary Science Reviews this week.
For more than a quarter-century, UMaine scientist George Denton, the article’s first author, has led research reconstructing the history of mountain glaciers in the Southern Hemisphere. In the late 1980s, he and Wallace Broecker, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory, noted that a key question remained unresolved: how shifts between cold and warm climates are linked to the orbital cycles affecting the length and strength of Earth’s seasons. Evidence showed that climate changes were more or less simultaneous in the northern and southern hemispheres, with rapid transitions from glacial to interglacial conditions. They concluded that existing theories could not adequately account for changes in seasonality, ice sheet size and regional climate.
Mountain glaciers are highly sensitive to climate, and well suited to climatic reconstruction, using moraine deposits that mark their former limits. In the 1990s, Denton led research teams that mapped and dated moraine sequences in South America and, more recently, in New Zealand’s Southern Alps, along with coauthor David Barrell, a geologist with the New Zealand government’s research institute GNS Science.
With advances in techniques to use isotopes for dating of moraines in the mid-2000s, Denton teamed up with Joerg Schaefer, who directs the cosmogenic nuclide laboratory at Lamont-Doherty. Along with Lamont-Doherty coauthor Michael Kaplan, and UMaine coauthor Aaron Putnam, the team developed a chronology of climate-induced glacier changes in New Zealand’s Southern Alps spanning many tens of thousands of years. The team then compared the moraines’ histories to paleoclimate data worldwide. They documented a general global synchronicity of mountain-glacier advances and retreats during the last ice age which ended about 15,000 years ago.
Insights into the climate dynamics came from coauthor Joellen Russell, a climate scientist at the University of Arizona. Her longstanding efforts at modeling the westerly winds showed that changes to the southern wind systems have profound consequences for the global heat budget, as monitored by glacier systems.
The “switch” takes its name from Zealandia, a largely submerged continental platform about a third of the size of Australia, with the islands of New Zealand being the largest emergent parts. Zealandia impedes ocean current flow. When the westerly wind belt is farther north, during glacial times, the southward flow of warm ocean water from the tropical Pacific is directed north of the landmass. When the wind belt is farther south, warm ocean water extends to the south of New Zealand; this is the warm, or interglacial mode. Computer modelling shows that global climate effects arise from the latitude at which the westerlies are circulating. A southward shift of the southern westerlies invigorates water circulation in the South Pacific and Southern oceans, and warms the surface ocean waters across much of the globe.
The researchers hypothesize that subtle changes in Earth’s orbit affect the behavior of the Southern Hemisphere westerly winds, and that this behavior lies at the heart of global ice-age cycles. This perspective is fundamentally different from the long-held view that orbital influences on the extent of Northern Hemisphere continental ice sheets regulate ice-age climates. Adding weight to the Zealandia Switch hypothesis: the Southern Hemisphere westerlies regulate the exchange of carbon dioxide and heat between the ocean and atmosphere, and, thus exert a further influence on global climate.
The last glacial termination was a global warming episode that led to extreme seasonality (winter vs. summer conditions) in northern latitudes by stimulating a flush of melt water and icebergs into the North Atlantic from adjoining ice sheets. All this fresh water (which freezes more easily than salt water) made it easier for winter sea ice to become more widespread in the North Atlantic. This caused very cold northern winters, and amplified the annual southward shift of the Intertropical Convergence Zone and the monsoonal rain belts. Although this created an impression of differing temperature responses between the polar hemispheres, the so-called “bipolar seesaw,” the researchers suggest this is due to contrasting inter-regional effects of global warming or cooling. The researchers suggest that a succession of short-lived, abrupt episodes of cold northern winters during the last ice age were caused by temporary shifts of the Zealandia Switch mechanism.
The southward shift of the Southern Hemisphere westerlies at the termination of the last ice age was accompanied by gradual but sustained release of carbon dioxide from the Southern Ocean, which may have helped to lock the climate system into a warm interglacial mode, they say.
The researchers suggest that the massive injection of fossil carbon dioxide into the atmosphere today by humans may be reawakening the same dynamics that ended the last ice age, potentially propelling the climate system into a new mode.
The research “provides an explanation [for] orbital-scale global shifts between glacial and interglacial climatic modes,” the research team writes. “Such behavior of the ocean-atmosphere system may be operative in today’s warming world, introducing a distinctly non-linear mechanism for accelerating global warming due to atmospheric CO2 rise.”
The work was funded by the Comer Family Foundation, the Quesada Family Foundation, the U.S. National Science Foundation and the New Zealand government.
Adapted from a press release by the University of Maine.
