Wind may speed climate change

Southern Ocean’s carbon effects concerns

The gigantic carbon sink below New Zealand that is the Southern Ocean might come to quicken the effects of climate change, due to a worrying feedback loop just identified by scientists.

The ocean current sucks up more than 40 per cent of the carbon dioxide we produce, acting as a temporary climate-change buffer by slowing accumulation of greenhouse gases in our atmosphere. Yet the same westerly winds that play a critical role in regulating its storing capacity now threaten its future as a CO2 bank, by bringing deep carbon-rich waters to the surface.

Many climate models predict the westerlies overlying the ocean to strengthen if atmospheric greenhouse gas concentrations keep rising.

A new international study suggests that in the past, strong westerlies have been linked to higher levels of atmospheric CO2 due to their impact on the Southern Ocean carbon balance. That meant stronger westerlies could actually speed up climate change if mankind continued to emit as much CO2 as it did today.

A team led by Australian Nuclear Science and Technology Organisation scientist Dr Krystyna Saunders travelled to subantarctic Macquarie Island, about 1100km south of Bluff, to reconstruct the behaviour of the Southern Hemisphere westerly winds over the past millennia.

“This is an exciting finding,” Saunders said. “Our new records of the Southern Hemisphere westerly winds suggest there have been large changes in wind intensity over the past 12,000 years.

“This is in marked contrast to climate model simulations that predict only relatively small wind speed changes over the same period.”

On Macquarie, where the winds were strongest, researchers got a clear picture of the westerlies’ behaviour. To reconstruct the changes, they used radioisotope dating methods to analyse ancient samples of accumulated sea spray and windblown minerals, which acted as telltale markers of wind strength over long periods.

They compared their reconstructed wind intensity data with other climate records and found that periods of intense wind strength were matched by marked increases in atmospheric carbon dioxide and temperature, as well as declines in sea ice levels.

“The connection between strong and persistent westerlies and rises in carbon dioxide at the beginning of our current warm interglacial period, about 12,000 years ago, and again from about 7000 years ago, suggests that the winds have determined whether the Southern Ocean acts as a net sink or source of carbon dioxide at different times in the past,” she said.

“Therefore, over a period of decades to millennia, further in- creases in wind strength will lead to faster accumulation of carbon dioxide in our atmosphere.”

Dr Sara Mikaloff-Fletcher, an atmosphere and ocean scientist at Niwa who wasn’t part of the study, said recent trends in the Southern Ocean carbon sink had been mysterious. Several lines of evidence suggested the ocean’s carbon uptake was weaker than expected from atmospheric CO2 changes in the 1990s.

This weaker carbon sink had been attributed to increasing westerlies — essentially through the same mechanism described in the new study. Yet sea surface carbon data suggested there was a reversal of this trend in the early 2000s, when the ocean began taking up carbon much more quickly, even though the westerlies didn’t slow.

“The mechanisms behind this change still aren’t fully explained, which makes it hard to predict whether this is a short-term effect or a long-term one.”

The new study followed a recent Niwa-led stocktake that found climate change was slowly shifting the chemistry of New Zealand’s oceans, threatening the multitude of life in our waters.

Ocean acidification, measured by a reduction in sea water pH, was mostly driven by oceans absorbing and storing increasing levels of CO2 in the atmosphere.

Between 1909 and 2009, New Zealand’s sea-surface temperatures had warmed by a statistically significant 0.71C, while pH levels of subantarctic waters had dropped by 0.0015 units per year since 1998.

Globally, oceans’ average pH was about 8.1 — 0.1 lower than 250 years ago. It might not sound much, but a decrease of one pH unit represents a 10-fold increase in acidity.

The effect is associated with decreases in nutrients in the surface ocean, where most marine organisms live.

This is an exciting finding . . . this is in marked contrast to climate model simulations. Dr Krystyna Saunders