Scientists blame ocean chain reaction for Antarctic ice spread

Scientists claim to have found the answer to why the extent of Antarctic sea ice has continued to grow.

Scientists claim to have found the answer to why the extent of ­Antarctic sea ice has continued to grow when computer models said it should be shrinking because of climate change.

A re-analysis of computer models by scientists from the US ­National Centre for Atmospheric Research, the Australian Bureau of Meteorology and Monash University has linked the ice growth in the Antarctic to natural cycles in ocean temperatures. Antarctic ice growth has been highlighted by climate sceptics and confounded climate scientists because it has not behaved as models predicted.

A new paper published in ­Nature Geoscience has brought the Antarctic ice growth conundrum back within the climate change science mainstream by connecting it to cooler-than-average sea surface temperatures in the tropical east Pacific. Scientists expect the Antarctic sea ice extent to show signs of retreat during the next decade following the change in the Interdecadal Pacific Oscillation from negative to positive in 2014. “The climate we experience during any given decade is some combination of naturally occurring variability and the ­planet’s ­response to increasing greenhouse gases,” NCAR scientist Gerald Meehl said.

“It’s never all one or the other, but the combination that is ­important to understand,” he said.

According to NCAR, sea ice surrounding Antarctica has been slowly increasing in area since the satellite record began in 1979.

But the rate rose nearly five- fold ­between 2000 and 2014 following the IPO transition to a negative phase in 1999.

The new study found that when the IPO changed phase, from positive to negative or vice versa, it set off a chain reaction of climate impacts that would affect sea ice formation in Antarctica.

Cooler sea surface temperatures cause changes to tropical precipitation which in turn drive large-scale changes to winds that extend to the Southern Ocean.

The scientists examined 262 climate model simulations and found 10 that correctly characterised the natural variability ­between 2000-2014, including the negative phase of the IPO.

All of these models showed a rise in Antarctic sea ice extent across all seasons. “When all the models are taken together, the natural variability is averaged out, leaving only the shrinking sea ice caused by global warming,” co-author Julie Arblaster from Monash said. “But the model simulations that happen to synch up with the observed natural variability capture the expansion of the sea ice area. We were able to trace these changes to the equatorial eastern Pacific in our model experiments.”