Tapping into nature’s archive of change

NIWA scientists are tapping into nature’s archives to understand our abrupt climate changes.

Nature provides the best archiving system available. NIWA’s Dr Helen Bostock is part of an international team tapping into it to learn more about Earth’s climate history during periods of abrupt change.

The history of Earth’s climate is stored in tiny variations in ice, ocean sediments, peat bogs and other natural archives over thousands of years. Recovering and deciphering how and why the climate has changed in the past is providing scientists with a way to understand climate systems.

Understanding abrupt climate changes

‘Abrupt climate change’ describes changes in climate that occur over years or decades, compared with human-caused changes that are occurring over decades and centuries.

Bostock, a NIWA marine biologist, joined fellow Kiwi scientists Dr Marcus Vandergoes and Dr Giuseppe Cortese from GNS Science as part of an international team that has documented duelling ocean and atmospheric heat transport during these periods of abrupt change.

It had previously been thought that changes in the amount of heat carried north by Atlantic Ocean currents were responsible for past periods of abrupt climate change.

Helen Bostock, NIWA
NIWA's fieldwork in Antarctica, such the 'Mertz Polyna' voyage in 2013, provides vital data to help understand climate change processes.

Ocean heat transport is part of the picture

Bostock and the team, which also included scientists from Denmark, Australia, the United States and France, found that changes in ocean heat transport were only part of the picture of abrupt climate change.

The scientists examined how the climate of the Southern Hemisphere behaved during a period of abrupt warming in Greenland and the North Atlantic. They compared climate records spanning Antarctic ice cores, marine sediment cores and even southern African rodent middens with climate model results. They were able to confirm previous ideas that increasing northward heat transport in the Atlantic warms the North Atlantic and Greenland at the expense of abrupt cooling in the Southern Ocean.

This concept is known as the ‘bipolar ocean seesaw’.

Atmospheric circulation is also a factor

The team’s new finding is that atmospheric circulation adjusts in an effort to compensate for the change in ocean heat transport. As the ocean transports more heat northward, the atmosphere responds by transporting more heat southward.

However, the compensation is imperfect; climate changes in different locations throughout the Southern Hemisphere reflect the battle between the opposing ocean and atmospheric heat fluxes. In the low latitudes, over the continents, the atmosphere wins out, driving abrupt drying and warming and shifting the location of the monsoon systems. In the South Atlantic and Southern Oceans, New Zealand and Patagonia, the ocean wins out, driving cooling that is amplified around Antarctica by expanding sea ice.

The research underlines the intimate coupling between the ocean and the atmosphere and helps to explain why past abrupt climate change unfolded so differently in different regions.

Read the research paper "The spatial extent and dynamics of the Antarctic Cold Reversal"

Helen Bostock on board Tangaroa. [NIWA]