Importance and scale of past climate change underestimated

The magnitude of past abrupt climate change events around the planet may have been underestimated, according to a major international study led by a scientist based at NUI Galway.

If the findings using new evaluation models are correct, the impact of current climate change on the planet may be larger than expected.

"Abrupt climate events that occurred during the last interglacial [warm] period, circa 125,000 years ago, have underestimated [temperature rise] by up to 4 degrees," said lead author Dr Audrey Morley from NUIG school of geography and archaeology.

“This is important because our current understanding of climate change and our predictions of future climate both rely on past examples from Earth’s climate history.”

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In the study published in the journal Geochemistry, Geophysics, Geosystems, Dr Morley collaborated with researchers from the University of California-Santa Cruz and Rutgers University New Jersey in the US, and the University of Bergen in Norway.

The researchers used an established geochemical tool for investigating sea surface temperatures in the past. In the modern ocean, observations have shown that marine plankton, known as foraminifera, will use more magnesium relative to calcium elements freely available in sea water when they form their shell in warmer waters.

This allows scientists to apply this modern relationship between magnesium, calcium and temperature to the past by measuring magnesium-to-calcium ratios in fossilised marine plankton deposited in seafloor sediments.

Amount of magnesium

However, there were limitations with using the relationship, because understanding of other processes that may influence the amount of magnesium in the shell was incomplete.

The researchers developed an innovative mathematical correction scheme that enabled the carbonate ion concentration effect to be isolated from the temperature signal recorded in marine plankton.

Dr Morley and her colleagues were able apply their models for a specific marine plankton species living in the subpolar North Atlantic Ocean.

“Applying the proposed correction scheme to past climate records reveals that we may have underestimated abrupt climate events by up to 4 degrees during past interglacial periods,” Dr Morley said.

Correcting for low carbonate ion concentration values improves the validity of temperature reconstructions and allows a reassessment of when the world suddenly warmed or cooled in the past.

This was particularly important for climate records from the subpolar and polar North Atlantic region that may have experienced abrupt changes in carbonate ion concentrations linked with abrupt climate events, Dr Morley said.

In such circumstances the availability of robust methods capable of processing large amounts of data from the Earth’s geological past are essential to deduce the magnitude of abrupt climate events.

Earlier this year, another study in Nature Climate Change, led by Dr Giovanni Sgubin of the University of Bordeaux in France, predicted abrupt climate events are much more likely to occur in the North Atlantic this century due to global warming.