Time travelling with
Dr Aidan Starr

The present day

Before arriving at Cambridge, this was Starr’s job - collecting deep sea sediment cores.

In the present day, Starr is a Postdoc Research Associate for the Department of Geography, and a Trevelyan Research Associate at Selwyn College. He works on processing the cores people bring back from the ocean bed.

Washing mud through a sieve like a gold rush prospector, he picks out microfossils from the mess. Some species are no larger than a grain of sand. The mud tells stories far-flung in space and time. He has sifted shells found outside Maine, from a beach which disappeared 20,000 years ago.

After practising for hundreds of hours, Starr has it down to a fine art.

“I see these fossils in my sleep. It’s quite meditative. Some species you always want - they’re the ‘faithful recorders’ of the carbon cycle. Whenever I see those guys in the corner of my eye, I’ll grab them.

“It’s remarkable how much information can be found in these deep sea cores. We can measure the chemistry and atomic character of plankton shells. We can tell how fast the ocean was flowing by looking at how coarse the silt grains are.”

Starr is looking for something very specific in these cores. He wants to scrutinise intervals of rapid climate change, when the world flipped between a glacial and interglacial period. These abrupt switches relate to changes in a mysterious pattern, which scientists have recently begun to unpick. The pattern is an ocean current with a two-sided nature, known as the AMOC.

Run AMOC

Meet the AMOC - the Atlantic Meridional Overturning Circulation. Like a giant conveyor belt, it carries water, salt and sediment across the oceans. Alongside these it brings heat from the south, across the equator and into the north, depositing that heat into the atmosphere.

“That’s why, in Cambridge, it’s relatively warm compared to other places at our latitude,” says Starr. “It's much colder in landlocked places on our latitude, like Siberia.”

Over the last 100 years, researchers have built a complex picture of how the AMOC functions. In 1961, oceanographer Henry Stommel realised that Earth’s oceans collectively circulate according to certain rules, governed by equilibrium states. These states are holding patterns that the oceans stabilise in. They only jump between states when certain conditions arise. In the 1980s, Wally Broeker discovered rapid warming intervals in the geological record. During these times, Greenland would warm 10 or 15 degrees in a couple of decades.

These rapid fluctuations became known as Dansgaard-Oeschger events - the flick of the switch from one equilibrium to another. Broeker recognised this switch-flick mechanism in the AMOC, in a paper called ‘Unpleasant surprises in the greenhouse.’ He would later coin the phrase ‘global warming’.

It turns out that the modern AMOC has two states - ‘strong’, which is stable and warmer, and ‘weak’, which is low energy and colder. Broeker realised that ocean fluctuations could totally change the climate in the North Atlantic: not gradually, but suddenly.

Starr has seen these sudden changes written in ice cores.

“In the ice, you can see glacial periods lasting around a hundred thousand years. But then if you zoom in, you see these jumps in the record. That’s a flicking of the AMOC switch.”

Currently, we’re in a stable, warm AMOC state. We have been for all our lifetimes, and for many generations before. Our interglacial is called the Holocene, and has kept us cosy for at least 11,000 years. But there are some signs that we might be heading towards a tipping point.

“The thing about a tipping point is, you might not know it’s crossed until it’s too late. Once you’re in the new equilibrium state, it takes a lot to throw the switch back.”