There are many things we don’t appreciate unless they slip away. Crops that don’t fail. Pleasant temperatures. The right amount of rain.
An under-appreciated helper with all those things is Amoc – the Atlantic Meridional Overturning Circulation. It moderates climate at a global scale, and its future seems dubious.
Amoc is a conveyor belt of ocean currents that drags immense volumes of warm water from the Southern Hemisphere to the northern Atlantic. In return, cold water flows south. The flow is driven by the classic rule that heavy stuff sinks and light stuff rises – and similarly, cold drops and heat rises.
As surface seawater travels over the equator it warms – and it becomes saltier because heat speeds up evaporation. It flows to the northernmost Atlantic coastlines, loses heat in the vicinity of Northern Europe, and becomes deeply chilled by Arctic winds and ice. Now, it’s cold, salty and dense, so it sinks deeply into the ocean. The conveyor belt has overturned. The cold deep water flows south to Antarctica then again overturns, or wells up, to head north atop the Atlantic on its warming return journey.
Amoc is immensely helpful, says Shaun Eaves, a physical geographer at Victoria University of Wellington. “It’s why northern Europe has a mild climate for its latitude.” But, he says, Amoc is at its weakest in at least a millennium, most likely due to more buoyant water in the Northern Atlantic. “Anything that causes water to become warmer, less salty and less dense means it’s less prone to sinking,”
Likely culprits are warmer temperatures, melting ice and more rain – caused by our continual emissions of greenhouse gases. Without enough force generated by sinking water, Amoc will continue to slow and then collapse.
It has shut down before, as the geological record shows. It would be catastrophic, dropping northern European temperatures by 10-15°C, apart from searing summer heatwaves. The globe’s tropical rain belt would move south, away from rainforests like the Amazon, leaving them dry and releasing their massive carbon stores. The last report of the Intergovernmental Panel on Climate Change said impassively that Amoc’s collapse “would very likely cause abrupt shifts in regional weather patterns, and large impacts on ecosystems and human activities”.
So far, so bad, but what would New Zealand be like without Amoc? Eaves co-authored a paper last year looking back to when Amoc suddenly strengthened 14,500 years ago. Back then, the strong Amoc was pulling more warm water northwards, so New Zealand got colder. That adds to other evidence showing we will heat up if the reverse happens and Amoc peters out. It’s called a thermal bipolar seesaw: when the northern Atlantic cools, the Southern Hemisphere warms, and vice versa.
“Even though New Zealand is about as far as you can get from the North Atlantic, we wouldn’t escape climatic impacts from Amoc collapse,” says Eaves. “The change here would be muted compared to the changes in the North Atlantic, perhaps a degree or two. But that’s on top of global warming.”
A recent flurry of papers suggests Amoc could shut down soonish. Former Nasa scientist James Hansen, who’s renowned for publicly sounding the alarm about climate change in 1988, estimates we have 20-30 years. Other models show shutdown could become unstoppable this century but won’t be complete until the next. The timing is hotly debated.
Everyone agrees that Amoc is one of the tipping points that human-induced global warming could trigger, and that, in turn, would vastly worsen the impacts of global warming. They agree its timing depends on how quickly we reduce greenhouse gas emissions. Hansen is sounding the alarm as never before, and he wishes people would take heed.
