Oxford Martin School

"Will the Gulf Stream shut down?"

David Marshall, co-director of the 21st Century Ocean Institute is asked that question by colleagues and friends frequently.  In his lecture as part of the 21st Century School's seminar series on Complexity and Systemic Risk, Marshall tackled that question, explained the background to the work of the his Institute, and presented some of the models he and his team have developed for understanding these complex natural systems.

At the 21st Century Ocean Institute, Marshall and co-director Gideon Henderson focus on the interaction between ocean chemistry and ocean circulation. Their work is quite future-focused, using computer modelling of ocean physics to observe the response of the oceans to climate change. 

To provide scientific background for his talk, Marshall presented an introduction to oceanic processes and dynamics, such as convection and diffusion - the latter with a live demonstration using a bowl of cold water, a kettle of boiling water and red food colouring. He also introduced a concept of global ocean circulation called AMOC  - "Atlantic meridional overturning circulation."  This large-scale ocean circulation is driven by differences in density caused by variations in temperature and salinity, and affects currents such as the Gulf Stream. Based on this circulation pattern, quite a bit of mixing occurs between the world's ocean basins. This makes ocean modelling considerably more challenging than atmospheric modelling due to its global reach. That said, the tools for modelling the world's ocean have improved considerably and have transformed the way we observe the ocean's changes. 

So, will the Gulf Stream collapse?

In answer to the question, Marshall cited Carl Wunsch, a physical oceanographer at MIT, who stated firmly that the Gulf Stream will continue as long as "the wind blows and the Earth turns."  A flip answer to be sure, but another way of saying that the Gulf Stream is mechanically driven by Southern Ocean winds that drive the warm water north and cooler water south, as well as the rotational motion of the planet.  In Marshall's model, if the Gulf Stream were to shut off due to global warming, the UK would experience a net loss of 3-4 degrees, not the ice age that the media have foretold.

Running AMOC

Marshall has his own reservations about the hyperbole and press coverage that has been given to the effects of climate change on the Gulf Stream, and publicly questions whether this is a good thing for the field of physical oceanography. From articles in the mainstream press to the Hollywood movie "The Day After Tomorrow," there is concern that amidst the hype, real issues will potentially get trivialised or overlooked.

To counteract the hype, Marshall concluded that there is much about the oceans we still don't know, and considerable variation that we cannot explain.  Modern observational techniques are giving us access to rich data for the first time; however, overreliance on models that vary as much as 40% in either direction leave the public feeling confused, misled and sceptical. Forward progress is within reach, but will require a careful combination of observation and simple models to test and develop physical understanding, before advancing numerical analysis and quantitative predictions.