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AGU Chapman: Meridional madness

abrupt_climate1_f.jpgToday’s theme at the AGU Chapman Conference on Abrupt Climate Change is that big baddie of climatic tipping points, the shutdown (and rebooting) of the Atlantic Meridional Overturning Circulation, or AMOC. Could this massive system go down again? Tom Delworth of NOAA took on that question and offered up some interesting new modelling evidence.

When running strong, AMOC carries heat from the Southern Hemisphere northward. It’s thought that some of the past coolings under scrutiny here stem from slowing or stopping of this conveyor belt. AMOC’s future change in response to greenhouse gas increases was recently considered in an assessment report on abrupt change by the US Climate Change Science Program, which Delworth helped author.

While some state-of-the-art models suggest the circulation could slow this century – a 25-30% decrease is the report’s best estimate – not a single one forecasts another shutdown in that time. That led the US panel to evaluate AMOC shutdown as “very unlikely”, in the parlance of the IPCC – meaning a less than 10% probability. The lack of support from models meant they couldn’t set the likelihood any higher, says Delworth – but on the other hand, the possibility of flawed simulations kept them from setting it lower, at “extremely unlikely”. But Delworth’s new work validates the model results.

Ocean models used for future projections don’t capture complex currents and eddies in detail: some oceanographers have said the simulated seas look more like molasses than water. Delworth and other NOAA scientists put a new, much higher-resolution model through its paces and found that it backs up its molasses-like counterparts. Both model types respond similarly to simple climate experiments – a 1% annual rise in CO2 concentration, and a dumping of freshwater into the North Atlantic (“water-hosing” is the term of art for this). These are equivalent to hitting the climate system with a hammer, says Delworth, but they still don’t break AMOC this century.

Even if AMOC shutdown is very unlikely, however, its high potential impact still makes it a risk worthy of attention. This came out vividly in a talk by Jeff Severinghaus of Scripps Institution of Oceanography based on research published a few days ago in Science (press release). Severinghaus and colleagues found new ice-core evidence that during a cold period about 15,000 to 18,000 years ago – arguably due to a sluggish AMOC – the rainfall belt at the tropics shifted significantly southward, along with much terrestrial vegetation. During the warming that followed, it flipped back to the north in a matter of mere decades. The graphic above shows ‘before and after’ pictures of the flip, as imagined by an illustrator – the reality may was probably somewhat less stark, says Severinghaus. (Click image to see full size.)

When the rain belt went south, monsoons that today water southeast Asia, India and Africa – and that billions of people depend on – dried up, as local cave stalagmite records had previously shown. What’s special about the ice data is that the changing rains are tracked by oxygen isotopes in trapped air bubbles, which means the change showed up in the well-mixed global atmosphere, which means it must have been extremely widespread.

Anna Barnett

Image: Zina Deretsky, National Science Foundation


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