A paper published in the journal Nature Geoscience has examined timescales of ocean warming in response to greenhouse gases in the Arctic.
Dr Tamsin Edwards, Lecturer in Environmental Sciences at The Open University, said:
“The Antarctic is a bit different to the rest of the planet. The warming we see elsewhere in the world is slowed by several unique factors: the vast cold ice sheet, the reflective sea ice that surrounds it, winds and ocean currents circling the continent that act as a kind of buffer, and cold water rising up from the deep after a very long journey south along the bottom of the ocean.
“This study looked at why the ocean waters closest to Antarctica are warming less quickly – and in many places cooling – while the band of waters just north of this are warming more rapidly. They concluded the main reason was the circulation bringing ancient cold water to the surface then taking any extra surface heat northwards. Other possible factors like wind patterns were less important. In other words, Antarctic waters are indeed taking up heat from a warming atmosphere, but like a bath with both the hot and cold taps on and the plug out, the overall temperature hasn’t changed.
“It’s important to understand ocean circulation patterns because they can have a big effect on the Antarctic ice sheet. We already see rapid ice loss from one region because relatively warm waters have made it onto the continental shelf and are melting the ice from below. If this starts to happen elsewhere too, as has been predicted under human-caused climate change, it could trigger more rapid sea level rise.”
Dr Emily Shuckburgh, Deputy Head of Polar Oceans at the British Antarctic Survey, said:
“The global average surface temperature has increased by about 0.6C over the decades since 1950. In general the land surface has warmed more than the ocean surface, and within the ocean some of the surface waters have warmed more than average (e.g. the Southern Ocean equatorward of 50S) and others less (e.g. the Southern Ocean poleward of 50S).
“This paper finds that the main reason for the pattern of surface temperature change in the Southern Ocean lies in the global circulation of the ocean. This takes water on a slow conveyor belt, sinking in the North Atlantic, travelling south at depth and then arriving at the surface in the Southern Ocean several centuries later. At the surface it is warmed by the atmosphere and then moves equatorward to be replenished by more cool water from below.
“The circulation is so slow that much of the water arriving there today originated at the surface in the North Atlantic in pre-industrial times and hence the particular region at the surface poleward of 50S shows little warming at present. It should be noted that as a whole the Southern Ocean has shown considerable warming, from the surface to depths of more than 1000m, and that changes to the circulation in the Southern Ocean has been bringing more relatively warm water up under the ice sheets, melting them from below.
“The result of this paper highlights the important role the ocean itself has in shaping the patterns and magnitude of surface temperature change and it emphasises the limitations of using surface temperature as a simple indicator of the accumulation of heat in the Earth system resulting from the anthropogenically-enhanced greenhouse effect.”
Prof. Andy Shepherd, Director of the NERC Centre for Polar Observation and Modelling at the University of Leeds, said:
“These climate model experiments suggest that the water around Antarctica is naturally slower to respond to planetary warming than other parts of the global ocean. Despite this, we have seen colossal ice losses from West Antarctica over recent decades, and so the prospect that additional warming is on the way is a real concern because it edges further towards the upper range of future sea level rise predictions.”
Dr Colin Summerhayes, Scott Polar Research Institute and former Director of NERC’s Institute of Oceanographic Sciences Deacon Laboratory, said:
“The surface waters of the Southern Ocean south of the Polar Front are several hundred years old. They originated by sinking in the Norwegian Greenland Sea, and then moved south as cold North Atlantic Deep Water, before welling up from the deep under the influence of the strong westerly winds around Antarctica. Arriving at the surface of the Southern Ocean, the winds then move these upwelled waters north – eventually to sink at the Polar Front where they meet warm waters from the north. The northward movement of these old cold surface waters prevents subtropical waters warmed by global warming from moving south to reach the continent.
“However, what we learn here is not new. It has long been known that upwelling driven by the wind brings cold subsurface water to the surface off Antarctica, just as it does off Peru and Namibia. Nevertheless, what we have begun to see is that more of that water, which has a temperature of just above 1 degree C, is now making its way up onto the continental shelf, where it can then penetrate beneath the ice shelves that surround the continent.
“That’s bad news because even water that cool is still warm enough to melt the ice at the base of the ice shelves, causing them to melt from below. That melting thins the ice shelves, which buttress the movement of ice streams from the interior. Less buttressing means that the ice streams, like the Pine Island Glacier (as wide as the English Channel at Boulogne), move faster, discharging more ice from the land into the sea. Eventually, as warming continues, those ice streams will drain much of the interior ice from West Antarctica into the sea, raising sea level by an estimated 3.3m.”
‘Southern Ocean warming delayed by circumpolar upwelling and equatorward transport’ by Kyle C. Armour et al. published in Nature Geoscience on Monday 30 May.