There is one climate topic that you’re likely to hear a lot about this year: tipping points.
In the context of climate science, a tipping point can occur when a relatively small change can have a large and irreversible effect on some of the Earth’s largest systems, such as the Antarctic ice sheets or the Amazon rainforest.
In the first post in our series on tipping points, we looked at the definition of tipping points. In the second of our series we literally go from pole to pole to examine the potential for huge change in the oceans and the cryosphere – the Earth’s wealth of ice.
Professor Tim Lenton – from the University of Exeter – is a world-renowned expert on tipping points. One tipping element (bits of the climate system that could pass a tipping point) that stands out as high risk for Professor Tim Lenton, is West Antarctica. Here there is physical evidence consistent with possibly having passed the tipping point for irreversible retreat of part of the ice sheet. Destabilization of the West Antarctica ice sheet could lead to about a three-metre sea-level-rise on a timescale of centuries to millennia. In a wider study Tim suggests that part of the East Antarctic ice sheet might similarly be unstable, with the potential to add another 3-4 m to sea level on timescales beyond a century.
He said: “We might already have committed future generations to living with sea-level rises of around 10 m over thousands of years. But that timescale is still under our control. The rate of melting depends on the magnitude of warming above the tipping point. More observational data will establish whether ice sheets are reaching a tipping point, and better developed models are needed to resolve how soon and how fast the ice sheets could collapse.” Tim.
Dr Ed Blockley, who leads the Polar Climate Group of the Met Office Hadley Centre, paints a similarly bleak picture for retreating Arctic sea ice. He explains that one challenge to understanding sea ice decline, is measuring the large seasonal and year-to-year variability against sustained long-term decline. However, over the past four decades, Arctic sea ice cover has reduced, on average, by 87,000 square kilometres, an area of more than four times the size of Wales each year. The Arctic has an important role to play in regulating climate, such as the albedo effect – where the expanses of white ice reflect the sun’s energy back into space – and atmospheric circulation patterns, which can influence the weather at lower latitudes such as in Europe.
Dr Blockley said: “One potential tipping point of Arctic sea ice is the ocean halocline, whereby cold, fresh water at the surface is less dense than warm, salty water below and currently prevents the warm water from reaching the surface and melting the sea ice. If the halocline were to collapse, this warm water, which contains enough heat to melt all the sea ice many times over, could mean that the Arctic would remain ice-free even if global warming were to be reversed.”
Tim Lenton added: “The Arctic is the place where a sort of cascade of unwelcome tipping point changes may be starting because it’s clearly the place that’s warming up two to three times as fast as the global average. We are also accumulating more evidence of casual interactions here as well, such as the role of Arctic sea-ice retreat and resultant warming in permafrost thawing. We appear to be approaching several tipping points.”
Perhaps one tipping element more than any other attracts regular media headlines: the collapse of the Atlantic Meridional Overturning Circulation (AMOC). This huge conveyor belt brings warm salty water from the tropics into the northernmost reaches of the Atlantic. A weakening or collapse of this current could have devastating impacts on the climate of the northern Atlantic region, potentially switching off the transport of warm conditions to northern Europe.
Dr Richard Wood, head of cryosphere and oceans group at the Met Office Hadley Centre, explains: “If we were to add fresh water to the North Atlantic – such as from melting glaciers or increased precipitation run-off, for example – you would make the surface water fresher and less dense, weakening the ‘pump’ that drives the ocean circulation.”
Warming of the surface waters due to greenhouse gases has a similar effect of making the surface waters less dense and so weakening the ocean circulation pump. So there is a ‘double whammy’ of warming and freshening conspiring to weaken the circulation.
Dr Wood added: “Because the AMOC is a circulation that spans the whole globe, it’s a fundamental part of our climate system. We don’t think a collapse is imminent in the next decade or so, but climate models do suggest that over the 21st century, the AMOC will weaken, as greenhouse gases increase. We need to monitor for any early warning signals that the AMOC is getting near a tipping point.”
The Met Office’s second episode of our Mostly Climate podcast on Tipping Points can be found here.
Next time in the lasts of our tipping points series we’ll be travelling to the Amazon.