Yesterday’s news that we now appear to be on course for the unstoppable and irreversible loss of the West Antarctic Ice Sheet (WAIS) – the largest ice body in the world (for now, anyway) – means that we’re now probably past two key climate tipping points, the other being the loss of summer Arctic sea ice. So this is probably a good time to post a quick overview of all of the main climate tipping points: if you’re not familiar with this list, you should be…
Below is a quick guide, adapted from a chapter by Exeter University’s Tim Lenton in his and Tim O’Riordan’s book on the same subject. The book was published last year; as you can see, scientific consensus at the time was that the West Antarctic Ice Sheet “appears to be further from a tipping point than its Greenland counterpart”. So much for that estimate.
- Arctic sea ice loss underwent a new record summer loss of area in 2012, breaking the previous record of 2007, and now covers only half the area of the late 1970s when satellites first began monitoring it. Current projections suggest we’re looking at complete loss of Arctic summer sea ice within decades. Already, the changing ice cover is changing air circulation patterns, and leading to cold winter extremes in Europe and North America.
- The Greenland ice sheet may be nearing a tipping point beyond which it is committed to shrink; here too, the summer of 2012 saw record melting. Ice loss would probably take place over centuries, so this form of change wouldn’t be abrupt – but it might well be irreversible, and could ultimately lead to 7 metres of sea level rise, including up to half a metre this century.
- The West Antarctic ice sheet appears to be further from a tipping point than its Greenland counterpart, but has the potential for more rapid change, and hence bigger impacts in the near term. If warming exceeds 4º Celsius, the current best guess is that the West Antarctic ice sheet is ‘more likely than not’ to collapse, causing sea level rise of 1 metre per century and 3-4 metres in total.
- The Yedoma permafrost in Siberia contains massive amounts of carbon – potentially up to 500 billion tonnes of it, about the same as has been emitted by human activity since the start of the industrial revolution. If decomposition inside the permafrost starts to generate enough heat, all of the permafrost could tip into irreversible, self-sustaining collapse, generating a “runaway positive feedback” with emissions of 2-3 billion tonnes of carbon a year, or about a third of current fossil fuel emissions. The current best guess is that this would not happen before regional warming of 9º Celsius, but that could be closer than we think: in 2007, for instance, Arctic surface temperatures jumped 3º Celsius.
- Ocean methane hydrates are thought to store up to 2,000 billion tonnes of carbon under the seabed. As the deep ocean warms, this frozen reservoir of methane could conceivably melt, causing an abrupt, massive release of the gas – which is four times as potent a greenhouse gas as carbon dioxide. This is probably at the least likely end of the spectrum, but the impacts if it happened would be extreme.
- The Himalayan glaciers could lose much of their mass over the coming century, with the change becoming self-amplifying as exposure of bare ground increases the amount of sunlight energy absorbed at the Earth’s surface rather than reflected back into space. Scientists are so far unsure as to whether a large-scale tipping point could apply to this trend.
- The Amazon rainforest experienced severe droughts in both 2005 and 2010 – in both cases, turning the forest into a source of carbon rather than a sink for absorbing it. If, as is currently happening, the dry season continues to lengthen, and droughts get more frequent and severe, then the rainforest could reach a tipping point, with up to 80% of trees dying off. Widespread dieback is expected at increases of over 4º Celsius, and could be committed to at much lower temperature increases – long before we notice it happening.
- Western Canada’s boreal forest is currently suffering from an invasion of mountain pine beetle, leading to widespread dieback which has already turned the forest from a source to a sink. More widespread die-off could happen in future at global average warming of more than 3º Celsius, further amplifying global warming.
- Tropical coral reefs are already experiencing bleaching as oceans warm up, and may be nearing a ‘point of no return’. Further risks come from the increasing acidification of the oceans, which could see up to 70% of corals in corrosive water by the end of this century.
- The Atlantic thermohaline circulation (THC) – in effect, a massive oceanic conveyer belt that drives the Gulf Stream and the North Atlantic Drift, and which helps keep northern Europe much warmer than it would otherwise be – could shut down altogether if enough freshwater dilutes the ocean’s salinity; current best guesses are that this could happen at global average warming of over 4º Celsius. A weakening THC could also drive an additional quarter metre of sea level rise along the north-eastern seaboard of the United States.
- The Sahel and West African Monsoon has experienced rapid changes in the past, including a catastrophic drought that lasted from the late 1960s through to the 1980s, and could be disrupted anew by changes to the THC. The Indian Summer Monsoon, meanwhile, is already being disrupted and rice harvests damaged, in particular by the cloud of brown haze that now sits semi-permanently over the Indian sub-continent.
Of course, the really big question in each case is exactly where the tipping points lie, and what level of temperature increase might push us over the threshold. Some research suggests that warning of 1º Celsius over the average temperature of the 1980s and 1990s would be dangerous; other recent work suggests that 4º could be where the danger zone begins.
But again, note that the best guess just one year ago was that it would take 4º of warming to cause the West Antarctic Ice Sheet to collapse; in the event, current warming of 0.7º above pre-industrial levels seems to have been enough to push us over the edge. It’s a salutary reminder of how little we know about climate tipping points – and hence just how risky and high stakes a situation we’re in.
And in all scenarios, the key point is that these kinds of temperature increases are exactly where our current trajectory is headed: current policies have us on track for 3.6-5.3 degrees Celsius of warming, according to the IEA. In the worst case scenario, Tim Lenton observes, we could slide into
“…’domino dynamics’, in which tipping one element of the Earth system significantly increases the probability of tipping another, and so on … on several occasions in the past, the planet was radically reorganized without there being any sign of a particularly large forcing perturbation.”
Wondering what you can do in the face of such relentlessly gloomy news? Get hold of any policymaker you can and ask them whether they support a binding global carbon budget, and fair – i.e. equal per capita – shares of it for all the world’s people. Ask whichever NGO you support whether they’re calling for the same thing at the Paris summit next year. And don’t take any shit from anyone about how technology and voluntary action are going to do the business without anyone having to take any tough decisions.
(See also this companion post, also published here today, for a slightly different take on the loss of the West Antarctic Ice Sheet.)