The targets to which many nations signed up at the 2015 Paris Climate Change meeting were designed to keep the rise in global average temperature to below 2°C, with a desire to keep it at 1.5°C above pre-industrial levels1. Given the governments of many countries, especially Australia, are attempting to fudge their targets, keeping the temperature increase to the 2°C targeted by Paris is looking like a forlorn hope.
The Intergovernmental Panel on Climate Change (IPCC) identified tipping points (at which feedbacks click in) two decades ago, but they did not expect them to kick in until the global average temperature increased by about 5°C. More recent reports from the IPCC had revised that analysis and expected them to kick in when the global average temperature hit 2 or 3°C, depending on the tipping point being discussed. The areas of most concern were the melting of Arctic sea ice, the thawing of the permafrost, decline of the Asian monsoon, decline of the west African monsoon, rainforest dieback, Boral forest dieback, increasing heatwaves, decline of grain-growing, and heat stress of tropical livestock2. These feedbacks were further clarified by Steffen and colleagues who estimated that some of these feedbacks would add about another half a degree but that they would show only a small or negligible impact before 21003,4. After alarming reports surfaced about thermokarst lakes forming in thawing permafrost, and relatively rapid increases in methane at mid-altitudes, I wondered if some of these tipping points had already been reached, as some of the indicators seemed to be increasing rather alarmingly5,6.
This continual revision of the effects of climate change to even worse scenarios is to be expected, because, as I have said numerous times, any compilations by groups of scientists tend to be conservative in their nature. This is because you can only make statements to which the most conservative of the authors is likely to agree, otherwise no agreement is possible. I have been involved in a few such compilations and, in my experience, this has always been the case. The fact that these IPCC compilations are getting fairly rapidly worse is alarming, and one wonders where it will end. Now a new paper has appeared which details what we now know about tipping points7. They have determined that several cryosphere (i.e. the frozen world) tipping points are “dangerously close”, and that some parts of the Antarctic ice sheet may have already passed their tipping point. In addition, the Greenland ice sheet is already melting at an accelerating rate and, as the melting decreases the elevation of the ice sheet, it melts further, as the surface is exposed to even warmer air. Models suggest that the Greenland ice sheet could be doomed at 1.5 °C of warming, which they estimate could happen as soon as 20307.
The latest IPCC models project a cluster of abrupt shifts between 1.5°C and 2°C, several of which involve sea ice. This ice is already shrinking rapidly in the Arctic, indicating that, at 2°C of warming, the region has a 10–35% chance of becoming largely ice-free in summer7. Ocean heatwaves have already led to mass coral bleaching and to the loss of half of the shallow-water corals on Australia’s Great Barrier Reef. A staggering 99% of tropical corals are projected to be lost if global average temperature rises by 2°C, the Paris Agreement target, owing to a combination of warming, ocean acidification and pollution. This would represent a profound loss of marine biodiversity as well as human livelihoods7, not least of which is tourism in Australia
The Arctic is warming at least twice as quickly as the global average, and the boreal forests in the subarctic are increasingly vulnerable. Already, warming has triggered large-scale insect disturbances and an increase in fires that have led to dieback of North American boreal forests, potentially turning some regions from a carbon sink to a carbon source. Permafrost across the Arctic is beginning to irreversibly thaw and release carbon dioxide and methane — a vastly more potent greenhouse gas than carbon dioxide7.
Lenton and colleagues estimate that the world’s remaining emissions budget for a 50:50 chance of staying within a 1.5°C temperature rise is only about 500 gigatonnes (Gt) of CO2. Emissions from melting permafrost could take an estimated 20% (100 Gt of CO2) off this budget, and that does not include methane from deep permafrost or undersea methane hydrates. If forests are close to tipping points, Amazon dieback could release another 90 Gt of CO2 and boreal forests a further 110 Gt of CO2. With global total CO2 emissions still at more than 40 Gt per year, the remaining budget could be all but erased already7. This suggests we have absolutely no possibility of keeping the rise in global average temperature of 1.5°C.
If current Paris Agreement national pledges to reduce greenhouse gas emissions are implemented, and that is a big ‘if’ given that the Australian government is already trying to fudge its emissions target by using leftover credits from Kyoto8, then these are likely to result in 3°C of heating7. That is well above where many of these extremely dangerous tipping points are likely to be reached.
This is just another demonstration of the fact that as we learn more about the climate and the effect of greenhouse gases on it, the predictions keep getting worse. Last year the IPCC said we may have only until 20309. Given the results of this recent study, that may be unduly optimistic. And we are lumbered with a government which denies there is a problem, and a prime minister who brought a lump of coal into parliament.
- Steffen, W., Rockström, J., Richardson, K., Lenton, T.M., Folke, C., Liverman, D., Summerhayes, C.P., Barnosky, A.D., Cornell, S.E., Crucifix, M., Donges, J.F., Fetzer, I., Lade, S.J., Scheffer, M., Winkelmann, R. & Schellnhuber, H.J., 2018. Trajectories of the Earth System in the Anthropocene. Proceedings of the National Academy of Sciences. https://doi.org/10.1073/pnas.1810141115