Continued emissions of greenhouse gases threaten to trigger climate tipping points. These are self-sustaining shifts in the climate system that would lock in destructive changes, such as sea level rise, even if all emissions were gone.
The first major assessment in 2008 identified nine parts of the climate system sensitive to inversion, including ice sheets, ocean currents, and major forests. Since then, massive advances in climate modeling, the flow of observations, and new records of paleoclimate change have given scientists a much better picture of these fluctuating elements. Additional types have also been suggested, such as the permafrost around the Arctic (permanently frozen land that can release more carbon if thawed).
Estimates of the levels of warming at which these elements could overturn have been declining since 2008. The collapse of the West Antarctic ice sheet was thought to be a risk when warming reached 3°C to 5°C above the Earth’s pre-industrial average temperature. Now it is believed to be possible at current levels of warming.
In our new assessment over the past 15 years of research, my colleagues and I have found that we cannot rule out five tipping points that are now triggered when global warming is around 1.2°C. Four of these five become more likely because global warming exceeds 1.5°C.
These are realistic conclusions. However, not all news coverage captured the nuances of our study. So that’s what our results actually mean.
We pooled the results of more than 200 studies to estimate warming thresholds for each inversion component. The best estimate was either multiple studies converged or a particularly reliable study reported. For example, past ice sheet retreat records and modeling studies indicate that the Greenland ice sheet is likely to collapse beyond 1.5°C. We also estimated minimum and maximum thresholds at which to break: estimates from the Greenland model range from 0.8 °C to 3.0 °C.
Within this range, tilt becomes more likely with increasing warming. We have identified the tendency as possible (but not yet likely) when warming is above the minimum but below the best estimate, and likely above the best estimate. We also judged how confident we were in each estimate. For example, we are more confident in our estimates of the collapse of the Greenland ice sheet than those of the sudden melting of permafrost.
This uncertainty means that we do not expect four climate tipping points in the first year that global temperatures reach 1.5°C (which climate scientists suggest is possible in the next five years), or even when temperatures average over the next five years. Several years to 1.5 ° C. Sometime in the next two decades. Instead, every bit of a degree makes reversal more likely, but we can’t be sure exactly when a tipping becomes inevitable.
This is especially true of the Greenland and West Antarctic ice sheets. While our assessment suggests that their collapse may exceed 1.5°C, the ice sheets are so massive that they are changing very slowly. The collapse could take thousands of years, and the processes driving it would require warming to remain outside the threshold for decades. If warming returns below the threshold before the inversion began, it may be possible for ice sheets to cross their thresholds temporarily without collapsing.
For some of the other tipping points, change is likely to be more dispersed. We estimate that the death of tropical corals and sudden thawing of permafrost is possible at the current level of warming. But thresholds differ between coral reefs and permafrost patches. Both are already occurring in some places, but in our assessment, these changes became more prevalent at a similar time above 1.5°C.
Elsewhere, small patches of the Amazon and boreal forests may overturn and transition into a savanna-like state first, withering a more catastrophic death through the entire forest. Model results that have not yet been published suggest that the Amazon core may occur in several regions at varying levels of warming rather than as a single large event.
There may also not be a well-defined limit for some items of tipping. Paleoclimatic records indicate that ocean currents in the North Atlantic could dramatically shift from being strong, as they are now, to weak as a result of rising temperatures and melting fresh water from Greenland disrupting circulation. Recent modeling suggests that the Atlantic circulation collapse threshold depends on how quickly warming increases along with other hard-to-measure factors, making it highly uncertain.
In the danger zone
There are signs that some critical points are already approaching. Degradation and drought have caused parts of the Amazon to become less resistant to disturbances such as fires and emit more carbon than they absorb.
The leading edge of some retreating glaciers in West Antarctica is just kilometers away from the unstoppable retreat. Early warning signals in climate monitoring data (such as larger and longer fluctuations in the amount of glacier melt each year) suggest that parts of the Greenland ice sheet and the Atlantic circulation are also destabilizing.
These signals cannot tell us exactly how close we are to tipping points, only that destabilization is underway and we may be approaching a tipping point. The most we can be sure of is that every bit of further warming will destabilize these transition elements and make the initiation of endogenous changes more likely.
This strengthens the case for ambitious emissions cuts in line with the Paris Agreement’s goal of capping warming at 1.5°C. This would reduce the chances of triggering many climate tipping points – even if we don’t rule out reaching some soon.
Risk of multiple climate tipping points escalates above 1.5°C of global warming
Introduction of the conversation
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