Climate tipping points can lead to unstoppable changes on the planet. How close are they?

Continued emissions of greenhouse gases risk triggering climate tipping points. These are self-sustaining changes in the climate system that will record disruptive changes such as sea level rise even if all emissions end.

The first major assessment in 2008 year nine parts of the art climate system which are susceptible to overturning, including ice sheets, ocean currents and large forests. Since then, huge advances in climate modeling and a flood of new observations and records of ancient climate change have given scientists a much better understanding of these overturning elements. Additional ones have also been proposed, such as permafrost around the Arctic (permanently frozen land that can release more carbon if it thaws).

Estimates of the level of warming to which these elements may be inclined have decreased since 2008. The collapse of the West Antarctic ice sheet was previously thought to be a danger if warming reached 3°C-5°C above the Earth’s pre-industrial average temperature. Now it is thought that possible at current levels of warming.

In ours new assessment over the past 15 years of research, my colleagues and I have found that we cannot rule out five tipping points that could be triggered right now when global warming is about 1.2°C. Four of these five become more likely if global warming exceeds 1.5°C.

These are sobering conclusions. Not all news coverage however, caught the nuances of our study. So here’s what our findings really mean.

Undefined thresholds

We synthesized results from more than 200 studies to estimate warming thresholds for each tipping element. The best estimate was either the one on which many studies converged, or the one that was found to be particularly reliable. For example, records of when ice sheets have retreated in the past and modeling studies show that the Greenland Ice Sheet is likely to collapse at temperatures above 1.5°C. We also estimated the minimum and maximum thresholds at which collapse is possible: model estimates for Greenland range from 0.8°C to 3.0°C.

Overturning becomes more likely in this range as it warms. We defined a rollover as possible (but not yet likely) if warming is above the minimum but below the best estimate and likely above the best estimate. We also rated 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 of the dramatic melting of permafrost.

This uncertainty means that we do not expect global temperatures to reach 1.5°C in the first year (which climate scientists suggest is possible next year). five years), or even if average temperatures reach 1.5°C sometime in the next few years couple of decades. Instead, every fraction of a degree makes tipping more likely, but we can’t be sure exactly when tipping becomes inevitable.

This is especially true for the Greenland and West Antarctic ice sheets. Although our estimate suggests that their collapse temperatures exceed 1.5°C, the ice sheets are so massive that they change very slowly. Collapse will take thousands of years, and the processes that cause it require warming to stay out of bounds threshold for several decades. If warming has returned below the threshold before the overturning began, it is possible that the ice sheets temporarily exceed their thresholds without collapsing.

For some other tipping points, the changes are likely to be more diffuse. We believe that at the current level of warming, both the death of tropical coral reefs and the rapid thawing of permafrost are possible. But the thresholds differ between reefs and permafrost areas. There are both already is happening in some locations, but we estimate that these changes become much more extensive at similar times above 1.5°C.

Elsewhere, small areas of the Amazon and boreal forests may tilt into a savanna-like state. the first, bypassing more catastrophic die-offs throughout the forest. model results which have not yet been published testify to this Amazon tips may occur in several regions at different levels of warming rather than as one large event.

There may also be no well-defined threshold for some tip elements. Ancient climate records show that ocean currents in the North Atlantic could change dramatically from strong, as they are now, to weak as a result of both warming and the melting of freshwater from Greenland, disrupting the circulation. A recent simulation suggests that the threshold for collapse of the Atlantic circulation depends on how fast warming increases, along with other factors that are difficult to measure, making it highly uncertain.

Into the danger zone

There are signs that some tipping points are already approaching. Degradation and drought have caused parts of the Amazon to transform less stable to obstacles such as fire and release more carbon what they absorb.

The leading edge of some retreating West Antarctic glaciers just in kilometers from incessant retreat. Early warning signals in climate monitoring data (such as larger and longer fluctuations in the amount of glacier melt each year) indicate that some parts Greenland ice sheet and Atlantic circulation also destabilize.

These signals cannot tell us exactly how close we are to tipping points, only that destabilization is underway and a tipping point may be approaching. The most we can be sure of is that each bit of further warming will further destabilize these tipping elements and make the onset of self-sustaining changes more likely.

This strengthens the case for ambitious emissions cuts in line with the Paris Agreement’s goal of ending warming at 1.5°C. This would reduce the likelihood of multiple climate tipping points—even if we cannot rule out that some of them will be reached in the near future.

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