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STOCKHOLM, 7 October (dialogue) Continued greenhouse gas emissions could trigger a climate tipping point. These are self-sustaining changes in the climate system that lock in devastating changes, such as sea level rise, even if all emissions are over.
The first major assessment in 2008 identified nine parts of the climate system that are sensitive to overturning, including ice sheets, ocean currents and major forests. Since then, huge advances in climate modeling and a host of new observations and records of ancient climate change have given scientists a better understanding of these triggers. Additional proposals have also been made, such as permafrost around the Arctic (permafrost, which could release more carbon if it thawed).
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Estimates of the level of warming at which these elements are likely to tilt have declined since 2008. The collapse of the West Antarctic ice sheet was once considered a risk when warming reached the Earth’s pre-industrial average of 3°C-5°C. It is now thought to be possible at current levels of warming.
In our new assessment of the past 15 years of research, my colleagues and I found that we cannot rule out that five tipping points are now triggered when global warming is about 1.2°C. Four of these five become more likely as global warming exceeds 1.5°C.
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These are sobering conclusions. Not all news stories captured the nuances of our study, though. That’s what our findings really mean.
indeterminate threshold
We synthesized the results of more than 200 studies to estimate the warming threshold for each triggering factor. The best estimates are either the results of multiple studies or a report that one study found to be particularly reliable. For example, records and modeling studies of when ice sheets have receded in the past suggest that the Greenland ice sheet is likely to collapse above 1.5°C. We also estimated minimum and maximum thresholds at which collapse could occur: Model estimates for Greenland range between 0.8°C and 3.0°C.
Within this range, tipping becomes more likely as warming increases. We define tipping 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 our confidence in each estimate. For example, our estimates of the collapse of the Greenland ice sheet are more confident than those of sudden permafrost thawing.
This uncertainty means we don’t expect to trigger four climate tipping points in the first year that global temperatures reach 1.5°C (which climate scientists think could be within the next five years), or even when average temperatures reach 1.5°C in several years sometime in the next few decades. Instead, every point in a degree increases the likelihood of tipping, but we can’t say for sure when tipping becomes unavoidable.
This is especially true of the Greenland and West Antarctic ice sheets. While our assessments suggest that their collapse could exceed 1.5°C, the ice sheets are so vast that they change very slowly. Collapse will take thousands of years, and the processes driving it will require warming to stay outside the threshold for decades. If warming falls back below the threshold before tipping begins, the ice sheet may temporarily exceed its threshold without collapsing.
For some other tipping points, changes may be more diffuse. We estimate that at current levels of warming, both tropical reef death and sudden permafrost thawing are possible. But the thresholds differ between reefs and permafrost. Both have already occurred in some places, but in our assessment these changes became more common at similar times above 1.5°C.
Elsewhere, small swathes of the Amazon and boreal forests may first slope and transition to a savannah-like state, bypassing more catastrophic dieback of entire forests. Modeling results, which have not yet been published, suggest that Amazon tipping is likely to occur in several regions with varying levels of warming, rather than as one big event.
Some tipping elements may also have no clear threshold. Ancient climate records suggest that ocean currents in the North Atlantic may have shifted sharply from their current strong currents to weak currents as warming and melting of freshwater in Greenland disrupted circulation. Recent models suggest that the threshold for collapse of the Atlantic circulation depends on the rate of warming and other factors that are difficult to measure, making it highly uncertain.
enter the danger zone
There are signs that some tipping point is approaching. Degradation and drought have made parts of the Amazon less resistant to disturbances such as fire, emitting more carbon than they absorb.
The fronts of some retreating West Antarctic glaciers are just a few kilometers away from the unstoppable retreat. Early warning signs in climate monitoring data, such as larger and longer fluctuations in annual glacier melt, suggest that parts of the Greenland ice sheet and the Atlantic circulation are also unstable.
These signals don’t tell us exactly how close we are to a tipping point, it’s just that instability is happening, and a tipping point may be approaching. The most we can be sure of is that each component of further warming makes these triggers less stable and makes self-sustaining changes more likely.
This reinforces the case for ambitious emissions reductions under the Paris Agreement’s target of limiting warming to 1.5°C. This would reduce the chance of triggering multiple climate tipping points – even if we cannot rule out that some will be reached soon. (conversation)
(This is an unedited and auto-generated story from the Syndicated News feed, the body of the content may not have been modified or edited by LatestLY staff)
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