Forces fast and slow: understanding our climate's change velocities

A new perspective on climate change thresholds suggest there are different paths global warming can take once critical thresholds are crossed

Today I would like to highlight new research published in the world's premier science journal, Nature. A paper in the latest edition by Paul D. L. Ritchie, Joseph J. Clarke, Peter M. Cox and Chris Huntingford offers an important revision to how we think about a future scenario in which we do not reduce the emissions currently heating the planet.

The conventional wisdom is that without concerted international efforts, there is some point when the average global temperature will cross a threshold of (more or less) 2 degrees Celsius above the pre-industrial period. It has been assumed that, at +2 degrees, we will cross a "tipping point" that will set us on an irreversible course to a new climate that will evolve steadily over time (and not in our favor).

This new research provides a more nuanced view of how climate change is playing out and what to expect in the future. The first concept the authors explain is the idea of a "tipping element," which is an environmental change that would take the world across the climate change tipping point:

Multiple strands of evidence indicate that components of the Earth system, called tipping elements, are capable of large and rapid changes in response to relatively small changes to forcings. Tipping elements are often irreversible over multiple human generations: the original system state is not recovered when the forcing is brought back to its original value. The point beyond which a tipping element changes state is called a tipping point. Tipping points are evident in palaeoclimate records, as well as in future projections made with Earth system models.

The researchers note that there are two kinds of tipping elements: fast-onset and slow-onset:

Once global warming passes a threshold for the system, the current state of the system starts to undergo a transition to an alternative state, and such a state might be vastly different. This transition may occur relatively quickly; we refer to these as having a fast-onset tipping point, and hypothetical examples include Amazon forest dieback and disruption to monsoons Other transitions may take much longer, and these slow-onset cases include ice sheet loss and the collapse of the Atlantic Meridional Overturning Circulation (AMOC).

Put another way, some tipping elements, once "tipped," will change the world's climate relatively quickly and other elements more slowly. This is an important point, because we are in a much worse situation if we are pushing fast-onset tipping elements across their change thresholds instead of slow ones. The distinction is critical for another reason, which has to with a proposed change in how we see tipping points themselves.

We tend to think of a tipping point as a "hard" threshold we should not cross, but the authors suggest that view is too simplistic:

Recently developed theory indicates that a threshold may be temporarily exceeded without prompting a change of system state, if the overshoot time is short compared to the effective timescale of the tipping element.

In other words, a tipping point is not really a tipping point if it is not exceeded for very long with respect to the type of tipping element (fast or slow-onset). This makes sense, of course. If we tip over something that falls quickly, we don't get a chance to stop it from crashing to the ground. If we tip over something that falls slowly, we may yet save it. The earth, the researchers argue, contains both kinds of environmental phenomena, so it's critical to understand precisely how global warming is affecting these different kinds of tipping elements.

Fig. 1: Comparison between slow- and fast-onset tipping elements

As the authors note:

Previous studies report low global warming thresholds above pre-industrial conditions for key tipping elements such as ice-sheet melt. If so, high contemporary rates of warming imply that exceeding these thresholds is almost inevitable, which is widely assumed to mean that we are now committed to suffering these tipping events. Here we show that this assumption may be flawed, especially for slow-onset tipping elements (such as the collapse of the Atlantic Meridional Overturning Circulation) in our rapidly changing climate.

There is both good and bad news in the paper. The good news is that if warming temperatures are affecting slow-onset tipping elements, we may not be doomed to a hotter world if and when we cross the +2-degree threshold (which seems inevitable at this moment to many scientists). The bad news is that if we push fast-onset tipping elements into a new state, we may have much less time to adapt than we currently believe.

As the paper explains:

The slower-onset tipping elements are ice cap melt and AMOC collapse, and it is possible to safely overshoot their thresholds for multiple centuries before returning and stabilizing at the 1.5 °C level. In contrast, for the faster-onset tipping elements of monsoon disruption and Amazon forest dieback, overshoot is possible only for decades or even just a few years before tipping would be induced.

In all, this vital study only highlights how ideal it would be not to push any tipping elements in the wrong direction. However, if that is no longer possible, we should at least try to limit the damage to slow-onset tipping points, which would buy us more time to solve the global warming problem. As the authors conclude:

This study highlights the importance of timescales for possible tipping points in a changing climate. Slow-onset tipping elements permit temporary overshoots of a threshold without triggering tipping to a different system state. Both the approach rate to any tipping point threshold, and any actions taken to reverse warming once over that threshold, will therefore determine whether our climate remains safe from unwelcome state changes.

In the end, both our approach to any tipping point threshold and any actions taken to reverse warming once we're over that threshold will determine the future climate in which human beings will have to survive.

The Research

Ritchie, P.D.L., Clarke, J.J., Cox, P.M. et al. Overshooting tipping point thresholds in a changing climate. Nature592, 517–523 (2021). https://doi.org/10.1038/s41586-021-03263-2