Today, a paper by my colleague, Barry Brook, appeared online in Trends in Ecology and Evolution. It’s bound to turn a few heads.
Let’s not get distracted by the title of the post, or the potential for a false controversy. It’s important to be clear that the planet is indeed ill, and it’s largely due to us. Species are going extinct faster than the would have otherwise. The planet’s climate system is being severely disrupted, so is the carbon cycle. Ecosystem services are on the decline.
But – and it’s a big ‘but’ – we have to be wary of claiming the end of the world as we know it or people will shut down and continue blindly with their growth and consumption obsession. We as scientists also have to be extremely careful not to pull concepts and numbers out of our bums without empirical support.
Specifically, I’m referring to the latest ‘craze’ in environmental science writing – the idea of ‘planetary tipping points‘ and the related ‘planetary boundaries‘. It’s really the stuff of Hollywood disaster blockbusters – the world suddenly shifts into a new ‘state’ where some major aspect of how the world functions does an immediate about-face.
Now there are plenty of localised examples of such ‘tipping points’, often characterised by something we call ‘hysteresis’. In their latest paper entitled Does the terrestrial biosphere have planetary tipping points?, Brook and colleagues define hysteresis as:
… “a situation where the current state of an ecosystem is dependent not only on its environment but also on its history, with the return path to the original state being very different from the original development that led to the altered state. Also, at some range of the driver, there can exist two or more alternative states.”
and tipping point as:
“the critical point at which strong nonlinearities appear in the relationship between ecosystem attributes and drivers; once a tipping point threshold is crossed, the change to a new state is typically rapid and might be irreversible or exhibit hysteresis.”
Some of these examples include state shifts that have happened (or mostly likely will) to the cryosphere, ocean thermohaline circulation, atmospheric circulation, and marine ecosystems, and there are many other fine-scale examples of ecological systems shifting to new (apparently) stable states. However, claiming that there is a major planetary boundary for our ecosystems (including human society) just over the horizon, where we witness such transitions simultaneously across the globe, is simply not upheld by evidence.
Brook and colleagues demonstrate that the latest fervour in crying the planetary boundary ‘wolf’ has been largely predicated on arbitrary thresholds, and ignores the fact that regional tipping points are unlikely to translate into planet-wide state shifts. The main reason is that our ecosystems aren’t that connected at global scales, despite all the ways in which we are disrupting the system.
In essence, the paper provides an elegant theoretical framework against which one can test the existence or probability of a planetary tipping point for any particular ecosystem function or state. To date, the application of the idea has floundered by the lack of specified criteria that would allow the terrestrial biosphere to ‘tip’. From a more sociological viewpoint, the claim of imminent shift to some (depauperate) state also risks alienating people from addressing the real problems (foxes), as they summarise nicely in their final statement:
“… framing global change in the dichotomous terms implied by the notion of a global tipping point could lead to complacency on the ‘safe’ side of the point and fatalism about catastrophic or irrevocable effects on the other.”
In other words, let’s be empirical about these sorts of politically charged statements instead of crying “Wolf!” while the hoards of foxes steal most of the flock.
CJA Bradshaw
P.S. A version of this post was published simultaneously at The Conversation.
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