In a paper published today in Trends in Ecology and Evolution, Barry Brook and colleagues argue against the idea of an ecological global-scale “tipping point”. Here, Professor Brook outlines the paper’s core argument, while Professor Corey Bradshaw (not an author on the study) explains what it means for conservation practice.
We argue that at the global-scale, ecological “tipping points” and threshold-like “planetary boundaries” are improbable. Instead, shifts in the Earth’s biosphere follow a gradual, smooth pattern. This means that it might be impossible to define scientifically specific, critical levels of biodiversity loss or land-use change. This has important consequences for both science and policy.
Humans are causing changes in ecosystems across Earth to such a degree that there is now broad agreement that we live in an epoch of our own making: the Anthropocene. But the question of just how these changes will play out — and especially whether we might be approaching a planetary tipping point with abrupt, global-scale consequences — has remained unsettled.
A tipping point occurs when an ecosystem attribute, such as species abundance or carbon sequestration, responds abruptly and possibly irreversibly to a human pressure, such as land-use or climate change. Many local- and regional-level ecosystems, such as lakes, forests and grasslands, behave this way. Recently however, there have been several efforts to define ecological tipping points at the global scale.
At a local scale, there are definitely warning signs that an ecosystem is about to “tip”. For the terrestrial biosphere, tipping points might be expected if ecosystems across Earth respond in similar ways to human pressures and these pressures are uniform, or if there are strong connections between continents that allow for rapid diffusion of impacts across the planet.
These criteria are, however, unlikely to be met in the real world.
First, ecosystems on different continents are not strongly connected. Organisms are limited in their movement by oceans and mountain ranges, as well as by climatic factors, and while ecosystem change in one region can affect the global circulation of, for example, greenhouse gases, this signal is likely to be weak in comparison with inputs from fossil fuel combustion and deforestation.
Second, the responses of ecosystems to human pressures like climate change or land-use change depend on local circumstances and will therefore differ between locations. From a planetary perspective, this diversity in ecosystem responses creates an essentially gradual pattern of change, without any identifiable tipping points.
This puts into question attempts to define critical levels of land-use change or biodiversity loss scientifically.
Why does this matter? Well, one concern we have is that an undue focus on planetary tipping points may distract from the vast ecological transformations that have already occurred.
After all, as much as four-fifths of the biosphere is today characterised by ecosystems that locally, over the span of centuries and millennia, have undergone human-driven regime shifts of one or more kinds.
Recognising this reality and seeking appropriate conservation efforts at local and regional levels might be a more fruitful way forward for ecology and global change science.
Let’s not get too distracted by the title of the this article - Does the terrestrial biosphere have planetary tipping points? - 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 they 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 thin air without empirical support.
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.
Don’t get me wrong: there are plenty of localised examples of such tipping points, often characterised by something we call “hysteresis”. Brook defines hysterisis 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 we are approaching a major planetary boundary for our ecosystems (including human society), where we witness such transitions simultaneously across the globe, is simply not upheld by evidence.
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.
The paper provides a 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 because of a lack of specified criteria that would allow the terrestrial biosphere to “tip”. From a more sociological viewpoint, the claim of imminent shift to some worse state also risks alienating people from addressing the real problems (foxes), or as Brook and colleagues summarise:
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 hordes of foxes steal most of the flock.