Planetary Boundaries and Tipping Points explained!

If you do any reading about "tipping points" you'll be bombarded with things like 'planetary boundaries', 'regime shifts', 'critical transitions' and just about every combination of these words. It can be a bit confusing! So I'm going to explain it in what I hope is a much clearer way....

Brook et al. (2013) describe the following:

· Planetary boundary: a concept developed to define a desired operating range for Earth-system features and processes. Crossing a boundary implies damage or loss of existing functions or services across the system.

· Regime shift: a large, relatively rapid reorganization of the state of an ecosystem that can be triggered by synergistic feedbacks. Regime shifts can result from crossing tipping points, and are often hard to anticipate and difficult to reverse.

· Tipping point: the critical point at which strong non-linearities 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.

Life on Earth has displayed abrupt and massive changes in the past, so we have no reason to expect that similar global regime shifts will not occur again. These shifts changed ecosystem dynamics worldwide by rising temperatures, changes in rainfall, retreat of polar ice and glaciers, and declining ocean pH amongst others, resulting in profound changes in ecosystem services, biodiversity, and aesthetic values. (Hughes et al. 2013). 

The problem is, not all these planetary boundaries and tipping points have been defined, and crossing these thresholds is unlikely to manifest as sudden and simultaneous collapses worldwide.What if we've already passed unrecognized and unanticipated tipping points as incremental changes accumulate, and are in a slow transition to a new regime? Once a transition occurs, it can be difficult or even impossible to return to the previous state (Barnosky et al, 2012).

This image from Barnosky et al (2012) shows a prediction of Earth ecosystem response as human population grows as does its impact on the environment. Here, the system crosses a tipping point and reaching an alternative stable state.

Human forcing includes transformation of 43% of land to agricultural or urban landscapes, with the remainder of natural landscapes broken up by roads. Even during the last global-scale critical transition, only 30% of Earth’s surface went from being covered by glacial ice to being ice free! Modelling suggests that for a third of Earth, plant species will not be able to migrate quick enough to keep up with climate change, and those that can will have to battle highly fragmented landscapes (Barnosky et al. 2012).

The scientific concept and potential policy implications of tipping points and their consequences have recently attracted considerable interest (Brook et al, 2013) and rightly so. Looking at past global-scale state shifts and the global forcings we continue to exert, suggests that another global-scale state shift is isn't far away, if it has not already begun (Barnosky et al, 2012). The extent and scale of human–biosphere interactions and disruption highlights the need to operate within safe planetary boundaries say Hughes et al, (2013). Regardless, we must address causes of human-driven global change and improve our management of the environment in order to prevent a global-scale state shift, or at least to monitor it as much as possible.

• Barnosky, A., D., Et Al. (2012) “Approaching a state shift in Earth’s Biosphere” Nature, 486, 52-58
• Brook, B.W. et al. (2013) Does the terrestrial biosphere have planetary tipping points? Trends Ecol. Evol. 28, 396–401
• Hughes, T., P., S. Carpenter, J. Rockstrom, M. Scheffer, and B. Walker (2013) “Multiscale regime shifts and planetary boundaries” Trends in Ecology & Evolution , 28, 7