Global change: the need for bathtub thinking

At present there is an immense research effort directed towards improving our understanding of the dynamics of the Earth system. Twenty-nine of the world’s leading Earth-system scientists have recently summarised the reasons why they are so worried about the future¹. They point out that we have entered an era where human activities have become the main drivers of global environmental change. They list ten critical boundaries in the Earth’s biophysical subsystems - thresholds beyond which the planet’s natural control mechanisms will have difficulty maintaining the unusually stable conditions that have persisted on Earth for the last 10,000 years. It is this period of stability, known as the Holocene, that has allowed human civilisations to develop worldwide. If we continue to push the planet out of the safe operating zone there is significant risk of “consequences that are detrimental or even catastrophic for large parts of the world”.

The Earth-system processes at threat are climate change, biodiversity loss, the nitrogen cycle, the phosphorus cycle, stratospheric ozone depletion, ocean acidification, global freshwater use, change in land use, atmospheric aerosol loading and chemical pollution. Professor Johan Rockström¹ and his colleagues identify three planetary thresholds that have already been passed. These are associated with climate change, the nitrogen cycle and the rate of biodiversity loss.

While scientists are deeply worried about global change, many others in our community are complacent. There are a number of factors behind this range of attitudes, but different assumptions about ‘how the world works’ play a crucial role. In this context we can usefully distinguish between two world views - ‘linear thinkers’ and ‘bathtub thinkers’. While the vast majority of world’s scientists are bathtub thinkers, significant numbers of our senior decision-makers are linear thinkers.

Consider atmospheric CO₂ concentrations: Linear thinkers assume that CO₂ concentration is proportional to the rate of CO₂ emissions. Increase emissions and concentration will increase. Decrease emissions and concentration will decrease. Linear thinking leads to the conclusion that atmospheric CO₂ concentrations can be stabilised by stabilising emissions and there are no delays. Linear thinking thus leads to a ‘wait-and-see’ approach - it is assumed that we can wait until we have clear evidence that rising concentrations cause climate disruption and then reduce concentration immediately by reducing emissions.

Bathtub thinkers assume that the relationship between CO₂ emissions and concentration is less immediate. The amount of CO₂ accumulated in the Earth’s atmosphere corresponds to the amount of water accumulated in the tub - such accumulations are called ‘stocks’. Then there are ‘flows’ that can change the levels of the stocks: CO₂ emissions correspond to water flowing into the tub, the removal of CO₂ from the atmosphere (by absorption in the planet’s surface) corresponds to water flowing out of the tub. This ‘stock-and-flow’ view leads to the conclusion that CO₂ concentrations will increase as long as emission rates exceed removal rates will decrease when emission rates are less than removal rates, and will stabilise only when emission rates equal absorption rates. Bathtub thinkers also see delays - changing atmospheric CO₂ concentrations takes hundreds of years.

These differences in thinking are more than a curiosity. They affect the way that problems of sustainability are framed and lead to different prescriptions for action. For this reason, reliable intuitions about the essential dynamics of complex systems - which require bathtub thinking - are of fundamental importance. But research over the last decade has demonstrated that more than 80% of well-educated people do not understand the dynamics of even very simple systems². The resulting poor understanding of Earth-system behaviour leads to confusion and related policy inertia that we can ill afford in this era of accelerating global change.

1 Rockström, J, et al (2009) A safe operating space for humanity, Nature, 461, 472-475.

2 Sterman, JD (2008) Risk Communication on Climate: Mental Models and Mass Balance, Science, 322, 532-533.

*Dr Barry Newell is a physicist who works to improve management understanding of the basic dynamics of complex social-ecological systems. He is Adjunct Associate Professor in the School of Engineering and Computer Science, and Visiting Fellow in the Fenner School of Environment and Society, at the Australian National University.