A silly title maybe, but I wanted to cover the various ways we can think of glacier change as an example of the behaviour of natural systems. Like many natural systems at the macroscale, glacier behaviour has elements of ‘Times Arrow‘, and elements of imperfect, hysteresis-laden ‘Times Cycle’.

These terms form the title of a book by Stephen J. Gould in which he unpacks the history and myth-making around the development of geological timscales, which was bound up in a debate between so called uniformitarianism and catastrophism, which respectively explain geological change to occur by the cumulative effects of very gradual and continuous processes or by rapid change in large scale events, and the proposition of cycling of earths elements and structures over geological deep time (leading to Hutton’s vision of an endlessly cycling Earth having “no vestige of a beginning” and “no prospect of an end”). All of these ideas are partly right and partly wrong, but were important in developing ideas of geological time, and evolution. It seemed to me like an apt place to start when thinking of (ir-)reversibility in natural systems.

My key questions in preparing my workshop contribution were:

• How perfect does reversibility have to be to ‘count’?
• How does our understanding of reversibility in the physical world relate to our perceptions of our ability to reverse the impact of our actions on Earths climate?

First I introduce the current setting of global anthropogenic warming, undeniably shown in Ed Hawkins now famous climate stripes, and describe how global temperature is defined as the equilibrium temperature required to solve the planetary energy balance, filtered through the major ‘spheres’ of Earths climate system.

I covered the main types of climate forcing and response; equifinality and hysteresis, and the different timescales of natural climate oscillations (related to tectonic processes, orbital changes, climate system oscillations, …), and discussed cascading effects and climate tipping points.

I then segued into considering how humanity still has the choice to be a stepwise forcing of climate, resulting in a fundamental change of our planets climate state, or through restorative practices our current impact could be converted into an event, from which the climate system can recover. I speculated on ways in which perceived sense of agency in tackling this problem may relate to our ideas of (ir-)reversibility.