Earth System Tipping Points: A Case Study from Mangrove Ecosystems

Abstract

The Earth has recently entered a new geological epoch, the Anthropocene, in which mankind is a force of nature equivalent to the climate, biosphere and oceans. The Anthropocene also brings with it rapid and sometimes irreversible changes to systems, accelerated by human action, such as rapidly dying rainforests and melting ice sheets, a phenomenon known as a regime shift. These transitions are characterised by tipping points which are notoriously difficult to predict and study due to complex and uncertain internal dynamics. In this thesis, I construct an abstract model of regime shifts in mangrove ecosystems, using a novel method known as generalised modelling from dynamical systems theory. Mangroves are critical coastal ecosystems which are under threat from sea level rise, and have previously been observed to exhibit tipping-like dynamics in mass dieback events, such as those seen in the Gulf of Carpentaria in 2015. By considering the dynamical interdependencies between mangroves, soil and salinity, I construct and analyse a generalised dynamical systems model of mangrove ecosystems, which calculates how certain factors influence stability and may induce tipping points. Various biophysically plausible ranges of parameters induce bifurcations in the generalised model, confirming that the constructed model does partially capture regime shift dynamics. Findings suggest that the most significant parameters for mangrove system stability are the sensitivity of inundation to soil height, along with soil accretion, subsidence rates and nonlinearity of salt flushing. Of all the parameters in the model, regime shifts were most sensitive to the relationship between the inundation period (hydroperiod) and peat soil height, indicating that mangrove hydrodynamics should be a high priority area for future research. Parameters related to soil accumulation and degradation processes such as sediment response to inundation appear to be pivotal in maintaining dynamical stability, which suggests monitoring and management of sedimentation should be a focus of mangrove management strategies. The risk of mangrove biomass collapse was greatly heightened once the proportion of mortality from salt stress exceeded 20%. If validated by more specific models, ecosystem managers may wish to monitor mangrove mortality for proximity to this threshold. The constructed generalised model may also provide a first step in more detailed dynamical systems modelling for particular regions, forming part of a broader effort to understand, manage and predict changes in mangroves ecosystems through the Anthropocene. I conclude that the tentative success of generalised modelling and relevant mathematical analysis suggests untapped potential for the further study of tipping points in Earth System Science. Show more