Abstract
We have used the Grid ENabled Integrated Earth system modelling framework to study the archetypal example of a tipping point in the climate system; a threshold for the collapse of the Atlantic thermohaline circulation (THC). eScience has been invaluable in this work and we explain how we have made it work for us. Two stable states of the THC have been found to coexist, under the same boundary conditions, in a hierarchy of models. The climate forcing required to collapse the THC and the reversibility or irreversibility of such a collapse depends on uncertain model parameters. Automated methods have been used to assimilate observational data to constrain the pertinent parameters. Anthropogenic climate forcing leads to a robust weakening of the THC and increases the probability of crossing a THC tipping point, but some ensemble members collapse readily, whereas others are extremely resistant. Hence, we test general methods that have been developed to directly diagnose, from time-series data, the proximity of a 'tipping element', such as the THC to a bifurcation point. In a three-dimensional ocean-atmosphere model exhibiting THC hysteresis, despite high variability in the THC driven by the dynamical atmosphere, some early warning of an approaching tipping point appears possible.
Highlights
The phrase ‘tipping point’ captures the intuitive notion that, for some systems under particular conditions, a small nudge or perturbation can make a big difference to their future state
Working Group 1 of the Intergovernmental Panel of Climate Change (IPCC 2007) recently gave a low probability of collapse in the thermohaline circulation (THC) this century
Within Grid ENabled Integrated Earth (GENIE), we have developed a large number of scripted workflows that orchestrate, compute and database activities to enable different types of study
Summary
The phrase ‘tipping point’ captures the intuitive notion that, for some systems under particular conditions, a small nudge or perturbation can make a big difference to their future state. Recognition that there are two stable regimes of flow for the THC (figure 1) dates back to the classic conceptual model of Stommel (1961) This has a region of bistability where both THC ‘on’ and ‘off’ states are stable for a range of boundary conditions. Under sufficient freshwater forcing of the North Atlantic, a bifurcation point is reached where the ‘on’ state disappears and the THC inevitably switches off, through an advective spindown that takes the order of a century. As freshwater forcing is reduced, the circulation remains ‘off’ until a second bifurcation point is reached, where the off state disappears and the THC inevitably switches on This produces the following three regimes for the THC as a function of climate boundary conditions: (i) only the on state is stable, (ii) both the on and off states are bistable, and (iii) only the off state is stable. Most ‘state-of-the-art’ models have only been subjectively tuned (to achieve, for example, a reasonable present strength of the THC) and have not been systematically constrained by the available data, or their ability to simulate, for example, past climate states
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