dictable) weather histories, led to collapse times of that varied between 250 and 500 years. These results have not been confirmed (or challenged) with more sophisticated models, and the random perturbations might have been unrealistically large, but the possibly chaotic nature of THC changes needs to be addressed.

Considering the known limitations of climate models, it is not currently possible to ascribe probabilities to future abrupt climate changes. Given the possible large impacts, simulation of abrupt climate change caused by a collapse in the THC constitutes a “Grand Challenge” problem in computational earth science. A dedicated supercomputer could be used to test whether the THC shows the possibility of threshold behavior, abrupt change, and hysteresis in a climate model that in its ocean components resolves the most important features of the ocean circulation. The conceptual models discussed here demonstrate that the outcome is open: on the one hand, high-resolution models are expected to have stronger gyre transport of heat and salinity, which appears to work in favor of the diffusive paradigm; on the other hand, a high-resolution model would be expected to exhibit a greater degree of internal variability, increasing the chance of self-induced threshold-crossing—if indeed a threshold exists. The need for substantially increased computational resources is clear.



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