Icehouse to greenhouse tipping points

icehouseA better understanding of the transition from an ice-free greenhouse to the current glaciated icehouse world suggests that there exist a tipping point dynamics due to hysteresis global ice cover dynamics. Lee Kump reflects on recent research by Liu et al in a the Science Magazine perspective Tipping Pointedly Colder:

For much of Earth history, the climate has been considerably warmer than it is today. But 33.7 million years ago, at the Eocene-Oligocene boundary, the world became trapped in the glacial state that continues to this day. Within just 200,000 years, Antarctica went from being a rather hospitable place to a polar continent buried under kilometers of ice. The transition was abrupt, but also overshot the new equilibrium with a super-glaciation–dubbed the Oi-1 climate event–that lasted a few hundred thousand years.

In an earlier study, Lear et al. (3) found evidence from deep-water sites that there was little if any cooling during the transition. This finding challenged common sense: Surely the planet needs to cool if ice sheets are to grow. Recent work on better preserved shallow-water sites by Lear and colleagues (4) and others (5), together with the results presented by Liu et al. from a geographically diverse suite of locations, shows that cooling did indeed accompany the growth of ice sheets on Antarctica.

Simultaneous cooling at high northern and southern latitudes points strongly to a greenhouse gas reduction. It is puzzling, though, that the tropics show little cooling. Climate model simulations, including those conducted by the present authors, cool both tropics and higher latitudes when atmospheric carbon dioxide concentrations are reduced from greenhouse climate conditions.

Using a climate model, Liu et al. can only reproduce the cooling indicated by the temperature proxies when they impose a reduction in atmospheric carbon dioxide concentrations from 8 to 2 times the preindustrial level across the transition. This reduction is much larger than that previously interpreted from atmospheric CO2 proxies, from 4 to 2 times the preindustrial level. If the atmospheric CO2 proxies are correct, then the models are missing something that amplifies the climate sensitivity to changes in atmospheric CO2.

Given the strong evidence that cooling accompanied the transition into the glacial world, the playing out of the onset of Antarctic glaciation follows the script of a tipping-point climate transition. …

In thinking about the future, we must recognize that threshold behavior in one direction–like the Eocene-Oligocene boundary studied by Liu et al.–is normally accompanied by threshold behavior in the reverse direction, although the barriers to transition can differ in size (see the figure). For example, modern climate/ice-sheet models exhibit considerable hysteresis, requiring atmospheric carbon dioxide concentrations to rise well above the original initiation level to melt the Antarctic ice sheet. Future tipping behavior into a warm Eocene-like climate state may thus be delayed, but if and when it does occur, the transition will likely be abrupt.

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