CLIMATE INSTABILITY IN A NOT-TOO-WARM CRETACEOUS

Mesozoic biotic crises have been linked to episodes of global warmth. Exactly how warm climates induce biotic crises is uncertain. It has been hypothesized that both rapid climate warming and marine anoxia through ocean stagnation may have played a role. Here we suggest another possibility—that climate stability may have been reduced during warm past intervals giving rise to more variable and stressful environmental conditions.

We have developed a series of mid-Cretaceous simulations with prescribed pre-industrial (280 ppmv; 1x) and elevated (10x and 16x PAL) atmospheric pCO₂ levels using the NCAR CCSM 3.0 coupled ocean-atmosphere model. Through >1600 years of model integration, the 1x and 16x cases converge toward equilibrium while exhibiting low (<20%) climate variability. In comparison, two 10x experiments with and without dynamic vegetation exhibit substantial climate variability on centennial and decadal timescales, respectively, that can exceed the differences between the 10 and 16x mean-climate states. The 10x climate variability includes high-latitude zonal-average annual surface temperature variations of 3°C, large fluctuations in sea ice area, and abrupt changes in basin-wide meridional overturning associated with the intensification/weakening of deep-water formation in the North Pacific Ocean.

Our model results indicate that a climate window exists within which climate stability is reduced and climate variability is enhanced. Climate stability is ultimately linked to northern hemisphere sea ice and its ability to modulate ocean-atmosphere heat exchange. In contrast to the 1x and 16x cases, which have large and (essentially) no sea ice amounts, the 10x experiments exhibit dynamic sea-ice lines. Finally, we note that our 10x experiments demonstrate that multiple overturning states can exist under constant high atmospheric CO₂ and that transitions between strong and weak overturning, and potentially oxic and anoxic conditions, can be geologically instantaneous.