THE RELATIVE ROLES OF GREENHOUSE GASES AND OCEAN HEAT TRANSPORTS IN MIDDLE PLIOCENE WARMING

That the middle Pliocene was a period of significant warmth is supported by both terrestrial and marine paleoclimate evidence as well as by several modeling studies. In fact, the middle Pliocene was very likely the last time the Earth experienced global temperatures that are comparable to the conditions we expect to face in the coming century. Specifically, the middle Pliocene climate was dominated by high-latitude warming and a reduced equator-to-pole temperature gradient.

On the one hand, the middle Pliocene has great potential to serve as an analogue for future climate change, since many climatic boundary conditions, such as the position of the continents, vegetation types and the intact state of the isthmus of Panama and the Indonesian through-flow were the same as, or similar to, the present-day. However, proxy data indicate only moderately higher levels of CO2 in the middle Pliocene than we find in the atmosphere at the beginning of the 21st century, which suggests that the Pliocene warming is unlike future global warming. Previous modeling studies, marine fauna and stable isotope records suggest that the middle Pliocene warming was due, in part, to altered ocean circulation, however, proxies also indicate that a modest atmospheric CO2 increase was also a forcing factor. Unfortunately, the links between the specific forcing and feedback mechanisms that allowed for this warm equilibrium climate are not yet well understood. Which means that it is impossible to rule out whether or not relatively modest greenhouse gas increases during the Pliocene played a role in triggering ocean changes that resulted in a greater warming than the radiative effects of CO2 alone.

One objective of our ongoing Pliocene modeling research is to use ocean parameterizations of varying complexity and radiative schemes that delineate key greenhouse gas forcings to distinguish the relative role of altered ocean circulation and the greenhouse effect in the middle Pliocene. We can narrow the potential levels of OHT and greenhouse gases to several unique combinations that yield middle Pliocene global temperatures. But, further research, using digital data sets of surface and deep sea temperatures to selectively eliminate more possibilities, will help us hone in on a unique set of forcings and feedbacks for the middle Pliocene warm period.