Pliocene Global Warming: Page 3 of 4
What Caused the Middle Pliocene Warming?
Sea surface temperature patterns such as of the Pliocene (e.g., large warming at mid and high latitudes with stable tropical temperatures) are inconsistent with the warming caused by increased CO2 as we understand it from GCM doubled-CO2 experiments. Well-mixed greenhouse gases tend to warm the tropics substantially as water vapor evaporated from tropical and subtropical oceans provides a positive feedback to the low latitude warming. However, it is possible that some combination of CO2 increase and ocean heat transport change may have resulted in the warmer Pliocene surface temperatures since altered ocean circulation could increase the divergence of heat from the tropics.
Our simulations of the Pliocene climate used near-modern levels of atmospheric carbon dioxide (315 ppm) but required a nearly 30% change in the implied meridional ocean heat transports to maintain Pliocene conditions. This additional heat transport implies substantial changes in the ocean's thermohaline circulation, wind-driven circulation, or both. Evidence of such thermohaline circulation changes comes from carbon isotopic data from deep-sea microfossils, which show that the strength of North Atlantic deep water production was increased during the middle Pliocene. Wind-driven changes, however, are not yet supported by the wind velocities indicated by model simulations or by geologic evidence.
We also conducted several Pliocene simulations with varying levels of increased atmospheric carbon dioxide. Simulated surface energy fluxes were collected from those simulations and were used to calculate the ocean heat convergence/divergence at each grid cell. From the convergences we calculated the implied ocean heat transports which would have been necessary to maintain the specified SST distribution; in this case the SSTs are those derived from Pliocene paleo observations. Figure 5 shows the poleward heat transports from this series of Pliocene experiments. The plot reveals that CO2 levels must be four times current values, and perhaps higher, before ocean heat transports could be reduced to modern levels. At lower levels of atmospheric CO2 the ocean heat transports must remain higher than modern in order to maintain anything close to the observed Pliocene SSTs.
Estimates based on carbon isotope measurements (Raymo and others, 1992; 1996) indicate that Pliocene atmospheric CO2 levels were, at most, 100 ppm greater than today. Moreover, if we compare Pliocene and modern ocean heat transport distributions (Figure 5) we find that a poleward shift in the peak ocean heat convergence would have been necessary to balance the Pliocene SSTs regardless of the CO2 level. Thus, neither simulation results or data support the conclusion that Pliocene warming was caused entirely by a large increase in atmospheric CO2 content. We cannot rule out, however, that some combination of the altered CO2 and altered ocean heat transport caused the warmer climate of the middle Pliocene.
Simulating past warm climates and identifying model/data contrasts for periods such as the Pliocene provide a test of the sensitivity of our primary tool for study future climate change: global climate models. At present, our results do not support the suggestion that Pliocene warming was caused by carbon dioxide increase since such changes are not consistent with the SST distributions derived from deep sea cores. There is evidence that changes in ocean circulation and the amount of heat oceans transport may be one potential cause of the warming.
Still, investigators have found evidence that minor increases in CO2 (up to 380 ppm) did occur in the Pliocene. This causes us to wonder whether it is possible that an, climate feedback, as of yet unknown, associated with small increases in CO2, could lead to the larger changes seen in the ocean circulation? Certainly the evidence for higher levels of CO2 and stronger thermohaline circulation challenges recent results from coupled ocean-atmosphere models, which suggest that thermohaline circulation weakens as global temperature rises. Perhaps the Pliocene warming is uncharacteristic of next century's expected warming, perhaps the causes are different but the effects will be similar, and perhaps the Pliocene is a warning that unkown factors still exist that could exacerbate or mitigate the CO2 increase and global warming.
Successful comparisons, while increasing our confidence in the basic approach, probably occur coincidentally in some cases and such errors would be difficult to identify. Nevertheless, mismatches between data interpretations and model results offer undeniable evidence that either the model, data, or both are innacurrate for a specific region and climate variable. Understanding this allows us to focus resources and efforts on areas that are likely to afford the most gain. Moreover, subsequent iterations, based on new treatments of the data or GCM, test the veracity of previous conclusions.
The GISS Pliocene GCM simulation and the PRISM reconstructions are a first step in the interative process of data collection and analysis, model experimentation and analysis, and data/model comparison; the gridded, boundary condition data sets are continuously being refined, updated, and extended into areas with scarce data. Additional modeling and sensitivity experiments involving new data sets and updated GCM versions will soon begin. Close cooperation between modeling and data groups can achieve an overall better understanding of global climate models, data, data collection and simulation strategies, and the climate changes our society and planet could face relatively soon.