Pliocene Global Warming: Page 2 of 4
A Warm Time in the Past
The Pliocene epoch covers the period from approximately 5 to 1.8 million years ago and, as such, spanned the period of time during which the Earth transitioned from relatively warm climates to the generally cooler climates of the Pleistocene. This transition included the emergence of the direct ancestors of humankind and contains the beginnings of cyclic Northern Hemisphere glaciation. The Pliocene epoch itself contains episodic climate fluctuations prior to the late Pliocene cooling, and our focus for study is a warm period in the middle Pliocene between 3.15 and 2.85 million years before present.
This middle Pliocene warming is, potentially, an analog of the future that may provide a means of gaining insight into the effects of global warming. Additionally, unlike many more ancient periods, which were also warmer than the present; the paleogeography of the Pliocene is similar to the present, many of the Pliocene plant and animal species are similar to those that remain today, and large numbers of ocean and land-based cores contain well-dated Pliocene sediments that are available for interpretation and mapping.
In our simulations of the middle Pliocene climate we use the GISS GCM and data generated and/or compiled by the PRISM (Pliocene Research, Interpretation, and Synoptic Mapping) project, part of the U.S. Geological Survey's Global Change Research Program. PRISM focuses on documenting climates of the middle to late Pliocene, with a primary goal of providing the climate modeling community with improved quantitative global paleoenvironmental information. Our Pliocene modeling, in turn, helps test the consistency of different sets of paleo observations, each of which has its own uncertainties.
GCM Simulations of the Middle Pliocene
Estimates of sea surface temperatures (SSTs), based on microfossils from deep ocean cores reveal a warm phase in the Pliocene between about 3.15 and 2.85 million years ago. Pollen records from land-based cores, although not as well-dated, also show evidence for a warmer climate at about this same time and further indicate that continental moisture levels were quite different from today. What caused the climate to be warmer is not known with certainty, but increased levels of greenhouse gases have been suggested (see below). Also, previous sensitivity experiments using the GISS GCM imply that warmer climates, such as those of the Pliocene, can be simulated with increased ocean heat transport. Recent evidence from North Atlantic deep sea records indicates that the oceans may very well have played a major role in the warming seen in the Pliocene.
As a test of this hypothesis we applied Pliocene SSTs, together with an estimate of the terrestrial vegetation cover, as boundary conditions in a GISS GCM simulation (see figures 1 and 2). The GCM provides the method for investigating the atmospheric processes that might have maintained the warmer Pliocene climate while consistency between independent palynological estimates of climate and the simulation results help verify the GCM's sensitivity to altered conditions.
In our experiments we have found both consistencies and inconsistencies between model and data-generated paleoclimate estimates. Temperature estimates show the greatest consistency, with both model and data indicating significantly warmer temperatures at high latitudes and diminished warming nearer to the equator (figure 3). The continental temperatures agree well with estimates from palynological studies, especially in the circum-North Atlantic region. This is not unexpected since that region is strongly influenced by the dramatically warmer North Atlantic SSTs. The GCM also yields temperature increases up to 10°C along the Arctic coasts and shows greatest warming in the winter. Although the original temperature increase is driven by warmer SSTs, much of the continental interior warming is generated by an ice-albedo feedback, as reduced snow cover in the warmer climate reflects less solar radiation away from the surface during winter months (see figure 4). Further warming at high latitudes comes from the increased levels of atmospheric water vapor (a greenhouse gas) which results from the warm, ice-free ocean conditions.
Despite the generally warmer climatic conditions, some areas show overall cooling. Notably, East Africa cools by 2 to 3°C due to increased low-level cloud cover, which reflects large amounts of incoming solar radiation back to space. Very few paleo observations are available for some remote parts of Africa, but our simulation is consistent with the single palynological record that exists for that region.
Estimates of hydrological values such as precipitation, soil moisture, and surface runoff show far less consistency between the simulation and data than do temperatures. This is not really a surprising result given that hydrologic processes are notoriously difficult to simulate using coarse-grid numerical models while terrestrial environments (what the data report) are usually quite heterogeneous.
The most common discrepancy seems to be an underestimation by the model of wetter conditions, as interpreted from pollen records, throughout the Northern Hemisphere. For example, the model predicts lower effective moisture (precipitation minus evaporation) in western North America, but geologic records indicate wetter conditions during the Pliocene. The root of the difference seems to lie in the northern summer season, where the model's ground hydrology responds to the warmer ground temperatures by drying out. Adding to the problem, the somewhat diminished intensity of the atmospheric circulation (a result of reduced latitudinal [i.e. equator-to-pole] temperature gradients) decreases the ability of the atmosphere to carry moisture evaporated from the ocean surface over the continents, where it could rain out and replenish the soil.
In the Arctic, Pliocene forests dominated where tundra exists today. In altering the specified vegetation cover to match this change, wetter soil moisture condtions were also assigned. Throughout the simulation, Pliocene Arctic soils remained wetter than the present day, fed by increased rainfall originating over the warmer Arctic ocean. The results indicate, at least, that these specified wet conditions are in equilibrium with the simulated climate.