Pliocene Global Warming

Traduction en final
Pliocene Global Warming
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.
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Fig. 1: Pliocene sea surface temperatures. Differences
from modern values values for two selected months.
Units are °C.
Fig. 2: Pliocene and modern vegetation global albedo
distribution. (Click on Fig. 1 or 2 to see a larger version
of the figure.)

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.
Fig. 3: Change in Northern Hemisphere surface air temperatures. Results
of a Pliocene simulation minus a current climate "control" simulation.
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Units are °C.
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.
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.
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Fig. 4: Feedback mechanisms in the Pliocene Northern
Hemisphere. All values are the zonally averaged
difference between the Pliocene and current climate
control simulation.

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.
Final Comments
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
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Fig. 5: Poleward ocean heat transport in the Northern
Hemisphere.

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.
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