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Phil. Trans. R. Soc. A (2009) 367, 3–17
doi:10.1098/rsta.2008.0205
Published online 13 October 2008
Introduction. Pliocene climate, processes
and problems
B Y A LAN M. H AYWOOD 1, * , H ARRY J. D OWSETT 2 , P AUL J. V ALDES 3 ,
D ANIEL J. L UNT 3 , J ANE E. F RANCIS 1 AND B RUCE W. S ELLWOOD 4
1
School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK
2
US Geological Survey, 926A National Center, Reston, VA 20192, USA
3
School of Geographical Sciences, University of Bristol, University Road,
Bristol BS8 1SS, UK
4
Postgraduate Research Institute for Sedimentology (PRIS ), University of
Reading, Whiteknights, Reading RG6 6AB, UK
Climate predictions produced by numerical climate models, often referred to as general
circulation models (GCMs), suggest that by the end of the twenty-first century global
mean annual surface air temperatures will increase by 1.1–6.48C. Trace gas records from
ice cores indicate that atmospheric concentrations of CO2 are already higher than at any
time during the last 650 000 years. In the next 50 years, atmospheric CO2 concentrations
are expected to reach a level not encountered since an epoch of time known as the
Pliocene. Uniformitarianism is a key principle of geological science, but can the past also
be a guide to the future? To what extent does an examination of the Pliocene geological
record enable us to successfully understand and interpret this guide? How reliable are the
‘retrodictions’ of Pliocene climates produced by GCMs and what does this tell us about
the accuracy of model predictions for the future? These questions provide the scientific
rationale for this Theme Issue.
Keywords: Pliocene; uniformitarianism; proxies; general circulation models, climate
1. The Pliocene epoch
The Pliocene epoch (Plio more, Cene recent) represents the uppermost
subdivision of the Tertiary Period. It spans a time frame from ca 5.3 to
1.8 Myr BP, according to the geological time scale of Gradstein et al. (2004). The
epoch incorporates the time interval in which the Earth experienced a transition
from relatively warm climates to the prevailing cooler climates of the Pleistocene
(Dowsett & Poore 1991; Mudelsee & Raymo 2005; Raymo et al. 2006; Lisieski &
Raymo 2007).
Climatically, the Pliocene can be crudely divided into three phases, (i) an Early
Pliocene warm period, (ii) a relatively short-lived ‘warm blip’ centred ca 3 Myr BP
referred to as the Mid-Pliocene warm interval, and (iii) a climatic deterioration
* Author for correspondence ([email protected]).
One contribution of 11 to a Theme Issue ‘The Pliocene. A vision of Earth in the late twenty-first
century?’.
3
This journal is q 2008 The Royal Society
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1.5
Pleistocene age
A. M. Haywood et al.
geomagnetic
polarity
benthic δ18O (‰)
C1r2r
C2n
C2r
2.5
3.5
Pliocene
age (Ma)
C2An
C2Ar
4.5
C3n
C3r
5.5
4.5
4.0
3.5
3.0
2.5
Figure 1. Pliocene magnetochronologic framework, after Berggren et al. (1995). Benthic d18O
record from Lisiecki & Raymo (2005). Vertical line through isotope curve represents presentday value.
during the Late Pliocene leading to the high-magnitude climate variability
associated with Pleistocene glacial/interglacial cycles (figure 1). Available higherresolution climate records for the Pliocene (e.g. Leroy & Dupont 1994; Lawrence
et al. 2006) suggest that even relatively short subdivisions of the epoch were
characterized by variations in precipitation and temperature, occurring on the time
scale of several thousand years. This climatic variability is only partially resolved
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Introduction
5
in terms of magnitude (Haywood et al. 2002a; Draut et al. 2003) but often occurred
with detectable periodicities of 41 000 and 19 000, 21 000 and 23 000 years linked to
obliquity and precessional cycles (e.g. Draut et al. 2003; Becker et al. 2006;
Sprovieri et al. 2006).
Even though a progressive cooling occurred during the Tertiary, the Pliocene
world appears to have been, on average, warmer than present day (Jansen et al.
2007). The ancient distribution of planktonic foraminifera along with terrestrial
fossil fauna and flora indicates that winter and summer temperatures in the midlatitudes were often several degrees higher than present (e.g. Thompson 1991;
Dowsett et al. 1996; Thompson & Fleming 1996; Salzmann et al. 2009). The
greatest warming appears to have been in the high latitudes where temperatures
were often elevated enough to allow species of animals and plants to exist at
higher latitudes than their nearest modern relatives (Adam 1994; Francis & Hill
1996; Ashworth & Kuschel 2003; Ashworth & Preece 2003; Ashworth &
Thompson 2003; Ashworth & Cantrill 2004; Ballantyne et al. 2006; Francis et al.
2007; Salzmann et al. 2009). Realization that significant warming took place at
high latitudes has potentially important ramifications for the behaviour and
extent of Pliocene sea ice, ice sheets and sea level (Dwyer & Chandler 2009; Lunt
et al. 2009; Naish & Wilson 2009).
2. The past is a guide to the future?
Uniformitarianism, the observation that fundamentally the same geological
processes that operate today also operated in the distant past, is a key principle
of modern geology. The methodological significance of the principle is
summarized in the statement: ‘The present is the key to the past.’
Uniformitarianism originated with the work of the geologist James Hutton but
was refined by John Playfair and popularized by Charles Lyell’s Principles of
geology (1830), but can the principle be reversed and extended so that the past
becomes the key to future?
The search for geological analogues for future climate change is not new, but
what is new is a realization that, given current and projected levels of greenhouse
gases (GHGs) in the atmosphere, we must travel far into the geological past to
find intervals of time in which GHGs, as well as global temperatures, were
comparable to what we predict will occur by the end of the twenty-first century.
Trace gas records from ice cores indicate that atmospheric concentrations of CO2
are already higher than at any time during the last 650 000 years, discounting
any period within that time as an analogue for late twenty-first century climate
(Siegenthaler et al. 2005).
Palaeoclimatology, therefore, faces a dilemma. The majority of scientific effort
has been, and remains, devoted to understanding (i) glacial and interglacial
climate variability during the Pleistocene, (ii) Holocene climate variability, and
(iii) the instrumental/historical record of climate change spanning the last 2000
years. For these periods our palaeoenvironmental datasets, and therefore our state
of knowledge, are at their best, yet no interval occurring during the last 650 000
years is extreme enough to be an indicator of late twenty-first century climate.
Indeed, for most of the last 650 000 years, climate has been very much colder than
today. So in terms of the relevance of palaeoclimate studies to understanding and
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A. M. Haywood et al.
predicting future climate change, has an emphasis on studying the Quaternary left
palaeoclimatology in a weak position? Quaternary palaeoclimate studies have
provided us with a unique opportunity to examine rapid high-magnitude climate
changes as well as given us the chance to test the robustness of climate models in
simulating dramatically different climate states; at the same time our knowledge
of warm climate intervals during the Tertiary has also grown significantly.
Estimates of atmospheric CO2 levels during the Pliocene have been derived
through analyses of the stomatal density of fossil leaves (e.g. Van der Burgh et al.
1993; Kürschner et al. 1996) and through analyses of d13C ratios of marine
organic carbon (Raymo & Rau 1992; Raymo et al. 1996). Both techniques
indicate that absolute CO2 levels during the warmest intervals of time, while
variable, may have been as high as 425 ppm by volume (Raymo et al. 1996),
meaning that the warmest intervals of the Pliocene may represent an equilibrium
climate response to atmospheric CO2 levels that will be attained approximately
by the middle of the twenty-first century.
The increasing importance of the Pliocene in the context of future climate
change is highlighted by the epoch’s inclusion in the recent IPCC fourth
assessment report, which, focusing on a particular interval of the Pliocene,
states that,
the mid-Pliocene is the most recent interval of geological time when mean global
temperatures were substantially warmer for a sustained period; w 2 8C to 3 8C above preindustrial temperatures. Therefore, the mid-Pliocene represents an accessible example of a
world that is similar in many respects to what models estimate could be the Earth of the late
twenty-first century. The mid-Pliocene is recent enough that the continents and ocean basins
had nearly reached their present geographic configuration. Taken together, the average of
the warmest times during the mid-Pliocene presents a view of the equilibrium state of a
globally warmer world, in which atmospheric CO2 concentrations were likely higher than
pre-industrial values.
( Jansen et al. 2007, ch. 6, pp. 440–442)
3. Climate processes and problems
Given that the Pliocene has now entered the political mainstream of climate
change science, we are fortunate to have a large body of existing work to refer to
and to use as the foundation for future scientific study. Decades of painstaking
research by individuals and small groups of scientists in a number of different
countries, working in a scientific area that was, until very recently, considered by
many to be marginal, have provided ever more detailed reconstructions of what
Pliocene environments and climates were like, and what climate processes may
have been responsible for the initiation, sustenance and termination of Pliocene
warm periods.
The work of the United States Geological Survey’s PRISM Group (Pliocene
Research Interpretation and Synoptic Mapping) in documenting Mid-Pliocene
environmental conditions around the world is a case in point. Twenty years of
detailed palaeoenvironmental reconstruction, using various proxies such as
planktonic foraminifera, diatoms, ostracods, pollen and leaves, has culminated
in the release of the PRISM 3D 28!28 digital dataset (Dowsett 2007). PRISM 3D
provides information on global Mid-Pliocene sea surface temperatures (SSTs) and
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Introduction
7
Atlantic/Pacific Ocean bottom water temperatures, vegetation cover, terrestrial
and sea ice cover, sea levels and topography specifically designed for integration
into general circulation models (GCMs) and/or as a means to evaluate outputs
from Pliocene climate modelling studies. As such, PRISM 3D represents the
most detailed palaeoenvironmental reconstruction available for any geological
time period older than the last glacial (see http://geology.er.usgs.gov/eespteam/
prism/prism3main.html for further details). This and earlier versions of the
PRISM dataset have facilitated a number of modelling exercises designed to
examine the dynamics of Mid-Pliocene climates (e.g. Chandler et al. 1994; Sloan
et al. 1996; Haywood et al. 2000a,b; Jiang et al. 2005; Haywood & Valdes 2006).
The climatic drivers behind warm intervals of the Pliocene have been widely
discussed (e.g. Raymo et al. 1996) and can be crudely summarized as relating to:
(i) palaeogeographic change, e.g. altered elevations of major mountain chains
such as the western cordillera of North and South America (Rind & Chandler
1991); (ii) altered atmospheric trace gas concentrations and water vapour
content (e.g. Raymo & Rau 1992; Van der Burgh et al. 1993); (iii) changes to
ocean circulation (Ravelo & Andresson 2000; Cane & Molnar 2001), ocean heat
transport (e.g. Dowsett et al. 1992; Haug & Tiedemann 1998; Kim & Crowley
2000; Lunt et al. 2008a), or the thermal structure of the oceans (e.g. Philander &
Fedorov 2003; Wara et al. 2005; Fedorov et al. 2006); and (iv) feedbacks
generated through altered land cover (including ice sheet extent), surface albedo,
cloud cover and temperature (Haywood & Valdes 2004, 2006; Salzmann et al.
2008, 2009).
Many previous studies have centred on the role of the atmosphere and oceans
(Raymo et al. 1996). As previously stated, specific proxies for atmospheric CO2
have identified elevated concentrations during warm intervals of the Pliocene.
The CO2 increases reconstructed for the Pliocene, up to 425 ppm by volume,
would produce a radiative forcing of approximately 2.5 W mK2. This may be
sufficient to explain the warmth of the Pliocene globally (depending on the
chosen climate sensitivity parameter, and whether there are any significant
changes in other aspects of the radiative forcing), but it has not been able on its
own to explain the regional changes in surface temperature suggested by
palaeoclimate proxies (Crowley 1996). Therefore, additional mechanisms working independently or combined with variations in CO2 concentrations in the
atmosphere are required to drive and maintain the warmer Pliocene conditions.
An alternative explanation invokes a change in the heat transport of the oceans.
SST estimates from diatom and planktonic foraminiferal assemblages, synthesized
as part of the PRISM project, originally showed no warming relative to present in
the tropics but significant warming at higher latitudes, especially in the North
Atlantic and Pacific oceans (Dowsett et al. 1996, 1999, 2009, see figure 2). This
pattern is indicative of enhanced meridional ocean heat transport, which may have
been a direct result of more vigorous North Atlantic Deep Water (NADW)
formation and thermohaline circulation (THC) during certain intervals of the
Pliocene compared with present day (Dowsett et al. 1996, 1999; Raymo et al. 1996).
Therefore, investigation of this period may provide critical information regarding
the nature and behaviour of NADW formation and THC under conditions of
greater global warmth that have characterized the recent geological past, and may
characterize the near future (Dowsett et al. 2009).
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A. M. Haywood et al.
(a)
T (°C)
0
1.5
equirectangular projection centred on 0.0° E
3.0
4.5
6.0
4.5
6.0
(b)
T (°C)
0
1.5
3.0
equirectangular projection centred on 0.0° E
Figure 2. PRISM SST anomalies for (a) February and (b) August. Anomalies derived by
subtracting present-day SST (Reynolds & Smith 1995) from PRISM2 SST (Dowsett et al. 1999).
Although the possibility of enhanced meridional ocean heat transport was
highlighted in early modelling studies (Rind & Chandler 1991), many difficulties
and unexplained issues remain. Paradoxically, a reduced latitudinal SST
gradient implies weaker atmospheric forcing of surface oceanic circulation, and
hence weaker oceanic heat transport from Equator to higher latitudes (Crowley
1996). This is a common problem associated with the dynamics of past warm
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Introduction
9
climates (Valdes 2000). Problems exist in any explanation of Pliocene warmth
that is based solely upon strengthening of the THC as it is difficult (i) to ascribe a
thermohaline coupling argument to all ocean basins and (ii) to generate the
correct reconstructed hemispheric temperature distribution (Crowley 1996;
Valdes 2000). However, it is clear that such an argument is too simplistic, as
climate processes operating at the regional scale are capable of overriding globalscale trends (Haywood et al. 2000a).
To test these and other ideas Haywood & Valdes (2004), using the HadCM3
GCM, carried out the first fully coupled ocean–atmosphere GCM simulation for
the Mid-Pliocene. The simulation resulted in a global surface temperature
warming of 38C compared with present day. In contrast to earlier modelling
studies (e.g. Chandler et al. 1994; Sloan et al. 1996; Haywood et al. 2000a,b),
which have used prescribed PRISM SSTs, surface temperatures increased in
most areas, including the tropics. Compared with a pre-industrial simulation, the
model predicted a general pattern of ocean warming in both hemispheres to a
depth of 2000 m, below which no significant differences were noted. Sea ice
coverage was massively reduced and the velocity of the Gulf Stream/North
Atlantic Drift was greater in the Pliocene simulation.
Analysis of the model-predicted ocean overturning suggested a global pattern
of reduced outflow of Antarctic bottom water, a shallower depth for NADW
formation and weaker THC. Model diagnostics for heat transport indicated that
neither the oceans nor the atmosphere transported significantly more heat in the
Mid-Pliocene case. Rather, the results indicated that the major contributing
mechanism to Mid-Pliocene warmth was the prescribed reduced extent of highlatitude terrestrial ice sheets and sea ice cover, resulting in a strong ice–albedo
feedback (Haywood & Valdes 2004).
The results from this modelling study conflict with two well-established
interpretations of Pliocene ocean conditions based on proxy data, which indicate
that tropical SSTs did not increase, and that warmer mid-latitude SSTs were
caused by enhanced meridional ocean heat transport/THC. So, are Pliocene
ocean data telling us that this model is wrong? This has yet to be determined.
A later study of tropical and low-latitude SSTs (Haywood et al. 2005) using
alkenone palaeothermometry indicated that SST warming in tropical upwelling
systems was at least possible during the Pliocene. Recent multi-proxy studies of
equatorial Pacific SST show that the east–west gradient was greatly reduced
during the Mid-Pliocene due to undeniable warming of the eastern equatorial
Pacific (EEP) and relative stability of the western equatorial Pacific warm pool
(Wara et al. 2005; Ravelo et al. 2006; Dowsett & Robinson 2009). Geochemical
data suggest modest warming in the west. While faunal-based methods, difficult
to apply in the west where present-day SST is at the limit of the calibration data
(approx. 308C), hint at the possibility of short periods within the Mid-Pliocene
where SST was higher than 308C (Dowsett & Robinson 2009). Resolution of the
Pliocene tropical SST debate is vital if we are to deconvolve the role of
atmosphere and oceans, and the carbon cycle in general, in Pliocene warmth and
test the efficacy of outputs from Pliocene ocean–atmosphere GCM studies.
The role of, and feedbacks related to, the cryosphere are likely to be central to
any explanation of Pliocene warmth. Unfortunately, our knowledge and ability
to reconstruct the Pliocene cryosphere is incomplete and limited. Proximal
geological records from the Antarctic and Greenland are few and far between, yet
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A. M. Haywood et al.
they provide the best opportunity to reconstruct ice sheet behaviour during the
Pliocene. The value of international projects that aim to study high-latitude
palaeoceanography via the collection of marine sediment cores is highlighted by
the success of the recent ACEX expedition to the Arctic (IODP Leg 302, see
Pagani et al. (2006) and Jakobsson et al. (2007)) and the current ANDRILL
project (Antarctic Drilling in McMurdo Sound), which has recovered a thick
sequence of marine deposits from underneath the Ross Ice Shelf providing a
unique window on ice shelf stability during Pliocene warm intervals. New
approaches in reconstructing Neogene ice sheet behaviour on land are also
yielding exciting results. For example, the study of Neogene ice–volcano
interactions on the Antarctic Peninsula and North Victoria Land is providing
a new way to examine the presence–absence of ice during the Neogene and
providing critical estimates of ice thickness (e.g. Smellie et al. 2006).
Indications of Pliocene ice sheet behaviour are available from distal sources via
oxygen isotope ratios of benthic foraminifera and ostracods combined with
Mg/Ca ratios (e.g. Lear et al. 2000; Dwyer & Chandler 2009; Naish & Wilson
2009). However, currently these combined benthic O18 and Mg/Ca records are
sparse and do not provide any information on how the ice was geographically
distributed. This situation must be remedied before an assessment of the
reproducibility of these distal geochemical records of global ice volume is possible
(Dwyer & Chandler 2009; Naish & Wilson 2009).
Traditionally, reconstructions of Pliocene ice sheets have been based solely
upon past sea-level estimates, derived from well-dated palaeo-shorelines
(Dowsett & Cronin 1990; Wardlaw & Quinn 1991), but which may have
significant uncertainties associated with the estimates. Owing to these
uncertainties, estimates of the Mid-Pliocene Antarctic ice sheet volume, for
example, range from near present-day values to greater than 33 per cent
reduction compared with modern. This uncertainty severely hampers modelling
efforts designed to investigate the cause of Pliocene warmth and restricts the
value of this period as a test-bed for numerical climate models. Therefore, refined
sea-level records with reduced uncertainties are urgently required (e.g. Dwyer &
Chandler 2009; Naish & Wilson 2009).
As geological evidence for the size and extent of Pliocene ice sheets and ice sheet
variation is currently inconclusive, it is important that the plausible extent of the ice
sheets during the Pliocene is assessed through coupled climate and ice sheet
modelling exercises. Outputs from Pliocene GCM modelling experiments are
currently being used to drive three-dimensional, thermomechanical ice sheet models
(Hill et al. 2007; Lunt et al. 2008a,b, 2009), which will enable us to estimate the
plausible state of the ice sheets during warm climate intervals in the Pliocene.
4. Recent developments and future challenges
One of the most exciting recent developments in Pliocene research has been the
examination of tropical climate dynamics and tropical high-latitude climate
linkages. Recent palaeoceanographic studies that have reconstructed SST
gradients across the tropical Pacific, using Mg/Ca and d18O analyses of
planktonic foraminifera, alkenone palaeothermometry and faunal methods,
indicate that the SST gradient across the Pacific during the Early and MidPhil. Trans. R. Soc. A (2009)
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Introduction
11
Pliocene was substantially lower than it is today (approx. 1.58C compared with
approx. 68C for the modern; Wara et al. 2005; Dowsett & Robinson 2009). Mean
SSTs in the EEP were considerably warmer than they are at present (by approx.
38C). This scenario of warmer EEP SSTs and a reduced SST gradient across the
tropical Pacific is akin to what occurs during a modern El Niño event, thus the
Early and Mid-Pliocene have been characterized as exhibiting permanent El
Niño-like conditions.
Barreiro et al. (2006) prescribed a permanent El Niño-like state in a GCM
simulation for present day and found that, given such a condition, the trade
winds along the Equator, and hence the Walker Circulation, collapse. Low-level
stratus clouds in the low latitudes diminish, reducing the albedo of the planet
and increasing global temperatures. The atmospheric concentration of water
vapour also increases. Since water vapour is responsible for approximately 70 per
cent of the known absorption of outgoing long-wave radiation, an increase in
atmospheric water vapour provides a further contribution to global warming.
Philander & Fedorov (2003) hypothesize that as surface temperatures warm in
the future, due to the anthropogenic emissions of GHGs, the thermocline in the
tropical Pacific may deepen, causing a reversion to a Pliocene permanent
El Niño-like state. The authors argue that the transition from uniformly warm
tropics to tropics with zonal SST gradients, ca 3 Myr ago, provided important
positive feedbacks for the amplification of glacial cycles and the onset of
Northern Hemisphere glaciation.
Haywood et al. (2007) and Lunt et al. (2008a) present two fully coupled
ocean–atmosphere GCM simulations for the Pliocene in which the behaviour of
the El Niño Southern Oscillation (ENSO) is examined. The first experiment
(MidPlioControl) was initialized with a full suite of Mid-Pliocene boundary
conditions derived from the PRISM dataset. The second experiment
(EarlyPlioControl) was identical to experiment MidPlioControl, except that the
Central American Seaway was specified as being open, allowing a connection to
be established between the western tropical Atlantic and eastern tropical Pacific
oceans, as was the case during the Early Pliocene. In both experiments,
statistical analyses of model-predicted Pacific SSTs, combined with the
examination of 100 years of model-predicted December, January and February
(DJF) surface air temperature variability for El Niño region 3.4 (central and
eastern Pacific), reveal a pattern of ENSO variability in both experiments
(figure 3). The standard deviation of DJF surface air temperatures in the central
and eastern Pacific suggests that the variability of ENSO in experiment
MidPlioControl is, in fact, greater than that for the pre-industrial experiment.
However, with a prescribed connection between the Atlantic and Pacific oceans
through the Central American Seaway (EarlyPlioControl), variability in surface
air temperatures decreased, underlining the importance of performing further
sensitivity experiments that explore the impact on ENSO behaviour of altered
geological boundary conditions (Bonham et al. 2009).
Given the temporal resolution of the palaeoceanographic data used in this
context, which is at best one sample spanning 10 000 years of actual time (Wara
et al. 2005), it is clear that no record is capable of proving or disproving the
existence of a permanent El Niño state during the Early or Mid-Pliocene, since
ENSO events occur over decadal to sub-decadal time scales. Ideally, an approach
capable of documenting SST variability at an annual or even sub-annual
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A. M. Haywood et al.
surface air temperature (°C)
during DJF
31
30
29
28
27
26
25
24
23
0
10
20
30
40
50
60
70
integration length (years)
80
90
100
Figure 3. One hundred years of DJF surface air temperature (8C) for El Niño region 3.4 (central
and eastern tropical Pacific) predicted by the HadCM3 GCM experiments MidPlioControl (thin
black curve), EarlyPlioControl (thick grey curve) and pre-industrial (dashed curve) (Haywood et al.
2007; Lunt et al. 2008a). Note the generally warmer temperatures for the Pliocene experiments and
the variability in temperatures compared with the pre-industrial experiment. Standard deviations
are pre-industrialZ0.69., MidPlioControlZ0.89, and EarlyPlioControlZ0.57. These results illustrate
the range of potential behaviours of Pliocene ENSO to changing geological boundary conditions
(e.g. ocean gateway configurations).
resolution would be used (for example, stable isotope or Mg/Ca analysis of
annual growth layers of mollusc shells or corals; Haywood et al. 2007). However,
the data do provide snapshots of the mean state of tropical Pacific SSTs at
specific geographical locations during the Pliocene, and it is entirely plausible
that a fundamental shift in the behaviour of ENSO during the Pliocene would be
reflected by a change in the mean state of SSTs in the EEP (Rickaby & Holloran
2005; Wara et al. 2005).
Molnar & Cane (2002) suggest that the pattern of temperature and
precipitation change derived from regional proxy data during the Pliocene is,
in many cases, similar to weather and climate patterns observed during a modern
El Niño event, thus providing far-field evidence in support of a permanent
El Niño state during the Pliocene. However, the far-field effects or signals of
modern El Niño events can be very weak and, as is shown by Bonham et al.
(2009), any apparent El Niño pattern in the proxy record can easily be explained
by other changes in the Pliocene Earth system (such as altered terrestrial
orography, global ice cover, global vegetation patterns and atmospheric trace gas
concentrations), making it impossible to support El Niño as a ‘unique solution’
that explains most of the significant regional differences in Pliocene climate
compared with present day.
Given the clear requirement to reconstruct and model Pliocene climates at
high temporal resolutions, new approaches to proxy reconstruction must be
adopted to provide the necessary data. One example of how the mean annual
range of temperature (MART) for localities during the Pliocene can be estimated
can be found in the inverse relationship between the size of cheilostome bryozoan
zooids and water temperature. Williams et al. (2009) have used MART data from
fossils collected from a range of latitudes to provide information about shelf SSTs
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Introduction
13
and to test Mid-Pliocene climate scenarios generated by numerical climate
models. The MART technique, developed by O’Dea & Okamura (2000), uses the
variation in zooid size within a colony to interpret the MART experienced by the
colony. To date, over 150 Pliocene colonies collected from fossil localities ranging
from Panama to the UK have been analysed to provide MART estimates,
providing a new way to quantify changes in surface and near-surface ocean
temperature seasonality.
The aims of this volume are threefold. Firstly, it is to synthesize the
current state of knowledge regarding warm climates and environments of the
Pliocene (e.g. papers by Dowsett & Robinson 2009; Dowsett et al. 2009; Dwyer &
Chandler 2009; Matthiessen et al. 2009; Williams et al. 2009). As such the
publication builds upon the foundations laid by two previous special issues on the
Pliocene published in the journals Quaternary Science Reviews in 1991 and
Marine Micropaleontology in 1996. Secondly, the focus of the papers presented is
to address fundamental questions about the workings of the Pliocene Earth
system, which have been raised in this introduction (e.g. Bonham et al. 2009;
Dowsett & Robinson 2009; Dowsett et al. 2009; Naish & Wilson 2009). Finally,
papers are included that make a direct assessment of how similar Pliocene
environments and climates may have been to climate and environmental
predictions for the late twenty-first century (e.g. Dowsett et al. 2009; Lunt et al.
2009; Salzmann et al. 2009), exploring the value of examining warm climate
states in Earth’s history as a potential analogue for future climate change.
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