Modelling Middle Pliocene Warm Climates of the Usa

Palaeontologia Electronica http://palaeo-electronica.org

MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA

Alan M. Haywood, Paul J. Valdes, Bruce W. Sellwood, Jed O. Kaplan, and Harry J. Dowsett Alan M. Haywood. Postgraduate Research Institute for Sedimentology, The University of Reading, PO Box 227, Whiteknights, Reading, RG6 6AB,UK and Department of Meteorology, The University of Reading, Earley Gate, PO Box 243,Whiteknights, Reading, RG6 6BB,UK Paul J. Valdes. Department of Meteorology, The University of Reading, Earley Gate, PO Box 243,Whitek- nights, Reading, RG6 6BB,UK Bruce W. Sellwood. Postgraduate Research Institute for Sedimentology, The University of Reading, PO Box 227, Whiteknights, Reading, RG6 6AB,UK Jed O. Kaplan. Max-Planck-Institute for Biogeochemistry, Postfach 10 01 64, D-07701 Jena, Germany Harry J. Dowsett. Eastern Earth Surface Processes, U.S. Geological Survey, 926A National Center, Reston, VA 20192, USA

ABSTRACT

The middle Pliocene warm period represents a unique time slice in which to model and understand climatic processes operating under a warm climatic regime. Palaeoclimatic model simulations, focussed on the United States of America (USA), for the middle Pliocene (ca 3 Ma) were generated using the USGS PRISM2 2° x 2° data set of boundary conditions and the UK Meteorological Office’s HadAM3 general circulation model (GCM). Model results suggest that conditions in the USA during the middle Pliocene can be characterised as annually warmer (by 2° to 4° C), less seasonal, wetter (by a maximum of 4 to 8 mm/day) and with an absence of freezing winters over the central and southern Great Plains. A sensitivity experiment suggests that the main forcing mechanisms for surface temperature changes in near coastal areas are the imposed Pliocene sea surface temperatures (SST’s). In interior regions, reduced Northern Hemisphere terrestrial ice, combined with less snow cover and a reduction in the elevation of the western cordillera of North America, generate atmospheric circulation changes and positive albedo feedbacks that raise surface temperatures. A complex set of climatic feedback mechanisms cause an enhancement of the hydrological cycle magnifying the moisture bearing westerly wind belt during the winter season (Dec., Jan., Feb.). Predic- tions produced by the model are in broad agreement with available geological evidence. However, the GCM appears to underestimate precipitation levels in the interior and central regions of the southern USA. KEY WORDS: Middle Pliocene, GCM, USA, evaluation. Copyright: Palaeontological Association, 22 June 2001 Submission: 28 February 2001 Acceptance: 15 May 2001

Haywood, Alan M., Valdes, Paul J., Sellwood, Bruce W., Kaplan, Jed O., and Harry J. Dowsett, 2001. Modelling Middle Pliocene Warm Climates of the USA. Palaeontologia Electronica, vol. 4, issue 1, art. 5: 21pp., 933KB. http://palaeo-electronica.org/2001_1/climate/issue1_01.htm. ALAN M. HAYWOOD, PAUL J. VALDES, BRUCE W. SELLWOOD, JED O. KAPLAN, AND HARRY J. DOWSETT: MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA INTRODUCTION al. 1996; Haywood et al. 1999, 2000a, 2000b). The middle Pliocene has been This paper describes the results of a described as the last period of geological new palaeoclimatic modelling study, time that displayed greater sustained focused on the USA, for the middle warmth and reduced climatic variability Pliocene using a state-of-the-art GCM. over a time period greater than any Qua- Previous GCM studies of the middle ternary interglacial (Dowsett and Poore Pliocene have been carried out by Chan- 1991; Dowsett et al. 1996). Geological dler et al. (1994), Sloan et al. (1996), and proxy evidence supporting this assertion Haywood et al. (1999, 2000a, 2000b). includes sea surface temperatures (SST) However, this is the first to examine, in reconstructed from planktonic foramin- detail, the nature and dynamics of the ifera (Dowsett and Poore 1991; Dowsett et palaeoclimate and the validity of model al. 1992; Dowsett et al. 1996; Raymo predictions on a regional scale. 1994), ostracods (Cronin 1991; Forester The geological record of the USA dur- 1991), siliceous microfossil records (Mor- ing the middle Pliocene is diverse and well ley and Dworestzky, 1991; Barron 1992, suited for the regional assessment of 1996), terrestrial vegetation records GCM predictions (Figure 1). A wealth of (Thompson 1991; Heusser and Morley geological data are available, derived from 1996; Thompson and Fleming 1996) and tectonic basins that contain long sedimen- numerous records of higher-than-present tary records of palaeoenvironmental sea levels (Dowsett and Cronin 1990; change. In addition, numerous localities Brigham-Grette 1994; Zubakov and display evidence for Pliocene lacustrine, Borzenkova 1990). As a result, the middle fluvial, and shallow marine environments Pliocene represents a unique time period (Thompson 1991; Thompson and Fleming in which to examine the behaviour of the 1996). Earth’s climate system under a regime of greater-than-present warmth. MODEL DESCRIPTION A relatively precise stratigraphy, wide geographical distribution of sample sites, The HadCM3 GCM and numerous extant climatic boundary The HadCM3 GCM used in this study conditions all combine to make the middle was developed at the Hadley Centre for Pliocene an ideal time period for palaeocli- Climate Prediction and Research, which is matic modelling studies using general cir- a part of the UK Meteorological Office. culation models (GCM; Dowsett et al. This GCM consists of a linked atmo- 1999). With anthropogenic greenhouse spheric model, ocean model,and sea ice gas emissions forcing greater warmth model. However, for the present study we (IPCC 1995), significant potential benefit use only the atmospheric component of exists in the careful investigation of past the model (HadAM3). warm periods in Earth’s history, such as The horizontal resolution of the model the middle Pliocene. Furthermore, palaeo- is 2.50 degrees of latitude by 3.75 degrees climatic modelling investigations of this of longitude. This gives a grid spacing in interval provide us with an opportunity to the mid-latitudes of 278 km in the North- assess the validity and robustness of pre- South direction and 417 km East-West, dictions produced by GCM that are being comparable to a T42 spectral resolution used to model past, present, and future cli- model. The HadAM3 atmospheric model mates alike (Chandler et al. 1994; Sloan et consists of 19 layers.

2 ALAN M. HAYWOOD, PAUL J. VALDES, BRUCE W. SELLWOOD, JED O. KAPLAN, AND HARRY J. DOWSETT: MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA

Figure 1. Summary of middle Pliocene palaeoclimatic/palaeoenvironmental evidence derived from the geological record of the United States.

Pope et al. (2000) describes the Boundary Conditions impact of the new physical parametriza- For this modelling study the required tions used in the HadAM3. The land-sea boundary conditions for the HadAM3 mask and orographic details used do not model were supplied by the United States differ from HadAM2 (Johns et al. 1997; Geological Survey’s (USGS) Pliocene Johns 1996). The model has a timestep of Research Interpretations and Synoptic 30 minutes and a new radiation scheme Mapping (PRISM) Groups, PRISM2 that can represent the effects of minor enhanced 2° x 2° digital data set, or they trace gases (Edwards and Slingo 1996). were specified as current values. Carbon Parametrizations of simple background dioxide was set at 315 ppm, a value in line aerosol climatology discussed by Cusack with previous modelling studies for the et al. (1998) are also included. The model middle Pliocene (i.e., Chandler et al. convection scheme has also been 1994). In practice, using prescribed SST improved according to Gregory et al. means that the model is relatively insensi- (1997). A new land-surface scheme tive to the precise choice of CO concen- includes the representation of the freezing 2 and melting of soil moisture. The repre- tration. Prescribed boundary conditions sentation of evaporation now includes the integrated into the model that are specific dependence of stomatal resistance on to the middle Pliocene include: (1) conti- temperature, vapour pressure, and CO nental configuration, modified by a 25 m 2 increase in global sea level, (2) modified concentration (Cox et al. 1999). The present-day elevations (elevation of the above changes have increased the ability western cordillera of North America of the HadAM3 model to perform a realis- reduced by 50 per cent; 950 m), (3) tic simulation of surface heat flux. reduced ice sheet size and height for

3 ALAN M. HAYWOOD, PAUL J. VALDES, BRUCE W. SELLWOOD, JED O. KAPLAN, AND HARRY J. DOWSETT: MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA Greenland (50%) and Antarctica (33%), Greenland and Antarctica by 50 percent (4) Pliocene vegetation distributions, and and 33 percent respectively, and raises (5) Pliocene SST and sea ice distributions the elevation of the western cordillera of (see Haywood et al. 2000a and Dowsett et North America by 50 percent. The aim of al. 1999 for more detailed descriptions of this approach is to investigate the relative PRISM2 boundary conditions and their importance of certain climatic boundary integration into the UKMO GCM). conditions in producing the climate The PRISM2 data set is the latest in a changes predicted by the HadAM3 model line of data sets generated by the USGS for the middle Pliocene over the USA. that reconstruct the palaeoenvironment of For both experiments the HadAM3, the middle Pliocene. The PRISM2 recon- GCM was integrated for 12 years and cli- struction consists of a series of 28 global matological means were compiled for the scale data sets on a 2° latitude by 2° longi- final 10 years only. Time-series analysis of tude grid, and as such, represents the all climate variables for the whole of the most complete and detailed global recon- 12-year simulation show that disregarding struction of climate and environmental the first two years of simulation is sufficient conditions older than the last glacial (Dow- for the model climatology to reach a sett et al. 1999). PRISM2 evolved from dynamic equilibrium. Furthermore, the numerous studies that summarised condi- slowest component to reach equilibrium is tions at a large number of marine and ter- soil moisture. An examination of global restrial localities (e.g., Cronin and Dowsett mean soil moisture showed no significant 1990; Poore and Sloan 1996). A detailed trend in the upper layers after two years. description of the new PRISM2 data set, However, there was a small residual trend that fully documents the construction of in the deepest layer that lasted for approx- PRISM2 and how it differs from earlier imately five years. Overall, the length of PRISM data sets, is now available as a the model run is comparable to previous USGS Open File Report (see Dowsett et modelling studies for the middle Pliocene al. 1999). (e.g., Sloan et al. 1996). In the results, statistical tests of the Pliocene control-run MODEL EXPERIMENTS model predictions are based on the Stu- dent t-test described by Chervin and Model Experiments, Integration, and Schneider (1976). Although this statistical Statistical Testing test has many limitations (see Wigley and Two modelling experiments are Santer 1990), it is commonly used and is a described in this article. The first (middle simple test. Moreover, the changes in cli- Pliocene control experiment) uses the full mate between the middle Pliocene and set of PRISM2 boundary conditions to present-day are generally large and so investigate the nature of the middle clearly exceed the statistical test. Pliocene palaeoclimate and its relative difference to the present-day climate of the BIOME 4 USA. The second experiment (Pliocene sensitivity experiment) utilises global An important feature of our simula- PRISM2 Pliocene SST estimates, sea tions with the HadAM3 model is that we level and vegetation distributions while use a Biome model (BIOME 4) to translate setting terrestrial ice cover and orographic climate model output from the Pliocene parameters to modern conditions. Specifi- control experiment, (specifically the sea- cally, this increases ice volume over sonal cycle of temperature, precipitation,

4 ALAN M. HAYWOOD, PAUL J. VALDES, BRUCE W. SELLWOOD, JED O. KAPLAN, AND HARRY J. DOWSETT: MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA and solar radiation) into biome distribution of the forest-grassland boundary in tions. Such an approach has two specific temperate and subtropical latitudes advantages. The first is that it condenses (Kaplan 2001). multivariate climate data into a small num- As an equilibrium vegetation model, ber of biomes producing an effective and BIOME 4 is an ideal tool for assessing informative summary of the middle vegetation distribution at a particular time Pliocene simulations produced by the in the past. However, plant assemblages HadAM3 model. Secondly, it greatly heavily influenced by short-term dynamic improves our ability to compare model- processes (e.g., fire, recurrent extreme simulated climate with estimations of weather events) may be incorrectly simu- palaeoclimate derived from fossil pollen. In lated in model. In this application, we have this study we use the BIOME 4 model, run BIOME 4 using a 2.5° by 3.75° grid which is the latest in a series of mechanis- corresponding to the same resolution as tically based models that were developed the HadAM3 GCM. Absolute values for from physiological considerations that monthly mean surface temperature, pre- place constraints on the growth and cipitation, and cloud cover derived from regeneration of different plant functional the ten-year climatological means from the types. These constraints were calculated HadAM3 Pliocene control run were used via the use of limiting factors for plant to provide the climatic information neces- growth. These factors include the mean sary for the BIOME 4 model. temperature of the coldest and warmest months, the number of growing degree MODEL RESULTS - PLIOCENE CONTROL days (GDD) above 0° and 5°C, and via the Surface Temperature Difference calculation of a coefficient (Priestley – Tay- lor coefficient) for the extent to which soil The HadAM3 model predicts a distinct moisture supply satisfies atmospheric warm anomaly over the USA during the moisture demand. GDD are calculated by middle Pliocene (Figure 2). This anomaly linear interpolation between mid-months is visible over the annual cycle and during and by a one-layer soil moisture balance winter (DJF) and summer (JJA) intervals. model (Prentice et al. 1993) independent The geographical distribution of the warm- of the HadAM3 model hydrology. ing is remarkably continuous. Annual The BIOME 4 model is representative mean temperature (∆T) changes show an of a gradual evolution of biome models average increase of 2° to 5°C. Surface (see Prentice et al. 1992; Prentice et al. temperature increases are highest during 1993; Prentice et al. 1998) and differs from the DJF period with DT values of + 5° to earlier BIOME models by the addition of 10°C common over most interior regions additional plant functional types for tundra of the USA. Temperature variations of climates and by factoring in the impor- above 20°C during DJF are also predicted tance that differing atmospheric CO2 con- for the Hudson Bay region of NE North centrations have on plant growth (Kaplan America. Surface temperature changes 2001). To minimise any model bias,the during JJA reveal a less consistent pattern BIOME 4 model, like its parent models, of change. Overall, a warming of 2° to 5°C has been validated and calibrated to vege- is predicted over many areas of the USA. tation distribution, productivity and other However, larger DT variations of 5° to biogeochemical data for the present-day 10°C are restricted to the Great Plains and the recent past. Known biases in the region. Predicted sporadic cooling over BIOME 4 model relate to the precise loca- isolated areas of the USA during the mid-

5 ALAN M. HAYWOOD, PAUL J. VALDES, BRUCE W. SELLWOOD, JED O. KAPLAN, AND HARRY J. DOWSETT: MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA

Figure 2. HadAM3 model prediction: difference in surface temperature (°C) during the middle Pliocene over North Amer- ica for both the winter (DJF) and summer (JJA) seasons.

dle Pliocene is caused by differences in have caused an increase in the absorption the assigned position of the coast speci- of solar radiation promoting further warm- fied in the PRISM2 data set. ing. The snow depth decrease occurs over the USA despite a general increase in win- Temperature Forcing and Feedbacks tertime precipitation. An increased propor- Lower terrestrial ice cover in the tion of precipitation during the warmer Northern Hemisphere and higher SST in middle Pliocene simulation falls as rain the North Atlantic-North Pacific provide the and not as snow. Comparison of absolute initial forcing mechanism behind the pre- temperature patterns for present-day and dicted surface temperature increases over the middle Pliocene over the annual cycle, the USA at 3 Ma B.P. However, reductions DJF and JJA, reveal interesting differ- in albedo, particularly during DJF gener- ences (Figure 3). During the middle ated by reduced snow cover and altered Pliocene, the freezing line over the annual Pliocene vegetation distributions, would period and DJF is positioned further to the

6 ALAN M. HAYWOOD, PAUL J. VALDES, BRUCE W. SELLWOOD, JED O. KAPLAN, AND HARRY J. DOWSETT: MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA

Figure 3. HadAM3 model prediction: present-day and middle Pliocene absolute surface temperatures (°C) over North America for the annual period, winter (DJF), and summer (JJA) seasons. north of the present-day position (~10°N). westerly winds over the USA during the Furthermore, the pattern of decreasing middle Pliocene. In the southern and cen- temperatures with increasing latitude dur- tral areas of the USA, particularly during ing the middle Pliocene is less spatially JJA, total precipitation rates decrease by variable than for present-day. This is a an average 2.00 mm/day. function of the prescribed reduction in ele- Changes in the pattern and amount of vation (by 50%) of the western cordillera of evaporation and humidity over the USA North America within the PRISM2 bound- are mainly a reflection of the altered pre- ary conditions. cipitation distribution and warmer surface temperatures. Areas experiencing Variations in Precipitation and Evaporation enhanced precipitation via moisture bear- Rates ing westerly winds in the northern USA A clear signal of changing precipita- have elevated evaporation rates by as tion rates over the USA during the middle much as 0.50 to 1.00 mm/day. This Pliocene is predicted by the HadAM3 reflects the greater moisture availability for model (Figure 4). Two zones of change evaporation and the warmer surface tem- are evident. Over the annual cycle and peratures. Conversely, central and south- during winter (DJF), northern parts of the ern parts of the USA have lower USA experience more rainfall (+ 4.00 to evaporation rates relative to present-day 8.00 mm/day). This increase in precipita- (by a maximum of 4.00 mm/day during tion occurs as a broad belt across North JJA) due to a decreased availability of America between 40° to 60°N. This moisture. Humidity values also show a increase of precipitation is consistent with corresponding decrease by as much as 30 an enhancement of the moisture bearing percent in this area during JJA.

7 ALAN M. HAYWOOD, PAUL J. VALDES, BRUCE W. SELLWOOD, JED O. KAPLAN, AND HARRY J. DOWSETT: MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA

Figure 4. HadAM3 model prediction: difference in the total precipitation rate (mm/day) during the middle Pliocene over North America for both winter (DJF) and summer (JJA) seasons.

Precipitation minus evaporation (P-E) south into the Gulf of Mexico. These P-E changes over the USA, for the Pliocene anomalies are a product of the predicted control simulation, indicates small anoma- Pliocene precipitation changes over the lies compared to present-day model simu- USA during DJF (see Figure 5). lations during JJA. However, significant Precipitation Forcing and Feedbacks anomalies are observed for the winter (DJF) season (Figure 5). The model sug- The primary forcing mechanism, pro- gests that for the northern USA, during the ducing the altered precipitation pattern middle Pliocene, a substantially greater over the USA during the middle Pliocene, (0.00 to 4.00 mm/day) precipitation flux is a strong increase in westerly wind existed compared to present-day. In con- strength (Figure 6). These enhanced west- trast, the southeast of the USA during DJF erly winds aid the transport of moisture has a greater evaporative flux (0.50 to that falls mostly as rain because of the 8.00 mm/day) compared to model simula- higher annual surface temperatures. The tions for the present, which extends further westerly wind increase is brought about by

8 ALAN M. HAYWOOD, PAUL J. VALDES, BRUCE W. SELLWOOD, JED O. KAPLAN, AND HARRY J. DOWSETT: MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA

Figure 5. HadAM3 model prediction: difference in P–E values (mm/day) during the middle Pliocene over North America during both winter (DJF) and summer (JJA) seasons.

a complex set of climatic feedbacks. magnitude changes in surface tempera- Higher SST in the North Atlantic-Pacific, ture and precipitation during the middle combined with reduced ice cover in the Pliocene over the USA are predicted for high latitudes of the Northern Hemisphere, the winter season. deepen the Aleutian and Icelandic low- The feedbacks generating the precipi- pressure systems. This generates an tation pattern in central and southern enhanced pressure gradient between the regions of the USA during the middle high latitude low pressure cells (Aleutian Pliocene are subtler. A possible explana- and Icelandic low-pressure systems) and tion for the drying in these areas is the the subtropical high pressure cells (i.e., reduced SST at lower latitudes prescribed Azores high-pressure system) intensifying during certain months in the PRISM2 digi- westerly wind strength in the Northern tal data set. This has the effect of decreas- Hemisphere, particularly during winter. ing southerly wind strength in the middle This aids in both the transport of heat and Pliocene simulation over the Gulf of Mex- moisture and explains why the highest ico while reducing the flow of moist mari-

9 ALAN M. HAYWOOD, PAUL J. VALDES, BRUCE W. SELLWOOD, JED O. KAPLAN, AND HARRY J. DOWSETT: MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA

Figure 6. HadAM3 model prediction: difference in surface wind strength (m/s-1) and vector mean winds during the middle Pliocene over North America for the winter (DJF) season. time air into the continental interior of the PLIOCENE SENSITIVITY EXPERIMENT USA, particularly during JJA. Surface Temperature Difference and Forcing The reduced height of the western cordillera of North America by 50% in the In response to the altered boundary middle Pliocene simulation should have conditions imposed in the Pliocene sensi- the effect of reducing the rain shadow tivity experiment, the HadAM3 model pre- effect to interior parts of the USA. Intu- dicts reduced surface temperatures itively, this should also increase precipita- compared to the Pliocene control over tion in this area. Discrimination of the wide regions of the USA during both winter effects of such a change from other fac- and summer seasons (Figure 7). The tors, such as increased SST that also magnitude of this cooling varies spatially increase precipitation in the region, will be with severest cooling (>5°C) concentrated addressed in the following section. over the western United States, associated with the western cordillera. However,

10 ALAN M. HAYWOOD, PAUL J. VALDES, BRUCE W. SELLWOOD, JED O. KAPLAN, AND HARRY J. DOWSETT: MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA

Figure 7. HadAM3 model prediction: difference in surface temperature (°C) over North America between the Pliocene sensitivity experiment and the Pliocene control simulation for both winter (DJF) and summer (JJA) seasons. areas in close proximity to the coast, pen- western and central parts of the USA insulas (e.g., Florida) and bays (e.g., Hud- observed in the Pliocene sensitivity exper- son Bay) do not exhibit a cooling trend. iment. Therefore, the primary forcing This is due to the warmer SST imposed mechanisms for the simulated warmer from the PRISM2 data set. This model conditions in the Pliocene – present-day suggests that in these regions the simu- control experiment are reduced Northern lated warming observed from the Pliocene Hemisphere terrestrial ice and snow cover – present-day control experiment are (via an ice albedo feedback) and the low- solely attributable to the warmer Pliocene ered elevation of the western cordillera of SST. Conversely, the increase in Northern North America. Hemisphere terrestrial ice and snow cover Precipitation/Wind Strength Difference and combined with the regional specified Forcing increase (~50%) in the elevation of the North American western cordillera prima- Overall, the HadAM3 model predic- rily drove the cooler temperatures over tions for the Pliocene sensitivity experi-

11 ALAN M. HAYWOOD, PAUL J. VALDES, BRUCE W. SELLWOOD, JED O. KAPLAN, AND HARRY J. DOWSETT: MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA

Figure 8. HadAM3 model prediction: difference in the total precipitation rate (mm/day) over North America between the Pliocene sensitivity experiment and the Pliocene control simulation for both winter (DJF) and summer (JJA) seasons.

ment suggest a pattern of reduced terrestrial ice cover and lower surface tem- precipitation levels over the USA for the peratures, which occur generally in the Pliocene sensitivity experiment (Figure 8). Northern Hemisphere, weaken the Icelan- The largest reductions are observed dur- dic low and Azores high-pressure sys- ing the winter (DJF) season in a latitudinal tems. This has the effect of weakening the range corresponding to the westerly wind moisture-bearing westerly wind belt in the belt. This reduction in precipitation is gen- Pliocene sensitivity experiment (Figure 9). erated by a number of feedback mecha- Therefore, increased precipitation levels nisms. Increased Northern Hemisphere simulated for the Pliocene – present-day snow and ice cover, combined with the control experiment for the region result increased elevation of the western cordil- from reduced snow and ice cover in the lera of North America, reduces surface Northern Hemisphere and the reduced temperatures, evaporation, and the mois- elevation of the western cordillera of North ture available for precipitation. Enhanced America.

12 ALAN M. HAYWOOD, PAUL J. VALDES, BRUCE W. SELLWOOD, JED O. KAPLAN, AND HARRY J. DOWSETT: MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA

Figure 9. HadAM3 model prediction: difference in surface wind strength (m/s-1) and vector mean winds over North America between the Pliocene sensitivity experiment and the Pliocene control simulation for the winter (DJF) season.

EVALUATION AND DISCUSSION OF are truly due to the imposed Pliocene MODEL RESULTS boundary conditions (PRISM2 data set), or inter-annual variability within the model Statistical Analyses of Pliocene Control itself. In this study, a threshold of 95 per- Results cent is considered significant. This does Statistical analyses of the model out- not automatically imply that any simulated put for the middle Pliocene control run is a changes that do not achieve a 95 percent necessary step in trying to determine how significance level are not true representa- much of the change in climatic variables tions of differences in the middle Pliocene

13 ALAN M. HAYWOOD, PAUL J. VALDES, BRUCE W. SELLWOOD, JED O. KAPLAN, AND HARRY J. DOWSETT: MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA

Figure 10. Statistical significance test results for middle Pliocene model simulations of (a) surface temperature (°C) change and (b) total precipitation rate change (mm/day) over North America. Shaded areas show the geographical coverage of statistically significant results (tested to a 95% significance level).

climate. Rather, they could represent changes is widespread over North Amer- either true variations or inter-annual vari- ica during DJF. This is, once again, a ability. Statistical results for surface tem- reflection of the magnitude of change in perature changes (°C), total precipitation these variables during winter that are (mm/day) are summarised in Figure 10. driven by changes to the high latitude, low Statistical results for surface tempera- and sub-tropical high-pressure cells. tures over North America as a whole sug- Comparison to Geological Record gest that much of the modelled differences in middle Pliocene surface temperatures Comparison of the simulated regional from present-day are significant, espe- climatic features produced by the GCM cially during DJF. The improved signifi- with Pliocene geological palaeoclimatic cance during DJF can be attributed to the data has been carried out in a semi-quan- higher magnitude of surface temperature titative fashion and is summarised as change predicted by the HadAM3 model Table 1. Broadly, the comparison suggests during that season. that the climatic predictions generated by Statistical results for changes in total the HadAM3 over the USA are robust and precipitation suggest that most of the pre- agree with the data. The geological evi- dicted changes are significant. During JJA dence of surface temperature warming of the geographical coverage of significant between 2° to 4°C and enhanced winter- results is diminished compared to DJF. warming over many areas of the USA (see The difference between seasons can boxes 1, 2, 3, 6, 8, and 10 on Figure 1) again be attributed to larger changes in closely approximates the simulations of precipitation occurring during DJF. surface temperature changes predicted by The geographical distribution of signif- the HadAM3 model. Furthermore, the sim- icant results for surface pressure (not ulated retreat of the freezing line during shown in Figure 10) and wind strength the middle Pliocene accords well with the

14 ALAN M. HAYWOOD, PAUL J. VALDES, BRUCE W. SELLWOOD, JED O. KAPLAN, AND HARRY J. DOWSETT: MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA

Table 1. Semi-quantitative comparison of middle Pliocene GCM outputs for areas 1 to 10 of the United States (see Fig- ure 1) compared to geological data for the same areas.

Annual Annual Area of U.S. model model (referring to temp. Statistically Geological precip. Statistically Geologica Quality of box change significant data Quality of change significant l data comparis numbers from PD to estimate comparison from PD to estimate on from Fig. 1). (°C) (%) (°C) (G/M/P) (mm/day) (%) (mm/day) (G/M/P)

Area 1 + 2 to 5 >99 + 2 - 4 G + 0.2 to 2 >99 Wetter M

Area 2 + 2 to 10 >99 Warmer M 0 to 2 >99 Wetter M

Area 3 + 2 to 10 >99 Warmer M 0 to 2 >99 Wetter M

Area 4 + 2 to 10 >99 Warmer M 0 to 2 >99 Wetter M

Area 5 Comparison not possible

Area 6 + 2 to 5 >99 Warmer M -1 to 0 >95 N/A -

Area 7 Comparison not possible

Area 8 + 2 to 10 >99 Warmer M - 2 to 0 >99 + 150 mm/ P yr.

Area 9 Comparison not possible

Area 10 + 2 - 10 >99 Warmer M - 2 to 0 <90 Wetter P fossil vertebrate record (Hibbard 1960; assertion of an enhanced hydrological Auffenburg 1974; Graham 1986; Thomp- cycle during the middle Pliocene (Thomp- son 1991) of the USA, which suggests that son 1991; Forester 1991; Thompson and freezing winters were absent on the cen- Fleming 1996). Moreover, the geographi- tral and southern Great Plains (Box 8: Fig- cal location of sites supporting higher pre- ure 1). A simulated northward regression cipitation levels in the northern USA of the freezing line is not a unique feature coincide with the model-predicted zone of produced by the HadAM3 model. Sloan et enhanced precipitation associated with the al. (1996) detected the same northward moisture bearing westerly wind belt. This shift of the freezing line using the NCAR suggests that the primary cause of the Genesis GCM and the PRISM1 data set. existence of large stable lakes or higher In contrast to surface temperature, lake levels in northern areas of the USA geological evidence for middle Pliocene suggested by the geological record (boxes hydrological characteristics exhibit great 1, 2, and 4, Figure 1) is related to the variability within the USA itself, thus mak- higher winter precipitation level generated ing the assessment of the GCM predic- by the enhancement of westerly winds. tions substantially harder. Furthermore, Evidence from fossil pollen, verte- the weakness of the statistical testing brates, and soil carbonates support a pat- results reduces our ability to evaluate the tern of enhanced precipitation in the model predictions. Nevertheless, geologi- southern USA on the eastern side of the cal evidence from northern offshore, cen- western Cordillera of North America as tral, and interior regions of the USA (boxes well as further north (Box 10, Figure 1). 1, 2, 4, 8, and 10: Figure 1) including This change in precipitation may have palaeolake levels and pollen support the resulted from a reduced rain-shadow

15 ALAN M. HAYWOOD, PAUL J. VALDES, BRUCE W. SELLWOOD, JED O. KAPLAN, AND HARRY J. DOWSETT: MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA effect, produced by the lowered elevation the atmosphere (Norris et al. 2000). Sum- of the mountain chain (Smith 1993; mer heating over a large mountain range Thompson 1991; Thompson and Fleming produces a rising, low-density air mass, 1996). Specifically, increases in rainfall low atmospheric pressure over the moun- have been suggested for Texas, Arizona, tains and thus strong winds blowing Nevada, and southern California. The towards the mountain front. In the North- HadAM3 model’s simulations of the middle ern Hemisphere, the winds blow counter Pliocene climate do not detect this trend of clockwise around the range due to the wetter conditions reconstructed from geo- Coriolis force. During winter, cooling of the logical evidence. mountain surfaces produces dense falling There are numerous possible expla- air masses that spread out away from the nations for this data-model inconsistency. mountain front. In this case, Northern Firstly, the GCM may be incorrectly simu- Hemisphere winds blow in a clockwise lating the small scale effects on condensa- direction (Norris et al. 2000). The strength tion and precipitation rates resulting from of the winds around mountains depends the change of orography associated with upon the breadth of the range and the lowering the western Cordillera within the strength of cooling produced by radiation Pliocene control simulation as specified in from the land surface. Thus, the pattern of the PRISM2 boundary conditions. How- air movements around the western cordil- ever, this is unlikely as the model correctly lera of North America will act to enhance simulates precipitation increases in the the movement of southerly winds into the northern parts of the USA. Therefore, the interior of the USA during JJA. However, simulated reduction in southerly winds that PRISM2 specifies a 50 percent reduction reduces the transport of moisture into in the elevation of this mountain chain. southern and central areas of the USA is This will reduce the strength of counter the most likely source of the data-model clockwise air motion around the mountains inconsistency. and potentially affect the strength of south- Present-day predictions of precipita- erly wind strength and hence precipitation tion amounts over the same area by the over southern and central areas of the model show no dry anomalies when the USA. The data-model inconstancy may outputs are compared to observed climatic therefore be a reflection of more sluggish data. Therefore, the reduced strength of counter clockwise air motion around the southerly winds and consequent moisture western cordillera of North America. Fur- transport in the southern and central areas thermore, it may indicate that the elevation of the USA must be linked to either an of this mountain chain has been reduced incorrect model response or deficiencies in the PRISM2 data set erroneously or by in PRISM2 boundary conditions. Any error a magnitude that is too great. in PRISM2 SST in the low latitudes and Data-model comparison of precipita- Gulf of Mexico region would lead to the tion characteristics over the Great Plains GCM incorrectly representing changes in region of the USA identifies another clear atmospheric circulation during the middle discrepancy between the geological Pliocene for the area in question. record (Box 8, Figure 1) and the predic- Heating or cooling air over mountain- tions of the HadAM3 model. While the tops can produce strong density contrasts model predictions mimic the geological in the atmosphere compared with air record in terms of temperature changes in masses over adjacent regions. This may this area, simulated precipitation changes result in strong rising or sinking motions in are opposite to the more humid conditions

16 ALAN M. HAYWOOD, PAUL J. VALDES, BRUCE W. SELLWOOD, JED O. KAPLAN, AND HARRY J. DOWSETT: MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA suggest by fossil vertebrates (Hibbard nomical time frame (Thompson 1996). 1960; Auffenburg 1974; Graham 1986; However, oscillations in the Milankovitch- Thompson 1991). The model simulations band frequencies, specifically the 41 kyr for precipitation change are not statistically obliquity cycle, have been detected for the significant over the Great Plains. However, Pliocene from marine records (Tiede- with the uncertainties surrounding the sta- mann et al. 1994) and also from terrestrial tistical technique, these results alone are diatom records at Tulelake in northern Cal- insufficient to ascribe the cause of the dis- ifornia (Adam et al. 1990). crepancy to the model and not to our inter- The palaeoclimate modelling results pretation of the geological record. Firstly, presented in this paper reflect a simple the chronological control on the Pliocene snap shot for the middle Pliocene. Clearly, vertebrate record of the USA is less robust this approach is insufficient in the light of than for other areas. Furthermore, the geological evidence suggesting climatic palaeoenvironmental interpretation variation within the middle Pliocene. derived from the Great Plains may be erro- Future modelling studies should aim to neous. Specifically, the existence of giant address the issue of climatic variability land tortoises on the Great Plains during within the middle Pliocene by the use of the Pliocene may not be a reflection of sensitivity experiments that alter orbital enhanced temperatures and moisture lev- parameters. Results from such model els. If the model predictions are robust, experiments can then be evaluated then this would suggest that temperature against geological evidence to better and the absence of freezing winters alone assess the predictions of the GCM. are the most significant driving forces in BIOME 4 Result allowing the advancement northward of giant land tortoises in the USA. If not, then The biome map for the North Ameri- the most likely cause of the error relates to can continent during the middle Pliocene, the deficiencies discussed in the last para- generated by BIOME 4 and the climatic graph, namely the model’s underestima- output of the HadAM3 model, can be com- tion of the strength of southerly winds that pared to the PRISM2 distribution of vege- transport moisture inland from the Gulf of tation types over the region. The aim of Mexico. such an approach is to assess how well High resolution core records taken the simulated climate produced by from the Glenns Ferry Formation in Idaho HadAM3-BIOME 4 for the middle Pliocene (Thompson 1996) and the Black Rock and mimics the boundary conditions imposed Pit of Death in Utah (Thompson et al. into it. If the model predictions are robust, 1995), all suggest that warmer and wetter then the biome distributions that are pre- conditions existed during the middle dicted by BIOME 4 should closely approxi- Pliocene in the USA, and as such support mate those in PRISM2 that were used as the simulations provided by the HadAM3 part of the boundary conditions for the (boxes 2 and 4, Figure 1). However, within GCM. the overall trend, significant and frequent Several difficulties are implicit with variations in moisture and temperature are such an approach. The first centres detected via pollen and magnetic suscepti- around the accuracy of the PRISM2 bility changes within the respective cores. boundary conditions imposed in the For the Glenns Ferry Formation, the peri- model. If certain boundary conditions odicities of these variations do not appear within the PRISM2 reconstruction are erro- to match any known Milankovitch astro- neous or out of phase with one another

17 ALAN M. HAYWOOD, PAUL J. VALDES, BRUCE W. SELLWOOD, JED O. KAPLAN, AND HARRY J. DOWSETT: MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA

Figure 11. Map of 12 biomes as simulated by BIOME 4 from HadAM3 model’s seasonal cycle of temperature, precipitation, and cloud cover over North America for the middle Pliocene.

then the resulting climate and biome simu- represent simplifications of reality and lation will be wrong. Under these circum- therefore are open to potential sources of stances it is very unlikely that the biome error just as any other models (Williamson distribution for North America produced by et al. 1998). BIOME 4 will match that described in At first sight, the HadAM3-BIOME 4 PRISM2. models predicted only a few biomes in Secondly, great care is required in locations generally similar to PRISM2 determining whether any observed dis- biome distributions (Figure 11). However, crepancies are related to the model, or dif- most of this discrepancy is attributed to the ferences in the biome classification differing classification schemes between schemes used in BIOME 4 and PRISM2. BIOME 4 and PRISM2. BIOME 4 does PRISM2 uses a simplified seven-stage correctly capture the expansion of forested classification of the Hansen et al. (1983) areas north and the concurrent retreat of biome scheme. Grassland, steppe, and barren and tundra areas that is a key char- savanna type vegetation are all incorpo- acteristic of the reconstructed middle rated into one classification due to difficul- Pliocene vegetation distribution, compared ties in distinguishing between them with with present-day (Thompson and Fleming pollen evidence (Thompson and Fleming 1996; Dowsett et al. 1999). Comparison of 1996). In contrast, BIOME 4 uses an the locations of biome boundaries advanced classification of 23 different between the BIOME 4 and PRISM2 is vir- biomes. It is important to remember that tually impossible due to the different clas- the BIOME models, just like the HadAM3, sification schemes used in both biome

18 ALAN M. HAYWOOD, PAUL J. VALDES, BRUCE W. SELLWOOD, JED O. KAPLAN, AND HARRY J. DOWSETT: MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA reconstructions. However, the latitude available evidence suggests that the boundary at which tundra climates PRISM2 biomes and SST could be out of become common is similar between both phase with one another leading to an biome reconstructions, as is the latitude underestimation of surface temperature when forest types switch from warm or change in the middle Pliocene control cool mixed to colder forms. experiment over this region by the Nevertheless, the BIOME 4 results HadAM3 model. show significant variation from the imposed PRISM2 biomes that cannot be CONCLUSIONS explained by the classification problem alone. Firstly, BIOME 4 predicts a signifi- The UK Meteorological Office’s cant advance of biomes representing HadAM3 GCM has been used to produce desert or arid areas compared with a regional climate simulation of the middle PRISM2 in eastern and central areas of Pliocene over the USA that is in broad the southern USA. Specifically, the geo- agreement with available geological graphical distribution of xerophytic shru- palaeoenvironmental evidence. Gener- bland moves northward and eastward ally, a warmer, less seasonal and wetter compared to the reconstructions of desert climate is simulated for the USA during the areas in PRISM2. We believe that this dis- middle Pliocene. However, according to crepancy reflects the dryer than expected the geological record and outputs from a conditions predicted by the HadAM3 for biome model (BIOME 4), the HadAM3 this region in the Pliocene control experi- model appears to misrepresent changes ment. to the intensity of the hydrological cycle in Secondly, BIOME 4 completely fails to the southern USA. This inconsistency is predict the presence of grassland in the related to the way in which the HadAM3 western USA. In contrast, PRISM2 sug- model represents changes to the dynamic gests that grasslands cover a significant circulation of the atmosphere (changes in geographical area of that region. This may southerly wind strength) during the middle suggest that the HadAM3 model could be Pliocene. This problem may arise due to over estimating precipitation levels in the an incorrect response of the model to the higher latitudes of the western USA. imposed boundary conditions. Alterna- Thirdly, BIOME 4 predicts temperate tively, incorrectly prescribed boundary conifer forest over parts of the southeast conditions (elevation of western cordillera USA where PRISM2 identifies deciduous of North America or SST) in the PRISM2 woodland. The results of our Pliocene data set could also be capable of generat- sensitivity experiment have demonstrated ing the data-model inconsistency. that, at least for this area of the USA, the Future work will continue to evaluate prescribed PRISM2 SST are primarily the robustness of the HadAM3 model pre- responsible for forcing the simulated dictions for the middle Pliocene. Specifi- changes in the middle Pliocene climate. cally, we aim to evaluate the reliability of PRISM2 SST reconstructions for coastal the model by comparing its outputs to areas around this region of the USA are proxy geological data from other regions slightly warmer (1° to 2°C), the same or and perhaps more significantly attempt to slightly cooler (0.5° to 1°C) than present model climate variability within the during certain months of the year (see PRISM2 time slice itself. Dowsett et al. 1999). In this case, the

19 ALAN M. HAYWOOD, PAUL J. VALDES, BRUCE W. SELLWOOD, JED O. KAPLAN, AND HARRY J. DOWSETT: MODELLING MIDDLE PLIOCENE WARM CLIMATES OF THE USA

ACKNOWLEDGEMENTS Dowsett, H.J. and Cronin, T.M. 1990. High eustatic sea level during the middle Pliocene: evidence from southeastern U.S. Atlantic coastal plain. Geology, The authors would like to thank the 18: 435-438. United States Geological Survey for pro- Dowsett, H.J. and Poore, R.Z. 1991. Pliocene sea sur- viding the PRISM2 digital data set. We face temperatures of the North Atlantic Ocean at 3.0 would also like to thank Dr. Mark Chandler Ma. Quaternary Science Reviews, 10: 189-204. Dowsett, H.J., Cronin, T.M., Poore, R.Z., Thompson, of the Goddard Institute of Space Science R.S., Whatley, R.C. and Wood, A.M. 1992. Micropal- (GISS/NASA) for his advice and experi- eontological evidence for increased meridional heat ence concerning aspects of middle transport in the North Atlantic Ocean during the Pliocene palaeoclimate modelling. This Pliocene. Science, 258: 1133-1135. Dowsett, H.J., Barron, J. and Poore, R.Z. 1996. Middle research was carried out in receipt of a Pliocene sea surface temperatures: A global recon- Reading University Endowment Trust struction. Marine Micropaleontology, 27, (1/4):13- Fund award, which is gratefully acknowl- 25. Dowsett, H.J., Barron, J.A., Poore, R.Z., Thompson, edged. R.S., Cronin, T.M., Ishman, S.E. and Willard, D.A. 1999. Middle Pliocene paleoenvironmental recon- REFERENCES struction: PRISM2. USGS Open File Report, 99- 535, http://pubs.usgs.gov/openfile/of99-535. Adam, D.P., Bradbury, J.P., Rieck, H.J. and Sarana- Edwards, J.M. and Slingo A. 1996. Studies with a flexible Wojcicki, A.M. 1990. Environmental changes in the new radiation code 1: Choosing a configuration for a Tulelake basin, Siskiyou and Modoc Counties, Cali- large-scale model. Q.J.R. Met. Soc., 122: 689-719. fornia, from 3 to 2 million years before present. U.S. Forester, R.M. 1991. 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