Climatic Classification Over Asia During the Middle Holocene Climatic Optimum Based on PMIP Models

Journal of Earth Science, Vol. 27, No. 1, p. 123–129, February 2016 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-016-0622-7

Climatic Classification over Asia during the Middle Holocene Climatic Optimum Based on PMIP Models

Hyuntaik Oh*1, Ho-Jeong Shin2 1. Center for Marine Environmental Impact Assessment, National Fisheries Research and Development Institute, Gijang-Gun, Busan 61975, Korea 2. Ocean Circulation and Climate Research Division, Korea Institute of Ocean Science and Technology, Sangrok-gu, Ansan-si 15627, Korea

ABSTRACT: When considering potential global warming projections, it is useful to understand the impact of each climate condition at 6 kyr before present. Asian paleoclimate was simulated by performing an integration of the multi-model ensemble with the paleoclimate modeling intercomparison project (PMIP) models. The reconstructed winter (summer) surface air temperature at 6 kyr before present was 0.85 ºC (0.21 ºC) lower (higher) than the present day over Asia, 60ºE–150ºE, 10ºN–60ºN. The seasonal variation and heating differences of land and ocean in summer at 6 kyr before present might be much larger than present day. The winter and summer precipitation of 6 kyr before present were 0.067 and 0.017 mm·day-1 larger than present day, respectively. The Group B climate, which means the dry climates based on Köppen climate classification, at 6 kyr before present decreased 17% compared to present day, but the Group D which means the continental and microthermal climates at 6 kyr before present increased over 7%. Comparison between the results from the model simulation and published paleo-proxy record agrees within the limited sparse paleo-proxy record data. KEY WORDS: paleoclimate modeling intercomparison project (PMIP), paleoclimate, global warming, Asian continent.

0 INTRODUCTION parison project (PMIP) was initiated in order to coordinate and The research on paleoclimate is important in order to under- encourage the systematic study of GCMs. And, the PMIP was stand the causes of climate changes, so that possible future cli- initiated to assess their ability to simulate large changes of mate condition can be projected. And, it is possible to test the climate such as those that occurred in the distant past (Bracon- capability of climate models for climate changes simulations not et al., 2007a). The PMIP participants agreed to focus in- (Joussaume et al., 1999; Texier et al., 1997). To understand and itially on a specific period in the past: the Middle Holocene predict the climate variation and climate system, researchers need climate occurring 6 000 years ago, which corresponds to global to use a high quality climate system model such as the general warming conditions that are relatively well known (Jansen et circulation model (GCM). The recent success of climate modeling al., 2008). Generally, the Holocene climate optimum was a by GCMs seems to offer an opportunity. In order to reveal the warm period during roughly the interval 9 000 to 5 000 years climate change during the Middle Holocene, the GCM is probably before present (BP) globally (Rossignol-Strick, 1999). The necessary to improve our understanding of climate, but other tools Middle Holocene was a period roughly from 7 000 to 5 000 are also necessary (Karl and Trenberth, 2003). Although there is a years ago (Zhou et al., 2004; Xiao et al., 2002). In this study, broad agreement among models, there are still many differences the Middle Holocene means roughly 6 000 before present, in the details of their predictions. Such models can also be used to which was warmer than present day (Joussaume and Taylor, simulate past climate conditions. Paleoclimate proxy record such 2000). The research on climate characteristics at 6 kyr BP with as pollen, foraminifera, and stable isotopes can be used to evaluate the model and observation data has actively been continued the results from models (Crowley, 2000; Haywood et al., 2000). (Braconnot et al., 2012; Harrison et al., 1998). Due to the above The GCMs have proven their usefulness to investigate mentioned reliability of climate data, many GCM groups have mechanisms of past climate changes (Braconnot et al., 2000; tried to reproduce the Middle Holocene climate using a GCM. Joussaume et al., 1999). The paleoclimate modeling intercom- Many simulated climate variables have been matched with the geological evidences in Asia (Chen et al., 2008; Braconnot et *Corresponding author: [email protected] al., 2007b; Yu et al., 2000), America (Harrison et al., 1998), © China University of Geosciences and Springer-Verlag Berlin Europe (Braconnot et al., 2000; Masson et al., 1998), North- Heidelberg 2016 west Austria and New Guinea (Miller et al., 2005; Wyrwoll and Miller, 2001), and the Pacific Ocean (Liu et al., 2003; Yu et al., Manuscript received April 14, 2014. 2000). Manuscript accepted November 25, 2014. The data used in this study are the result of PMIP activi-

Oh, H., Shin, H.-J., 2016. Climatic Classification over Asia during the Middle Holocene Climatic Optimum Based on PMIP Models. Journal of Earth Science, 27(1): 123–129. doi:10.1007/s12583-016-0622-7. http://en.earth-science.net 124 Hyuntaik Oh and Ho-Jeong Shin ties, which involves 18 GCMs, using an integration of the mul- over Asia including China and Japan (Harrison et al., 2001; ti-model ensemble. This study focuses on the simulation of Takahara et al., 2000; Yu et al., 2000). climatic features during the Middle Holocene climatic optimum and Köppen climate classification. For comparison of model 2 RESULTS AND DISCUSSION outputs, paleoenvironmental proxy data was used. This study 2.1 Temperature and Precipitation may not only provide a chance to simulate climate features In annual mean downward shortwave radiation at the top of during the Middle Holocene climatic optimum and climate atmosphere, the values of 6 kyr BP and PD are 343.84 and 344.61 classification over Asian continent, but also provide a chance to W·m-2 over study area respectively (60ºE–150ºE, 10ºN–60ºN), examine the climate impacts of orbital forcing on global warm- primarily due to the precession of the equinox. The difference in ing climate change. annual mean insolation between 6 kyr BP minus PD is -0.77 W·m-2. The mean insolation during winter at 6 kyr BP is dimi- 1 DATA AND METHOD nished regionally by about 2–19 W·m-2, while it increases by We use 18 PMIP outputs which have been focused on cli- about 17–22 W·m-2 during summer compared to the insolation at mate simulation during the Middle Holocene, 6 kyr BP. The si- PD. These changes are equivalent to about 0–7% during summer, mulations at 6 kyr BP differ from the present day (PD) in at least compared to the insolation at PD. The Asian monsoon during the two respects: orbital parameters and atmospheric CO2 (Braconnot Middle Holocene links to the solar changes and climate variability et al., 2004; Joussaume et al., 1999). The experimental design was (Wang et al., 2005; Mayewski et al., 2004). These differences in simplified in order to isolate particular aspects of the model re- solar forcing may influence climate change significantly and sponse while giving the forcing conditions during 6 kyr BP same make a stronger monsoon during the Middle Holocene (Bond et as those value of PD for sea surface temperature (SST), sea ice al., 2001; van Geel et al., 1999). cover, snow free albedo, ice sheets, and topography. Due to the The annual mean surface air temperature (SAT) over precession of the equinox, the change of insolation pattern consi- study area of 6 kyr BP and PD is 13.05 and 13.34 ºC, respec- dered the most important change between PD (present insolation) tively. The reconstructed SAT shows a similar pattern between and 6 kyr BP (6 kyr BP insolation). Thus, the PMIP experiment 6 kyr BP and PD. In comparison with the reconstructed climate on the Middle Holocene cannot be expected to yield complete using paleo proxy data, the models produce well the warm aggreement with paleodata. For the CO2 concentration at 6 kyr BP, summer over study area in the Middle Holocene (Fig. 1a). The the preindustrial level of 280 ppm was used for the model run mean SAT differences show about a 0.5 ºC increase above 50º (Lorenz and Lohmann, 2004). latitude. Except regions above 50º latitude, the mean SAT at 6 In this research, we use the standard output, which is sug- kyr BP was 0.4–1.4 ºC lower than PD. Especially it decreased gested by the PMIP. Most simulations have now been com- in the northern Indian monsoon area and land area of East Asia. pleted by the PMIP modeling groups and have been stored at The reconstructed winter surface air temperature at 6 kyr BP the Program for Climate Model Diagnosis and Intercomparison was lower than PD due to the reduced shortwave radiation at (PCMDI) (Covey et al., 2000). The database version of the TOA. The mean winter (December–January–February) 28/01/2004 is used for all figures. The output has been inte- surface air temperature of the 6 kyr BP is 0.85 ºC lower than grated anywhere from 8 to 30 years with the orbital parameters PD, owing to the land differences, especially in the northern at 6 kyr BP and PD. PMIP experiments have been performed Indian and China (Fig. 1b). The reason for considerable tem- by 18 groups, including the YONU (Yonsei University) GCM perature differences in land area compared with those of the in South Korea (Iorio and Guilderson, 2002). Each group cal- ocean was due to the reflection of model configuration that culated the climatological statistics, with an averaging period fixed the same SST between 6 kyr BP and PD (Crucifix et al., of data ranging between 8 and 30 years. Each model performed 2002; Joussaume and Taylor, 2000). This response is consistent a spin up run prior to the climatological period of 1 to 10 years. with the proxy-derived temperature changes in winter, inferred Each model has its own grid resolutions. Re-gridded data reso- from deep sea sedimentary cores at 6 kyr BP (Wanner et al., lutions are all 2.5º longitude×2.5º latitude. These re-gridding 2008; Shin et al., 2006). Such cooling may be the primary programs are designed for preserving the mass mean values. cause of the slightly delayed with respect to temperature re- We used a climate classification devised by Wladimir sponse over land areas. Köppen. As a tool for representing the general pattern of cli- The mean summer (June–July–August) surface air tem- mates, the Köppen classification has been the best-known and perature 6 kyr BP is 0.21 ºC higher than that of PD; primarily still widely used (Peel et al., 2007). It uses only easily obtained due to the temperature differences in the continent above 40ºN monthly and annual mean values of temperature and precipita- latitude (Fig. 1c). The simulated Middle Holocene temperature tion. Furthermore, the criteria are specific, relatively simple to during July is much warmer than the simulated value of July at apply, and practically dividing the world into climate regions. PD due to the increased insolation compared to the PD. It is We used the Global Paleovegetation Mapping Project (BIOME reported that Kuroshio warm current’s activity was very active 6000) to evaluate the model simulations of the 6 kyr BP over at the Middle Holocene (Razjigaeva et al., 2002; Korotky et al., Asia with a realistic paleoclimate record (Harrison and Prentice, 2000). The paleo proxy records such as pollen distribution 2003). Based on the paleo proxy data of pollen, BIOME gives a patterns also showed that the warm climate indicator species global distribution of the vegetation between 6 kyr BP and PD. occurred at the higher latitude in Asia (Yu et al., 2000, 1998). Some species which are sensitive to climate change have been In particular, significant temperature differences are shown in studied in lake and coastal areas during the Middle Holocene the Far East Asia (Fig. 1). The summer surface air temperature

Climatic Classification over Asia during the Middle Holocene Climatic Optimum Based on PMIP Models 125 of Middle Holocene and present shows a similar pattern (Mac- system was asynchronous among different regions in East Asia Cracken, 2008; Yuan et al., 2004; Kattenberg et al., 1996). The (Zhao et al., 2009; Chen et al., 2008; He et al., 2004). We as- SAT differences in summer at 6 kyr BP were 0.1–0.5 ºC, sume from our model results that the monsoonal activity at 6 slightly lower than the PD because of the plentiful precipitation kyr BP was more active than PD similar to the proxy record of of central China. The abundant rainfall had the effect of shift- the Middle Holocene (Sun et al., 2005; An et al., 2000). The ing the northern boundary of temperate deciduous forests simulated results can be useful tool to the understanding and shifted to the north about 800 km, and regions of the broad- interpretation of regional paleoenvironmental proxy data, when leaved evergreen forest also shifted about 300–400 km to the those records are asynchronous among different regions. north (Yu et al., 2000). The reconstructed SAT at 6 kyr BP In the reconstructed annual precipitation field, the amount simulates an increased seasonal cycle of temperature than PD. of precipitation at 6 kyr BP is nearly the same as PD in most The Asian monsoon system caused asynchronous Holocene study area above 35ºN latitude (Fig. 2a). The enhanced precipi- climate over East Asia, proxy records suggest the climate tation regions in the Middle Holocene were the northern Indian,

Figure 1. Distributions of the climatological mean surface air temperature Figure 2. Distributions of the climatological mean precipitation differences differences (ºC) between 6 kyr BP minus PD in Asia, differences in annual (mm/day) between 6 kyr BP minus PD in Asia, indicates differences in annual SAT (a), winter SAT (b), and summer SAT (c). precipitation (a), winter precipitation (b), and summer precipitation (c).

126 Hyuntaik Oh and Ho-Jeong Shin show that the reconstructed precipitation of 6 kyr BP over and present climate, became somewhat diminished over the ocean, several regions above 50ºN latitude. Especially, the results Tailand and Burma (12ºN–16ºN, 95ºE–105ºE). northern Indian Peninsula was about 0.5–1.0 mm·day-1 more abundant than the value of PD. However, the precipitation 2.2 Climate Classification decreased at the low latitude around 10ºN during the 6 kyr BP In this study, Köppen climate classification was applied to compared to that of PD. the output of multi-model ensemble with PMIP models as a The mean winter precipitation of 6 kyr BP is 0.067 diagnostic tool for GCM (Kottek et al., 2006; Shin et al., 2004; mm·day-1 larger than the value at PD. However, central China, Lohmann et al., 1993). We analyzed global scale climate classi- Taklimakan and Gobi desert areas show -0.2– -0.4 mm·day-1 fication and Asian continent in this study. At present climate, lower precipitation values than PD (Fig. 2b). The mean sum- some of the grid-regions of the Sahara Desert classified in mer precipitation of the 6 kyr BP is 0.017 mm·day-1 larger than Class BS, BWh, and BWk climate were divided into Class Cw PD. It especially increased in the northern Indian monsoon area and Cf climate at 6 kyr BP (Fig. 3a). The African continent and northern regions over the Asia. The tropical belt, including adjacent to Morocco and Egypt was classified as Class C cli- the Indian Ocean from the Indian Peninsula to the Korean Pe- mate at 6 kyr BP instead of Class B climate at PD. The area of ninsula, moved to further northwards regions and strengthened the Steppe and Savanah biomass also reduced toward the north at 6 kyr BP from model results (Fig. 2c). We assume that this at 6 kyr BP. The region adjacent to the Sahara Desert at 6 kyr caused more precipitation than PD over the northern India Pe- BP was more humid than PD. A comparison between the re- ninsula. The precipitation near Indonesia below 20°N latitude, sults from the model simulation and published paleo proxy one of the most abundant rainfall region in the earth at the records well agree within the limited sparse paleo proxy record

Figure 3. Distributions of the climate class according to the Köppen climate classification in Asian continent, global (a) and over Asian continent (b) climate class at 6 kyr BP.

Climatic Classification over Asia during the Middle Holocene Climatic Optimum Based on PMIP Models 127

(Gupta et al., 2003; Prell and van Campo, 1986). observed tree distributions at 6 kyr BP over Japan were rather The reconstructed Class B climate such as desert regions, similar to PD (Gotanda et al., 2002; Takahara et al., 2000). including Class BS, BWh, and BWk climate, was reduced to- Therefore we suggest that the changes in the climate and bio- ward the west during the Middle Holocene due to an increase climate of Japan and southern part of China have been small of summer precipitation over desert regions (Fig. 3b). The since the Middle Holocene. The reconstructed climate at 6 kyr Class C climate was systematically shifted toward the north at BP over the Korean Peninsula shows that Class Dfa climate oc- 6 kyr BP due to the increase of summer precipitation. The for- cupied central and northern regions in the Korean Peninsula, but est biomass in China at 6 kyr BP was shifted towards the north Class Cfa dominated southern regions in the Korean Peninsula. It and extended westwards compared to the present (Yu et al., is reported that the cool-temperate central and montane forest 2000; Prentice and Webb III, 1998). The summer precipitation existed during the Middle Holocene in the eastern area of the change between 6 kyr BP minus PD represents about 20%– Korean Peninsula (Yi, 2011). Because biomass distributions in 40% increase in desert regions and about 10% increase in the the Korean Peninsula were different from Japan at 6 kyr BP (Ta- central part of China. In general, classes B and C climates are kahara et al., 2000), climate and bioclimate between the Korean greatly affected by summer precipitation and temperature. Peninsula and Japan have been different since at least 6 kyr BP. Therefore, Class B climate decreased, while Class C climate Because winter insolation in the northern mid-latitude was systematically extended toward the north. However, winter less than today, a warm winter at 6 kyr BP in Asia continent is precipitation decreased over land areas. contrary to what would be expected in terms of the radioactive The Class B climate during 6 kyr BP was diminished about effects of orbital changes during the Middle Holocene (Prentice 2.1% globally, and 17% over study area than PD (Fig. 4). In this and Jolly, 2000; Shi et al., 1993). A weakening of the Asian study, we more focused on the Asian continent, because it is a continent winter monsoon was related to the persistent exis- vast area reaching polar regions to equatorial regions and contains tence of the strengthened greenhouse effect caused by increas- diverse climate type (Shin et al., 2004). However, the areal ratio ing water resources in summer at 6 kyr BP. In the reconstructed of Class D climate was extended to the south about 6.5% globally, winter evaporation field, the amount of evaporation at 6 kyr BP and about 7 % over Asian continent compared with PD. Therefore, was about 0.13 mm·day-1 more abundant than the value of PD, the decrease in Class B climate over Asian continent at 6 kyr BP it caused the winter precipation at 6 kyr BP is nearly the same was bigger than that of the global decrease at the same period as PD in most of Asian continent. Because realistic topographi- compared to the PD. This was related to the occupied areal ratio cal features are not simulated in PMIP models, climate classifi- of Class B climate over Asian continent being greater than the cation may be less detailed. More systematic and wide applica- global value during Middle Holocene. tions of the PMIP output should be done in the future to ade- The reconstructed climate at 6 kyr BP over Japan and the quately assess its performance, and to provide information on southern part of China were similar to the modern climate. The the effect over Asian continent.

A (a) A (b)

B B

C C

D D

Koeppen climate class climate Koeppen 6FIX Climate class climate Koeppen 6FIX Climate E 0FIX Climate E 0FIX Climate

01020304001020304050 Areal ratio (%) Areal ratio (%)

Figure 4. Areal ratio (%) in each climate class according to the Köppen climate classification the areal ratio between 6 kyr BP and PD in globe (a) and in Asian continent (b).

3 CONCLUSION tinent was 0.85 ºC (0.21 ºC) lower (higher) than the PD. The This study focuses on the simulation of climate features decrease of SAT in summer at 6 kyr BP over strong monsoonal during the Middle Holocene climatic optimum based on the regions was caused by the increase of precipitation. The mean PMIP models. For comparison of model outputs, paleoenvi- winter (summer) precipitation of the 6 kyr BP is 0.067 ronmental proxy record was used. The value of insolation at 6 mm·day-1 (0.017 mm·day-1) larger than the value at PD. This kyr BP and PD are 343.84 W·m2 and 344.61 W·m2 over study indicates that these differences in solar forcing make a stronger area (60ºE–150ºE, 10ºN–60ºN). The mean insolation during Asian monsoon during the Middle Holocene. The climate at 6 winter at 6 kyr BP is diminished regionally by about 2–19 kyr BP reconstructed based on Köppen climate classification. W·m-2, while it increases by about 17–22 W·m-2 during sum- The Class B climate was systematically reduced toward the mer compared to the insolation at PD. The reconstructed winter north over Asian continent. The Class D climate shifted syste- (summer) surface air temperature of 6 kyr BP over Asian con- matically toward the south compared to PD. The Class B cli-

128 Hyuntaik Oh and Ho-Jeong Shin mate at 6 kyr BP was diminished about 2.1% globally, and Modern-Day and Late Quaternary Samples. Quaternary 17% over Asian continent compared to the PD due to the occu- Science Reviews, 21(4): 647–657 pied areal mean of Class B climate being greater than the glob- Gupta, A. K., Anderson, D. M., Overpeck, J. T., 2003. Abrupt al value. Changes in the Asian Southwest Monsoon during the Ho- locene and Their Links to the North Atlantic Ocean. Na- ACKNOWLEDGMENT ture, 421(6921): 354–357 This study was funded by the National Institute of Fisheries Harrison, S. P., Jolly, D., Laarif, F., et al., 1998. Intercompari- Science of Korea (No. RP-2016-ME-036). The final publication is son of Simulated Global Vegetation Distributions in Re- available at Springer via http://dx.doi.org/10.1007/s12583-016- sponse to 6 kyr BP Orbital Forcing. Journal of Climate, 0622-7. 11(11): 2721–2742

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