Spatio-temporal Variation Of Sea-ice And Surface Temperature Since The Middle Holocene:Implications For The Holocene Temperature Conundrum

The Holocene is the youngest geological period.The reconstruction of the global mean sea surface temperature during the Holocene shows that the highest temperature occurred during the early to middle Holocene,followed by a decreasing trend;while the results based on transient climate simulations show that the global mean annual temperature(GMAT)has been increasing during the Holocene.This data-model discrepancy regarding GMAT trends is known as the“Holocene Temperature Conundrum”.Previous studies suggested that the“Holocene Temperature Conundrum”might be caused by seasonal bias of temperature reconstruction or by modeled bias of internal climate feedbacks.As one of the internal feedback factors,sea ice can effectively regulate global temperature changes.Therefore,biased sea ice simulations can distort global temperature simulations.Based on the above findings,the research investigated the characteristics of global and hemispheric temperature and sea ice reconstructions.Then the temporal and spatial variations of temperature and sea ice in CCSM3,FAMOUS,LOVECLIM and MPI-ESM transient experiment and slice experiments in the Fourth Paleoclimate Modelling Intercomparison Project(PMIP4)are analyzed.Finally,by comparing the differences in temperature,sea ice and their reconstructed records,as well as the differences in the statistical relationship between sea ice and SST in local areas and those change in climate models,this study explores the possible reasons for the deviations of sea ice simulation in climate models.1)The modeled results show a global and southern hemisphere warming trend consistently,while large difference among these models is found in the northern hemisphere.In order to investigate the causes of global temperature simulations,the spatial and temporal variation characteristics of the simulation results were analyzed by empirical orthogonal decomposition(EOF).The first principal component of the models showed increasing trend of the GMAT,and the second principal component of EOF shows a millennial scale oscillation.This study used Ensemble Empirical Mode Decomposition(EEMD)to study the effect of temperature oscillations over millennium scales on long-term trends since the Middle Holocene.The last Intrinsic Mode Function(IMF)of EEMD of those models show that the trend of global temperature increased,and some IMFs show an oscillation on millennium time scale,but the amplitude was low and did not have a fundamental impact on the trend of temperature change during a certain period.2)Spatiotemporal changes and causes of sea ice in the northern and southern hemispheres revealed.The reconstruction of sea ice in the northern hemisphere shows that the sea ice in the West Arctic has been increasing since the Middle Holocene;The sea ice in the southern hemisphere has shown and expansion trend.However,the modeled sea ice of climate models shows different trends in the northern hemisphere,while the sea ice in the southern hemisphere is decreasing(MPI-ESM sea ice shows an increasing trend,but the total amount is small).Sea ice changes and their causes in CCSM3 and FAMOUS are listed as follows:(1)There is no evident long-term trend of Arctic sea ice in CCSM3,which may be caused by the synergistic effect of the increase of atmospheric CO₂ and the decrease of solar radiation in summer(June,July and August);(2)The abrupt reduction of Antarctic sea ice at 5kyr BP was related to the abrupt cessation of freshwater forcing in the southern ocean,which triggers the rapid reduction of sea ice by activating the positive feedback between sea surface and sea subsurface;(3)The increase of CO₂ leads to the decrease of sea ice in both hemispheres in the FAMOUS.The multi-model average of PMIP4 shows that the decrease of solar radiation in summer(June,July and August)in the northern hemisphere and spring(September,October and November)in the southern hemisphere controled the change of sea ice,leading to more sea ice during PI than 6kyr.