Selection of prior distributions and multiscale analysis in Bayesian temperature reconstructions based on fossil assemblages

It is possible to reconstruct the past variation of an environmental variable from measured historical indicators when the modern values of the variable and the indicators are known. In a Bayesian statistical approach, the selection of a prior probability distribution for the past values of the environmental variable can then be crucial and the selection therefore should be made carefully. This is particularly the case when the data are noisy and the statistical model used is complex since the influence of the prior on the results can then be especially strong. It can be difficult to elicit the prior probability distribution from the available information, since usually there are no measured data on the past values of the variable one wants to reconstruct and different reconstructions are typically consistent with each other only at a coarse level. To overcome these difficulties we propose to use a non-informative smoothing prior, possibly in combination with an informative prior, that simply penalizes for roughness of the reconstruction as measured by the variability of its values. We believe that it can sometimes be easier to set an overall prior distribution on the roughness than to agree on a prior for the actual values of the reconstructed variable. Note that by using a smoothing prior one incorporates into the model itself the smoothing step usually done before or after the actual numerical reconstruction. Another idea proposed in this paper is to integrate the reconstruction model with a multiscale feature analysis technique known as SiZer. Multiscale analysis of the posterior distribution of the reconstructed variable makes it possible to infer its statistically significant features such as trends, maxima and minima at several different time scales. While only temperature is considered in this paper, the technique can be applied to other environmental variables.     

TEMPERATURE PROXY RECORDS COVERING THE LAST TWO MILLENNIA: A TABULAR AND VISUAL OVERVIEW

Proxy data are our only source of knowledge of temperature variability in the period prior to instrumental temperature measurements. Until recently, very few quantitative palaeotemperature records extended back a millennium or more, but the number is now increasing. Here, the first systematic survey is presented, with graphic representations, of most quantitative temperature proxy data records covering the last two millennia that have been published in the peer-reviewed literature. In total, 71 series are presented together with basic essential information on each record. This overview will hopefully assist future palaeoclimatic research by facilitating an orientation among available palaeotemperature records and thus reduce the risk of missing less well-known proxy series. The records show an amplitude between maximum and minimum temperatures during the past two millennia on centennial timescales ranging from c. 0.5 to 4°C and averaging c. 1.5–2°C for both high and low latitudes, although these variations are not always occurring synchronous. Both the Medieval Warm Period, the Little Ice Age and the 20th century warming are clearly visible in most records, whereas the Roman Warm Period and the Dark Age Cold Period are less clearly discernible.     

The relationship between air and water temperatures in lakes of the Swiss Plateau: a case study with pal\sgmaelig;olimnological implications

In pal\sgmaelig;olimnological studies, inference models based on aquatic organisms are frequently used to estimate summer lake surface water temperatures. However, the calibration of such models is often unsatisfactory because of the sparseness of measured water temperature data. This study investigates the feasibility of using air temperature data, usually available at much higher resolution, to calibrate such models by comparing regional air temperatures with surface water temperatures in 17 lakes on the Swiss Plateau. Results show that altitude-corrected air temperatures are sufficiently uniform over the entire Swiss Plateau to allow local air temperatures at any particular lake site to be adequately estimated from standard composite air temperature series. In early summer, day-to-day variability in air temperature is reflected extremely well in the temperature of the uppermost metre of the water column, while monthly mean air temperatures correspond well, with respect to both absolute value and interannual variations, with water temperatures in most of the epilimnion. Standardised altitude-corrected air temperature series may therefore be a useful alternative to surface water temperatures for the purposes of calibrating lake temperature inference models. In Northern Hemisphere temperate regions, mean air and water temperatures are likely to correspond most closely in July, suggesting that calibration and reconstruction efforts be concentrated on this month.     

2007—2020年西藏草面温度时空分布特征

利用2007—2020年西藏38个气象站点平均草面温度（简称草温）、平均气温、平均地表温度、云量、降水量等观测资料,采用气候统计诊断方法分析了西藏草面温度的时空分异特征及其影响因素,以期科学研究当地草地生态系统和开展专业气象服务。结果表明：西藏年平均草温呈自东南向西北递减的分布。草温与海拔高度存在显著的负相关,海拔高度每升高100 m,季平均草温降低0.44~0.70 ℃,年平均草温降低0.58 ℃;与纬度有着显著的曲线关系,29.3°N以南（北）地区,随着纬度增加,草温随之升高（降低）。各站草温呈一峰一谷的日变化特征,日最低值出现在07:00—08:00（北京时间）,日最高值均出现在14:00;草温月平均最低值都出现在1月,月平均最高值出现在6月或7月;76%的站点草温的变化为夏季>春季>秋季>冬季的气候特征。西藏草温年较差为21.4 ℃,较气温年较差偏大3.1 ℃;草温日较差达35.7 ℃,远高于气温日较差,偏大21.6 ℃。草温与气温之差以夏季最大,其次是春季、冬季两者比较接近;草温与地表温度之差以春季最大,夏季次之,冬季最小。在空间分布上,月平均草温与气温、地表温度均呈显著的正相关,与平均风速、积雪日呈显著的负相关;积雪深度对草温的影响,除冬季外二者存在显著的负相关;大部分月份平均草温与总云量、低云量、降水量的关系不显著。86.8%的站点5—9月平均逐小时草温与降水量存在显著的负相关关系。     

中国中东部秋季PM10时空变化及其与日气温的关系

利用2000-2010年秋季中国中东部83个重点城市的PM10浓度数据以及其中63个城市的逐日气象资料,分析了PM10浓度的时空变化以及晴空条件下PM10浓度与日气温之间的关系,讨论了不同云量条件下二者关系的稳定性以及辐射的相应变化.结果表明:(1)近11年来,秋季PM10浓度呈现下降趋势,全部天气条件下和晴空条件下的线性趋势值分别为-2.87 μg·m-3/年、-4.92 μg·m-3/年;空间分布上,中国中东部重点城市的秋季PM10浓度普遍下降,其中华北地区的下降最快最显著.(2)秋季PM10浓度与日气温的波动之间存在显著相关,定量统计表明:当PM10浓度偏高10 μg·m-3时,日最高气温、日最低气温和日平均气温分别偏低0.15℃、0.14℃和0.16℃,同时气温日较差减小0.01 ℃.(3)秋季日气温的上述变化可能主要与气溶胶的直接效应有关.PM10增多会造成地面总辐射和地表净辐射的显著减少,进而造成日最高气温、日平均气温的显著下降;同时,PM10增多对近地面的影响总体上是致冷效果.     

Traveltime measurements from noise correlation: stability and detection of instrumental time-shifts

We test the feasibility of using Green's functions extracted from records of ambient seismic noise to monitor temporal changes in the Earth crust properties by repeated measurements at regional distances. We use about 11 yr of continuous recordings to extract surface waves between three pairs of stations in California. The correlations are computed in a moving 1-month window and we analyse the temporal evolution of measured interstation traveltimes. The comparison of the arrival times in the positive and negative correlation time of Rayleigh and Love waves allows us to separate time-shifts associated with any form of physical change in the medium, those resulting from clock drift or other instrumental errors, and those due to change in the localization of the noise sources. This separation is based on the principle of time symmetry. When possible, we perform our analysis in two different period bands: 5–10 and 10–20 s. The results indicate that significant instrumental time errors (0.5 s) are present in the data. These time-shifts can be measured and tested by closure relation and finally corrected independently of any velocity model. The traveltime series show a periodic oscillation that we interpret as the signature of the seasonal variation of the region of origin of the seismic noise. Between 1999 and 2005, the final arrival time fluctuations have a variance of the order of 0.01 s. This allows us to measure interstation traveltimes with errors smaller than 0.3 per cent of the interstation traveltime and smaller than 1 per cent of the used wave period. This level of accuracy was not sufficient to detect clear physical variation of crustal velocity during the considered 11 yr between the three stations in California. Such changes may be more easily detectable when considering pairs of stations more closely located to each other and in the vicinity of tectonically active faults or volcanoes.     

1984—2020年辽宁省极端气温时空变化及粮食产量响应研究

通过选取极端气温指数对1984—2020年辽宁省极端气温时空变化进行分析,结果表明：① 近37 a极端气温指数的时间变化具有一致性,表现为暖指数（年极端高温、暖昼日数、暖夜日数）上升和冷指数（年极端低温、冷昼日数、冷夜日数）下降。受城市化进程影响,突变主要集中在1995—2005年。② 极端气温指数具有空间差异性,具体表现在城镇化水平高的城市极端气温差值变化较小,沿海地区相对指数变化幅度相对较小。③ 结合辽宁省粮食与气象灾害数据,得出1984—2020年粮食实际产量与趋势产量呈波动上升变化。相对产量与气象产量波动趋势一致,近37 a气候丰年10个、气候歉年7个,其他为正常年份。气象产量受极端高温、极端低温的影响较大,风雹、冷冻成为影响粮食产量的关键气象灾害。     

Geodetic inversion for space–time distribution of fault slip with time-varying smoothing regularization

We have developed a new geodetic inversion method for space–time distribution of fault slip velocity with time-varying smoothing regularization in order to reconstruct accurate time histories of aseismic fault slip transients. We introduce a temporal smoothing regularization on slip and slip velocity through a Bayesian state space approach in which the strength of regularization (temporal smoothness of slip velocity) is controlled by a hyperparameter. The time-varying smoothing regularization is realized by treating the hyperparameter as a time-dependent stochastic variable and adopting a hierarchical Bayesian state space model, in which a prior distribution on the hyperparameter is introduced in addition to a conventional Bayesian state space model. We have tested this inversion method on two synthetic data sets generated by simulated aseismic slip transients. Results show that our method reproduces well both rapid changes of slip velocity and steady-state velocity without significant oversmoothing and undersmoothing, which has been hard to overcome by the conventional Bayesian approach with time-independent smoothing regularization. Application of this method to transient deformation in 2002 caused by a silent earthquake off the Boso peninsula, Japan, also shows similar advantages of this method over the conventional approach.     

Comparison of theoretical and observed temperature profiles in Devon Island ice cap, Canada

Summary. A non-steady-state theoretical model is used to predict the present variation of temperature with depth in two boreholes in the Devon Island ice cap, Arctic Canada. The boreholes are 300 m apart and one of them reaches bedrock. The heat transfer equation is solved numerically with the record of past temperatures obtained from measurements of the variations of oxygen—isotope ratio with depth in the cores as surface boundary condition. The effects of ice advection, refreezing of meltwater percolating from the surface (the amount of which is recorded in the cores), heating due to firn compaction and ice deformation, and heat flow in the bedrock below the ice sheet are all included in the model. The free parameters are geothermal heat flux, present surface temperature and heat loss at the surface which depends on the depth of meltwater penetration and other factors. Agreement between observed and predicted temperature—depth profiles is very close. Latent heat released by percolating meltwater is the predominating factor in determining the temperature distribution in the upper half of each borehole. The temperature distribution is insensitive to the value of the factor used to convert oxygen—isotope ratio to temperature.     

Late quaternary paleoclimatic reconstructions for interior Alaska based on paleolake-level data and hydrologic models

Hydrologic models are developed for two lakes in interior Alaska to determine quantitative estimates of precipitation over the past 12,500 yrs. Lake levels were reconstructed from core transects for these basins, which probably formed prior to the late Wisconsin. Lake sediment cores indicate that these lakes were shallow prior to 12,500 yr B.P. and increased in level with some fluctuation until they reached their modern levels 4,000-8,000 yr B.P. Evaporation (E), evapotranspiration (ET), and precipitation (P) were adjusted in a water-balance model to determine solutions that would maintain the lakes at reconstructed levels at key times in the past (12,500, 9,000 and 6,000 yr B.P.). Similar paleoclimatic solutions can be obtained for both basins for these times. Results indicate that P was 35-75% less than modern at 12,500 yr B.P., 25-45% less than modern at 9,000 yr B.P. and 10-20% less than modern at 6,000 yr B.P. Estimates for E and ET in the past were based on modern studies of vegetation types indicated by fossil pollen assemblages. Although interior Alaska is predominantly forested at the present, pollen analyses indicate tundra vegetation prior to about 12,000 yr B.P. The lakes show differing sensitivities to changing hydrologic parameters; sensitivity depends on the ratio of lake area (AL) to drainage basin (DA) size. This ratio also changed over time as lake level and lake area increased. Smaller AL to DA ratios make a lake more sensitive to ET, if all other factors are constant.     

Crosshole seismic waveform tomography – I. Strategy for real data application

The frequency-domain version of waveform tomography enables the use of distinct frequency components to adequately reconstruct the subsurface velocity field, and thereby dramatically reduces the input data quantity required for the inversion process. It makes waveform tomography a computationally tractable problem for production uses, but its applicability to real seismic data particularly in the petroleum exploration and development scale needs to be examined. As real data are often band limited with missing low frequencies, a good starting model is necessary for waveform tomography, to fill in the gap of low frequencies before the inversion of available frequencies. In the inversion stage, a group of frequencies should be used simultaneously at each iteration, to suppress the effect of data noise in the frequency domain. Meanwhile, a smoothness constraint on the model must be used in the inversion, to cope the effect of data noise, the effect of non-linearity of the problem, and the effect of strong sensitivities of short wavelength model variations. In this paper we use frequency-domain waveform tomography to provide quantitative velocity images of a crosshole target between boreholes 300 m apart. Due to the complexity of the local geology the velocity variations were extreme (between 3000 and 5500 m s⁻¹), making the inversion problem highly non-linear. Nevertheless, the waveform tomography results correlate well with borehole logs, and provide realistic geological information that can be tracked between the boreholes with confidence.     

全球升温对中国区域温度纬向梯度的影响

利用中国气象中心160站点的实际观察资料,对中国半个世纪的气温变化分时间段进行了计算分析。用各个站点52年的气温序列斜率与纬度和海拔分别做了相关分析,发现气温变化幅度和纬度有着很好的相关性,特别是在冬季;但气温变化和海拔没有相关性。另外,通过对不同季节的温度变化情况进行计算和纬向分析,发现随着全球气温的持续上升,气温的纬向梯度在变小,变化的幅度是冬季大、夏季小,高纬度地区大、低纬度地区小。最后指出造成气温纬度梯度减小的主要原因是大陆度随纬度的不同分布。     

经验模态分解下中国气温变化趋势的区域特征

用经验模态分解(EMD) 方法对中国700 多个站(1951-2001 年) 月平均气温进行了分析, 提取气温变化趋势项, 作空间分型, 并计算各站气温变化率, 以地理信息系统为数据处理平台, 以1km×1km 分辨率的DEM 数据作为地形的综合反映, 得到了中国平均气温空间分型和变化率精细化分布图。结果表明: 近50 年来中国北方大部分地区气温变化率多在0.4 ^oC/10a 以上, 西南和长江中下游部分地区气温变化率较小, 气温变化率为负值区零星状散落 在西南等地区。同时, 1881-2001 年中国9 个区域的气温资料分析表明, 近百年来中国气温变 化趋势以东北、华北、华东、华南、西北和新疆区是持续上升, 西南区呈下降型; 华中区呈倒"V" 型变化, 西藏区趋势不明显。     

Variation in vegetation greenness in spring across eastern China during 1982-2006

Vegetation greenness is a key indicator of terrestrial vegetation activity. To understand the variation in vegetation activity in spring across eastern China (EC), we analysed the variation in the Normalised Difference Vegetation Index (NDVI) from April to May during 1982-2006. The regional mean NDVI across EC increased at the rate of 0.02/10yr (r²=0.28; p=0.024) prior to 1998; the increase ceased, and the NDVI dropped to a low level thereafter. However, the processes of variation in the NDVI were different from one region to another. In the North China Plain, a cultivated area, the NDVI increased (0.03/10yr; r²=0.52; p<0.001) from 1982 to 2006. In contrast, the NDVI decreased (-0.02/10yr; r²=0.24; p=0.014) consecutively from 1982 to 2006 in the Yangtze River and Pearl River deltas, two regions of rapid urbanisation. In the eastern region of the Inner Mongolian Plateau and the lower reaches of the Yangtze River in East China, the NDVI increased prior to 1998 and decreased thereafter. In the Hulun Buir area and the southern part of the Yangtze River Basin, the NDVI increased prior to 1998 and remained static thereafter. The NDVI in the grasslands and croplands in the semi-humid and semi-arid areas showed a significant positive correlation with precipitation, while the NDVI in the woodlands in the humid to semi-humid areas showed a significant positive correlation with temperature. As much as 60% of the variation in the NDVI was explained by either precipitation or temperature.     

中国东北夏季温度年代际变化特征

东北地区夏季温度变化主要有4个独立模态,其中EOF1呈全区一致型为第一模态,占总方差贡献的72%左右,其时间系数呈现多年代际尺度变化;其他3个模态仅分别占总方差贡献的10%、4%和3%,表现出受海洋、地形及纬度差异的影响,呈南北向、东西向和山脉与平原地区的反向异常分布型;突变和小波分析进一步揭示EOF1模态的多年代际尺度变化具有突变性质,2007~2010年受20 a左右周期影响,东北地区温度变化全区一致的模态时间系数处于负值为主的阶段,值得关注。东北地区夏季温度变EOF1模态与北半球500 hPa高度场和对流层中上层纬向风场(西风急流)在30oN~40oN呈环球尺度带状显著正相关分布,在东北亚地区纬度较高,可达60oN附近,而且该区500 hPa高度场与东北夏季温度EOF1模态多年代际变化近于同步。EOF1模态与靠近亚洲大陆沿岸和西太平洋暖池(包括印度洋的中部和北部)、南太平洋环澳大利亚、靠近北美大陆及北大西洋北部的漂流区和北赤道漂流区的海表温度存在显著的同步正相关,而与太平洋海表温度的年代际振荡模态亦有较显著的正相关;表明东北地区夏季温度的EOF1模态与北半球海-气系统多年代际变化联系密切,同时也受到全球变暖的影响。     

哈萨克斯坦东北部310年来初夏温度变化的树轮记录

在哈萨克斯坦东北部的阿尔泰山南坡,位于森林上限的西伯利亚落叶松的树轮宽度对生长季初期温度敏感,且在近年来气候变暖的背景下对温度的响应较为稳定,可以作为该区域温度变化的良好替代材料.利用森林上限区的树轮资料,从树轮宽度中提取初夏温度信息,建立了树轮宽度年表与卡通卡拉盖气象站6月平均温度的转换方程,重建了这一区域310年来的初夏温度变化历史,重建方程的方差解释量达到42.7％.由于校准期较长,利用独立检验方法对重建方程进行检验,各项检验的参数表明重建方程是稳定可靠的.重建序列与相邻的中国阿勒泰地区西部和阿尔泰山北坡树轮反映的温度变化序列的冷暖阶段是一致的,其中19世纪的温度波动较为明显,持续时间最长的冷期(1842-1871年)和暖期(1872-1906年)都出现在这个阶段.重建温度序列存在11a左右的周期,与太阳活动的周期一致.     

1960—2015年山西省“数九”时节气温时空变化特征

基于山西省24个气象站点1960—2016年逐日最高、最低、平均气温数据,运用线性倾向估计法、Mann-Kendall非参数检验法、Morlet小波分析等方法,分析了山西省近56 a"数九"时节气温的时空变化特征。结果表明:(1)近56a山西"数九"时节气温均呈上升趋势,其中平均最高气温上升趋势最大,约为0.5℃·(10 a)^-1。就平均气温变化而言,"五九"变化趋势最大,约为0.703℃·(10 a)^-1,就分布而言,"四九"值最低,"五九"开始回暖,"六九"回暖幅度最大。20世纪90年代前后各"九"各项平均气温对比发现,最低值延后,回暖时间提前,"数九"冷期缩短。(2)"数九"时节气温高值区主要集中在晋南山间盆地、河谷区,以运城盆地最为明显,低值区主要分布于以晋东北五台山为中心的山地丘陵区,增温幅度分布情况大致与此相反。(3)在1990年左右气温均出现暖突变,平均最高气温突变前后变化最大,约为1.73℃。(4)气温周期变化明显,均存在4~8 a短时间尺度周期。本研究填补山西省对"数九"时节气温变化研究的空白,为山西省气候资源评价和农业生产布局提供科学依据。     

类型变更的相邻气象观测站的日气温资料整合

利用日气温资料,分析了因气象观测站类型变更而致的气候资料连续性和均一性问题及其影响,提出了订正相邻观测站日气温并将因观测站类型变更而致的相邻站不连续日气温资料整合的方法。主要结论有:(1)气象观测站类型变更而致的气候数据不连续既影响资料的均一性,也影响气候变化研究的结果。在考虑或不考虑测站类型变更(仅1980年以后)时,对我国过去60年1月气温趋势变化的估计结果差别达6.0%。(2)虽然海拔差异明显影响相邻站气温,但城乡差别等下垫面及其周边环境差异因素的作用极为显著;可导致两个地理位置相近的测站最大月气温差别超过0.50℃。(3)利用相邻测站的月气温差异进行各月的日气温订正可以消除海拔、台站下垫面及其周边环境差异对观测资料的均一性影响;使订正后的序列能更好地反映出气候的年际变化特征;从而可为我国正在开展的气候区划新方案、气候变化对区划影响及冷暖期环境格局变化等研究工作提供更均一的气候观测基础资料。     

The use of high-resolution gridded climate data in the development of chironomid-based inference models from remote areas

The development of chironomid-based air temperature inference models in high latitude regions often relies on limited spatial coverage of meteorological data and/or on punctual measurements of water temperature at the time of sampling. The use of simple linear regression to relate air temperature and latitude was until recently the best method to characterize the air temperature gradient along a latitudinal gradient. However, recent studies have used high-resolution gridded climate data to develop new chironomid-based air temperature inference models. This innovative approach has, however, never been further analyzed to test its reliability. This study presents a method using ArcGIS^® to extract air temperatures from a high-resolution global gridded climate data set (New et al. 2002) and to incorporate these new data in a variety of chironomid-based air temperature inference models to test their performance. Results suggest that this method is reliable and produces better estimates of air temperature and will be helpful in the development of further quantitative air temperature inference models in remote areas.     

Deciphering chrysophyte responses to climate seasonality

Climate change involves alterations in seasonality as well as shifts in mean annual temperature. Cold temperate lakes show strong seasonality, with winter ice cover and alternating mixing and stratification periods during the ice-free season. These physical changes are ultimately related to seasonal weather variation and also drive annual phytoplankton succession. Therefore, phytoplankton remains in lake sediment records are potentially useful for reconstructing past seasonal climate signals. With the exception of investigations on varved sediments, however, little research has been carried out on the subject. Here we present two lines of evidence demonstrating that chrysophyte stomatocysts can be useful for inferring past climatic seasonality. First, we show that marked seasonal stomatocyst replacement is related to periods of the main physical processes in the lake. Second, using instrumental climate data and microfossils in sediment cores, we show that two main components of stomatocyst variability over the last ~150 years are related to seasonal fluctuations. The first of these components is related to stomatocysts present during summer stratification that respond to general warming trends, particularly in July and August. The second component relates to stomatocysts typically present in the spring and autumn mixing periods. Numbers of these two stomatocyst types vary inversely in response to variation in spring (April–May) temperatures. The number of stomatocyst types affected by spring temperatures is much greater than those related to summer temperatures. We provide evidence that chrysophyte stomatocysts are an excellent proxy for spring air temperature reconstructions, with little influence from summer or general annual trends. We also show that with relatively little effort, it is possible to categorise the chrysophyte stomatocyst assemblage of a lake to carry out detailed investigation of the record of seasonal changes preserved within the sediment.