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11.
A portion of the debate about climate reconstructions of the past millennium, and in particular about the well-known Mann-Bradley-Hughes
(“MBH” 1998, 1999) reconstructions, has become disconnected from the goal of understanding natural climate variability. Here,
we reflect on what can be learned from recent scientific exchanges and identify important challenges that remain to be addressed
openly and productively by the community. One challenge arises from the real, underlying trend in temperatures during the
instrumental period. This trend can affect regression-based reconstruction performance in cases where the calibration period
does not appropriately cover the range of conditions encountered during the reconstruction. However, because it is tied to
a unique spatial pattern driven by change in radiative balance, the trend cannot simply be removed in the method of climate
field reconstruction used by MBH on the statistical argument of preserving degrees of freedom. More appropriately, the influence
from the trend can be taken into account in some methods of significance testing. We illustrate these considerations as they
apply to the MBH reconstruction and show that it remains robust back to AD 1450, and given other empirical information also
back to AD 1000. However, there is now a need to move beyond hemispheric average temperatures and to focus instead on resolving
climate variability at the socially more relevant regional scale. 相似文献
12.
The Mann et al. (1998) Northern Hemisphere annual temperature reconstruction over 1400–1980 is examined in light of recent
criticisms concerning the nature and processing of included climate proxy data. A systematic sequence of analyses is presented
that examine issues concerning the proxy evidence, utilizing both indirect analyses via exclusion of proxies and processing
steps subject to criticism, and direct analyses of principal component (PC) processing methods in question. Altogether new
reconstructions over 1400–1980 are developed in both the indirect and direct analyses, which demonstrate that the Mann et
al. reconstruction is robust against the proxy-based criticisms addressed. In particular, reconstructed hemispheric temperatures
are demonstrated to be largely unaffected by the use or non-use of PCs to summarize proxy evidence from the data-rich North
American region. When proxy PCs are employed, neither the time period used to “center” the data before PC calculation nor
the way the PC calculations are performed significantly affects the results, as long as the full extent of the climate information
actually in the proxy data is represented by the PC time series. Clear convergence of the resulting climate reconstructions
is a strong indicator for achieving this criterion. Also, recent “corrections” to the Mann et al. reconstruction that suggest
15th century temperatures could have been as high as those of the late-20th century are shown to be without statistical and
climatological merit. Our examination does suggest that a slight modification to the original Mann et al. reconstruction is
justifiable for the first half of the 15th century (∼+0.05∘), which leaves entirely unaltered the primary conclusion of Mann et al. (as well as many other reconstructions) that both
the 20th century upward trend and high late-20th century hemispheric surface temperatures are anomalous over at least the
last 600 years. Our results are also used to evaluate the separate criticism of reduced amplitude in the Mann et al. reconstructions
over significant portions of 1400–1900, in relation to some other climate reconstructions and model-based examinations. We
find that, from the perspective of the proxy data themselves, such losses probably exist, but they may be smaller than those
reported in other recent work.
The National Center for Atmospheric Research is sponsored by the National Science Foundation, USA.
The authors contributed equally to the development of the research presented. 相似文献
13.
Simon A. Hunt Alex Lindsay-Scott Ian G. Wood Michael W. Ammann Takashi Taniguchi 《Physics and Chemistry of Minerals》2013,40(1):73-80
Orthorhombic post-perovskite CaPtO3 is isostructural with post-perovskite MgSiO3, a deep-Earth phase stable only above 100 GPa. Energy-dispersive X-ray diffraction data (to 9.4 GPa and 1,024 K) for CaPtO3 have been combined with published isothermal and isobaric measurements to determine its P–V–T equation of state (EoS). A third-order Birch–Murnaghan EoS was used, with the volumetric thermal expansion coefficient (at atmospheric pressure) represented by α(T) = α0 + α1(T). The fitted parameters had values: isothermal incompressibility, $ K_{{T_{0} }} $ = 168.4(3) GPa; $ K_{{T_{0} }}^{\prime } $ = 4.48(3) (both at 298 K); $ \partial K_{{T_{0} }} /\partial T $ = ?0.032(3) GPa K?1; α0 = 2.32(2) × 10?5 K?1; α1 = 5.7(4) × 10?9 K?2. The volumetric isothermal Anderson–Grüneisen parameter, δ T , is 7.6(7) at 298 K. $ \partial K_{{T_{0} }} /\partial T $ for CaPtO3 is similar to that recently reported for CaIrO3, differing significantly from values found at high pressure for MgSiO3 post-perovskite (?0.0085(11) to ?0.024 GPa K?1). We also report axial P–V–T EoS of similar form, the first for any post-perovskite. Fitted to the cubes of the axes, these gave $ \partial K_{{aT_{0} }} /\partial T $ = ?0.038(4) GPa K?1; $ \partial K_{{bT_{0} }} /\partial T $ = ?0.021(2) GPa K?1; $ \partial K_{{cT_{0} }} /\partial T $ = ?0.026(5) GPa K?1, with δ T = 8.9(9), 7.4(7) and 4.6(9) for a, b and c, respectively. Although $ K_{{T_{0} }} $ is lowest for the b-axis, its incompressibility is the least temperature dependent. 相似文献
14.
The effect of pressure on ionic diffusion in orthorhombic MgSiO3 perovskite has been investigated using density functional theory. An intensive investigation of possible silicon pathways
revealed new positions of the saddle-points and an enthalpy of migration at 26.2 GPa of 4.7 eV that is in fair agreement with
the experimental values of about 3.5 eV at 25 GPa. This is much lower than found in previous studies (~9 eV) and removes the
need to explain silicon diffusion by a complicated process involving coupled oxygen vacancies, as has been previously proposed.
Our migration enthalpies for oxygen and magnesium are in excellent agreement with experiments. We find that oxygen diffusion
occurs via a chain of several inequivalent jumps along the octahedron edges, and that magnesium occurs via two inequivalent
[110] jumps and one [001] jump. We also present activation volumes for all three species at 25 and 135 GPa. 相似文献
15.
K.E. Fishbaugh C.S. Hvidberg S. Clifford A. Haldemann J.W. Head M. Koutnik W.J. Ammann 《Icarus》2008,196(2):305-317
As an introduction to this Icarus special issue for the 4th Mars Polar Science and Exploration Conference, we discuss five key questions in Mars polar science, gleaned from plenary discussions and presentations held at the conference. These questions highlight major unknowns in the field. (1) What are the physical characteristics of the polar layered deposits (PLD), and how are the different geologic units within, beneath, and surrounding the PLD related? (2) How old are the PLD? And what are their glacial, fluvial, depositional and erosional histories? (3) What are the mass and energy budgets of the PLD, and what processes control these budgets on seasonal and longer timescales? (4) What chronology, compositional variability, and record of climatic change is expressed in the stratigraphy of the PLD? (5) How have volatiles and dust been exchanged between polar and non-polar reservoirs? And how has this exchange affected the past and present distribution of surface and subsurface ice? 相似文献
16.
Brigitta Ammann 《Journal of Paleolimnology》1988,1(4):269-277
In the sediment of three Swiss lakes at a range of altitude from 514 to 2017 m, the Bölling and the Preboreal are recognized as two periods of rapid biotic changes. The main reason is rapid climatic change that triggered shifts in different groups of aquatic and terrestrial organisms. Although these groups would be expected to have very different response times, e.g., to increasing summer temperature, their assemblages responded with surprizing synchroneity. For extracting climatic signals from stratigraphies, the quickly responsive indicators like oxygen isotopes or beetles are useful. For understanding ecological dynamics under a changing climate, the comparison of biota with various response time are important.This is the sixth of a series of papers to be published by this journal that was presented in the paleolimnology sessions organized by R. B. Davis and H. Löffler for the XIIth Congress of the International Union for Quaternary Research (INQUA), which took place in Ottawa, Canada in August 1987. Drs. Davis and Löffler are serving as guest editors of this series. 相似文献
17.
18.
准噶尔盆地南缘新生代构造特征及其与砂岩型铀矿成矿作用初析 总被引：5，自引：0，他引：5
准噶尔盆地南缘盆山结合部中新生界沉积巨厚,新生代变形强烈,是研究新盆山耦合的理想场所,也是我国砂岩型铀矿找矿的远景区段。本文在综合前人资料和野外观测分析基础上,根据新生代构造活动特征,将准噶尔盆地南缘划分为博格达山前和西部断褶带两个构造分区,博格达山前以强烈的逆冲推覆为特征,发育多条活动的逆冲推覆断裂; 乌鲁木齐以西至乌苏南的西部断褶带则发育三-四排的褶皱-逆冲断裂构造带。对采自博格达山前逆冲推覆断裂带内的方解石和断层泥,利用电子自旋共振测年手段,推测博格达山前的富康-吉木萨尔断裂带和北三台断裂带分别在0.7~1Ma和0.25Ma期间,经历了一期重要的逆冲推覆作用。结合盆地南缘砂岩型铀矿的展布规律及其成矿条件的分析,探讨了新生代构造运动对砂岩型铀矿成矿的控制作用,认为西部断褶带的第一排构造带具有较好的成矿前景,而博格达山前由于新生代构造活动强烈而相对成矿不利,为此提出了准噶尔盆地南缘砂岩型铀矿成矿“构造优先权”的构造控矿模式,进而指出了区域找矿的优选区段。 相似文献
19.
N. E. Graham C. M. Ammann D. Fleitmann K. M. Cobb J. Luterbacher 《Climate Dynamics》2011,37(5-6):1217-1245
Widely distributed proxy records indicate that the Medieval Climate Anomaly (MCA; ~900–1350 AD) was characterized by coherent shifts in large-scale Northern Hemisphere atmospheric circulation patterns. Although cooler sea surface temperatures in the central and eastern equatorial Pacific can explain some aspects of medieval circulation changes, they are not sufficient to account for other notable features, including widespread aridity through the Eurasian sub-tropics, stronger winter westerlies across the North Atlantic and Western Europe, and shifts in monsoon rainfall patterns across Africa and South Asia. We present results from a full-physics coupled climate model showing that a slight warming of the tropical Indian and western Pacific Oceans relative to the other tropical ocean basins can induce a broad range of the medieval circulation and climate changes indicated by proxy data, including many of those not explained by a cooler tropical Pacific alone. Important aspects of the results resemble those from previous simulations examining the climatic response to the rapid Indian Ocean warming during the late twentieth century, and to results from climate warming simulations—especially in indicating an expansion of the Northern Hemisphere Hadley circulation. Notably, the pattern of tropical Indo-Pacific sea surface temperature (SST) change responsible for producing the proxy-model similarity in our results agrees well with MCA-LIA SST differences obtained in a recent proxy-based climate field reconstruction. Though much remains unclear, our results indicate that the MCA was characterized by an enhanced zonal Indo-Pacific SST gradient with resulting changes in Northern Hemisphere tropical and extra-tropical circulation patterns and hydroclimate regimes, linkages that may explain the coherent regional climate shifts indicated by proxy records from across the planet. The findings provide new perspectives on the nature and possible causes of the MCA—a remarkable, yet incompletely understood episode of Late Holocene climatic change. 相似文献
20.
人为强迫和自然强迫对近年来对流层顶高度变化的贡献 总被引：1，自引：0，他引：1
杨双艳 《沙漠与绿洲气象(新疆气象)》2009,3(5):58-60
观测表明,自1979年以来对流层顶（平流层与对流层之间的界限）高度增加了数百米。在气候模式实验中,等量的增加也是显而易见的。气候模式实验表明,人为因素导致臭氧和均匀混合温室气体的变化,对1979-1999年对流层顶高度增加的影响占所有因素的80％。臭氧对平流层的冷却作用和均匀混合的温室气体对对流层的加热作用,使对流层顶高度增加。 相似文献