Modelling the climate of the last millennium: what causes the differences between simulations?

An ensemble of simulations performed with a coarse resolution 3-D climate model driven by various combinations of external forcing is used to investigate possible causes for differences noticed in two recent simulations of the climate of the past millennium using General Circulation Models (GCMs). Our results strongly suggest that differences in sensitivity (equilibrium and transient climate response) could be responsible for temperature changes that differ by more than a factor of two between two models. In addition, the spin-up procedure could explain some differences between the simulations during the first centuries of the second millennium. The choice of the forcing reconstruction is found to play a smaller role for the differences in the simulated climate, in the model configurations analyzed here. Furthermore, at decadal scale, internal climate variability can mask the differences associated with different forcing reconstructrions. Copyright 2005 by the American Geophysical Union

Continuous Quantum Measurement and the Quantum to Classical Transition

While ultimately they are described by quantum mechanics, macroscopic mechanical systems are nevertheless observed to follow the trajectories predicted by classical mechanics. Hence, in the regime defining macroscopic physics, the trajectories of the correct classical motion must emerge from quantum mechanics, a process referred to as the quantum to classical transition. Extending previous work [Bhattacharya, Habib, and Jacobs, Phys. Rev. Lett. {\bf 85}, 4852 (2000)], here we elucidate this transition in some detail, showing that once the measurement processes which affect all macroscopic systems are taken into account, quantum mechanics indeed predicts the emergence of classical motion. We derive inequalities that describe the parameter regime in which classical motion is obtained, and provide numerical examples. We also demonstrate two further important properties of the classical limit. First, that multiple observers all agree on the motion of an object, and second, that classical statistical inference may be used to correctly track the classical motion.Comment: 12 pages, 4 figures, Revtex

The delta-function-kicked rotor: Momentum diffusion and the quantum-classical boundary

We investigate the quantum-classical transition in the delta-kicked rotor and the attainment of the classical limit in terms of measurement-induced state-localization. It is possible to study the transition by fixing the environmentally induced disturbance at a sufficiently small value, and examining the dynamics as the system is made more macroscopic. When the system action is relatively small, the dynamics is quantum mechanical and when the system action is sufficiently large there is a transition to classical behavior. The dynamics of the rotor in the region of transition, characterized by the late-time momentum diffusion coefficient, can be strikingly different from both the purely quantum and classical results. Remarkably, the early time diffusive behavior of the quantum system, even when different from its classical counterpart, is stabilized by the continuous measurement process. This shows that such measurements can succeed in extracting essentially quantum effects. The transition regime studied in this paper is accessible in ongoing experiments.Comment: 8 pages, 4 figures, revtex4 (revised version contains much more introductory material

Experimental study of the quantum driven pendulum and its classical analogue in atoms optics

We present experimental results for the dynamics of cold atoms in a far detuned amplitude-modulated optical standing wave. Phase-space resonances constitute distinct peaks in the atomic momentum distribution containing up to 65% of all atoms resulting from a mixed quantum chaotic phase space. We characterize the atomic behavior in classical and quantum regimes and we present the applicable quantum and classical theory, which we have developed and refined. We show experimental proof that the size and the position of the resonances in phase space can be controlled by varying several parameters, such as the modulation frequency, the scaled well depth, the modulation amplitude, and the scaled Planck's constant of the system. We have found a surprising stability against amplitude noise. We present methods to accurately control the momentum of an ensemble of atoms using these phase-space resonances which could be used for efficient phase-space state preparation

Testing the Effect of Relative Pollen Productivity on the REVEALS Model : A Validated Reconstruction of Europe-Wide Holocene Vegetation

Reliable quantitative vegetation reconstructions for Europe during the Holocene are crucial to improving our understanding of landscape dynamics, making it possible to assess the past effects of environmental variables and land-use change on ecosystems and biodiversity, and mitigating their effects in the future. We present here the most spatially extensive and temporally continuous pollen-based reconstructions of plant cover in Europe (at a spatial resolution of 1° × 1°) over the Holocene (last 11.7 ka BP) using the 'Regional Estimates of VEgetation Abundance from Large Sites' (REVEALS) model. This study has three main aims. First, to present the most accurate and reliable generation of REVEALS reconstructions across Europe so far. This has been achieved by including a larger number of pollen records compared to former analyses, in particular from the Mediterranean area. Second, to discuss methodological issues in the quantification of past land cover by using alternative datasets of relative pollen productivities (RPPs), one of the key input parameters of REVEALS, to test model sensitivity. Finally, to validate our reconstructions with the global forest change dataset. The results suggest that the RPPs.st1 (31 taxa) dataset is best suited to producing regional vegetation cover estimates for Europe. These reconstructions offer a long-term perspective providing unique possibilities to explore spatial-temporal changes in past land cover and biodiversity

Statistical distributions of ice core sulfate from climatically relevant volcanic eruptions

International audienceQuantitative knowledge of external climate forcing is required for accurately attributing past climatic changes. Information on volcanic activity over the past millennium has primarily been drawn from high-latitude ice cores. A few large events with distinct signatures in the ice are well known and they are commonly used as marker events to synchronize time scales in individual ice cores. Over the past decade different efforts have been undertaken to systematically identify lesser known eruptions and to develop time series of past volcanic forcing. Here we mathematically quantify the distribution of the magnitude of volcanic events that have a climatic relevance during the past millennium. Volcanic sulfate magnitudes of such events clearly exhibit a ''heavy tailed'' extreme value distribution. Indeed, the climatically relevant eruptions are only the extremes of global volcanic activity. This characterization of volcanic amplitude is a fundamental step in detection and attribution studies of past natural forcing and of its effects on climate

Evidence for global climate reorganization during medieval times

A synthesis of global climate model results and inferences from proxy records suggests an increased sea surface temperature gradient between the tropical Indian and Pacific Oceans during medieval times

Medieval climate anomaly to little ice age transition as simulated by current climate models

Inter-model differences and model/reconstruction comparisons suggest that simulations of the Medieval Climate Anomaly either fail to reproduce the mechanisms of climate response to changes in external forcing, or that anomalies during this period are largely influenced by internal variability.Peer reviewe

Perinatal transmission of human immunodeficiency virus type 1 by pregnant women with RNA virus loads <1000 copies/mL

In a collaboration of 7 European and United States prospective studies, 44 cases of vertical human immunodeficiency virus type 1 (HIV-1) transmission were identified among 1202 women with RNA virus loads <1000 copies/mL at delivery or at the measurement closest to delivery. For mothers receiving antiretroviral treatment during pregnancy or at the time of delivery (or both), there was a 1.0% transmission rate (8 of 834; 95% confidence interval [CI], 0.4%–1.9%), compared with 9.8% (36 of 368; 95% CI, 7.0%–13.4%) for untreated mothers (risk ratio, 0.10; 95% CI, 0.05–0.21). In multivariate analysis adjusting for study, transmission was lower with antiretroviral treatment (odds ratio [OR], 0.10; P < .001), cesarean section (OR, 0.30; P = .022), greater birth weight (P = .003), and higher CD4 cell count (P = .039). In 12 of 44 cases, multiple RNA measurements were obtained during pregnancy or at the time of delivery or within 4 months after giving birth; in 10 of the 12 cases, the geometric mean virus load was >500 copies/mL. Perinatal HIV-1 transmission occurs in only 1% of treated women with RNA virus loads <1000 copies/mL and may be almost eliminated with antiretroviral prophylaxis accompanied by suppression of maternal viremia