Search

Description, Subpolar Atlantic dynamics drive Arctic climate at the onset of the 20th century., The recent expansion of Atlantic waters into the Arctic Ocean represents undisputable evidence of the rapid changes occurring in this region. Understanding the past variability of this “Atlantification” is thus crucial in providing a longer perspective on the modern Arctic changes. Here, we reconstruct the history of Atlantification along the eastern Fram Strait during the past 800 years using precisely dated paleoceanographic records based on organic biomarkers and benthic foraminiferal data. Our results show rapid changes in water mass properties that commenced in the early 20th century—several decades before the documented Atlantification by instrumental records. Comparison with regional records suggests a poleward expansion of subtropical waters since the end of the Little Ice Age in response to a rapid hydrographic reorganization in the North Atlantic. Understanding of this mechanism will require further investigations using climate model simulations.

Modes of climate variability affect global and regional climates on different spatio-temporal scales, and they have important impacts on human activities and ecosystems. As these modes are a useful tool for simplifying the understanding of the climate system, it is crucial that we gain improved knowledge of their long-term past evolution and interactions over time to contextualise their present and future behaviour. We review the literature focused on proxy-based reconstructions of modes of climate variability during the Holocene (i.e., the last 11.7 thousand years) with a special emphasis on i) proxy-based reconstruction methods; ii) available proxy-based reconstructions of the main modes of variability, i.e., El Nino Southern Oscillation, Pacific Decadal Variability, Atlantic Multidecadal Variability, the North Atlantic Oscillation, the Southern Annular Mode and the Indian Ocean Dipole; iii) major interactions between these modes; and iv) external forcing mechanisms related to the evolution of these modes. This review shows that modes of variability can be reconstructed using proxy-based records from a wide range of natural archives, but these reconstructions are scarce beyond the last millennium, partly due to the lack of robust chronologies with reduced dating uncertainties, technical issues related to proxy calibration, and difficulty elucidating their stationary impact (or not) on regional climates over time. While for each mode the available reconstructions tend to agree at mutidecadal timescales, they show notable disagreement on shorter timescales beyond the instrumental period. The reviewed evidence suggests that the intrinsic variability of modes can be modulated by external forcing, such as orbital, solar, volcanic, and anthropogenic forcing. The review also highlights some modes experience higher variability over the instrumental period, which is partly ascribed to anthropogenic forcing. These features stress the paramount importance of further studying their past variations using long climate-proxy records for the progress of climate science., A.H. is supported by a Beatriu de Pinos -Marie Curie Cofund programme fellowship (2016 BP 00023) and HOLMODRIVE -North Atlantic Atmospheric Patterns influence on Western Iberia Climate: From the Lateglacial to the Present (PTDC/CTA-GEO/29029/2017) project funded by the Fundacao para a Ciencia e a Tecnologia, Portugal (FCT). A.H., S.G., and S.P-R. thank the Spanish research project PaleoModes (CGL2016-75281-C2-1-R) which provided some of their financial support. E.M.-C. contribution was funded by the project PARAMOUR (30454083) from the EOS program by the F.R.SFNRS. CMP is supported by the Royal Society (ref: DH150185). P.O. contribution has been supported by the Ramon y Cajal senior tenure programme from the Spanish Ministry of Economy and Competitiveness. L.C-B. acknowledges support from the ERC-funded project GC2.0 (Global Change 2.0: Unlocking the past for a clearer future, grant number 694481). The Spanish PTI PolarCSIC (https://www.polarcsic.es) is also acknowledged for providing partial financial support to S.G. R.M. was partially supported by the Swedish Research Council (grant DNR20138421). A. P.'s work was supported by a Science Foundation Ireland Career Development Award (17/CDA/4695), a research centre award (12/RC/2289_P2), an investigator award (16/IA/4520), and a Marine Research Programme funded by the Irish Government, cofinanced by the European Regional Development Fund (Grant-Aid Agreement No. PBA/CC/18/01). A.A.S. was supported by the Met Office Hadley Centre Climate Programme funded by BEIS and Defra. J.S. is supported by the strategic research program of Modelling the Regional and Global Earth system (MERGE) hosted by the Faculty of Science at Lund University. D.S. is supported by Blue-Action (European Union's Horizon 2020 research and innovation program, Grant Number: 727852) and EUCP (European Union's Horizon 2020 research and innovation programme under grant agreement no 776613) projects as well as by the French national program LEFE/INSU with VADEMECUM project. G. X. thanks the funding from the Chinese Scholarship Council (201704910171). This study is partly based on discussions held during the joint workshop of the CLIMOVAR group and IBCC-lo2k project, Barcelona, 25-27 September 2013. PAGES and IBCC-lo2k are thanked for supporting this workshop.

Modes of climate variability affect global and regional climates on different spatio-temporal scales, and they have important impacts on human activities and ecosystems. As these modes are a useful tool for simplifying the understanding of the climate system, it is crucial that we gain improved knowledge of their long-term past evolution and interactions over time to contextualise their present and future behaviour. We review the literature focused on proxy-based reconstructions of modes of climate variability during the Holocene (i.e., the last 11.7 thousand years) with a special emphasis on i) proxy-based reconstruction methods; ii) available proxy-based reconstructions of the main modes of variability, i.e., El Nino Southern Oscillation, Pacific Decadal Variability, Atlantic Multidecadal Variability, the North Atlantic Oscillation, the Southern Annular Mode and the Indian Ocean Dipole; iii) major interactions between these modes; and iv) external forcing mechanisms related to the evolution of these modes. This review shows that modes of variability can be reconstructed using proxy-based records from a wide range of natural archives, but these reconstructions are scarce beyond the last millennium, partly due to the lack of robust chronologies with reduced dating uncertainties, technical issues related to proxy calibration, and difficulty elucidating their stationary impact (or not) on regional climates over time. While for each mode the available reconstructions tend to agree at mutidecadal timescales, they show notable disagreement on shorter timescales beyond the instrumental period. The reviewed evidence suggests that the intrinsic variability of modes can be modulated by external forcing, such as orbital, solar, volcanic, and anthropogenic forcing. The review also highlights some modes experience higher variability over the instrumental period, which is partly ascribed to anthropogenic forcing. These features stress the paramount importance of further studying their past variations using long climate-proxy records for the progress of climate science. A.H. is supported by a Beatriu de Pinos -Marie Curie Cofund programme fellowship (2016 BP 00023) and HOLMODRIVE -North Atlantic Atmospheric Patterns influence on Western Iberia Climate: From the Lateglacial to the Present (PTDC/CTA-GEO/29029/2017) project funded by the Fundacao para a Ciencia e a Tecnologia, Portugal (FCT). A.H., S.G., and S.P-R. thank the Spanish research project PaleoModes (CGL2016-75281-C2-1-R) which provided some of their financial support. E.M.-C. contribution was funded by the project PARAMOUR (30454083) from the EOS program by the F.R.SFNRS. CMP is supported by the Royal Society (ref: DH150185). P.O. contribution has been supported by the Ramon y Cajal senior tenure programme from the Spanish Ministry of Economy and Competitiveness. L.C-B. acknowledges support from the ERC-funded project GC2.0 (Global Change 2.0: Unlocking the past for a clearer future, grant number 694481). The Spanish PTI PolarCSIC (https://www.polarcsic.es) is also acknowledged for providing partial financial support to S.G. R.M. was partially supported by the Swedish Research Council (grant DNR20138421). A. P.'s work was supported by a Science Foundation Ireland Career Development Award (17/CDA/4695), a research centre award (12/RC/2289_P2), an investigator award (16/IA/4520), and a Marine Research Programme funded by the Irish Government, cofinanced by the European Regional Development Fund (Grant-Aid Agreement No. PBA/CC/18/01). A.A.S. was supported by the Met Office Hadley Centre Climate Programme funded by BEIS and Defra. J.S. is supported by the strategic research program of Modelling the Regional and Global Earth system (MERGE) hosted by the Faculty of Science at Lund University. D.S. is supported by Blue-Action (European Union's Horizon 2020 research and innovation program, Grant Number: 727852) and EUCP (European Union's Horizon 2020 research and innovation programme under grant agreement no 776613) projects as well as by the French national program LEFE/INSU with VADEMECUM project. G. X. thanks the funding from the Chinese Scholarship Council (201704910171). This study is partly based on discussions held during the joint workshop of the CLIMOVAR group and IBCC-lo2k project, Barcelona, 25-27 September 2013. PAGES and IBCC-lo2k are thanked for supporting this workshop.

© 2019 Elsevier B.V. We propose a new forecasting procedure which particularly explores opportunities for improving the precision of medium and long-term forecasts of the Niño3.4 time series that is linked with the well-known El Niño phenomenon. This important climatic time series is subject to an intricate dynamic structure and is interrelated to other climatological variables. The procedure consists of three steps. First, a univariate time series model is considered for producing prediction errors. Second, signal paths of the prediction errors are simulated via a dynamic factor model for the errors and explanatory variables. From these simulated errors, ensemble time series for Niño3.4 are constructed. Third, forecasts are generated from the ensemble time series and their sample average is our final forecast. As part of these dynamic factor simulations, we also obtain the forecast of the El Niño event which is a categorical variable. We present empirical evidence that our procedure can be superior in its forecasting performance when compared to other econometric forecasting methods.

D.E. and H.P.A. acknowledge support from Yale University and the Yale Analytical and Stable Isotope Center and from Grant 171/16 of the Israel Science Foundation. W.R. and L.J.C. were supported by NWO Grant ALW 822 01 009. The Tanzania Commission for Science and Technology (COSTECH) and the Tanzania Petroleum Development Corporation supported the Tanzania Drilling Project (TDP) which recovered the Tanzanian specimens. Laser-ablation ICPMS work at RHUL was partly funded by a 2014 Natural Environment Research Council (NERC) Capital Equipment Grant (Ref. CC073). The Research Foundation Flanders is acknowledged for financial support (to P.S.). M.Z. acknowledges support from the Netherlands Earth System Science Center and Horizon 2020 Grant MSCA-IF-2014 655073. Past greenhouse periods with elevated atmospheric CO2 were characterized by globally warmer sea-surface temperatures (SST). However, the extent to which the high latitudes warmed to a greater degree than the tropics (polar amplification) remains poorly constrained, in particular because there are only a few temperature reconstructions from the tropics. Consequently, the relationship between increased CO2, the degree of tropical warming, and the resulting latitudinal SST gradient is not well known. Here, we present coupled clumped isotope (Δ47)-Mg/Ca measurements of foraminifera from a set of globally distributed sites in the tropics and midlatitudes. Δ47 is insensitive to seawater chemistry and therefore provides a robust constraint on tropical SST. Crucially, coupling these data with Mg/Ca measurements allows the precise reconstruction of Mg/Casw throughout the Eocene, enabling the reinterpretation of all planktonic foraminifera Mg/Ca data. The combined dataset constrains the range in Eocene tropical SST to 30–36 °C (from sites in all basins). We compare these accurate tropical SST to deep-ocean temperatures, serving as a minimum constraint on high-latitude SST. This results in a robust conservative reconstruction of the early Eocene latitudinal gradient, which was reduced by at least 32 ± 10% compared with present day, demonstrating greater polar amplification than captured by most climate models. Postprint

It is well established that greenhouse conditions prevailed during the Cretaceous Period (~ 145–66 Ma). Determining the exact nature of the greenhouse-gas forcing, climatic warming and climate sensitivity remains, however, an active topic of research. Quantitative and qualitative geochemical and palaeontological proxies provide valuable observational constraints on Cretaceous climate. In particular, reconstructions of Cretaceous sea-surface temperatures (SSTs) have been revolutionised firstly by the recognition that clay-rich sequences can host exceptionally preserved planktonic foraminifera allowing for reliable oxygen-isotope analyses and, secondly by the development of the organic palaeothermometer TEX86, based on the distribution of marine archaeal membrane lipids. Here we provide a new compilation and synthesis of available planktonic foraminiferal δ18O (δ18Opl) and TEX86-SST proxy data for almost the entire Cretaceous Period. The compilation uses SSTs recalculated from published raw data, allowing examination of the sensitivity of each proxy to the calculation method (e.g., choice of calibration) and places all data on a common timescale. Overall, the compilation shows many similarities with trends present in individual records of Cretaceous climate change. For example, both SST proxies and benthic foraminiferal δ18O records indicate maximum warmth in the Cenomanian–Turonian interval. Our reconstruction of the evolution of latitudinal temperature gradients (low, ±48°, palaeolatitudes) reveals temporal changes. In the Valanginian–Aptian, the low-to-higher mid-latitudinal temperature gradient was weak (decreasing from ~ 10–17 °C in the Valanginian, to ~ 3–5 °C in the Aptian, based on TEX86-SSTs). In the Cenomanian–Santonian, reconstructed latitudinal temperature contrasts are also small relative to modern (< 14 °C, based on low-latitude TEX86 and δ18Opl SSTs minus higher latitude δ18Opl SSTs, compared with ~ 20 °C for the modern). In the mid-Campanian to end-Maastrichtian, latitudinal temperature gradients strengthened (~ 19–21 °C, based on low-latitude TEX86 and δ18Opl SSTs minus higher latitude δ18Opl SSTs), with cooling occurring at low-, middle- and higher palaeolatitude sites, implying global surface-ocean cooling and/or changes in ocean heat transport in the Late Cretaceous. These reconstructed long-term trends are resilient, regardless of the choice of proxy (TEX86 or δ18Opl) or calibration. This new Cretaceous SST synthesis provides an up-to-date target for modelling studies investigating the mechanics of extreme climates.