161 research outputs found
Southern Hemisphere high-resolution palaeoclimate records of the last 2000 years
No full textThis study presents a comprehensive assessment of high-resolution Southern Hemisphere (SH) paleoarchives covering the last 2000 years. We identified 174 monthly to annually resolved climate proxy (tree ring, coral, ice core, documentary, speleothem and sedimentary) records from the Hemisphere. We assess the interannual and decadal sensitivity of each proxy record to large-scale circulation indices from the Pacific, Indian and Southern Ocean regions over the twentieth century. We then analyse the potential of this newly expanded palaeoclimate network to collectively represent predictands (sea surface temperature, sea level pressure, surface air temperature and precipitation) commonly used in climate reconstructions. The key dynamical centres-of-action of the equatorial Indo-Pacific are well captured by the palaeoclimate network, indicating that there is considerable reconstruction potential in this region, particularly in the post AD 1600 period when a number of long coral records are available. Current spatiotemporal gaps in data coverage and regions where significant potential for future proxy collection exists are discussed. We then highlight the need for new and extended records from key dynamical regions of the Southern Hemisphere. Although large-scale climate field reconstructions for the SH are in their infancy, we report that excellent progress in the development of regional proxies now makes plausible estimates of continental- to hemispheric-scale climate variations possible
Multi-proxy temperature and precipitation field reconst-ructions for southern South America over the past centuries
No full textHigh-resolution multiproxy climate reconstruction for southern South America since 1000 AD: LOTRED-SA, a new IGBP-PAGES initiative
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Inconsistent comparison of temperature reconstructions over the Common Era
No full textBüntgen et al. (2020) present a new reconstruction of extra-tropical summer temperatures based on updated versions of a large number of summer temperature sensitive tree-ring width chronologies from the Northern Hemisphere (NH), which cover the full Common Era (CE). This new dataset allows the authors to draw conclusions about NH temperature history and its relation to climate forcing, marking an important contribution to our understanding of past climate changes. While we have no issues with the main conclusions of B20, here we show that their comparison with PAGES 2k reconstructions is flawed: B20′s reconstruction focused on regional, summertime temperature, while the PAGES 2k reconstruction targeted global, annual mean temperature. For their reconstruction intercomparisons, B20 rescale all six tree-ring based reconstructions to their regional observational target but fail to do this same processing step with the PAGES 2k reconstructions. This inconsistent comparison leads B20 to incorrectly conclude that the PAGES 2k reconstructions severely lack variance and are therefore unreliable. In this contribution, we present a consistent comparison of the B20 and PAGES 2k reconstructions, and we highlight the importance of careful illustrations for interpreting scientific results both in the literature and in the public discussion. Our results show that, if more accurate methods for comparisons are applied, the temperature history and low-frequency amplitudes of the different proxy selection approaches and reconstruction products are not at odds, but actually consistent with the differences between their targets over the pre-industrial CE
The variable European Little Ice Age
No full textThe Little Ice Age (LIA), which lasted from about 1250 to 1860 AD, was likely the coldest period of the last 8000 years. Using new documentary data and analyses of alpine glacier fluctuations, the complex transition from the Medieval Climate Anomaly to the LIA and the ensuing high variability of seasonal temperatures, are described and interpreted for Europe. The beginning of the LIA was likely different in both hemispheres. The low temperature average of the LIA is primarily due to the high number of cold winters. Conversely many summers were warm and dry.
Important triggers of the lower temperatures were, primarily, the numerous clusters of volcanic eruptions and the weak solar irradiance during the four prominent Grand Solar Minima: Wolf, Spörer, Maunder, and Dalton. The drop in temperature triggered the sea-ice–albedo feedback and led to a weakening of the Atlantic overturning circulation, possibly associated with a trend towards negative North Atlantic Oscillation indices.
The statistics of extreme events show a mixed picture. Correlations with forcing factors are weak, and can only be found in connection with the “Years without a Summer”, which very often occurred after large volcanic eruptions
Teleconnection stationarity, variability and trends of the Southern Annular Mode (SAM) during the last millennium
No full textThe Southern Annular Mode (SAM) is the leading mode of atmospheric interannual variability in the Southern Hemisphere (SH) extra-tropics. Here, we assess the stationarity of SAM spatial correlations with instrumental and paleoclimate proxy data for the past millennium. The instrumental period shows that temporal non-stationarities in SAM teleconnections are not consistent across the SH land areas. This suggests that the influence of the SAM index is modulated by regional effects. However, within key-regions with good proxy data coverage (South America, Tasmania, New Zealand), teleconnections are mostly stationary over the instrumental period. Using different stationarity criteria for proxy record selection, we provide new austral summer and annual mean SAM index reconstructions over the last millennium. Our summer SAM reconstructions are very robust to changes in proxy record selection and the selection of the calibration period, particularly on the multi-decadal timescale. In contrast, the weaker performance and lower agreement in the annual mean SAM reconstructions point towards changing teleconnection patterns that may be particularly important outside the summer months. Our results clearly portend that the temporal stationarity of the proxy-climate relationships should be taken into account in the design of comprehensive regional and hemispherical climate reconstructions. The summer SAM reconstructions show no significant relationship to solar, greenhouse gas and volcanic forcing, with the exception of an extremely strong negative anomaly following the AD 1257 Samalas eruption. Furthermore, reconstructed pre-industrial summer SAM trends are very similar to trends obtained by model control simulations. We find that recent trends in the summer SAM lie outside the 5–95% range of pre-industrial natural variability.Fil: Dätwyler, Christoph. University of Bern. Centre for Climate
Change Research. Institute of Geography and Oeschger ; SuizaFil: Neukom, Raphael. University of Bern. Centre for Climate
Change Research. Institute of Geography and Oeschger ; SuizaFil: Abram, Nerilie J.. Australian National University; AustraliaFil: Gallant, Ailie J. E.. Monash University; AustraliaFil: Grosjean, Martin. University of Bern. Centre for Climate
Change Research. Institute of Geography and Oeschger ; SuizaFil: Jacques-Coper, Martín. Universidad de Concepción; ChileFil: Karoly, David J.. University of Melbourne; AustraliaFil: Villalba, Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentin
Early-twentieth-century cold bias in ocean surface temperature observations
No full textThe observed temperature record, which combines sea surface temperatures with near-surface air temperatures over land, is crucial for understanding climate variability and change1,2,3,4. However, early records of global mean surface temperature are uncertain owing to changes in measurement technology and practice, partial documentation5,6,7,8, and incomplete spatial coverage9. Here we show that existing estimates of ocean temperatures in the early twentieth century (1900–1930) are too cold, based on independent statistical reconstructions of the global mean surface temperature from either ocean or land data. The ocean-based reconstruction is on average about 0.26 °C colder than the land-based one, despite very high agreement in all other periods. The ocean cold anomaly is unforced, and internal variability in climate models cannot explain the observed land–ocean discrepancy. Several lines of evidence based on attribution, timescale analysis, coastal grid cells and palaeoclimate data support the argument of a substantial cold bias in the observed global sea-surface-temperature record in the early twentieth century. Although estimates of global warming since the mid-nineteenth century are not affected, correcting the ocean cold bias would result in a more modest early-twentieth-century warming trend10, a lower estimate of decadal-scale variability inferred from the instrumental record3, and better agreement between simulated and observed warming than existing datasets suggest2
Teleconnections and relationship between the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) in reconstructions and models over the past millennium
No full textThe climate of the Southern Hemisphere (SH) is strongly influenced by variations in the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). Because of the limited length of instrumental records in most parts of the SH, very little is known about the relationship between these two key modes of variability over time. Using proxy-based reconstructions and last-millennium climate model simulations, we find that ENSO and SAM indices are mostly negatively correlated over the past millennium. Pseudo-proxy experiments indicate that currently available proxy records are able to reliably capture ENSO–SAM relationships back to at least 1600CE. Palaeoclimate reconstructions show mostly negative correlations back to about 1400CE. An ensemble of last-millennium climate model simulations confirms this negative correlation, showing a stable correlation of approximately −0.3. Despite this generally negative relationship we do find intermittent periods of positive ENSO–SAM correlations in individual model simulations and in the palaeoclimate reconstructions. We do not find evidence that these relationship fluctuations are caused by exogenous forcing nor by a consistent climate pattern. However, we do find evidence that strong negative correlations are associated with strong positive (negative) anomalies in the Interdecadal Pacific Oscillation and the Amundsen Sea Low during periods when SAM and ENSO indices are of opposite (equal) sign
El Niño–Southern Oscillation variability, teleconnection changes and responses to large volcanic eruptions since AD 1000
No full textThe El Niño–Southern Oscillation (ENSO) is the earth’s dominant mode of inter-
annual climate variability. It alternates between warm (El Niño) and cold (La Niña) states, with global impacts on climate and society. This study provides new ENSO reconstructions based on a large, updated collection of proxy records. We use a novel reconstruction approa ch that employs running principal components, which allows us to take covariance changes between proxy records into account and thereby identify periods of likely teleconnection changes. Using different implementations of the principal component analysis enables us to identify periods within the last millennium when quantifications of ENSO are most robust. These periods range from 1580 to the end of the 17th century and from 1825 to present. We incorporate an assessment of consistency among our new and existing ENSO reconstructions leading to five short phases of low agreement among the reconstructions between 1700 and 1786. We find a consistent spatial pattern of proxy covariance during these four phases, differing from the structure seen over the instrumental period. This pattern points towards changes in teleconnections in the west Pacific/Australasian region, compared to the present state. Using our new
reconstructions, we find a significant response of ENSO towards more La Niña-like conditions 3–5 years after major volcanic events. We further show that our new reconstructions and existing reconstructions largely agree on the state of ENSO during volcanic eruptions in the years 1695 and 1784, which helps put into perspective the climatic response to these events. During all other large volcan ic eruptions of the last 1000 years, there is no reconstruction coherency with regard to the state of ENSO
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