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1. Understanding drivers of glacier-length variability over the last millennium

   https://doi.org/10.5194/tc-15-1645-2021  

   Huston, Alan; Siler, Nicholas; Roe, Gerard H.; Pettit, Erin; Steiger, Nathan J. (January 2021, The Cryosphere)

   null (Ed.)

   Abstract. Changes in glacier length reflect the integrated response to local fluctuations in temperature and precipitation resulting from both external forcing (e.g., volcanic eruptions or anthropogenic CO2) and internal climate variability. In order to interpret the climate history reflected in the glacier moraine record, the influence of both sources of climate variability must therefore be considered. Here we study the last millennium of glacier-length variability across the globe using a simple dynamic glacier model, which we force with temperature and precipitation time series from a 13-member ensemble of simulations from a global climate model. The ensemble allows us to quantify the contributions to glacier-length variability from external forcing (given by the ensemble mean) and internal variability (given by the ensemble spread). Within this framework, we find that internal variability is the predominant source of length fluctuations for glaciers with a shorter response time (less than a few decades). However, for glaciers with longer response timescales (more than a few decades) external forcing has a greater influence than internal variability. We further find that external forcing also dominates when the response of glaciers from widely separated regions is averaged. Single-forcing simulations indicate that, for this climate model, most of the forced response over the last millennium, pre-anthropogenic warming, has been driven by global-scale temperature change associated with volcanic aerosols. 

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2. Multidecadal Changes in Wet Season Precipitation Totals Over the Eastern Amazon

   https://doi.org/10.1029/2020GL087478  

   Granato‐Souza, Daniela; Stahle, David W.; Torbenson, Max C. A.; Howard, Ian M.; Barbosa, Ana Carolina; Feng, Song; Fernandes, Katia; Schöngart, Jochen (April 2020, Geophysical Research Letters)

   Abstract Instrumental observations indicate that Amazon precipitation and streamflow extremes have increased during the last 40 years, possibly due to anthropogenic changes and natural variability. How unprecedented these changes might be is difficult to determine because some paleoclimatic, instrumental, and climate model simulations suggest that Amazonian precipitation and streamflow may be subject to multidecadal variability with return intervals longer than most direct observations. A new 258‐yearlong tree‐ring chronology ofCedrela odoratahas been developed in the eastern Amazon and has been used to reconstruct wet season precipitation totals from 1759–2016. Reconstructed drought extremes are associated with significant sea surface temperature anomalies over the tropical Pacific and Atlantic Oceans. Strong multidecadal variance is identified in the reconstruction that may reflect a component of natural rainfall variability relevant to forest ecosystem dynamics and suggesting that recent hydroclimate changes over the eastern Amazon may not be unprecedented over the past 258 years. 

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3. Quantifying Contributions of Internal Variability and External Forcing to Atlantic Multidecadal Variability Since 1870

   https://doi.org/10.1029/2020GL089504  

   Qin, Minhua; Dai, Aiguo; Hua, Wenjian (November 2020, Geophysical Research Letters)

   Abstract Identifying the mechanisms behind the Atlantic Multidecadal Variability (AMV) is crucial for understanding and predicting decadal climate change. However, what is behind the AMV is still debated. A key issue is the relative role of internal variability (IV) and external forcing in causing the AMV. By analyzing observations and a large number of climate model simulations, here we show that IV and volcanic and anthropogenic aerosols all influenced the AMV over the last ~150 years. Although the AMV since 1870 resulted mainly from IV, decadal variations in aerosol forcing happen to be in phase with the IV‐induced AMV and thus enlarged its amplitudes, especially since the late 1920s. Our results support the notion that the AMV resulted from both internal climate variability and decadal changes in aerosols but are inconsistent with the conclusion that the recent AMV is mainly a direct response to external forcing. 

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4. Revisiting a Constraint on Equilibrium Climate Sensitivity From a Last Millennium Perspective

   https://doi.org/10.1029/2023GL104126  

   Cropper, S.; Thackeray, C_W; Emile‐Geay, J. (October 2023, Geophysical Research Letters)

   Abstract Despite decades of effort to constrain equilibrium climate sensitivity (ECS), current best estimates still exhibit a large spread. Past studies have sought to reduce ECS uncertainty through a variety of methods including emergent constraints. One example uses global temperature variability over the past century to constrain ECS. While this method shows promise, it has been criticized for its susceptibility to the influence of anthropogenic forcing and the limited length of the instrumental record used to compute temperature variability. Here, we investigate the emergent relationship between ECS and two metrics of global temperature variability using model simulations and paleoclimate reconstructions over the last millennium (850–1999). We find empirical evidence in support of these emergent relationships. Observational constraints suggest a central ECS estimate of 2.5–2.7 K, consistent with the Intergovernmental Panel on Climate Change's consensus estimate of 3K. Moreover, they suggest ECS “likely” ranges of 1.7–3.3 K and 1.9–3.5 K. 

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5. A multimillennial climatic context for the megafaunal extinctions in Madagascar and Mascarene Islands

   https://doi.org/10.1126/sciadv.abb2459  

   Li, Hanying; Sinha, Ashish; Anquetil André, Aurèle; Spötl, Christoph; Vonhof, Hubert B.; Meunier, Arnaud; Kathayat, Gayatri; Duan, Pengzhen; Voarintsoa, Ny Riavo; Ning, Youfeng; et al (October 2020, Science Advances)

   null (Ed.)

   Madagascar and the Mascarene Islands of Mauritius and Rodrigues underwent catastrophic ecological and landscape transformations, which virtually eliminated their entire endemic vertebrate megafauna during the past millennium. These ecosystem changes have been alternately attributed to either human activities, climate change, or both, but parsing their relative importance, particularly in the case of Madagascar, has proven difficult. Here, we present a multimillennial (approximately the past 8000 years) reconstruction of the southwest Indian Ocean hydroclimate variability using speleothems from the island of Rodrigues, located ∼1600 km east of Madagascar. The record shows a recurring pattern of hydroclimate variability characterized by submillennial-scale drying trends, which were punctuated by decadal-to-multidecadal megadroughts, including during the late Holocene. Our data imply that the megafauna of the Mascarenes and Madagascar were resilient, enduring repeated past episodes of severe climate stress, but collapsed when a major increase in human activity occurred in the context of a prominent drying trend. 

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