Browsing by Author "Cheng, Hai"
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Item Open Access Modern anthropogenic drought in Central Brazil unprecedented during last 700 years(Nature Research, 2024-02-26) Misailidis Stríkis, Nicolas; Silva Melo Buarque, Plácido Fabrício; Cruz, Francisco William; Bernal, Juan Pablo; Vuille, Mathias; Tejedor, Ernesto; Simões Santos, Matheus; Harumi Shimizu, Marília; Ampuero, Angela; Du, Wenjing; Sampaio, Gilvan; Reis Sales, Hamilton dos; Campos, José Leandro; Toshie Kayano, Mary; Apaéstegui Campos, James Emiliano; Fu, Roger R.; Cheng, Hai; Edwards, R. Lawrence; Chavez Mayta, Victor; Silva Francischini, Danielle da; Zezzi Arruda, Marco Aurélio; Felipe Novello, ValdirA better understanding of the relative roles of internal climate variability and external contributions, from both natural (solar, volcanic) and anthropogenic greenhouse gas forcing, is important to better project future hydrologic changes. Changes in the evaporative demand play a central role in this context, particularly in tropical areas characterized by high precipitation seasonality, such as the tropical savannah and semi-desertic biomes. Here we present a set of geochemical proxies in speleothems from a well-ventilated cave located in central-eastern Brazil which shows that the evaporative demand is no longer being met by precipitation, leading to a hydrological deficit. A marked change in the hydrologic balance in central-eastern Brazil, caused by a severe warming trend, can be identified, starting in the 1970s. Our findings show that the current aridity has no analog over the last 720 years. A detection and attribution study indicates that this trend is mostly driven by anthropogenic forcing and cannot be explained by natural factors alone. These results reinforce the premise of a severe long-term drought in the subtropics of eastern South America that will likely be further exacerbated in the future given its apparent connection to increased greenhouse gas emissions.Item Restricted Precipitation changes over the eastern Bolivian Andes inferred from speleothem (δ¹⁸O) records for the last 1400 years(Elsevier, 2018-07-15) Apaéstegui Campos, James Emiliano; William Cruz, Francisco; Vuille, Mathias; Fohlmeister, Jens; Espinoza, Jhan Carlo; Sifeddine, Abdelfettah; Strikis, Nicolas; Guyot, Jean Loup; Ventura, Roberto; Cheng, Hai; Edwards, R. LawrenceHere we present high-resolution δ¹⁸ records obtained from speleothems collected in the eastern Bolivian Andes. The stable isotope records are related to the regional- to large-scale atmospheric circulation over South America and allow interpreting changes in δ¹⁸ during the last 1400 yr as a function of changes in precipitation regimes over the southern tropical Andes. Two distinct phases with more negative δ¹⁸ values, interpreted as periods of increased convective activity over the eastern Andean Cordillera in Bolivia are observed concomitantly with periods of global climate anomalies during the last millennium, such as the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA) respectively. Changes in the Bolivian δ¹⁸ record during the LIA are apparently related to a southward displacement of the Intertropical Convergence Zone (ITCZ), which acts as a main moisture driver to intensify convection over the tropical continent. During the MCA, however, the increased convective activity observed in the Bolivian record is likely the result of a different mechanism, which implies moisture sourced mainly from the southern tropical Atlantic. This interpretation is consistent with paleoclimate records further to the north in the tropical Andes that show progressively drier conditions during this time period, indicating a more northerly position of the ITCZ. The transition period between the MCA and the LIA shows a slight tendency toward increased δ¹⁸ values, indicating weakened convective activity. Our results also reveal a non-stationary anti-phased behavior between the δ¹⁸ reconstructions from Bolivia and northeastern Brazil that confirms a continental-scale east–west teleconnection across South America during the LIA.Item Restricted Speleothem records decadal to multidecadal hydroclimate variations in southwestern Morocco during the last millennium(Elsevier, 2017-10-15) Ait Brahim, Yassine; Cheng, Hai; Sifeddine, Abdelfettah; Wassenburg, Jasper A.; Cruz, Francisco W.; Khodri, Myriam; Sha, Lijuan; Pérez-Zanón, Núria; Hassane Beraaouz, El; Apaéstegui Campos, James Emiliano; Guyot, Jean-Loup; Jochum, Klaus Peter; Bouchaou, LhoussaineThis study presents the first well-dated high resolution stable isotope (δ¹⁸O and δ¹³C) and trace element (Mg and Sr) speleothem records from southwestern Morocco covering the last 1000 yrs. Our records reveal substantial decadal to multidecadal swings between dry and humid periods, consistent with regional paleorecords with prevailing dry conditions during the Medieval Climate Anomaly (MCA), wetter conditions during the second part of the Little Ice Age (LIA), and a trend towards dry conditions during the current warm period. These coherent regional climate signals suggest common climate controls. Statistical analyses indicate that the climate of southwestern Morocco remained under the combined influence of both the North Atlantic Oscillation (NAO) and the Atlantic Multidecadal Oscillation (AMO) over the last millennium. Interestingly, the generally warmer MCA and colder LIA at longer multidecadal timescales probably influenced the regional climate in North Africa through the influence on Sahara Low which weakened and strengthened the mean moisture inflow from the Atlantic Ocean during the MCA and LIA respectively.