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1.
Nature ; 613(7943): 292-297, 2023 01.
Artículo en Inglés | MEDLINE | ID: mdl-36631651

RESUMEN

The recovery of long-term climate proxy records with seasonal resolution is rare because of natural smoothing processes, discontinuities and limitations in measurement resolution. Yet insolation forcing, a primary driver of multimillennial-scale climate change, acts through seasonal variations with direct impacts on seasonal climate1. Whether the sensitivity of seasonal climate to insolation matches theoretical predictions has not been assessed over long timescales. Here, we analyse a continuous record of water-isotope ratios from the West Antarctic Ice Sheet Divide ice core to reveal summer and winter temperature changes through the last 11,000 years. Summer temperatures in West Antarctica increased through the early-to-mid-Holocene, reached a peak 4,100 years ago and then decreased to the present. Climate model simulations show that these variations primarily reflect changes in maximum summer insolation, confirming the general connection between seasonal insolation and warming and demonstrating the importance of insolation intensity rather than seasonally integrated insolation or season duration2,3. Winter temperatures varied less overall, consistent with predictions from insolation forcing, but also fluctuated in the early Holocene, probably owing to changes in meridional heat transport. The magnitudes of summer and winter temperature changes constrain the lowering of the West Antarctic Ice Sheet surface since the early Holocene to less than 162 m and probably less than 58 m, consistent with geological constraints elsewhere in West Antarctica4-7.

2.
Nature ; 611(7935): 295-300, 2022 11.
Artículo en Inglés | MEDLINE | ID: mdl-36352132

RESUMEN

The Pacific cold tongue annual cycle in sea surface temperature is presumed to be driven by Earth's axial tilt1-5 (tilt effect), and thus its phasing should be fixed relative to the calendar. However, its phase and amplitude change dramatically and consistently under various configurations of orbital precession in several Earth System models. Here, we show that the cold tongue possesses another annual cycle driven by the variation in Earth-Sun distance (distance effect) from orbital eccentricity. As the two cycles possess slightly different periodicities6, their interference results in a complex evolution of the net seasonality over a precession cycle. The amplitude from the distance effect increases linearly with eccentricity and is comparable to the amplitude from the tilt effect for the largest eccentricity values over the last million years (e value approximately 0.05)7. Mechanistically, the distance effect on the cold tongue arises through a seasonal longitudinal shift in the Walker circulation and subsequent annual wind forcing on the tropical Pacific dynamic ocean-atmosphere system. The finding calls for reassessment of current understanding of the Pacific cold tongue annual cycle and re-evaluation of tropical Pacific palaeoclimate records for annual cycle phase changes.

3.
Proc Natl Acad Sci U S A ; 111(47): 16688-93, 2014 Nov 25.
Artículo en Inglés | MEDLINE | ID: mdl-25368154

RESUMEN

Heinrich Events, the abrupt changes in the Laurentide Ice Sheet that cause the appearance of the well-observed Heinrich Layers, are thought to have a strong effect on the global climate. The focus of most studies that have looked at the climate's response to these events has been the freshwater flux that results from melting icebergs. However, there is the possibility that the varying height of the ice sheet could force a change in the climate. In this study, we present results from a newly developed coupled climate/ice sheet model to show what effect this topographic change has both on its own and in concert with the flux of freshwater from melting icebergs. We show that the topographic forcing can explain a number of the climate changes that are observed during Heinrich Events, such as the warming and wettening in Florida and the warm sea surface temperatures in the central North Atlantic, which freshwater forcing alone cannot. We also find regions, for example the tropical Atlantic, where the response is a mixture of the two: Here observations may help disentangle the relative importance of each mechanism. These results suggest that the simple paradigm of a Heinrich Event causing climate change via freshwater inputs into the North Atlantic needs to be revised.

4.
Nat Commun ; 14(1): 5697, 2023 Sep 14.
Artículo en Inglés | MEDLINE | ID: mdl-37709741

RESUMEN

The winter and summer monsoons in Southeast Asia are important but highly variable sources of rainfall. Current understanding of the winter monsoon is limited by conflicting proxy observations, resulting from the decoupling of regional atmospheric circulation patterns and local rainfall dynamics. These signals are difficult to decipher in paleoclimate reconstructions. Here, we present a winter monsoon speleothem record from Southeast Asia covering the Holocene and find that winter and summer rainfall changed synchronously, forced by changes in the Pacific and Indian Oceans. In contrast, regional atmospheric circulation shows an inverse relation between winter and summer controlled by seasonal insolation over the Northern Hemisphere. We show that disentangling the local and regional signal in paleoclimate reconstructions is crucial in understanding and projecting winter and summer monsoon variability in Southeast Asia.

5.
Science ; 372(6546): 1097-1101, 2021 06 04.
Artículo en Inglés | MEDLINE | ID: mdl-34083489

RESUMEN

Water-stable isotopes in polar ice cores are a widely used temperature proxy in paleoclimate reconstruction, yet calibration remains challenging in East Antarctica. Here, we reconstruct the magnitude and spatial pattern of Last Glacial Maximum surface cooling in Antarctica using borehole thermometry and firn properties in seven ice cores. West Antarctic sites cooled ~10°C relative to the preindustrial period. East Antarctic sites show a range from ~4° to ~7°C cooling, which is consistent with the results of global climate models when the effects of topographic changes indicated with ice core air-content data are included, but less than those indicated with the use of water-stable isotopes calibrated against modern spatial gradients. An altered Antarctic temperature inversion during the glacial reconciles our estimates with water-isotope observations.

6.
Sci Rep ; 10(1): 16482, 2020 10 05.
Artículo en Inglés | MEDLINE | ID: mdl-33020586

RESUMEN

The interpretation of palaeoclimate archives based on oxygen isotopes depends critically on a detailed understanding of processes controlling the isotopic composition of precipitation. In the summer monsoonal realm, like Southeast Asia, seasonally and interannually depleted oxygen isotope ratios in precipitation have been linked to the summer monsoon strength. However, in some regions, such as central Vietnam, the majority of precipitation falls outside the summer monsoon period. We investigate processes controlling stable isotopes in precipitation from central Vietnam by combining moisture uptake calculations with monthly stable isotope data observed over five years. We find that the isotopic seasonal cycle in this region is driven by a shift in moisture source from the Indian Ocean to the South China Sea. This shift is reflected in oxygen isotope ratios with low values (- 8 to - 10‰) during summer and high values during spring/winter (0 to - 3‰), while 70% of the annual rainfall occurs during autumn. Interannual changes in precipitation isotopes in central Vietnam are governed by the timing of the seasonal onset and withdrawal of the Intertropical Convergence Zone, which controls the amount of vapour contributed from each source.

7.
Sci Adv ; 5(10): eaax1697, 2019 10.
Artículo en Inglés | MEDLINE | ID: mdl-31692956

RESUMEN

The East Asian monsoon plays an integral role in human society, yet its geological history and controlling processes are poorly understood. Using a general circulation model and geological data, we explore the drivers controlling the evolution of the monsoon system over the past 150 million years. In contrast to previous work, we find that the monsoon is controlled primarily by changes in paleogeography, with little influence from atmospheric CO2. We associate increased precipitation since the Late Cretaceous with the gradual uplift of the Himalayan-Tibetan region, transitioning from an ITCZ-dominated monsoon to a sea breeze-dominated monsoon. The rising region acted as a mechanical barrier to cold and dry continental air advecting into the region, leading to increasing influence of moist air from the Indian Ocean/South China Sea. We show that, apart from a dry period in the middle Cretaceous, a monsoon system has existed in East Asia since at least the Early Cretaceous.

8.
Laryngoscope ; 128(5): 1200-1206, 2018 05.
Artículo en Inglés | MEDLINE | ID: mdl-28940574

RESUMEN

OBJECTIVES/HYPOTHESIS: To assess balloon dilation of the Eustachian tube with Eustachian tube balloon catheter in conjunction with medical management as treatment for Eustachian tube dilatory dysfunction. STUDY DESIGN: In this prospective, multicenter, randomized, controlled trial, we assigned, in a 2:1 ratio, patients age 22 years and older with Eustachian tube dilatory dysfunction refractory to medical therapy to undergo balloon dilation of the Eustachian tube with balloon catheter in conjunction with medical management or medical management alone. METHODS: The primary endpoint was normalization of tympanogram at 6 weeks. Additional endpoints were normalization of Eustachian Tube Dysfunction Questionaire-7 symptom scores, positive Valsalva maneuver, mucosal inflammation, and safety. RESULTS: Primary efficacy results demonstrated superiority of balloon dilation of the Eustachian tube with balloon catheter + medical management compared to medical management alone. Tympanogram normalization at 6-week follow-up was observed in 51.8% (72/139) of investigational patients versus 13.9% (10/72) of controls (P < .0001). Tympanogram normalization in the treatment group was 62.2% after 24 weeks. Normalization of Eustachian Tube Dysfunction Questionaire-7 Symptom scores at 6-week follow-up was observed in 56.2% (77/137) of investigational patients versus 8.5% (6/71) controls (P < .001). The investigational group also demonstrated substantial improvement in both mucosal inflammation and Valsalva maneuver at 6-week follow-up compared to controls. No device- or procedure-related serious adverse events were reported for those who underwent balloon dilation of the Eustachian tube. CONCLUSIONS: This study demonstrated superiority of balloon dilation of the Eustachian tube with balloon catheter + medical management compared to medical management alone to treat Eustachian tube dilatory dysfunction in adults. LEVEL OF EVIDENCE: 1b. Laryngoscope, 128:1200-1206, 2018.


Asunto(s)
Dilatación/métodos , Enfermedades del Oído/terapia , Trompa Auditiva/fisiopatología , Adulto , Femenino , Pruebas Auditivas , Humanos , Masculino , Persona de Mediana Edad , Estudios Prospectivos , Encuestas y Cuestionarios , Resultado del Tratamiento , Estados Unidos
9.
Sci Rep ; 7(1): 9382, 2017 08 24.
Artículo en Inglés | MEDLINE | ID: mdl-28839263

RESUMEN

The latitude of the tropical rainbelt oscillates seasonally but has also varied on millennial time-scales in response to changes in the seasonal distribution of insolation due to Earth's orbital configuration, as well as climate change initiated at high latitudes. Interpretations of palaeoclimate proxy archives often suggest hemispherically coherent variations, some proposing meridional shifts in global rainbelt position and the 'global monsoon', while others propose interhemispherically symmetric expansion and contraction. Here, we use a unique set of climate model simulations of the last glacial cycle (120 kyr), that compares well against a compilation of precipitation proxy data, to demonstrate that while asymmetric extratropical forcings (icesheets, freshwater hosing) generally produce meridional shifts in the zonal mean rainbelt, orbital variations produce expansion/contractions in terms of the global zonal mean. This is primarily a dynamic response of the rainbelt over the oceans to regional interhemispheric temperature gradients, which is opposite to the largely local thermodynamic terrestrial response to insolation. The mode of rainbelt variation is regionally variable, depending on surface type (land or ocean) and surrounding continental configuration. This makes interpretation of precipitation-proxy records as large-scale rainbelt movement challenging, requiring regional or global data syntheses.

10.
Int J Surg ; 4(3): 153-9, 2006.
Artículo en Inglés | MEDLINE | ID: mdl-17462339

RESUMEN

BACKGROUND: It has been shown that abrupt re-exposure of ischemic myocardium to oxygen can lead to increased peroxidative damage to myocytes (oxygen paradox). Controlled cardiac reoxygenation, as an adjunct to substrate-enhanced cardioplegia, has been shown to improve myocardial function and limit reperfusion injury when utilizing standardized hyperoxic cardiopulmonary bypass (CPB). The objective of our study was to evaluate the effect of controlled reoxygenation on myocardial function following global ischemia employing normoxic CPB. STUDY DESIGN: Nineteen female swine (30-40kg) were placed on vented, normoxic CPB. They were subjected to 45-50min of unprotected global ischemia (aortic cross clamping) followed by 30min of controlled cardiac reperfusion utilizing substrate-enhanced cardioplegia. Group 1 maintained normoxic pO(2) (O(2) tension of 90-110mmHg). In Group 2, reoxygenation was titrated gradually and increased from venous to arterial levels (O(2) tensions from 40 to 110mmHg over 15min). We measured coronary sinus blood samples for CK, CK-MB, nitric oxide, and conjugated dienes at baseline, 5min into the cardioplegic resuscitation, 5min after the cross clamp removal, and just prior to the termination of the study. Hearts were pathologically studied and scored for evidence of tissue peroxidation. RESULTS: Although not significantly different, Group 1 (normoxic reperfusion) animals were more likely to wean from CPB (p=0.141) and had a higher mean arterial pressure (p=0.556). In Group 1, conjugated dienes were significantly higher 5min into the resuscitative protocol (p=0.018) and at the termination of bypass (p=0.035). Five of six animals in Group 1 eventually attained normal sinus rhythm as opposed to three out of 13 in Group 2 (p=0.041). There was no significant difference in histology scoring between the two groups for tissue peroxidation. CONCLUSION: This study of controlled cardiac reoxygenation in a lethal ischemic swine model failed to demonstrate that the use of controlled reoxygenation on the myocardial function following global ischemia was better with maintained normoxic pO(2) (with O(2) tensions of 90-110mmHg) than when reoxygenation was titrated gradually and increased from venous to arterial levels (O(2) tensions from 40 to 110mmHg over 15min).

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