| 英文摘要: | The Earth’s climate system is driven by a complex interplay of internal chaotic dynamics and natural and anthropogenic external forcing. Recent instrumental data have shown a remarkable degree of asynchronicity between Northern Hemisphere and Southern Hemisphere temperature fluctuations, thereby questioning the relative importance of internal versus external drivers of past as well as future climate variability1, 2, 3. However, large-scale temperature reconstructions for the past millennium have focused on the Northern Hemisphere4, 5, limiting empirical assessments of inter-hemispheric variability on multi-decadal to centennial timescales. Here, we introduce a new millennial ensemble reconstruction of annually resolved temperature variations for the Southern Hemisphere based on an unprecedented network of terrestrial and oceanic palaeoclimate proxy records. In conjunction with an independent Northern Hemisphere temperature reconstruction ensemble5, this record reveals an extended cold period (1594–1677) in both hemispheres but no globally coherent warm phase during the pre-industrial (1000–1850) era. The current (post-1974) warm phase is the only period of the past millennium where both hemispheres are likely to have experienced contemporaneous warm extremes. Our analysis of inter-hemispheric temperature variability in an ensemble of climate model simulations for the past millennium suggests that models tend to overemphasize Northern Hemisphere–Southern Hemisphere synchronicity by underestimating the role of internal ocean–atmosphere dynamics, particularly in the ocean-dominated Southern Hemisphere. Our results imply that climate system predictability on decadal to century timescales may be lower than expected based on assessments of external climate forcing and Northern Hemisphere temperature variations5, 6 alone. From over 25 hemispheric-scale temperature reconstructions published in recent decades, only three cover the ocean-dominated Southern Hemisphere7. These Southern Hemisphere temperature reconstructions include only seven8 or fewer9 proxy datasets for the entire Southern Hemisphere, or were provided as peripheral components of Northern Hemisphere and global reconstruction efforts4 with the caveat that ‘more confident statements about long-term temperature variations in the Southern Hemisphere and globe on the whole must await additional proxy data collection’4. Consequently, attribution of temperature changes to external forcings10, 11 and investigations of the coupling between temperature and greenhouse gas concentrations5, 6 have focused on the Northern Hemisphere. Data spanning inter-annual to multi-millennial timescales suggest limited temperature coherence between the two hemispheres. The degree of independence in Northern Hemisphere and Southern Hemisphere temperature trends over the past 150 years2 indicates that responses to external forcing may be modulated by ocean–atmosphere variability, reducing predictability of the climate system in twenty-first century model projections1, 3. Patterns of late Quaternary deglaciation have also demonstrated high inter-hemispheric variability, attributed to a coupling of orbital forcing, ice-albedo feedbacks and the Atlantic Meridional Overturning Circulation12, 13. Finally, a recent evaluation of multi-centennial reconstructions from seven continents also suggests stronger regional temperature coherence within the hemispheres than between them14. Yet, the preliminary nature of existing annually resolved Southern Hemisphere temperature reconstructions has hindered knowledge of the existence and driving mechanisms of inter-hemispheric climate variability on the societally relevant multi-decadal to centennial timescales. Here, we introduce a Southern Hemisphere temperature reconstruction ensemble and assess inter-hemispheric temperature variability over the past millennium in both empirical reconstructions and state-of-the-art climate model simulations. We use an extensive Southern Hemisphere palaeoclimate data network from more than 300 individual sites15 yielding 111 temperature predictors (Supplementary Section 1). This proxy collection nearly doubles the number of records considered in the most advanced previous reconstruction attempt4, now allowing the development of an annually resolved and well-verified Southern Hemisphere temperature reconstruction for the past millennium (Fig. 1 and Supplementary Section 2) which is insensitive to moderate changes in reconstruction methodology or proxy network composition (Supplementary Section 3).
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