| 英文摘要: | Temperature reconstructions of the past millennium rely heavily on Northern Hemisphere data. Now a Southern Hemisphere temperature reconstruction is available and sheds light on the complexity of the interhemispheric temperature relationship. For far too long the climate science community has grappled with an inconvenient truth: the vast majority of the datasets used to constrain temperature trends of the recent past come from the Northern Hemisphere. Over a dozen reconstructions of Northern Hemisphere temperature spanning the past millennium exist and have played a critical role in distinguishing natural from anthropogenic climate change. However, the extent to which recent temperature variations in the Northern Hemisphere resemble those in the Southern Hemisphere remains unclear. Such information is critical to a complete understanding of the mechanisms of global, rather than hemispheric, climate change. Writing in Nature Climate Change, Raphael Neukom and co-authors1 present a new, millennium-long reconstruction of Southern Hemisphere temperature by combining information from a wide variety of palaeoclimate sources. Although the new reconstruction resembles the Northern Hemisphere reconstructions in some key aspects — the anomalous nature of twentieth century warming being one of them — it also suggests that temperatures in the two hemispheres may have differed more than they have agreed over the past millennium. The best-dated, highest-resolution records of past climate variability rarely extend beyond the past millennium, making this time period an important test bed for quantitative comparisons between climate field reconstructions and numerical climate model simulations of past climate2, 3. Yearly temperature can be reconstructed from archives, such as corals, ice cores, tree rings, lake sediments and cave stalagmites, by calibrating their geochemical or physical signals against the instrumental record of climate, where they overlap over the past century. In this regard, extremely poor data coverage for Southern Hemisphere ocean temperature observations makes this calibration more difficult (Fig. 1). Scientists use a variety of advanced statistical techniques to extract the shared signals across a given network of palaeoclimate records. The uncertainties associated with reconstructed temperature estimates inevitably increase further back in time, as the number of available records decreases, but can be quantified using a variety of approaches.
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