Tree rings circle an abrupt shift in climate

doi:10.1126/science.abf1700

GSTDTAP > 气候变化

DOI	10.1126/science.abf1700
	Tree rings circle an abrupt shift in climate
	Qi-Bin Zhang; Ouya Fang
	2020-11-27
发表期刊	Science
出版年	2020
英文摘要	An abrupt and substantial change in a climate system from one steady state to another—called a climate regime shift—has a considerable impact on ecosystems and society because of the abbreviated time period for adaptation ([ 1 ][1], [ 2 ][2]). Recognizing the occurrence of regime shifts in climate is crucial for the timely development of effective policy responses to cope with the change. On page 1095 of this issue, Zhang et al. ([ 3 ][3]) reveal a recent regime shift to a drier and hotter climate throughout inner East Asia. The shift was detected with tree ring–based reconstructions of heat waves and soil moisture over the past 260 years. Scientists have found it difficult to accurately identify a threshold over which the climate will irreversibly step into a new regime. The threshold can only be determined by understanding the natural range of climate variability over a time scale that is much longer than the new regime. Yet, long climate records are rarely directly available for a specific region of concern. In addition, onset of the regime shift is usually caused by various complex factors that interact in poorly understood ways. This lessens a scientist's ability to judge whether a change in climate is indeed a regime shift or merely an extreme climate event that will return to a normal state afterward. Tree rings have long been recognized as a useful proxy for past climate variations because of their special characteristics, such as precise dating, annual resolution, long time series, and climate sensitivity ([ 4 ][4]). The International Tree-Ring Data Bank (ITRDB) archives a wealth of tree-ring records contributed by scientists around the world. However, the use of these data for reconstruction of a specific climate variable over a large geographical region is not a straightforward procedure. Because a variety of environmental characteristics (such as macroclimate, local habitats, environmental disturbances, and tree physiology) influence tree growth, the signals from the macroclimate must be extracted while removing the nonclimate noise embedded in the rings. The nonclimate noises from local habitats and disturbances are usually random in nature; thus, these noises can be canceled out by averaging tree-ring series from a large number of trees. Sustained growth reduction in trees at a sampling site should be interpreted carefully, because it might be the result of either prolonged adverse climate or damage of tree health after site disturbances or climate extremes ([ 5 ][5]). Also, trees do not merely respond passively to adverse climate but develop ecophysiological resilience to resist and recover from the influence; this makes the macroclimate signals more difficult to extract ([ 6 ][6]). In addition, sites where tree growth is not strongly sensitive to macroclimate should be excluded from regional climate reconstruction. Zhang et al. compiled ITRDB tree-ring width data from 76 sites throughout inner East Asia and screened them for signals of two climate variables: summer heat-wave frequency and soil moisture content. They evaluated the linear relationships between series of tree-ring widths and observed climate records at interannual (2 to 4 years) and above-interannual (more than 4 years) time scales for each site. The authors selected two groups of tree-ring width data, each independently composed of 10 climate-sensitive sites, for reconstructing the history of the two climate variables. In the multicentury context of the reconstruction, the authors observed an unusual “drier-hotter” climate that emerged in the 1990s and continues to the present day. This finding opened a window through which one could tackle the physical mechanisms that trigger the onset of a climate regime shift. Most studies on climate regime shifts have concentrated primarily on one climate variable at a time, to detect when and how it persistently exceeds the threshold (also called “tipping point”) of its natural variability. Real-world climate systems involve many variables whose complex interactions could either cause negative feedbacks that reduce the probability that any single variable crosses its tipping point or cause positive feedbacks that increase the probability that multiple variables cross their tipping points ([ 7 ][7]). A challenge for scientists is to discover what circumstances cause the interaction of climate variables to generate negative or positive feedbacks. By investigating the characteristics of land-atmosphere interactions over inner East Asia, Zhang et al. demonstrated that the coupling of summer heat waves and droughts has been intensified by positive feedback loops since the late 20th century. This positive feedback was fueled by an enhanced soil-moisture deficit that might have nudged the climate over a tipping point. Much remains to be discovered before scientists can fully understand the occurrence and mechanisms of climate regime shifts. Despite the concurrence of extreme droughts and heat waves reported in the literature ([ 8 ][8]–[ 10 ][9]), it remains unknown whether these phenomena resulted from a regime shift associated with global warming. In addition, the nature of the land-atmosphere interactions differs among geographical regions. For example, in addition to the shift to a drier-hotter climate in inner East Asia described by Zhang et al. , other researchers have reported a hotter-wetter climate in the Tibetan Plateau ([ 11 ][10]). Given that climate systems involve feedback among multiple variables, it is crucial to be able to predict when each variable will exceed its tipping point and when changes in these variables will lead to a domino effect, which has more severe deleterious consequences for ecosystems and society than do individual variable changes ([ 12 ][11], [ 13 ][12]). Development of trustworthy long-term climate records is a critical prerequisite for accurate detection of temporal changes in the interactions of climate variables, for discovering past climate regime shifts, and for predicting thresholds of potential future shifts. 1. [↵][13]1. T. M. Lenton et al ., Nature 575, 592 (2019). [OpenUrl][14] 2. [↵][15]1. S. Trumbore, 2. P. Brando, 3. H. Hartmann , Science 349, 814 (2015). 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领域	气候变化 ; 资源环境
URL	查看原文
引用统计
文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/304871
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Qi-Bin Zhang,Ouya Fang. Tree rings circle an abrupt shift in climate[J]. Science,2020.
APA	Qi-Bin Zhang,&Ouya Fang.(2020).Tree rings circle an abrupt shift in climate.Science.
MLA	Qi-Bin Zhang,et al."Tree rings circle an abrupt shift in climate".Science (2020).