NOAA/WDS Paleoclimatology - Cleaveland - Krause Springs - TADI - ITRDB TX053

Short instrumental climatic records prevent appropriate statistical and historical characterization of extreme events such as the extent, duration, and severity of multiyear droughts. The best solution is to extend climatic records through well-understood proxies of climate. One of the best such proxies is climate-sensitive annual tree rings, which can be dated precisely to the year, are easily sampled, and are widely distributed. We created 3 new baldcypress chronologies in South Central Texas and used them, along with existing Douglas-fir chronologies from West Texas and a composite post oak chronology in Central Texas,to calibrate 1931-2008 and reconstruct June Palmer Drought Severity Index (PDSI) in Texas climate divisions 5 (Trans Pecos), 6 (Edwards Plateau), 7 (S. Central), and 8 (Upper Coast) 1500-2008. We validated the reconstructions against observed data not used in calibration. Most water planners in Texas at present use the drought of the 1950s, 1950-1956, as a worst-case scenario. Our reconstructions show, however, that a number of extended droughts of the past were longer and/or more intense than the 1950s drought. Furthermore, extended droughts have been a consistent feature of southwestern climate since the 800s, including at least 4megadroughts 15- to 30-years long centered in central or northern Mexico (Stahle et al. 2009; 2011b). This and previous studies indicate that severe decadal-scale droughts have occurred in Texas at least once a century since the 1500s. Current use by water planners of the 1950s drought as a worst-case scenario, therefore, is questionable. When water managers consider past droughts, population growth, and climate change, it becomes highly probable that the future poses unprecedented challenges.

有限的器测气候记录难以对极端气候事件（如多年干旱的范围、持续时长与严重程度）开展精准的统计与历史特征分析。破解这一难题的最优方案，是借助学界认知充分的气候代用指标（proxy）拓展气候记录序列。其中最优的代用指标之一便是对气候敏感的年际树木年轮：其可精确厘定至年份，采样便捷且分布范围广泛。本研究于得克萨斯州中南部新建了3组落羽杉树轮年表，并结合得克萨斯州西部已有的花旗松树轮年表，以及得克萨斯州中部的复合糙皮栎树轮年表，对1931至2008年的帕默尔干旱指数（Palmer Drought Severity Index, PDSI）进行校准，并重建了得克萨斯州气候分区5（跨佩科斯地区）、6（爱德华兹高原）、7（中南部）与8（上海岸地区）1500年至2008年的6月帕默尔干旱指数。本研究采用未参与校准的实测数据对重建结果开展验证。目前，得克萨斯州多数水资源规划者均以1950至1956年的1950年代干旱作为极端干旱的最坏情景。但本研究的重建结果显示，历史上多场持续干旱的时长与/或严重程度均超过1950年代干旱。此外，自公元9世纪以来，持续干旱始终是美国西南部气候的典型特征，其中至少包含4场持续15至30年、以墨西哥中部或北部为核心区域的超级干旱（Stahle等，2009；2011b）。本研究与既往研究均表明，自16世纪以来，得克萨斯州至少每百年便会发生一次十年尺度的严重干旱。因此，水资源规划者当前以1950年代干旱作为最坏情景的做法尚存争议。当水资源管理者综合考量历史干旱、人口增长与气候变化等因素时，未来极有可能面临前所未有的挑战。