NOAA/WDS Paleoclimatology - Brahmaputra River 700 Year Monsoon Season Streamflow Reconstruction

The lower Brahmaputra River in Bangladesh and Northeast India often floods during the monsoon season, with catastrophic consequences for people throughout the region. While most climate models predict an intensified monsoon and increase in flood risk with warming, robust baseline estimates of natural climate variability in the basin are limited by the short observational record. Here we use a new seven-century (1309-2004 C.E) tree-ring reconstruction of monsoon season Brahmaputra discharge to demonstrate that the early instrumental period (1956-1986 C.E.) ranks amongst the driest of the past seven centuries (13th percentile). Further, flood hazard inferred from the recurrence frequency of high discharge years is severely underestimated by 24-38% in the instrumental record compared to previous centuries and climate model projections. A focus on only recent observations will therefore be insufficient to accurately characterise flood hazard risk in the region, both in the context of natural variability and climate change.

孟加拉国与印度东北部的布拉马普特拉河（Brahmaputra River）下游河段常在季风季暴发洪涝，给区域内民众带来灾难性后果。尽管多数气候模型预测，随全球变暖，季风活动将加剧、洪涝风险随之升高，但该流域自然气候变率的可靠基准估算受限于较短的观测记录而难以获得。本研究依托一项新的、覆盖7个世纪（公元1309年至2004年）的树轮重建（tree-ring reconstruction）数据，重建了布拉马普特拉河季风期径流量序列。分析显示，早期仪器观测时段（公元1956年至1986年）属于过去7个世纪以来最干旱的时期之一，仅处于第13百分位。进一步研究表明，相较于前几个世纪以及气候模型预估结果，基于仪器记录推算的高径流量年份重现频率所对应的洪涝灾害危险性（flood hazard）被严重低估了24%至38%。因此，仅依赖近期观测数据，无法准确刻画该区域的洪涝灾害风险，无论从自然气候变率还是气候变化的视角分析均是如此。