Here’s what one team of scientists thinks happened in Nepal’s deadly flood wave

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The catastrophic August flooding on the Nepal-China border was influenced by a complex cascade of factors, many of which were worsened by human-driven climate change, according to a new scientific analysis by more than 20 international experts.

The region is still grappling with the aftermath of the devastating rock-ice avalanche that tore through narrow mountain valleys in late August, wiping out towns, roads and bridges, killing at least 1,300 people and leaving more than 5,000 missing.

As rescue attempts transition into recovery operations, and the country begins to reckon with the mammoth task of rebuilding, scientists all over the world have been working to understand what caused the tragedy.

Here’s what we already knew : on August 26, a vast chunk of mountain rock and glacier ice around 2,000 feet wide sheared off Langtang Lirung mountain, plunging roughly 7,000 feet into the river below, releasing the same amount of energy as a magnitude 5.2 earthquake.

It fell with enough force to melt the ice almost instantaneously. The torrent also picked up moisture-laden sediment and likely crashed into buried ice on the valley floor, adding to a raging flood wave that thundered more than 20 miles downstream at devastating speed.

It was “an unprecedented disaster” in the Himalayas, said Walter Immerzeel, a mountain hydrologist at Utrecht University in the Netherlands.

But why did the avalanche happen in the first place?

The team of glaciologists and climate scientists with World Weather Attribution, or WWA, a network which examines the role climate change plays in extreme weather events, released its analysis Wednesday in a comprehensive attempt to answer the question.

The disaster was not triggered by a single extreme weather event, but rather by a slew of processes acting together, compounded by decades of climate warming caused by fossil fuel pollution, according to the report, which used peer-reviewed methods but is not yet itself peer reviewed.

The scientists found the mountain may have been preconditioned for collapse by a 7.8 magnitude earthquake in April 2015 , which triggered a rock and ice avalanche from the south side of the mountain, potentially weakening the slope. Since then, the mountain has experienced increased rates of landslides, the report found.

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The scientists then looked at the role climate change could have played and found several processes acting together.

Climate change is pushing up the “zero degree” line, the point in the atmosphere where temperatures hit 0 degrees Celsius (32 Fahrenheit), by more than 320 feet a decade in the Himalayas.

Rising temperatures are weakening the permafrost in Nepal’s high mountains — the mix of ice, rock, sand and soil that can act like glue — especially around the elevation where the collapse happened, the report found.

Thawing permafrost “reduces the rock strength and allows more water to penetrate into the slope,” Immerzeel said.

Glacier change is another important factor. The Langtang-Lirung Glacier has undergone substantial thinning and shrinking over recent decades, particularly after 2010, making the slope less stable and producing meltwater able to seep into fractures in the rock, the report found.

The researchers also looked at the weather preceding the collapse. Last fall saw exceptional amounts of snow in the region in October and early November. The following July and August were the warmest on record locally, which could have created large amounts of meltwater, further weakening the slope.

Using historical data and climate models, the researchers found that July and August temperatures in the region are now around 1.5 degrees Celsius (2.7 Fahrenheit) warmer because of climate change. And the region’s annual temperatures have increased by 2 degrees Celsius (3.6 Fahrenheit), significantly above the global average.

The report found less of a clear link between global warming and increased snow and rainfall, due in part to the challenges of measuring and monitoring across the complex, rugged Himalayas.

Overall, however, “there is absolutely no doubt that human-induced climate change played a role here in the preconditioning of the disaster,” said Friederike Otto, professor of climate science at Imperial College London.

The scientists concluded that this collapse was fundamentally different in its size, speed and complexity from anything the region has experienced before. “No existing early warning system could have provided sufficient lead time,” they wrote.

It is “one of the types of events that are absolutely outside of the limits of what we can possibly adapt to,” Otto said. Unless the world rapidly cuts fossil fuel pollution, she added, “we will inevitably see more disasters of this scale.”

阿卜杜勒·戈尼/阿纳多卢/盖蒂图片社

20多位国际专家进行的一项新的科学分析表明，尼泊尔-中国边境8月份发生的灾难性洪水是由一系列复杂的因素造成的，其中许多因素因人为造成的气候变化而加剧。

该地区仍在努力应对 8 月下旬席卷狭窄山谷的毁灭性冰岩雪崩的后果，这场雪崩摧毁了城镇、道路和桥梁，造成至少 1300 人死亡，5000 多人失踪。

随着救援行动过渡到灾后重建，以及该国开始着手应对重建的艰巨任务，世界各地的科学家都在努力了解造成这场悲剧的原因。

我们之前已经知道：8 月 26 日，朗唐里隆山上一块宽约 2000 英尺的巨大山岩和冰川冰块崩落，坠入下方约 7000 英尺深的河流中，释放出的能量相当于 5.2 级地震。

洪水倾泻而下，势不可挡，几乎瞬间融化了冰层。湍急的河水还裹挟着富含水分的泥沙，很可能撞击到谷底埋藏的冰层，汇聚成汹涌澎湃的洪水，以惊人的速度向下游奔涌了20多英里。

荷兰乌得勒支大学的山地水文学家沃尔特·伊默泽尔表示，这是喜马拉雅山脉“前所未有的灾难”。

但雪崩究竟是如何发生的呢？

世界天气归因组织（WWA）是一个研究气候变化在极端天气事件中所起作用的网络，其冰川学家和气候科学家团队于周三发布了分析报告，试图全面回答这个问题。

报告指出，这场灾难并非由单一极端天气事件引发，而是由一系列因素共同作用造成的，再加上数十年来化石燃料污染导致的气候变暖，最终酿成灾难。该报告采用了同行评审的方法，但其本身尚未经过同行评审。

科学家发现，2015年4月发生的7.8级地震可能已经为这座山的崩塌埋下了隐患。当时，地震引发了山体南侧的岩石和冰崩，可能削弱了山坡的稳定性。报告指出，自那以后，这座山的滑坡发生频率有所增加。

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普拉宾·拉纳巴特/法新社/盖蒂图片社

科学家们随后研究了气候变化可能发挥的作用，并发现有多种过程共同作用。

气候变化正使喜马拉雅山脉的“零度线”（大气层中温度达到 0 摄氏度（32 华氏度）的点）每十年上升超过 320 英尺。

报告发现，气温上升正在削弱尼泊尔高山的永久冻土层——这种由冰、岩石、沙子和土壤混合而成的物质可以起到粘合剂的作用——尤其是在崩塌发生的海拔附近。

伊默泽尔说，永久冻土融化“会降低岩石强度，并允许更多的水渗入斜坡”。

冰川变化是另一个重要因素。报告指出，朗塘-里隆冰川在近几十年，特别是2010年以后，经历了显著的消融和萎缩，导致冰川坡面稳定性下降，融水渗入岩石裂缝中。

研究人员还分析了崩塌发生前的天气情况。去年秋季，该地区在10月和11月初经历了异常多的降雪。随后的7月和8月是当地有记录以来最温暖的月份，这可能导致大量融水汇聚，进一步削弱了山坡的强度。

研究人员利用历史数据和气候模型发现，由于气候变化，该地区7月和8月的平均气温比以往高出约1.5摄氏度（2.7华氏度）。而且，该地区的年平均气温也上升了2摄氏度（3.6华氏度），远高于全球平均水平。

报告发现，全球变暖与降雪和降雨量增加之间的联系并不那么明显，部分原因是测量和监测复杂崎岖的喜马拉雅山脉面临着诸多挑战。

但总体而言，“毫无疑问，人为造成的气候变化在这次灾难的发生前起到了推波助澜的作用，”伦敦帝国理工学院气候科学教授弗里德里克·奥托说道。

科学家们得出结论，这次崩塌在规模、速度和复杂性上都与该地区以往经历的任何崩塌截然不同。“现有的任何预警系统都无法提供足够的预警时间，”他们写道。

奥托说，这是“完全超出我们适应能力范围的事件之一”。她补充说，除非世界迅速减少化石燃料污染，“否则我们必将看到更多此类规模的灾难”。