A potential glacial lake outburst flood from Lake Palcacocha (Cordillera Blanca, Peru) threatens Huaraz, a city of 120,000 people. In 1941, an outburst flood destroyed one-third of the city and caused at least 1,800 fatalities. Since pre-industrial times, Lake Palcacocha has expanded due to the retreat of Palcaraju glacier. Here we used observations and numerical models to evaluate the anthropogenic contribution to the glacier’s retreat and glacial lake outburst flood hazard. We found that the magnitude of human-induced warming equals between 85 and 105% (5–95% confidence interval) of the observed 1 °C warming since 1880 in this region. We conclude that it is virtually certain (>99% probability) that the retreat of Palcaraju glacier to the present day cannot be explained by natural variability alone, and that the retreat by 1941 represented an early impact of anthropogenic greenhouse gas emissions. Our central estimate is that the overall retreat is entirely attributable to the observed temperature trend, and that the resulting change in the geometry of the lake and valley has substantially increased the outburst flood hazard.

来自帕尔卡科查湖（秘鲁科迪勒拉布兰卡）的潜在冰川湖爆发洪水威胁着拥有 12 万人口的城市瓦拉斯。1941 年，一场突如其来的洪水摧毁了该市的三分之一，造成至少 1,800 人死亡。自前工业时代以来，帕尔卡科查湖因帕尔卡拉茹冰川的退缩而扩大。在这里，我们使用观测和数值模型来评估人为对冰川退缩和冰川湖溃决洪水灾害的贡献。我们发现，自 1880 年以来在该地区观察到的 1 °C 变暖的人为引起的变暖幅度等于 85% 到 105%（5-95% 置信区间）。我们得出的结论是，几乎可以肯定（>99% 的概率）Palcaraju 冰川退缩到今天不能仅用自然变化来解释，到 1941 年的撤退代表了人为温室气体排放的早期影响。我们的中心估计是，整体撤退完全归因于观察到的温度趋势，并且由此产生的湖泊和山谷几何形状的变化大大增加了爆发洪水的危险。