氣候變化對水循環的影響(英語:Effects of climate change on the water cycle)很深遠,被描述會對水循環(也稱水文循環)產生“強化”(或是整體的“加強”)效果。[1]:1079這種現象至少從1980起就被觀察到。[1]:1079其中一例是加劇的強降水事件。這情況對淡水資源可用性,以及海洋、冰蓋、大氣和陸地表面等儲水的所在產生重要連鎖反應。水循環對地球上的生命非常重要,對氣候和洋流發揮重要作用。有多種原因讓地球變暖,導致水循環隨之發生變化。[2]例如較暖的大氣中含有更多的水氣,而會影響蒸發和降水。海洋可吸收93%來自太陽的熱量,也發揮重要的作用。自1971年以來海洋熱含量即在增加,對海洋本身和循環產生重大影響。[3]
氣候模型尚無法順利把水循環作模擬。[15]原因之一是降水量很難量化,它本質上是種間歇性發生的現象。[6]:50通常引用的僅為平均數量。[16]人們傾向於使用“降水”一詞,就好像它與“降水量”的意思相同。在描述地球降水模式的變化時,真正重要的並非僅為總量:還與強度(下雨,或是下雪的強度)、頻率(次數)、持續時間(時間長度)和類型(是雨,或是雪)有關[6]:50紐西蘭氣候學家凱文·E.·川伯斯(英语:Kevin E. Trenberth)針對降水的特性做研究,發現極端事件的頻率和強度很重要,而這些在氣候模型中很難估算。[15]
全球氣候變化“會同時增加確保水安全的複雜度和成本”。[37]產生新的威脅和調適挑戰。[38]原因是氣候變化導致水文變率和極端事件增加。氣候變化對水循環造成許多影響,產生更多的氣候和水文變異性,而威脅到水安全。[39]:1[40]: vII 水循環變化威脅到現有和未來的水基礎設施。因為水循環的未來變率有太多不確定性,對規劃水利基礎設施的投資而言將更會有困難度,[38]而讓社會更易受到與水有關極端事件的影響,水安全因而被降低。[39]:vII
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