都市化與極端天氣雙重衝擊，台灣面臨新一輪水災威脅

近日，台灣受到熱帶性低氣壓及低壓帶的雙重影響，新北市、基隆市等地區多處發生嚴重淹水。根據中央氣象局統計，新北市平溪在過去24小時內累積降雨量達426毫米，成為全台最高，其他如瑞芳、雙溪、萬里、汐止等地區的累積雨量也突破300毫米。這樣的極端天氣現象不僅威脅台灣城市的基礎設施，也讓全球「城市濕島效應」的問題再次受到關注。

Recently, Taiwan has been affected by both tropical depressions and low-pressure belts, causing severe flooding in many areas such as New Taipei City and Keelung City. According to statistics from the Central Meteorological Bureau, the cumulative rainfall in Pingxi, New Taipei City, reached 426 millimeters in the past 24 hours, becoming the highest in Taiwan. The cumulative rainfall in other areas such as Ruifang, Shuangxi, Wanli, and Xizhi also exceeded 300 millimeters. Such extreme weather phenomena not only threaten the infrastructure of Taiwanese cities, but also draw attention once again to the global issue of the "urban wet island effect."

城市濕島效應：降雨增加的新威脅

隨著城市人口密度的增加以及建築物不斷擴展，不僅帶來了我們熟知的「城市熱島效應」，即城市地區的溫度比周邊鄉村高，還衍生出了另一個被稱為「城市濕島效應」的現象。根據德州大學奧斯汀分校科學家的研究，許多城市的年降雨量遠超過附近的農村地區，這是由於城市結構和人類活動改變了氣流和降雨的模式。

Urban wet island effect: a new threat from increased rainfall

The increase in urban population density and the continuous expansion of buildings have not only brought about the well-known "urban heat island effect", that is, the temperature in urban areas is higher than that in the surrounding countryside, but also derived another phenomenon called the "urban wet island effect". Many cities receive far more annual rainfall than nearby rural areas because urban structures and human activities change airflow and rainfall patterns, according to research by scientists at the University of Texas at Austin.

該研究發現，過去20年來，全球1,056個城市中的63％出現了年降雨量異常增加的情況。這些城市包含胡志明市、吉隆坡、拉各斯，台北到桃園的大台北都會區更是排名全球第六。這一研究表明，隨著城市化的加劇，城市的降水量異常增多，導致了水災風險大幅提升。

The study found that 63% of 1,056 cities around the world have experienced abnormal increases in annual rainfall over the past 20 years. These cities include Ho Chi Minh City, Kuala Lumpur, and Lagos. The Greater Taipei Metropolitan Area from Taipei to Taoyuan ranks sixth in the world. This study shows that with the intensification of urbanization, urban precipitation has increased abnormally, leading to a significant increase in flood risks.

為何城市會下更多雨？

氣象專家指出，城市濕島效應與熱島效應有相似之處，都是由城市獨特的結構和環境所引發。城市中的高層建築物會阻擋和減慢風速，促使空氣在市中心積聚，這些建築物還提供了上升氣流，使地表的溫暖濕氣被推入冷空氣層，從而形成雨雲。此外，來自汽車和工業排放的氣溶膠，作為雲中的凝結核，也加速了降雨過程。

熱島效應的存在，使城市上空的氣溫比周圍地區更高，這進一步促使了不穩定的空氣上升，冷卻並形成降雨雲。研究表明，人口多、氣溶膠排放量高以及熱島效應強烈的城市，年降水量更可能異常增長。這些結果警示我們，隨著全球氣候變遷，這種效應將對城市地區造成更大威脅。

Why does it rain more in cities?

Meteorological experts pointed out that the urban wet island effect is similar to the heat island effect, and both are caused by the city's unique structure and environment. Tall buildings in cities block and slow wind speeds, causing air to accumulate in city centers. These buildings also provide updrafts that push warm moisture from the surface into layers of cold air, forming rain clouds. In addition, aerosols from automobile and industrial emissions also accelerate the rainfall process by acting as condensation nuclei in clouds.

The existence of the heat island effect makes the temperature above the city higher than the surrounding area, which further promotes the unstable air to rise, cool and form rain clouds. Research shows that cities with large populations, high aerosol emissions, and strong heat island effects are more likely to experience abnormal increases in annual precipitation. These results warn us that as global climate changes, this effect will pose a greater threat to urban areas.

The Current Situation in Taiwan: Urban Expansion Brings Challenges

台灣的現況：都市擴張帶來挑戰

在台灣，特別是大台北地區，過去幾年的降雨異常情況也令人擔憂。隨著都市化的快速發展，防洪排水系統面臨更大壓力。基隆、新北等地頻繁的淹水事件，正是全球城市濕島效應的具體體現。由於建設不透水的鋪面，降雨無法有效滲透到地下，加劇了城市內澇的風險。

對此，專家建議台灣政府應借鑑國際經驗，推動「海綿城市」建設，提升城市對暴雨的應對能力。同時，應增加城市綠化，種植更多的樹木和增加可透水的土壤層，這不僅有助於降溫，還能提升地面吸收雨水的能力，減少洪災發生的機率。

In Taiwan, especially the greater Taipei area, the abnormal rainfall in the past few years is also worrying. With the rapid development of urbanization, flood control and drainage systems are facing greater pressure. Frequent flooding events in Keelung, New Taipei and other places are a concrete manifestation of the global urban wet island effect. Due to the construction of impermeable pavements, rainfall cannot effectively penetrate into the ground, exacerbating the risk of urban waterlogging.

In this regard, experts suggest that the Taiwanese government should learn from international experience to promote the construction of "sponge cities" and improve the city's ability to respond to heavy rains. At the same time, urban greening should be increased, more trees should be planted and the water-permeable soil layer should be increased. This will not only help to cool down, but also improve the ground's ability to absorb rainwater and reduce the chance of flooding.

未來展望與應對策略

Future prospects and coping strategies

聯合國政府間氣候變遷專門委員會（IPCC）預測，全球城市化進程仍將持續，至2050年全球70％的人口將居住在城市。隨著城市面積擴大，城市熱島和濕島效應將更加顯著，這需要各國政府在城市規劃和基礎設施建設上，充分考慮到這些新興的氣候風險。

The United Nations Intergovernmental Panel on Climate Change (IPCC) predicts that the global urbanization process will continue, and 70% of the world's population will live in cities by 2050. As urban areas expand, the urban heat island and wet island effects will become more significant, which requires governments of all countries to fully consider these emerging climate risks in urban planning and infrastructure construction.

台灣作為一個高度都市化的島國，未來必須更積極地應對這些挑戰。推動防洪設施升級、改善城市排水系統以及加強環境教育，都是台灣邁向氣候適應型城市的必要步驟。通過科學的規劃和政策的引導，台灣可以減少城市濕島效應帶來的風險，並保障城市居民的安全與福祉。

As a highly urbanized island country, Taiwan must respond more actively to these challenges in the future. Promoting the upgrade of flood control facilities, improving urban drainage systems, and strengthening environmental education are all necessary steps for Taiwan to move towards a climate-resilient city. Through scientific planning and policy guidance, Taiwan can reduce the risks caused by the urban wet island effect and ensure the safety and well-being of urban residents.