墨尔本：在东南亚潮湿的热带地区，午后雷阵雨几乎每天都会出现。白天的高温促使高耸且蓬松的积雨云形成并不断膨胀，随后便给新加坡、香港和曼谷等城市带来倾盆大雨。
MELBOURNE: In the humid tropics of Southeast Asia, afternoon thunderstorms roll through almost every day. Daytime heat causes towering, fluffy cumulonimbus clouds to form and swell before unleashing torrential rain on cities such as Singapore, Hong Kong and Bangkok.
这些地区的排水系统原本就是为应对短暂而剧烈的降雨而设计的。然而，上周曼谷的降雨情况却完全超出了常规。
In these places, drainage systems are designed to cope with short, heavy downpours of rain. But the rains in Bangkok over the last week were far from typical.
9月本是泰国这座繁华首都的雨季，月平均降雨量约为344毫米，通常分布在18天里。但就在上周，曼谷在48小时内降雨量就达到了约300毫米，远远超出了当地排水系统的承受能力。
September is the wettest month in Thailand’s bustling capital, with a monthly average of 344mm usually spread across 18 rainy days. But just last week, roughly 300mm of rain fell in Bangkok over 48 hours, overwhelming the city’s drainage systems.
如此巨大的降雨量促使当局宣布全城进入灾害状态。随着浑浊的褐色积水淹没整座城市，至少已有31人丧生，数千名居民被迫撤离。
This extraordinary volume of water prompted authorities to declare a city-wide disaster zone. As stagnant, brown water has inundated the city, at least 31 people have died and thousands have been forced to evacuate.
这种突如其来的极端强降雨是由一个在曼谷地区滞留了约三天的低压系统引发的。在低压系统中，空气上升，其中的水汽凝结形成云层并降下雨水。如果低压系统长时间停滞不动，就会在同一地区持续降雨，进而引发严重洪涝。
NO STRANGER TO FLOODING This sudden and extreme downpour was caused by a low-pressure system that stayed over the Bangkok region for about three days. In a low-pressure system, air rises and moisture in the air condenses to form clouds and rain. If a low-pressure system stalls, it can dump rain over the same location for days, leading to severe flooding.
在邻近的泰国湾，海面温度接近30摄氏度，比当地平均水平高出数度。这种温暖的海水为风暴的持续提供了充足的水汽与能量。
In the nearby Gulf of Thailand, sea surface temperatures are sitting close to 30°C, several degrees above the local average. These warm conditions provide ample moisture and energy for the storm to persist.
泰国并非第一次遭遇洪水。2011年，大量降雨、多场热带风暴、海平面升高以及季风季节提前到来相互叠加，引发了持续数月的特大洪水，导致800多人丧生。世界银行的数据显示，那次洪水造成的经济损失估计高达465亿美元，当地制造业受到的冲击尤为严重。
Thailand is no stranger to flooding. In 2011, more than 800 people died when high rainfall, multiple tropical storms, high sea levels and an early start to the monsoon season collided. This triggered widespread flooding that lasted months. World Bank figures show these floods caused an estimated US$46.5 billion in damage, with the local manufacturing industry worst hit.
曼谷坐落于湄南河入海口附近的冲积平原上，这条河流最终汇入泰国湾。整座城市地势平坦，缺乏能够减缓或分流大量水流的自然地形条件。
Bangkok sits on a floodplain near the mouth of the Chao Phraya river, which drains into the Gulf of Thailand. The city is largely flat, without the natural topography to slow or redirect vast volumes of water.
数十年来，曼谷的地表一直在下沉。20世纪中叶，由于大量抽取地下水，曼谷的地基逐渐下沉，仅1988年一年地表就下降了10多厘米。
Bangkok has also been sinking for decades. Extensive groundwater extraction in the mid-20th century caused the ground beneath Bangkok to progressively sink, dropping by more than 10cm in 1988 alone.
后来通过调整政策，比如对地下水实行定价机制并加强监测，地表下沉速度已降至每年约2至3厘米。但随着海平面不断上升，这座处于海岸带的城市越来越难以将洪水排入泰国湾，发生持续性洪水的风险也随之增加。
Policy changes, such as groundwater pricing and extensive monitoring, have reduced this rate of sinking to about 2cm to 3cm per year. But as sea levels rise, this sinking coastal city is less able to drain floodwater into the Gulf of Thailand, heightening the risk of prolonged floods.
与此同时，曼谷的城市化进程也在加速，高楼大厦林立，人口持续增长。城市中大量松软的土壤被混凝土取代，导致降雨时地面吸水能力大幅下降。过去30年间，曼谷的城市化进程使得极端降雨期间的地表径流增加了25%。
Bangkok is also rapidly becoming more urban, characterised by high rises and a booming population. Concrete is replacing spongy soils across much of the city, meaning the ground absorbs less water when it rains. The past 30 years of urbanisation in Bangkok have increased surface water runoff by 25 per cent during bouts of extreme rainfall.
气候变化的痕迹目前尚难以准确评估气候变化对此次降雨事件的具体影响。不过，从全球极端降雨的趋势来看，气候变化的痕迹已清晰可见。
FINGERPRINT OF CLIMATE CHANGE It’s too early to know the exact effect of climate change on this specific rainfall event. However, the fingerprint of climate change is detectable in global extreme rainfall trends.
在曼谷，相关研究显示极端降雨日的降水量还在持续增加：每年降雨最多的那一天的降水量每十年就会增加6至12毫米。
In Bangkok, research suggests extremely rainy days are getting even more wet, with the heaviest rainfall day of the year increasing by 6mm to 12mm every decade.
另一项针对2011年洪水的研究也发现，泰国雨季前的降雨量呈明显上升趋势。泰国的大部分降雨都集中在5月至10月的雨季，这种趋势可能使得土壤在强降雨来临前就处于极易发生洪水的状态。
Another study of the 2011 flood identified a strong increasing trend in pre-monsoon rainfall across Thailand. Most of Thailand’s rain happens in the monsoon months of May to October. This could prime the soils for flooding even before the heaviest rain occurs.
随着全球气温升高，大气能够容纳的水分也更多，这意味着未来降雨强度很可能会超过没有人为气候变暖情况下的水平。令人担忧的是，今年8月地球大气中的水汽含量创下了历史最高纪录。
The atmosphere is able to hold more water as global temperatures rise, meaning future rainfall will likely be more intense than if humans weren’t warming the planet. Worryingly, Earth’s atmospheric water vapour reached a record high in August.
目前我们还不清楚气候变化是否会影响那些停滞不前的低压系统。不过一旦这些系统形成，预计降雨量也会随之增加。
We don’t yet know if climate change will affect stalling low-pressure systems. But when these systems do form, we can expect more rain to fall.
未来降雨或将加剧：气象预报显示，曼谷未来一周将迎来更多降雨。尽管如此，当地居民早已开始清理工作，并继续过着日常生活；甚至还有人创办了水上市场来售卖农产品。
PREPARE FOR INTENSIFYING RAIN Forecasts suggest Bangkok is in for more rain over the next week. However, residents have already started the clean-up and are getting on with everyday life, even creating a floating market to sell produce.
但从长远来看，受气候、地理条件及城市化进程的共同影响，曼谷无疑还会遭遇极端洪水。
In the long-term, however, extreme floods will undoubtedly strike Bangkok again due to a combination of climate, geography and urbanisation.
关键在于，城市排水系统的设计不仅要能应对当前的极端降雨，还要能适应未来可能愈发频繁的强降雨。推行绿色基础设施与透水路面等城市规划措施，有助于减少暴雨径流并提升地面的吸水能力。
Crucially, urban drainage systems must be designed to cope not just with present-day extreme rainfall, but future projections of intensifying rain. Urban planning that promotes green infrastructure and permeable pavements can reduce storm run-off and allow the ground to absorb more water.
完善预警系统对保护弱势群体至关重要——有曼谷居民反映，他们直到积水已没过腰部才收到手机预警信息。加大对全球气象观测及高分辨率气象模型的研究投入，则能提升极端天气的预报精准度。
Improving early warning systems is vital to protecting vulnerable residents, with some Bangkok locals reporting they didn’t receive phone warnings until the water was already waist-high. Investing in global weather observations and high-resolution models will provide better forecasts of extreme weather.
金伯利·里德是墨尔本大学的大气科学领域博士后研究员。本文最初发表于《对话》网站。
Kimberley Reid is Postdoctoral Research Fellow in Atmospheric Sciences at The University of Melbourne. This commentary first appeared on The Conversation.