"Polder Effect" Puts Shanghai, Dhaka, Karachi at Risk of "Watery Grave" by 2100

• Coastal megacities like Shanghai, Dhaka, and Karachi risk catastrophic flooding and disappearing by 2100 due to sinking land and rising sea levels.
• The "polder effect" describes how floodwater becomes trapped in sinking delta cities when protective barriers fail, as it cannot drain naturally.
• A study identified 40 major delta cities at risk, including Shanghai, Dhaka, Karachi, and New Orleans, with China and Southeast/South Asia being high-risk regions.
• Current reliance on sea walls is insufficient; experts warn that climate change increases storm threats, and wall failures could lead to widespread, trapped flooding.
• Scientists recommend layered protection strategies, including natural barriers (wetlands, mangroves), "sponge cities," and relocating development from vulnerable zones.

Why It Matters: Coastal megacities face an underestimated "polder effect" flood risk, threatening millions.