London's $850 million Thames Barrier is one of the largest movable flood barriers in the world and it features 13-story-high gates designed to protect the city from flooding caused by storm surges and abnormally high tides. Proposals to build larger barriers are being debated to help the city prepare for future sea level rise. "The key thing is an adaptable plan," said Tim Reeder, regional climate change program manager for the U.K. Environment Agency. "We know that science will continue to change in terms of storm surge."