predicts entire cities could be submerged in the coming decades. and that includes london as cnn's phil black reports. >> reporter: london has long respected the power of the sea. straddling the city's famous river, you'll find this, the thames barrier. finished in the early '80s, it's a mighty, mechanized fortification. this sped-up video of a recent test shows how it works. fully extended, the gates stand five stories high. it's designed to hold back the north sea on its angriest days to protect lives and the vast riches of one of the world's wealthiest cities. >> if there was flooding, it would be absolutely catastrophic, particularly if it happened at night. if something was to go wrong, the city would absolutely grind to a halt. you know, flooding of subway stations, flooding of the tubes. >> reporter: we meet oceanographer ivan hague on a day the thames is swollen by a