"The biggest indicator of change that is already happening is usage of the Thames Barrier," King told the conference late on Monday.
"The Thames Barrier was designed to be used once every two or three years and in that period after it was built in 1980 it was indeed used once every two or three times a year. We’re now using it six times a year."
The direct cost of a breach in the flood defences would exceed 30 billion pounds ($52.70 billion) directly, King added, not including the "indirect" damage to people’s lives.
Scientists predict that heat-trapping carbon dioxide (CO2), produced by burning fossil fuels, can raise the temperature of the Earth, melt ice caps and cause flooding.
The most recent time that CO2 levels were higher than now was 55 million years ago, when CO2 was more than double current levels and the sea about 100 meters higher, King said.
King saw climate change posing the biggest threat to Britain from changes to rainfall patterns and storm damage to coastlines.