LONDON — Prime Minister David Cameron walked the flooded streets of York on Monday as Britain’s Environment Agency warned that the country needed “a complete rethink” of its flood defenses.

Thousands of people in the north of England spent another day dealing with what they called unprecedented flooding, with roads in York and in nearby Leeds still underwater and some electricity cut off. David Rooke, the deputy chief executive of the Environment Agency, said that “we are moving from known extremes to unknown extremes.”

Some scientists speculated that the effects of climate change could be evident in a year of record flooding. “We are having more severe floods in the U.K. than 10 years ago,” said Reza Ahmadian, a lecturer on water management at Cardiff University. “This is not something just happening in the U.K. — and we will see more and more of this.”

He added, “We need to be more creative about flood defenses.”

Mr. Ahmadian suggested that building temporary reservoirs for flood relief in less populated areas could be more effective than building ever-higher and more expensive flood walls, which “just create further problems downstream.”