Climate change may have been behind the decline of some Bronze Age "megacities" built by a civilization in Pakistan and India, scientists say. An abrupt weakening of the summer monsoon affecting northwest India 4,100 years ago and the resulting drought coincided with the beginning of the decline of the metropolis-building Indus Civilization, researchers at Britain's Cambridge University reported Wednesday. An analysis of oxygen isotopes in shells preserved in ancient lake bed sediments revealed how much rain fell in the lake thousands of years ago, with strong evidence of drought, they said. "We think that we now have a really strong indication that a major climate event occurred in the area where a large number of Indus settlements were situated," David Hodell of Cambridge's Department of Earth Sciences said. Large parts of modern Pakistan and much of western India were home to a great Bronze Age urban society, the researchers said. "The major cities of the Indus civilization flourished in the mid-late 3rd and early 2nd millennium B.C.," Cambridge archaeologist Cameron Petrie said. "Large proportions of the population lived in villages, but many people also lived in 'megacities' that were 80 hectares [200 acres] or more in size -- roughly the size of 100 football pitches. "But, by the mid 2nd millennium B.C., all of the great urban centers had dramatically reduced in size or been abandoned." Evidence suggests the drought affecting he area was long-lived, the researchers said, which could have resulted in changes in the location of settlements and a demographic shift. "We estimate that the climate event lasted about 200 years before recovering to the previous conditions, which we still see today, and we believe that the civilization somehow had to cope with this prolonged period of drought," Hodell said.