Summary: If humans don't act, earth could share this distant planet's fate.
“This world was a lot like earth a few millennia ago,” the Doctor told Bill as they stood atop a high ridge dotted with stunted trees, looking out across a vast expanse of water. “Now all the cities and the rich farmland are gone. The survivors live in scattered settlements, trying to farm the uplands. It’s a hard life.”
“What happened?”
“It’s a familiar story. Pollution and carelessness altered the planet’s climate, the icecaps melted, and the lowlands flooded. The cities still exist, beneath the water; an entire civilisation is now home to fish. The same could happen on earth.”