For years, climate change activists have faced a wrenching dilemma: how to persuade people to care about a grave but seemingly far-off problem and win their support for policies that might pinch them immediately in utility bills and at the pump. But that calculus may be changing at a time when climatic chaos feels like a daily event rather than an airy abstraction, and storms powered by warming ocean waters wreak havoc on the mainland United States. Americans have spent weeks riveted by television footage of wrecked neighborhoods, displaced families, flattened Caribbean islands and submerged cities from Houston to Jacksonville. "The conversation is shifting," said Senator Brian Schatz, Democrat of Hawaii. "Because even if you don't believe liberals, even if you don't believe scientists, you can believe your own eyes."