Californians rejoiced this week when big drops of water started falling from the sky for the first time in any measurable way since the spring, an annual soaking that heralds the start of the rainy season following some of the hottest and driest months on record.

But as the rain was beginning to fall on Tuesday night, Gov. Gavin Newsom did a curious thing: He issued a statewide drought emergency and gave regulators permission to enact mandatory statewide water restrictions if they choose.

Newsom’s order might seem jarring, especially as forecasters predict up to 7 inches of rain could fall on parts of the Northern California mountains and Central Valley this week. But experts say it makes sense if you think of drought as something caused not by the weather, but by climate change.

For decades, California has relied on rain and snow in the winter to fill the state’s major rivers and streams in the spring, which then feed a massive system of lakes that store water for drinking, farming and energy production. But that annual runoff from the mountains is getting smaller, mostly because it’s getting hotter and drier, not just because it’s raining less.

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