One scorching summer doesn't confirm that climate change is real any more than a white Christmas proves it's a hoax. What matters is the trend—a decades-long march toward hotter and wilder weather. But with more than 26,000 heat records broken in the last 12 months and pervasive drought turning nearly half of all U.S. counties into federal disaster areas, many data-driven climate skeptics are reassessing the issue.

Respected Republican leaders like Govs. John Kasich of Ohio and Chris Christie of New Jersey have spoken out...