Anonymous gifts are common. But a climate group says the $10 million donation is a big mystery

On a Friday morning in April, Dan Stein, the founder of Giving Green, a climate philanthropy organization, found big news in a surprising email. An anonymous donor had given his fund $10 million.

"I haven't quite processed the number of zeros," Stein said, adding that he was "excited, awestruck and surprised" by the gift.

Giving Green collects and distributes donations to a handful of nonprofits that it believes have the potential to make a significant difference in preventing climate change and reducing greenhouse gas emissions. The $10 million donation is by far the largest gift the nonprofit has ever received ...