If you guessed that the University of California system would champion “sustainable” investing, you’d be correct. The catch is, the shelves in the sustainable investing aisle aren’t especially well-stocked with opportunities. U.S. college endowments altogether hold more than $500 ­billion in assets, a growing part of which they’ve allocated to sustainable or “impact” investing. Yet they also face something of a paradox: Where, exactly, does one invest that money? The easy answer, and one popular among smaller endowments, is to buy funds of stocks deemed green. Another option is real assets, says Jagdeep Bachher, who oversees the University of California’s...