The European Union’s policies could become significantly greener as the bloc’s decision-makers absorb the lessons of the recent European Parliament elections.

Green parties were among the main beneficiaries as the May 23-26 elections saw voters switch from the usually dominant center-right and center-left parties in a number of EU countries.

Parties affiliated to the European Parliament’s Green/European Free Alliance increased the number of seats they hold from 52 to 69. The newly elected Parliament will be formally inaugurated July 2 and will serve a five-year term.

But the European Parliament is a big place, housing 751 lawmakers in total...