In this commentary, we argue that social capital theory, the idea that membership in a group creates opportunities to acquire valuable information and resources from other group members, is a useful framework in which to consider ways to increase diversity in the Earth System Sciences (ESS) and in the science, technology, engineering, and mathematics fields more broadly. Existing literature documents numerous barriers to underrepresented groups' participation in the sciences. These include a sense of isolation, a lack of visible role models, and a lack of trust in mentors or teachers. We discuss how these challenges impact acquisition of social capital and how lack of social capital affects career success and satisfaction. We conclude with recommendations for increasing diversity in the ESS through careful attention to building trustworthy professional relationships. In particular, the community should (1) recognize that trust must be built in order for students to feel connected to the larger community; (2) provide explicit guidance to students on different types of ties, how to build each type, and the value of each type in career development; and (3) train professionals to recognize their own social capital and best practices for imparting capital to students.