Expanding the Spatial Reach and Human Impacts of Critical Zone Science

Two major barriers hinder the holistic understanding of subsurface critical zone (CZ) evolution and its impacts: (a) an inability to measure, define, and share information and (b) a societal structure that inhibits inclusivity and creativity. In contrast to the aboveground portion of the CZ, which is visible and measurable, the bottom boundary is difficult to access and quantify. In the context of these barriers, we aim to expand the spatial reach of the CZ by highlighting existing and effective tools for research as well as the "human reach" of CZ science by expanding who performs such science and who it benefits. We do so by exploring the diversity of vocabularies and techniques used in relevant disciplines, defining terminology, and prioritizing research questions that can be addressed. Specifically, we explore geochemical, geomorphological, geophysical, and ecological measurements and modeling tools to estimate CZ base and thickness. We also outline the importance of and approaches to developing a diverse CZ workforce that looks like and harnesses the creativity of the society it serves, addressing historical legacies of exclusion. Looking forward, we suggest that to grow CZ science, we must broaden the physical spaces studied and their relationships with inhabitants, measure the "deep" CZ and make data accessible, and address the bottlenecks of scaling and data-model integration. What is needed-and what we have tried to outline-are common and fundamental structures that can be applied anywhere and used by the diversity of researchers involved in investigating and recording CZ processes from a myriad of perspectives. The "critical zone" is the zone of the Earth from treetops to belowground water. It is where crops are grown, water is drawn for drinking and industry, and waste generated by humans ends up. Understanding how deep this section of the Earth is, and how it is changing, is key to being able to determine how impactful changes in land use or climate will be to human systems. That said, it is difficult to get below-ground information, and scientists in different subfields of the Earth sciences define the depth of the critical zone differently. Here, we describe the tools and language we use to make those decisions. We also note that Earth scientists are not a diverse group, which means we miss out on the ideas, solutions, and impacts that those from historically excluded groups might have to address the big problems humans must solve. To advance critical zone science, we need to broaden the physical areas we study, access to these sites, and the inclusion and sense of belonging of people studying them. We also need to measure deep into the Earth where we can, make our data accessible, and tackle scientific obstacles in integrating data and models. CZ depth is defined by multiple processes key to human existence, yet is undermeasured and thus poorly understood Scientific jargon can make defining and communicating CZ processes difficult to stakeholders and even scientists in other fields Legacies in western science, upon which CZ science is based, can be countered to promote progress in the Earth sciences