Critical Zone Science and the Path Ahead

The Critical Zone is often described as the “living skin” of the planet—the region extending from the top of the vegetation canopy down through the soil, weathered rock, and underlying groundwater. Within this narrow zone, vital processes such as soil formation, water filtration, and nutrient cycling occur. Though these processes have been studied independently for centuries, scientists started bringing them together in the early 2000s to understand the interconnected system more holistically. This shift in perspective gave birth to what we now call Critical Zone Science.

One milestone in shaping this field was the establishment of the Critical Zone Observatories (CZOs) by the National Science Foundation (NSF) in 2007. These observatories were designed as natural laboratories where hydrologists, geochemists, ecologists, soil scientists, and others could collaborate to unravel how the interactions among air, water, rock, soil, and living organisms shape the Earth’s surface over time. Early research in these observatories underscored how understanding the Critical Zone is essential for managing resources such as clean water, fertile soils, and stable ecosystems.

Historically, geologists studied bedrock and mountain-building processes, while soil scientists focused on the uppermost layers. Hydrologists investigated rivers and groundwater systems, and ecologists looked at flora and fauna. Yet, these individual disciplines often lacked opportunities for cohesive interaction. The concept of the Critical Zone began to bridge these gaps, showing that the surface of the Earth is not just a set of isolated components but an integrated system. By focusing on how water, energy, and materials flow through this dynamic interface, researchers gained insights into problems that transcend scientific boundaries—such as how climate change might alter soil processes or how land-use shifts could impact water storage in the subsurface.

Fast-forward to December 2024 at the American Geophysical Union (AGU) Fall Meeting (#AGU24). A dedicated town hall titled “The Future of Critical Zone Science” convened researchers to share updates, discuss challenges, and chart next steps. Kamini Singha, from the Colorado School of Mines, initiated the discussions by highlighting that defining the Critical Zone remains a challenge. The term can encompass a wide variety of processes and scales, and sometimes researchers become siloed in their specific techniques or topical interests. Singha’s earlier writings, such as her March 2024 paper published in Earth’s Future, illustrate the diversity of research questions being tackled under the Critical Zone umbrella—from moisture retention in fractured rock to the influence of microbial communities on elemental cycling.

Next, Asmeret Asefaw Berhe of the University of California Merced spoke on the importance of understanding soil health within the Critical Zone, referring to her 2015 publication linking soil and human health. Soil serves as a foundation for agriculture, a filter for groundwater, and a complex habitat for microorganisms. Berhe’s work has shown that small-scale shifts in soil chemistry can have significant downstream impacts—from greenhouse gas emissions to nutrient availability for crops. By placing soil research within the broader Critical Zone framework, scientists can better see how changes in precipitation or temperature—factors that might once have been considered separately—affect overall ecosystem resilience.

During open discussion, attendees learned about a newly announced NSF program called Water, Landscape, and Critical Zone Processes (WaLCZ). This initiative aims to support interdisciplinary research dedicated to unraveling how water flows through landscapes and interacts with the biotic and abiotic components of the Critical Zone. Such a program serves as both a funding opportunity and a strategic move to continue building connections among diverse fields. Panelists emphasized the value of conducting work in established research sites, which may already have data, instrumentation, or local expertise. However, as climate change shifts precipitation patterns and land use expands into previously undeveloped areas, identifying ideal future study locations could become more complex.

One recurring theme was the need for continued integration—scientific, institutional, and interpersonal. Numerous participants at the town hall echoed that no single discipline can solve Critical Zone problems alone. Researchers must look beyond their individual specialties to form teams that tackle questions on scales ranging from microscopic to global. Studies that combine hydrology, geology, biogeochemistry, and social sciences are increasingly crucial. Yet panelists also acknowledged the inherent difficulties of managing “team science,” including coordination challenges, data sharing, and reconciling different methodologies or jargon.

In concluding remarks, participants reiterated that to ensure a bright future for Critical Zone Science, the community needs robust collaboration and a broad, systems-thinking approach. Simple guidelines emerged: step beyond your discipline, link processes across the Earth’s surface, and foster partnerships that bring in new perspectives—be they from across the hallway or across the globe. With continued support from programs like WaLCZ, the research community has tools to unite efforts and address pressing environmental questions.

Critical Zone Science arose to meet a growing realization: the intricate processes that shape life-sustaining functions on Earth cannot be fully understood in disciplinary silos. By looking back at key moments—like the formation of the CZOs and the rise of transdisciplinary collaborations—scientists today can appreciate how far we’ve come and what challenges remain. Just as the Critical Zone itself is a mosaic of interconnected environments, the future of its study depends on equally interconnected teams of researchers dedicated to preserving and understanding this vital region of our planet.