The history of critical loads in North America could be described as a large group scavenger hunt, where many dedicated scientists, land managers and environmental specialists sought to discover multiple pathways which could be used to determine “how much deposition is too much” for sensitive ecosystems. Critical loads are valuable because they can be used in policy and management contexts to determine whether current policies and programs are protecting ecosystems from harm or damage or, if the loading threshold has been reached, to assist in developing goals and strategies for recovery.

These investigations and explorations began in earnest in North America in the 1990s, following the European models, and were largely centered on ozone in the US, and on acidification of surface waters and soils in Canada. The U.S. research on critical loads began in a relatively uncoordinated fashion through multiple efforts affiliated with many different agencies and universities. This resulted in a variety of different methods, some overlap and duplication of effort, and an insufficient linkage between research and policy needs.

In the early 2000s, critical loads in North America began to gain momentum as federal agencies and the research community held several workshops to coordinate and plan research, modeling, monitoring and policy efforts. These workshops determined that items needed in the critical loads scavenger hunt could span a vast portfolio of pollutants (nitrogen, sulfur, ozone, mercury); pathways and mechanisms (airborne, soil and water mediated, direct environmental uptake by plants), synergistic effects such as acidification from both nitrogen and sulfur; and ecological receptors sensitive to air pollutants (e.g., certain species of fish and other aquatic biota, lichen, plants, trees, and soil microorganisms).

In 2006, the critical loads research community gathered at the NADP annual meeting to consider a coordinated multi-stakeholder effort to advance critical loads science in the U.S. Ultimately, Critical Loads Atmospheric Deposition (CLAD) was formed as NADP’s first science subcommittee to provide a forum for information sharing, coordination and planning between North American agencies, groups and individuals interested in critical loads development and use. CLAD has worked successfully for over 12 years to systematically prioritize and implement critical loads research and projects, an important task given the immense complexity involved in all the facets of critical loads development. In the early days of CLAD, easier problems were tackled (such as using simple surface water models to predict changes in water chemistry at levels harming fish), later moving successively towards building more complex models (such as tens of thousands of data points representing the impacts of deposition on lichen, tree and herb species); and building comprehensive critical loads data bases and interactive web-based  tools.  Critical loads data is now increasingly being used to support regulatory development (Nox/Sox secondary standards), resource management decisions (FS and NPS land management and park planning), and stakeholder collaboration (e.g., government-business centered on agricultural emissions) forums. Critical loads has also been instrumental in making over 1000 scientific linkages between air pollutant deposition and ecosystem services such as clean water, healthy fisheries, timber, fiber, grazing, traditional medicine, and the enjoyment of nature. The field of critical loads will continue to evolve and build upon new in