Citizen science is a term most often employed to describe projects for which volunteers collect data for use in organized scientific research. This usage of the term emerged from the Cornell Lab of Ornithology in 1994 when the lab desired a new name for its rapidly growing assemblage of data collection projects focused on birds. At that time, volunteer data collection efforts were relatively few in number, and most of the ones that did exist focused on monitoring the quality of lakes, streams, and rivers. Twenty years later, data-driven citizen science projects number in the thousands, and their participants number in the many hundreds of thousands. Projects cover a breadth of topics ranging from native bees to invasive species, from urban birds to arctic glaciers, and from pollen to stardust. Some projects engage a handful of participants in one small watershed, while others enroll many thousands of observers dispersed across several continents. Although projects vary in the degree of collaboration between volunteer participants and science researchers, in most projects volunteers receive some degree of guidance in project procedures to ensure consistency in data collection and accuracy in data analysis. The scientific impact of these projects, which yield knowledge by collecting and analyzing vast quantities of data at unprecedented scales, is easily measured by the rapidly growing number of publications based on volunteer-collected data (listings of projects and published papers are available at www.citizenscience.org).

While citizen science is sometimes considered a recent phenomenon, amateur scientists have been studying the world for much of recorded history, usually by noting observations of the environment around them. Also known as “volunteer monitoring” and “community science,” citizen science efforts have yielded important datasets, specimen collections, and scientific insights since the seventeenth century and probably before. Much of our current information about the distributions of plants and animals, the timing of events in nature such as plant budding and bird nesting, the quality of water in streams and rivers, and the impacts of climate change on organisms around the world is derived from data collected by members of the public.

Although citizen science as a concept has a long history, the strategy of involving the public in scientific research as a method for increasing public science literacy is relatively recent. In the late 1980s, a group of educators pondering innovations in science education realized that by providing participants in volunteer monitoring projects with materials to support learning – for example, information about why a project was started, what scientific questions it was investigating, how a participant’s data would be combined with data from others to answer those questions, and details about the organisms or phenomena being studied – the participants might learn scientific facts and concepts and also begin to understand how scientists conduct investigations that yield evidence-based results. For example, for The Birdhouse Network – which began in 1995 and is now part of Project NestWatch (www.nestwatch.org) – participants kept track of the birds nesting in birdhouses in their yards and communities. They noted the species, number of eggs laid, timing of hatching and fledging, and overall nesting success and then submitted their data to a centralized project database. The data were then analyzed by scientists to determine information such as the influence of latitude on nesting success. At the same time, through the process of learning about cavity-nesting birds and studying their breeding behavior – which was supported by instructional booklets, posters, and simple data forms – project participants increased their knowledge of a number of aspects of bird biology.

As the twentieth century got under way, the idea that public participation in organized research could yield “hands-on” science learning took hold rapidly, and the number of projects intended to achieve goals for increasing both science knowledge and public science literacy began to multiply. The expansion of complex citizen science projects was further fueled by the development of the Internet, which allowed project participants to submit data to online databases and, in some cases, to be able to access project data for their own interpretation. Also, some citizen science projects, such as the University of Minnesota’s Monarch Larva Monitoring Project, began to develop science curricula specifically designed for K-12 teachers who wished to incorporate citizen science into their classroom activities. Such curricula have been shown to help students learn many different aspects of science such as content knowledge and understanding of key features of scientific investigations and the nature of scientific research.

In response to the burgeoning field, the US National Science Foundation funded a workshop in 2007 that assembled 50 citizen science project leaders to discuss “best practices” for citizen science project design. The workshop yielded the “Citizen Science Toolkit,” which provided guidelines for developing, implementing, sustaining, and evaluating projects designed to achieve outcomes for both science and education. The NSF funded a second citizen science conference in 2011; this one focused on how citizen science projects could advance the field of biological conservation. The proceedings of these two conferences, both available at www.citizenscience.org, are a rich introduction into the field of citizen science and its outcomes for a wide range of project types. And in 2012, an open conference on citizen science held in Portland, Oregon, attracted nearly 300 professional scientists and educators who discussed a wide range of project models and who launched an International Association for Citizen Science (reports from this conference also are available at www.citizenscience.org).

In the early 2000s, a new form of data-driven citizen science began to emerge, born of developing technology and the concept of crowdsourcing. At the vanguard was a project called Galaxy Zoo, which employed the power of the Internet to enable members of the public to classify images of space captured by the Hubble Space Telescope. This form of citizen science became very popular as new projects were developed to explore the surface of the moon, model Earth’s climate using historic ship logs, and explore the ocean floor (www.zooniverse.org). Like the earlier monitoring projects, many of these data classification projects were intended not only to achieve scientific goals but also to help participants learn scientific information and develop positive attitudes toward science while participating in the scientific process. For example, participants in a project called “Citizen Sky” have demonstrated a positive change in scientific attitudes, apparently related to their engagement in the project’s social activities.

In 2009, a group of researchers working under the auspices of CAISE (Center for Advancement of Informal Science Education) produced a document that described different models of citizen science for which participants collect or classify data. These authors introduced the term “Public Participation in Scientific Research” (PPSR) as an umbrella concept to refer to a range of project types that engage participants in the scientific process to varying degrees. The authors found that different PPSR models yielded different types of learning outcomes and suggested that project developers be deliberate in their project designs, carefully matching design to desired outcomes.

An additional form of citizen science also exists as described by Alan Irwin in his 1995 book Citizen Science: A Study of People, Expertise, and Sustainable Development. In contrast to the definition of citizen science as the engagement of volunteers and professionals in collaborative research to generate new science-based knowledge, the concept of citizen science that Irwin champions aims to bring the public and science closer together, to consider possibilities for a more active “scientific citizenship,” and to involve the public more deeply in issues related to risk and environmental threat. Some data-driven citizen science projects do have objectives for achieving better linkages between science and society and even “democratizing” science, such as work currently being conducted in Europe by the Extreme Citizen Science group (ExCiteS: http://www.ucl.ac.uk/silva/excites).

With its goal of transforming the world through the bottom-up creation of knowledge, the future of the citizen science field seems nearly boundless. The ultimate success of the field will be measured by the ability of citizen science to empower members of the public to invoke transformative change for themselves, society, and the environment, blending concepts and ideas from all forms of public participation into powerful societal change.

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