by L.L. Robbins (USGS) and J.A. Kleypas (National Center for Atmospheric Research)

Research findings of the past decade have led to mounting concern that rising atmospheric carbon dioxide (CO2) concentrations will cause changes in the ocean's carbonate chemistry system, and that those changes will affect some of the most fundamental biological and geochemical processes of the sea.

—Impacts of Ocean Acidification on Coral Reefs and Other Marine Calcifiers

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Currently, the ocean absorbs about 30 percent of the carbon dioxide (CO₂) released to the atmosphere by fossil-fuel combustion. As the atmospheric concentration of CO₂ rises, so does absorption of CO₂ by the ocean, causing chemical changes that include a decrease in seawater pH. This "ocean acidification" reduces the availability of carbonate ions (CO₃^2-) required for building the shells and skeletons of calcifying marine organisms. Such changes, which could have far-reaching effects, have intensified researchers' interest in the study of seawater carbon—particularly CO₂ and other forms of inorganic carbon—and the need for tools to gather and manage increasing amounts of chemical data.

Since the late 1990s, those who conduct research on the chemical behavior of inorganic carbon in seawater have had several software packages for calculating CO2-system chemistry. Ernie Lewis of the Brookhaven National Laboratory and Doug Wallace of the Leibniz Institute of Marine Sciences at Kiel University (Germany) first undertook the monumental effort of sorting through the original literature and equations to produce CO2SYS, a Windows-based program that has been an invaluable and well-documented service to the research community. The equations and documentation of CO2SYS have since been adapted for use within Microsoft Excel and coded for use within such programming languages as MatLab and "R". These programs are not particularly user friendly, however, and in some cases require users to have a fair amount of skill in a computer language.

While walking with our iPhones at the Ocean Carbon Biogeochemistry Short Course on Ocean Acidification in November 2009, we decided that an iPhone application (app) for CO2SYS would be a valuable tool for many researchers, particularly for new students in the field. Development of this app would also enable the more rapid analysis of carbon data from two USGS projects: "Response of Florida Shelf Ecosystems to Climate Change" and "Arctic Ocean Acidification."

With the excellent programming skills of Mark Hansen (USGS, St. Petersburg, Florida) and Stephan Meylan (Jacobs Technology, contracted to the USGS), we developed CO2calc, an easy-to-use CO₂-system calculator designed for anyone with a PC, Mac, or iPhone. The app is described in USGS Open-File Report 2010-1280.

Like the other calculators, CO2calc is based largely on CO2SYS, but it incorporates several new developments in CO2-system calculations, including options to use the dissociation constants of Tim Lueker and others, as well as the constants of Frank Millero, which are a better fit for estuarine waters. An entirely new feature is the option to calculate air-sea CO₂ fluxes according to the gas-transfer-velocity formulations of Rik Wanninkhof, Philip Nightingale and others, and David Ho and others. This last feature has enabled a rapid, more comprehensive approach to analyzing USGS CO₂ flux data from the west Florida shelf and the Arctic. Currently, USGS scientists and collaborators are using the app to develop carbon budgets in the Gulf of Mexico.

CO2calc has an intuitively designed graphical user interface for choosing constants, entering data, and displaying results. It also includes many additional features, such as the capability of tagging data with date, time, latitude/longitude (all of which can be automatically retrieved on iPhone 3, 3GS, 4, and Windows hosts that have an NMEA [National Marine Electronics Association]-enabled GPS), as well as sample name and comments. It also allows batch-file processing, as well as an option to save sample information, data input, and calculated results as a comma-separated value (CSV) file for use with Microsoft Excel, ArcGIS, or other applications. Finally, it includes an option to export points with geographic coordinates as a KMZ file for viewing and editing in Google Earth. CO2calc documentation is provided as a PDF file, which on the iPhone version is organized into separate tab-based folders. CO2calc versions for Mac, PC, and iPhone can be downloaded from USGS Open-File Report 2010-1280 and from the Carbon Dioxide Information Analysis Center (CDIAC) site, which offers the other CO₂-system calculation programs as well. The iPhone version is also available for free in the Education section of the iTunes App store.

An important lesson learned during the development of CO2calc is that the small format of the iPhone forced the design of the interface to be simple and intuitive, so much so that the format was adapted for the PC and Mac interfaces. This new application is not only novel but also very handy for both inexperienced and experienced researchers. Since the launch of CO2calc in mid-December, we have received positive feedback from educators and scientists in many countries who are or will be using it in their classes and research.  Statistics from the USGS publication site and iPhone App store indicate that researchers in more than 17 countries have already downloaded this app, either for Mac, PC, or iPhone.

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