Using a coupled global atmospheric chemistry and climate model we have predicted the evolution of tropospheric concentrations of chemical species along with climate parameters, based on a set of economic model predictions for anthropogenic emissions of chemically and radiatively important trace gases in the next 120 years. In particular, our predictions for tropospheric hydroxyl radical (OH) concentrations indicate the potential for substantial future changes affecting both atmospheric chemistry and climate. OH is arguably the most important free radical in the troposphere because it is the primary removal mechanism for most gases entering the atmosphere, and therefore, determines the lifetimes of these species. Our research indicates that if CH4 and CO emissions continually increase as expected through the next century, the tropospheric concentration of OH could decrease by as much as 29% from its current value. As a result, the lifetime of CO in the year 2100 is predicted to lengthen by 0.6 months beyond its current value of 2 months, and the CH4 lifetime in 2100 would exceed its current value (9 years) by 2.5 years in our reference case. / Includes bibliographical references. / Abstract in HTML and technical report in HTML and PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/)