We developed a simplified plant-soil model (PSM) composed of four coupled differential equations that simulate the effects of climate change on major stocks and fluxes of carbon (C) and nitrogen (N) in terrestrial ecosystems. Here we use the model to examine past, present, and future changes in C storage in arctic Alaska, a region undergoing rapid climate change. Model parameters were initialized to simulate the buildup of C and N stocks from the beginning of the current postglacial period (~10,000 years BP) to present-day levels for tussock tundra at Toolik Lake, Alaska. For projected rates of warming during the next century, the model predicts an increase in aboveground plant biomass and a net loss of soil carbon, resulting in almost no net change in total ecosystem C. The simplified model structure serves to clarify several important issues that have not been adequately addressed in previous studies. These issues include altered residence times of C and N in soils and plants, decreased synchrony of above and belowground processes, and the relationship between a model's initial conditions and the ecosystem's trajectory at the point of initialization.