Rapid climate change—such as recent changes in climate that are occurring more rapidly than at any time since the last glacial maximum—presents two particularly formidable challenges for national parks and society in general. First, we must improve our understanding of the effects of climate change and how to manage them. Second, we must communicate the science of climate change in a concrete, noncontroversial (or minimally controversial) way that promotes understanding and action. In the National Park Service (NPS), many efforts are under way to address these two challenges (http://www.nps.gov/climatechange). Here, we describe one promising approach that addresses both challenges simultaneously: studying climate-driven changes in phenology—the timing of seasonal biological events, such as flowering and migrations.

Phenology has played an important role in the lives of people, plants, and animals through history. Human subsistence has depended on knowing when food plants are available and when game species arrive or depart on migrations. Much of ecological theory and many of our management practices recognized this, but assumed that phenology was relatively stable from one year to the next, in part because climate, which drives the timing of many phenological events, was long thought to be fairly stable, or “stationary” (Milly et al. 2008).

In a period of rapid climate change, though, understanding phenology becomes even more important. Almost every ecological relationship and process—including predator-prey and plant-pollinator interactions, the spread of disease, pest outbreaks, and water and carbon cycling—depends on the timing of phenological events (Forrest and Miller-Rushing 2010). As climatic conditions change, phenology changes, and so do these ecological relationships and processes. These shifts are further complicated because the phenologies of different species change at different rates and in different directions, some occurring earlier, others later (Sherry et al. 2007; Thackeray et al. 2010). In some cases this may lead to mismatches, as has occurred in parts of Europe where pied flycatchers (Ficedula hypoleuca) are now breeding too late relative to when their primary food source, winter moth caterpillars, is available; where this mismatch is most severe, populations of pied flycatchers are declining by up to 90% (Both et al. 2006). Changes in phenology also vary across space, as is evident in the earlier-than-average spring green-up and flowering of most plants in the northern United States, but later in southern regions (Zhang et al. 2007; Von Holle et al. 2010). Right now we are ill-equipped to predict the impacts of phenological changes on species and ecosystems because of a dearth of data describing the phenology of most species and the role of timing in regulating species interactions and ecological processes.

In addition to its role in ecosystem functions, phenology provides one of the most fundamental ways people relate to nature. Phenological events mark the changing of seasons: the emergence of leaves and butterflies and the sounds and activities of birds, frogs, and other animals herald the arrival of spring; fall foliage and crop harvest mark the onset of autumn and winter in much of the country. Because phenology is tightly coupled with climate and is changing wherever climate is changing, it provides a way that people can “see” climate change and its impacts wherever they are.