Modeling Phenological Change at Global-Scales Using Climate Data

Phenology is the study of recurring plant and animal life cycle stages, such as leafing and flowering, maturation of agricultural plants, emergence of insects, and migration of birds, especially their timing and relationship with weather and climate. Recently, the value of phenological research for understanding Earth systems interactions and facilitating global change studies has been realized. As a simple expression of seasonal biology, phenology offers another independent measure (along with climate records and remote sensing observations) of the extent and impact of climate change. However, phenological data are still not collected and recorded in spatially comprehensive and comparable ways around the world. Thus for now, phenological models can allow simulation of general plant responses, facilitating testing of broad hypotheses in locations and at times when actual phenological data are not available, but with more detail than possible when using remote sensing-derived measures. One set of phenological models that have been successfully applied to assess impacts of climate change on the onset of the spring growing season across temperate regions around the Northern Hemisphere are the Spring Indices (SI). This suite of metrics includes several sub-models and associated measures, all of which can be calculated using daily maximum/minimum surface temperatures and station latitude. SI models process weather data into a form mimicking the spring growth of plants that are not water limited, and are responsive to temperature increases. This paper summarizes earlier SI results from station data that have confirming a nearly universal quicker onset of early spring warmth (SI first leaf date, -1.2 days/decade), late spring warmth (SI first bloom date, -1.0 days/decade), and last spring freeze date (-1.5 days/ decade) across most temperate Northern Hemisphere land regions over the 1955–2002 period. Further, more recent work using longer and denser station data since 1900 across the continental USA has shown: 1) the SI onset of spring growing earlier since the late 1950s, including a dramatic shift in the mid-1980s; 2) regional differences in the Southeast USA; 3) 2012, the earliest year on record; and 4) 2013, among the latest years on record. Finally, preliminary results from on-going work will be presented that uses gridded air temperature data and SI to assess changes in the spring phonological response around the globe in both the past and the future at the century time-scale.