Understanding atmosphere-biosphere interactions is a crucial part of efforts to improve global change simulation models, monitor variations in the growing season, and calculate the carbon budget. Satellite-derived information has a role to play in the development of global biospheric databases, but no comprehensive surface phenology network exists to calibrated these data. One type of phenological measure, the first appearance of Spring foliage (commonly called the "green wave,” “start of season,” or “onset of Spring”), is particularly important because it is crucial for accurate assessment of many processes, and is among the most sensitive plant-response measures of climate change.

Since satellite data are available for only several decades, and may not provide the details needed for many studies, and a global phenology network is not yet functional, alternatives must be employed to measure changes in the onset of Spring at the global spatial-scale and century timescale. The Spring Indices phenology models have been developed to simulate the Spring phenology of representative understory shrubs, using only daily maximum-minimum temperature data as input. They have been rigorously tested in a variety of regions and continents. While not capable of reproducing all the detailed information that would be obtained from multi-species phenology data, they can process weather data into a form where it can be applied as a baseline assessment of some aspects of a location’s phenological response over time.

In this study, an extensive mid-latitude daily maximum-minimum temperature database is used to assess the onset of Spring in all appropriate areas during the 20th century. Length of data records varies considerably by location. Some regions have temperature records available only back to the 1970's, but most locations have information from mid-century, and North American data covers the entire century and earlier. Previous work has shown trends toward earlier Spring onsets since mid-century in portions of Europe and North America, but no such trend in China. This study will look at these changes in more detail, and establish baselines and general trends for the onset of Spring in all regions.