Seasonal timing has myriad impacts on plants and animals, biospheric processes, and human systems, and is critical for formulating adaptive responses to both climate variability and change. In the U.S., and especially, the timing of seasonal transitions varies widely from year to year and is also changing directionally, yet the climatic drivers, patterns, and consequences of these variations are not well understood. I will discuss day-of-year (DOY) metrics that define spring onset in the U.S. These DOY metrics exhibit secular trends consistent with both natural variability and greenhouse warming, with abrupt advances spring for most regions clustered in the mid-1980’s and abrupt delays in fall clustered in the mid-1990. Exceptions include “warming holes,” with delayed spring onset abruptly ~1958 and advanced autumn onset in the High Plains gradually since the 1950’s. In the West, both snowmelt and accumulated heat needed to bring plants out of winter dormancy track Pacific Ocean variability, but experience a step change due to springtime warming around 1984. In the atmosphere, spring onset variations also appear linked to the Pacific North American (PNA) pattern and the Northern Annular Mode (NAM). By contrast, last spring frost, first fall frost, and the duration of the growing season in the coterminous U.S. follows Indian and North Atlantic Ocean variability. My presentation will discuss different interpretations of large-scale drivers, opportunities for long-range forecasting, and implications for continental-scale phenological monitoring in the U.S.
Julio Betancourt is a Senior Scientist with the U.S. Geological Survey in Reston VA. He is broadly trained and has published two books and more than 160 technical papers in a wide variety of scientific journals. Julio studies how climate variability and change affects terrestrial ecosystems at scales critical for understanding ecological and evolutionary processes. His research helps inform rational approaches to managing water and other natural resources under an uncertain and changing climate. Two of his favorite achievements include his role launching a regional non-profit to coordinate assessment and mitigation of buffelgrass invasion in southern Arizona (www.buffelgrass.org) and establishing the USA National Phenology Network (www.usanpn.org). He has received prestigious awards from the American Water Resources Association, the Ecological Society of America, the Geological Society of America, the American Geophysical Union, the Department of Interior, and the White House.