Relationships between phenological trends (positive = later, or delayed, occurrence; negative = earlier, or advanced, occurrence) and degrees north latitude (A), length of the time series upon which trend estimates are based (B), and the first year of observation in the time series upon which trend estimates are based (C). Non-significant (P > 0.05) trend estimates are shown in gray; black dots indicate significant trend estimates in all panels. Trend lines in each panel apply to scatter plots of significant trend estimates only. In panel A, the slope of the relationship is −0.50 ± 0.04 for pooled significant and non-significant phenological trends, and −0.53 ± 0.03 for significant phenological trends only. Graphic: Post, et al, 2018 / Scientific Reports

By Kat Kerlin
2 March 2018
(UC Davis) – Spring is arriving earlier, but how much earlier? The answer depends on where on Earth you find yourself, according to a study led by the University of California, Davis.The study, published in Nature’s online journal Scientific Reports, found that for every 10 degrees north from the equator you move, spring arrives about four days earlier than it did a decade ago. This northward increase in the rate of springtime advance is roughly three times greater than what previous studies have indicated.For example, at southern to mid latitudes such as Los Angeles, New Orleans, or Dallas, the study suggests spring might be arriving a mere one day earlier than it did a decade ago. Farther north, in Seattle, Chicago, or Washington D.C., it might be arriving four days earlier. And if you live in the Arctic, it might be arriving as much as 16 days earlier.“This study verifies observations that have been circulating in the scientific community and popular reports for years,” said lead author Eric Post, a fellow of the John Muir Institute and polar ecologist in the UC Davis Department of Wildlife, Fish and Conservation Biology. “Yes, spring is arriving earlier, and the Arctic is experiencing greater advances of spring than lower latitudes. What our study adds is that we connect such differences to more rapid springtime warming at higher latitudes.”

Accelerated spring

The study is the most comprehensive analysis to date of springtime advance, or phenology, as you move north with latitude. Such signs include birds migrating, flowers blooming, amphibians calling and the emergence of leaves.The researchers analyzed 743 previously published estimates of the rate of springtime advance from studies spanning 86 years across the Northern Hemisphere, as well as rates of springtime warming over the same range of years and latitude. Even after accounting for differences in the length, time and location of those previous studies, the relationship between earlier springs and higher latitudes was strong.

Unknowns for birds, migratory species

Springtime provides important biological cues for many plant and animal species, and it is unclear how an accelerated spring could play out for these species across the planet.The study notes that impacts to migratory birds are a potential concern. Many birds move from tropical zones to higher latitudes, such as the Arctic, to breed.“Whatever cues they’re relying on to move northward for spring might not be reliable predictors of food availability once they get there if the onset of spring at these higher latitudes is amplified by future warming,” Post said. “The springtime emergence of the plants and insects they’ll eat when they arrive is happening faster than the changes at the lower latitudes those birds are departing from.” The study’s co-authors are Byron Steinman from the University of Minnesota Duluth, and Michael Mann from Pennsylvania State University.The research received financial support from the National Science Foundation.

Contact

Kat Kerlin, UC Davis News and Media Relations, 530-752-7704, 530-750-9195 (cell), kekerlin@ucdavis.edu

Spring Is Springing Earlier in Polar Regions Than Across the Rest of Earth

ABSTRACT: In the Northern Hemisphere, springtime events are frequently reported as advancing more rapidly at higher latitudes, presumably due to an acceleration of warming with latitude. However, this assumption has not been investigated in an analytical framework that simultaneously examines acceleration of warming with latitude while accounting for variation in phenological time series characteristics that might also co-vary with latitude. We analyzed 743 phenological trend estimates spanning 86 years and 42.6 degrees of latitude in the Northern Hemisphere, as well as rates of Northern Hemisphere warming over the same period and latitudinal range. We detected significant patterns of co-variation in phenological time series characteristics that may confound estimates of the magnitude of variation in trends with latitude. Notably, shorter and more recent time series tended to produce the strongest phenological trends, and these also tended to be from higher latitude studies. However, accounting for such variation only slightly modified the relationship between rates of phenological advance and latitude, which was highly significant. Furthermore, warming has increased non-linearly with latitude over the past several decades, most strongly since 1998 and northward of 59°N latitude. The acceleration of warming with latitude has likely contributed to an acceleration of phenological advance along the same gradient.

Acceleration of phenological advance and warming with latitude over the past century