Not only is spring springing earlier, but how early depends on how far from the equator you are, according to a study out of the University of California, Davis. Spring arrives four days earlier for every 10 degrees north you may find yourself, which is about three times the variation previously thought. The implications are potentially concerning for birdlife in particular, the researchers say.

According to the study, cities at mid latitudes like New Orleans (30 degrees north), Dallas (32 degrees north) or Los Angeles (34 degrees north) will be seeing spring just one day sooner than they were a decade ago. For international context, cities at similar latitudes include Lahore (31 degrees north), Baghdad (33 degrees north) and Tokyo (35 degrees north).

But pop farther north to Washington DC (39 degrees north), Chicago (41 degrees north) or Seattle (47 degrees north), and spring could be arriving four days earlier than a decade ago. Outside America, cities like Beijing (40 degrees north), Bucharest (44 degrees north) and Milan (45 degrees north) lie at similar latitudes.

As far north as the arctic, the researchers think that spring could be arriving 16 days sooner than was the case 10 years ago.

The research, which looked exclusively at the Northern hemisphere, examined 743 estimates on the rate of springtime advance from 86 years of related studies, drawing on springtime indicators like the appearance of new leaves, blooming flowers and migratory birds.

"This study verifies observations that have been circulating in the scientific community and popular reports for years," says Eric Post, lead author of the research. "What our study adds is that we connect such differences to more rapid springtime warming at higher latitudes."

Perhaps unsurprisingly, the study says climate change is likely a contributing factor in this variation across latitudes. The study notes that the degree of land surface warming is generally greater further from the equator, which chimes with prior research from NASA. The report suggests that, left unchecked, the variation may continue to widen.

The research raises particular questions for migratory birds which fly north 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 explains. "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."

Previous research from the University of Edinburgh (PDF link) found that birds are migrating earlier as a result of climate change and warmer springtimes, potentially arriving north too early for food supply. Affected species include swallows, flycatchers, lapwings and wagtails.

This new study is available online in the Nature journal Scientific Reports.