The Rocky Mountain wildflower season has lengthened by over a month since the 1970s, according to a study published Monday that found climate change is altering the flowering patterns of more species than previously thought.

Flowers used to bloom from mid-May to early September, but the season now lasts 35 days longer, from April to mid-September, according to researchers who collected 39 years of data at the Rocky Mountain Biological Laboratory near Crested Butte, Colo.

Earlier spring snowmelt and other climate shifts have changed the timing of blooms for more than two-thirds of 60 species of native wildflowers in mountain meadows, stands of Aspen trees and conifer forest that were surveyed from 1974 to 2012, according to the study published in the Proceedings of the National Academy of Sciences.

The scientific paper is the latest to document one of the strongest signs that global warming is shaking up the natural world. Scientists studying phenology – the timing of seasonal events in nature – are observing rapid shifts in when flowers bloom, trees leaf out and bees, birds and butterflies appear in the spring. Scientists have documented the trend using historical records from writers and naturalists, including Henry David Thoreau, who in the 1850s began recording in his journal the first blooms of the season around Concord, Mass.