Global Warming

Frost Season Arriving Later Each Year - USA

Across the United States, the year's first freeze has been arriving further and further into the calendar, according to more than a century of measurements from weather stations nationwide.

The trend of ever later first freezes appears to have started around 1980, according to an analysis by The Associated Press of data from 700 weather stations across the U.S. going back to 1895 compiled by Ken Kunkel, a meteorologist at the National Oceanic and Atmospheric Administration's National Centers for Environmental Information.

To look for nationwide trends, Kunkel compared the first freeze from each of the 700 stations to the station's average for the 20th century. Some parts of the country experience earlier or later freezes every year, but on average freezes are coming later.

The average first freeze over the past 10 years, from 2007 to 2016, is a week later than the average from 1971 to 1980, which is before Kunkel said the trend became noticeable.

Overall the United States freeze season of 2016 was more than a month shorter than the freeze season of 1916. It was most extreme in the Pacific Northwest. Oregon's freeze season was 61 days - two months - shorter than normal.

A shorter freeze season means a longer growing season and less money spent on heat. But it also hurts some plants that require a certain amount of chill, such as Georgia peaches, said Theresa Crimmins, a University of Arizona ecologist. Crimmins is assistant director of the National Phenology Network . Phenology is the study of the seasons and how plants and animals adapt to timing changes.

Pests that attack trees and spread disease aren't being killed off as early as they normally would be, Crimmins said.

In New England, many trees aren't changing colors as vibrantly as they normally do or used to because some take cues for when to turn from temperature, said Boston University biology professor Richard Primack.

Clusters of late-emerging monarch butterflies are being found far further north than normal for this time of year and are unlikely to survive their migration to Mexico.