The growing season in Washington and California has lengthened by 40 to 50 days since 1900, the largest change in the Lower 48, according to a new climate report by the Environmental Protection Agency.

The Oregon and Idaho growing seasons also increased, but not quite as much. The report defined the growing season as the number of days between the last spring frost and the first fall frost.

The EPA relied on a study led by North Carolina State scientist Kenneth Kunkel. Researchers looked at temperature records dating back to 1895.

The frost-free season grew by a week in the U.S. between 1910 and 1930. After a 50-year pause, the frost-free season resumed growing in 1980, particularly in the West, researchers found.

Natural fluctuations likely caused the frost-free season to grow in the first half of the 20th century, Kunkel said in an interview Tuesday.

“I’m confident what we’re seeing since (1980) is coming from overall global warming from greenhouse gases,” he said.

The EPA’s report on “climate change indicators,” its fifth since 2010, also cited research reporting declining snowpacks and larger wildfires in the West.

Warming since 1900 has been most pronounced in the West and northern tier of the U.S., while temperatures in parts of the Southeast have changed little or declined, the EPA reported.

The frost-free season has lengthened in every state, except Georgia. Longer growing seasons could diversify crops, but also bring pressures from insects and weeds, and stretch water supplies, according to the report.

The frost-free season has grown by 30 to 40 days in Oregon, Arizona, North Dakota, Utah and Wyoming, the second-most after Washington and California.

The season has increased by 20 to 30 days in Idaho and seven other states. Many states had smaller increases, while the season was unchanged in Oklahoma, Alabama and Mississippi.

Throughout Western states, the frost-free season has grown by an average of 19 days, compared to only three days in the East. As to why the difference, “that’s still very much a research topic,” Kunkel said.

The report doesn’t distinguish between frost-free seasons west and east of the Cascade Range in Washington and Oregon.

Small changes in temperatures could have a big impact on the timing of frosts on the coast, where temperatures rarely drop below freezing, Kunkel said. “You essentially have two climates there,” he said.

The report also didn’t look at the change in frost-free seasons by National Weather Service climate divisions. The weather service divides Washington into nine divisions, for example.

“If you went down to the climate division level, you would have a different view,” Kunkel said.

Researchers were faced with collecting enough temperature records. They discarded sites with too many missing dates. Records were unavailable for much of Eastern Oregon and Idaho.

“I think we still have enough to get the basic changes correct,” Kunkel said.