AUDIE CORNISH, HOST:
Spring is here, sort of.
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UNIDENTIFIED MAN #1: That is the warmest temperature we've ever had in the month of February...
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UNIDENTIFIED MAN #2: Take a look at this, man. Our temperature is nearing 70 degrees across central Indiana.
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UNIDENTIFIED WOMAN: A historically warm and record-breaking month.
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UNIDENTIFIED MAN #3: Break the record for the most 60-degree days in February.
CORNISH: Turns out that there is a spring index, which, for the last six years, has kept track of regional temperatures and when certain plants bloom. And it's showing that parts of the country throughout the southeast and as far north as Indiana are seeing spring already. To talk more, we have Jack Weltzin on the line. He's a program manager at the U.S. Geological Survey, where he runs the National Phenology Network. Welcome to the program.
JAKE WELTZIN: Thanks, Audie.
CORNISH: Help us better understand this spring index. I mentioned that you keep track of temperatures and plants. What else goes into figuring out when the spring should or will arrive?
WELTZIN: We actually have access to a long-term data set of leafing and blooming of lilacs and honeysuckles. And we relate that to the temperature conditions where the leafing and blooming has occurred to create a simple model that helps us forecast when leafing and blooming might happen.
CORNISH: And so what does that mean? What are the temperatures because I've lived in Tennessee, and, to me, it just always felt warm?
WELTZIN: From the biology side, it's the plants and the animals and, of course, human activity. We're starting to kind of emerge from our winter torpor. There's physical signs, too. So we have melting snow, and we have rivers starting to run, especially in colder regions where that ice is starting to thaw.
CORNISH: But this is the kind of story that usually ends with someone saying that, you know, long-term spring is moving up. It's climate change. Everybody panic.
WELTZIN: Weather varies from day to day. And the timing of the onset of spring varies from year to year. If you are, say, in the northern United States or the upper Midwest, we do know that in many places when you chart the onset of spring through time, you actually do see a general trend towards an earlier spring. Other areas in the United States, though, don't show those patterns. Like, in the southeast, it hasn't really changed a lot from one year to the next when you look at a bunch of years all strung together and look for a trend.
CORNISH: Are there any downsides to an early onset of spring?
WELTZIN: Well, yes. There are issues with invasive species like invasive plants that can get an earlier start in the year. Some plants are more responsive to that warm condition, and they can get established and get growing early and get a real head start. And farmers are always tracking the variation in the weather conditions. They need to be able to get the tractors out into the fields. So that does affect how farmers are making - planning and decisions, and that happens on a daily to weekly basis.
CORNISH: So in the end, with all these variations, it sounds like there's not a kind of a straight-line trend. You've called it more of a squiggle.
WELTZIN: More of a squiggle. If you average all the conditions across the entire nation, we did see that 2012 was a really early year, and as - looking like right now that 2017 maybe even earlier. So that's what we're sort of watching to see how spring rolls up like a big green wave across the country.
CORNISH: Jack Weltzin of the U.S. Geological Survey, thank you for speaking with us.
WELTZIN: Thanks, Audie. Transcript provided by NPR, Copyright NPR.