Check out this graph of America's "Growing Season" — it measures the number of continuous days and nights when it never gets below 32 degrees. You could call this our "frost-free" time of year. In many places, the frost-free season begins in the spring and ends somewhere in October.
As you can see, over the 20th century, it's been staying frost-free longer...and longer...and longer...
...about two weeks longer, on average, with a pronounced spike over the last 30 years. Which means your sweater stays folded in the dresser a little longer these days — on average. Averages, of course, can deceive. According to a new book Global Weirdness by Climate Central, a nonprofit science and journalism group:
...the change has been much greater in the West than the East. In fact, the western United States has gained an average of 19 [frost-free] days, while the growing season in the East has lengthened by just three days.
So the East is still chilly-ish. The West is warmer.
But what if we check out a smaller region, say the Sonoma/Napa wine country in California? There the data say, "The growing season has lengthened by a full 66 days, from 254 to 320, since 1950 — much to the delight of winemakers in the region."
So this warming trend is welcome — if you grow grapes. But if you are a winter wheat farmer, and you need cool winter temperatures for your wheat to grow — then this is not a happy development.
Now comes the big question: Why is this happening? The Climate Central folks don't jump to any quick conclusions. "Natural variations in climate account for some of these changes," they say.
But here's a clue: daytime has always been warmer than nighttime, (obviously, because the sun is up). But that difference is narrowing. The first frosts usually come at night, and sub-freezing nights are rarer now. Why would the nights be getting warmer?
The folks at Climate Central say here's why:
This is just what scientists expect to see as greenhouse gases in the atmosphere increase: the heat they trap keeps the surface warm even when the sun isn't shining. If the thermometer stays just a little above freezing at night, it doesn't affect average overall temperatures very much, but it does have an impact on how many frost free days there are. Studies have clearly shown that these changes in the growing season worldwide are linked to warming from greenhouse gases during the second half of the twentieth century.
At night, heat rises off the earth and some of it escapes into space. That's one reason the nights are colder; daytime heat literally radiates off our planet. But if there are more aerosols and greenhouse gases in the atmosphere, those greenhouse gasses behave like a blanket, trapping the heat and keeping it here.
Consider The Moon...
The moon, for example, has no atmosphere and no greenhouse gases. That's why it can be 118 degrees Celsius in the lunar sunshine and plunge to 168 degrees below zero in the lunar night; because on the moon, daytime heat just dissolves into space at night. On Earth, the night air holds on to some of our daytime heat, but lately, it seems to be holding on to more. Nighttime temperatures have been creeping up, (see this study) and that points to changes in our atmosphere.
So if your sweater stays in the drawer till late October, even when it's 11 p.m. and you're out walking the dog — that's a hint. Something's different. And the difference may be those invisible, man-made gasses gathering above you.
Climate Central's new book, written by Emily Elert and Mike Lemonick, based on data gathered and reviewed by five scientists, is a careful, sober look at worldwide climate change. It's short, point-by-point specific, and makes great bathroom reading, because each chapter is only a few pages long. (If you don't mind being troubled while you sit.) It's called Global Wierdness.