In Boston, the weather has been unseasonably warm this December. And it’s not just here. All over the Northeast US, parts of the Western US, in the UK, and beyond are still waiting for winter to arrive. The odd weather is caused by a combination of two global phenomena. First, there’s the Arctic Oscillation. This is pattern of air pressure in the Arctic that varies, but isn’t predictable. When there is low air pressure in the Arctic (relative to further south), then winds that blow west-to-east across the U.S. and Eurasia trap the colder Arctic air up north. That’s what’s going on right now. It’s the opposite of the Polar Vortex we’ve heard about the last couple years.
Second, there’s El Niño. El Niño is the warm phase of temperature oscillations in the Pacific ocean near the equator. In particular, it means that temperatures there are warmer than usual. In strong El Niño years, like this one, those west-to-east winds blow much of that warmth across the United States, and so we tend to have warmer winters.
The combination of the two means not just warmer weather, but much warmer weather. And the effects this weather has on plants can be substantial.
In mid-December, I was out at the Arnold Arboretum when it reached 60 degrees Farenheit. At the top of one of the hills there sits a Korean rhododendron (Rhododendron mucronulatum), and it was in full bloom. Not just a flower open here and there — the whole shrub was covered in pink flowers.
This rhododendron is one of the earliest species at the arboretum to bloom each spring. And it must have assumed that all the warm weather meant spring. This is one of the dangers early blooming species face: accidentally blooming too early. In this case, as normal winter temperatures return, the flowers will all die and the plant will miss out on a year’s worth of reproduction.
If early winters become warmer and warmer on average, which they likely will, it will be interesting to see if early-blooming species are able to adjust and hold off on flowering a bit longer, or whether they will have fewer and fewer “good” years of early cold and more and more “bad” years of flowering before winter cold even hits. It may well be that some species will adjust and some won’t. This is one of many questions that phenology researchers are working on.