With temperatures up into the upper 50s yet again as I sit down to write this, my topic can only be the oddly pleasant weather of the past few weeks. But I’ll start by noting that, in addition to the warmth, we’ve also had episodes of unseasonable cold. Temperatures have already dipped into the low 20s a couple of times on our porch in Oak Bluffs. We’ve certainly been warmer than average, but the real story this season may have been the unusually wide range in conditions.
Such volatility is a key prediction of climate change models, such as the ones that inspired the multinational agreement on climate change arrived at in Paris this past week. Temperature is really just a measure of energy, and a warming atmosphere is a more energetic one. With more energy stored in it, the atmosphere tends to produce more dramatic contrasts and events: stronger storms, more extremes of drought and precipitation, more unusual patterns of heat and cold.
Climate scientists avoid attributing any particular event solely to climate change. Weather has always varied, and it’s hard to distinguish such short-term variability from a longer-term trend. But statistically speaking, unusual weather is predicted to grow more likely as time passes; seasons like the one we’re having will become the rule rather than the exception.
For humans, the main risks of climate change appear to be rising sea levels and disruption of agriculture and water availability. These factors will affect wildlife, too, but altered weather even on a modest and local scale will also favor some species while harming others. We’re headed into a period of ecological change, which will eventually see the rise of new habitat types, different interactions among species, and likely increased dominance by a subset of species well adapted to the new normal.
First, though, it’s worth pointing out that for many species, the effect of generally warm December weather is outweighed by the fact that we’ve had even a little true cold. Tender annual and perennial plants need only a few hours of hard frost to be killed or top-killed, and the same is true for many species of insects. We’ve had nights cold enough to coat vegetation with a layer of ice crystals, with the cold penetrating down to ground level. If that’s sufficient to kill you, it doesn’t make a bit of difference if temperatures rebound into the 60s the next day.
But for somewhat hardier species, a mild late autumn may be significant. Usually gone by mid-November, red-legged grasshoppers were still active in my yard at least up to this past weekend — and they were actually mating as late as Nov. 26, when I photographed a pair industriously copulating in our yard.
While mere persistence to a late date isn’t biologically meaningful, an extended reproductive season might be; it seems very possible that more of these grasshoppers had time to mate and lay eggs than in a typical year, and that may very well translate into larger numbers hatching and maturing next summer. And for every species that experiences enhanced (or inhibited) reproduction, there will be secondary effects — some good, some bad — in other species that relate to the first one as predator or prey.
On the bird front, likewise, a mild late autumn can have fairly dramatic implications. Most of our breeding birds, especially the long-distance migrants, appear to have left on schedule, their movements probably governed more by day length than by temperature. But for incoming winter residents, the mild weather seems to have discouraged migration. Numbers of sea ducks, for example, or of hard-core winter songbirds like tree sparrows, seem far lower than I’d expect for this point in the season.
They’ll get here eventually, I’m sure. But in the meantime, mild conditions have allowed them to linger along their migration routes. This may work to their benefit: When true winter weather hits, these birds will likely be in better condition as a result of their leisurely migration, and the result may be higher than average survival.
But increasingly volatile weather also raises the risk that birds lingering in the north will be caught by a sudden onset of cold weather or snow. If that happens, delayed migration may prove to be a bad mistake, exposing large numbers of birds to unsurvivable conditions.
For many species, then, this bizarre run of warmth makes no difference at all. For others, this weather is likely beneficial, given added opportunities to reproduce or store reserves with which to survive the winter. And for a few, unseasonable warmth may represent a trap, fooling organisms into expending their resources at the wrong time.
A season like this one is a foreshadowing of decades to come, as a changing global climate will result in generally milder (though more volatile) weather. Some species will be harmed; others will benefit — with the benefits, no doubt, unevenly distributed, so the net result will be an ecosystem substantially reshaped in unpredictable ways.