As much as spring is welcome when it arrives, it seems to feel better after a long winter. This year, winter never really started. December and January both got off to a cold start, but that quickly changed through the end of those months. By mid-February, we saw March flowers pop out of the ground. Winter is dead.
According to the National Phenology Network, spring is running 20 days or more ahead of schedule in parts of the Ohio River Valley and the Mid-Atlantic. That will soon be the case in the Midwest and the Northeast.
This is not surprising. In fact, it is exactly what we should expect as the climate warms, according to myriad peer-reviewed studies summarized succinctly by the 2017 National Climate Assessment. In technical terms, the growing season in North America is getting longer. You may also see it referred to as “frost-free” days, since the growing season is the span of time between the last frost and the first frost.
A longer growing season sounds great, especially given the dire warnings of food shortages resulting from climate change. Hang on, though, because a longer growing season is not always a good thing. The longer growing season is inherently related to food shortages. Really.
We can see it happening even now. “Plant productivity has not increased” alongside the number of growing season days, according to the National Climate Assessment. There are a number of reasons for this.
Freeze damage caused by late-season frosts
This is straightforward. Just ask any citrus grower in the Southeast — a late frost or freeze can wipe out an entire crop, depending on the timing. Once plants have reached a certain phase of development, which happens earlier when winter is warm and spring is early, they are extremely fragile and susceptible to freezing temperatures. If, say, half of the plant’s blossoms are killed off by a freeze, it essentially cuts the plant’s productivity by half.
Limits to growth because of lack of sunlight in early fall
Longer summer temperatures will keep the environment hospitable for crops, but they need sunlight first and foremost. As the sun angle decreases in the fall, plants lose the light necessary for photosynthesis.
Plants need winter to thrive
It sounds counterintuitive, but they basically need a break. They need a period of time each year to turn off and conserve energy. Shorter winters puts a lot of stress on plants that are not acclimated to the new climate we have thrust them into.
Each plant evolved specifically for a known climate — when, on average, growing season started and ended at certain times. In those conditions, they can shut down their metabolic processes and enter dormancy before the harsh conditions begin. Plants use the steady decrease in temperature in the fall to predict that winter is coming, so they know to go dormant.
In an uncertain climate, plants are sometimes forced into dormancy. Instead of being able to predict when winter will arrive, it shows up without warning in the form of a hard freeze while the plant is still happily metabolizing and doing its thing. It is kind of like shutting off a computer — there is a process, and it is not good for the system if you simply pull the plug out of the wall.
They literally run out of water
A longer growing season does not necessarily mean there are enough resources for the plant to survive that whole time. It means they need water over a longer period, and we know that — in this new climate — water is often hard to come by. Whether by natural drought or our own depletion of water resources, the plants are getting less and less.
Climate scientists and biologists are cautious about saying they do not know exactly how plant production will change in the future. Given that we have not been through this kind of change before, it is hard to say with certainty whether productivity will continue to decline. Risk management would suggest we should not bank on a comeback anytime soon.