Seasons

Tim Iverson

(11/2018) The Voyager 1 satellite is more than a staggering 13 million miles away. Out of all the things we’ve hurled deep into the cosmos, it’s the furthest and it’s still going. In February of 1990, under the request of astronomer Carl Sagan, NASA briefly turned the ship around to take a picture of the Earth. At that moment, as Voyager 1 slipped out of our solar system and into the great expanse of the universe forever, it snapped one last shot of home. The image sent back home is slightly unremarkable at first glance. It’s a grainy dark image with just a speck of light blue, that you’d be forgiven if you thought was dust. However, it’s not dust. It’s us. It’s all of us.

All of us are currently hurtling through space on this pale blue dot. We’re getting pulled in a lot of different directions. For instance we’re whipping around the sun at a spritely 67 thousand miles per hour, giving us a year to get back to this date. Meanwhile, our dot is also spinning in circles at about 1000 miles per hour. Try not to get dizzy.

While all this spinning is going on the Earth is also wobbling. Our axis, the invisible line bisecting the planet, wobbles back and forth like a spinning top struggling to remain upright. This slow motion wobble tilts regions of the planet towards and away from the sun. This axial tilt is what causes seasons to change. As one hemisphere tilts away from the sun another leans in. The region falling away experiences cooler temperatures and shortening days as the sunlight becomes less consistent. The region more exposed to the direct light rises in temperature and is illuminated in longer daylight hours as summer crescendos into a fever pitch. Then just as you get settled the wobble shifts back. Summer becomes autumn. Autumn falls into winter. Winter springs forward. Spring bounds into summer. Before you know it we’ve come one full 67,000 mile per hour circle back to where we were.

All of this spinning and swaying is fairly standard as far as planets go. Mercury’s is barely noticeable at all, tilting as little as .03 degrees. Uranus, on the other hand, practically goes sideways tilting over as much as 82 degrees. Earth tilts about 24 degrees one way before swaying back. However, that slight tilt along the axis is just enough of a change in light and temperature to give us our unique seasons, which sets the scene for the natural world around us.

During the summer leaves are little factories for trees. Green tinted chlorophyll allows leaves to capture sunlight and make energy for the tree. The chlorophyll is sensitive to cold temperatures, so when the air gets crisp trees respond by producing less of it. Hidden within the leaves all along were yellow and orange color pigments called carotenoids, also found in carrots, that finally have their moment to shine through. In some leaves while the chlorophyll is breaking down it causes other chemical reactions to occur in the leaf. Anthocyanins are then created, which in turn make brilliant reds out of the once verdant leaves.

As the colors begin to fade another process, called abscission, begins to take place. Abscission means to "cut away," and that’s exactly what happens to the leaf. Where a leaf stem attaches to the tree a special layer of cells gradually develops severing the connective tissue. As a gust of wind comes along it blows the leaf off and the body of the tree is sealed shut by this new layer of tissue.

During the winter, like many animals, deciduous leafy trees go through a dormant hibernation type of state. Lack of water and frigid temperatures are just as dangerous to them as they are for any other living thing. Deciduous trees have a few adaptations and tricks up their sleeves to help combat these issues. During the summer those broad leaves are essential for survival, but in the winter those leaves become a lethal liability. The leaves are a costly source of water loss and consumption, and during the winter months water is hard to come by. Losing leaves is one such way to avoid excessive water loss. Bark acts as a tightly woven blanket that prevents ice from penetrating the internal organs of the tree. Coniferous, or evergreen, trees on the other hand are much better suited for winter weather.

Conifer needles have less surface area and retain water better than leaves because of a waxy coating and smaller pores, which can allow for year round photosynthesis. Conifer trees also have a higher density of foliage, which during winter means snow and ice accumulates faster on them. This makes them more susceptible to breakage, especially with the addition of high winds. To offset this problem conifers tend to be more flexible and the cone shape of the tree helps to shed snow easier. The dense foliage also makes them more attractive to animals. Birds, deer, mice, and more eat the foliage and fruit that these trees produce during the winter months. Mice, rabbits, and a few others chew bark for the nutrients stored within, and of course seek shelter within the branches or groves throughout the winter.

As the climate continues to change delays in seasonal cycles for plants and animals will ultimately be upset. They take their cues from changes in daylight and temperature. Temperatures continue to rise due to atmospheric emissions while axial tilt remains the same. These, now competing, environmental triggers for migrations, reproduction, and other adaptive behaviors will be upset and fall out of sync. Even favorite seasonal events like leaf color changes will be delayed, disrupted, or might not occur at all.

Hypothetically, warming temperatures can indicate to plants that it’s time to sprout and bloom. These temperatures also indicate to insect species like butterflies it’s time to migrate north. These butterflies fly north and begin to lay eggs, but because it’s still so early in the season a frost settles in destroying the plants and leaving the eggs nothing to eat should they survive the frost. Migratory hummingbirds could also face similar issues with food scarcity caused by frosts destroying plants and their nectar. Other birds having exhausted food supplies in the south and being signaled that it’s time to move north due to increasing temperatures might find that the warming pace hasn’t ushered in the required prey species. The issue is that these aren’t hypothetical situations. These are documented cases involving the Mormon Fritillary Butterfly and the Red Knot (a migratory bird that stops mid trip in Delaware to feast on horseshoe crab eggs). Breaking any link in these chains can be critical for these species survival and overall productivity.

Ultimately the wobble, or tilt, of the Earth’s axis is consistent and is still the primary driver of seasonal change. Our spinning little ball dips about 24 degrees, which is what creates the seasons we all endear meaning and affection to. However, it’s becoming increasingly more common that emission driven temperatures, and not axial tilt, are disrupting the seasonal timing of events like leaf changes and blooms. Whichever is your favorite season, it’s those fleeting ephemeral moments that make them so special. Sip your apple cider. Donn the yule tide apparel. Plant some flowers. Soak up the sun. Just don’t let the spin get you dizzy.