Today, Groundhog Day is just another reason to have a party. No complaints about that from me, I was at one last night and we had a pretty good time. But, in antiquity, the equivalent of our modern-day ritual was serious business.
Before there were supermarkets with food supplies, people had to make careful plans on how to get through the winter. A mistake could prove fatal and often did. It was vitally important to know about the change of seasons.
This is how astronomy got started because the seasons are a result of the planet's tilt on its axis and rotation around the Sun. They didn't know about those things in ancient times, but they knew the stars, Sun and Moon changed in the sky on a regular basis. By observing over a period of time, virtually every ancient culture throughout the world figured out the two solstices and two equinoxes and how the seasons changed relative to those four days. We find ancient observatories everywhere devoted to identifying these days. In addition, the four days between, the cross-quarter days, also became important. People with knowledge of these eight days were powerful and respected. They were the ones who told everyone when to plant, when to harvest, when to do just about everything according to the seasons. This is probably how ancient priesthoods got started.
The winter and summer solstices (about December 21 and June 21) as well as the vernal and fall equinoxes (about March 21 and September 21) are well known today, with some small variation in the exact day. The cross-quarter days are not as well known, but are still mostly observed under many different names. The best known one is probably Halloween, midway between the fall equinox and the winter solstice. The cross-quarter day between the summer solstice and the fall equinox occurs in the first week of August and was referred to as first harvest. There are still some cultures that observe this day, but not many. In olden times, villagers would tithe a portion of their crop to the village to help everyone get through winter. The cross-quarter day between the spring equinox and the summer solstice occurs at the beginning of May, is known as May Day in modern times and is observed as a celebration of spring and fertility. That leaves the cross-quarter day between the winter solstice and the spring equinox - now known as Groundhog Day.
When we track the winter months we find the coldest days occur around the end of January and the beginning of February - right at the cross-quarter day. As a result, some cultures said spring started on the cross-quarter day (known as Imbolc in Germanic countries). Other cultures said it started on the equinox in March. To resolve this, the cultures in Norther Europe began a system of weather prognostication involving badgers or bears. If the day was cloudy and a shadow was not seen on Imbolc, spring had arrived. Otherwise, spring wouldn't start until the equinox. This, of course, was very serious business for these people. If they ran out of food before spring they were in real trouble. Starvation and famine were not unusual.
When the Germans immigrated to United States, they brought the tradition with them, but began using groundhogs instead of badgers. Now, with modern supply systems in place, it is just another way to have fun.
But, there is something amiss - spring is coming earlier.
As we have seen, there are many ways to define 'spring.' One way is when the dormant plants come back to life. This is particularly convenient because the active/dormant cycle with plants results in an up and down pattern to CO2 levels in the atmosphere. This allows us to measure onset of plant active and dormant phases by examining the rise and fall of CO2.
For the period of 1955 - 2002 we see the spring decrease of CO2 level is occurring earlier and this indicates spring plant activity is starting earlier and earlier. The observed rate of change is about -1.2 days per decade earlier for first leaf date, -1.0 days per decade earlier for first bloom, -1.4 days per decade for the last day below 5 degrees C and -1.5 days per decade for last spring freeze date. Consistently, we are seeing spring moving backwards by between 1.0 and 1.5 days earlier ever decade. For the six decades since 1955 this translates into an earlier spring onset of between 6 and 9 days. This is known as 'season creep.'
You might be tempted to say, 'Less winter! Wonderful news.' But, that would be a mistake. For instance, we need winter snowpack to get us through the dry seasons in summer. Less winter means less snowpack and that translates into problems with water supply. Also, the population of many pests are not being knocked down by winter temperatures the way the used to be.
Also, not all plants and animals adjust to the change equally. This is a real problem when two different species depend on each other. If plants depend on certain birds to cross-pollinate them, and the birds depend on the nectar of the flowers, they need to be together when the plants are flowering. If the plants start flowering earlier and the birds don't adjust their migration pattern, the flowers may be dying off by the time the birds get there. This, obviously, would be bad for both plants and birds - and anything that depends on those plants and birds later on.
In fact, we see this is actually happening.
Once again, climate change rears its head to remind us, even when we engaging in a relic ritual, that things are changing. The good news is Groundhog Day is based on astronomy and not climate, so we don't have to ever worry about it getting earlier.