The coming of spring is an astronomical and biological phenomenon.
The moment of the vernal equinox – the start of spring in the northern hemisphere – is the moment where the length of days is changing fastest. Biologically, spring is characterized by the re-emergence of life. In particular, it is characterized by the resumption of productive photosynthesis as leaf-bearing plants deploy their solar collectors to take advantage of longer days. This process, along with organic decay during the darker months, gives the Keeling Curve its characteristic wiggles.
In a way, each spring is a dawn for renewable energy. Aside from a few chemical-eating bugs in the ocean, pretty much all life on Earth is ultimately powered by sunlight as processed by photosynthetic organisms. Spring shows how the vast energy output of the sun can be used in the service of life.