Early in the book I am currently reading, I found these words:
"If a Black Swallowtail caterpillar turns into a pupa on October 2, it really doesn't matter whether it happens in 1908 or 2008. Black swallowtail caterpillars have been pupating in the same way and at about the same time for eons."This is followed by a footnote about us being the "last generation to enjoy the timelessness of nature", presumably because of global warming. What rot!
Nature seems timeless to most people because our lives are short compared to geologic and climatic cycles. This era, the last 10,000 to 11,000 years, called the Holocene by archaeologists and geologists, is actually the latest of about twenty interglacial periods of the Pleistocene, the most recent set of Ice Ages. This cold period is somewhat short (only about two million years) and rather less severe than some of the earlier ones.
It actually began in the Pliocene, in which a dozens of warm-cold cycles, each about 50,000 years long, gradually slowed and got more severe, so that by two million years ago the Earth's climate would enter a new icing event every 100,000 years, which would end after 80,000-95,000 years and be followed by five-to-twenty thousand years of interglacial warming. Of the past twenty interglacial periods, some have been cooler than this one, and some have been warmer. Within each, there have been smaller gyrations of a few degrees plus or minus.
The Medieval Warm Period, which peaked about a thousand years ago, was comparable to the pleasant period of 1950-1980. In between, things were cooler by about two degrees, bottoming out with the Little Ice Age that caused so much hardship during the American Revolution and for a couple decades before and after it.
These relatively minor climate cycles have shifted the indicators of Spring and Fall by a month or two, back and forth, and the Swallowtails and other critters have managed to survive. The Black Swallowtail doesn't pupate on a certain calendar date; it doesn't have a calendar. It pupates when two things prompt it: the shortening day and the falling nighttime temperature. During the 1990s, early October was typically the time of the first post-Equinox frost, at least in the New York City area. While a frost can occur before September 21, the caterpillar doesn't seem to get the message until the sun begins to set before 6PM. Caterpillars, as most creatures, do have a diurnal clock that can be quite sensitive.
Just a hint, folks: Fifty million years ago, and also 65 million years ago, there was tropical vegetation growing in Antarctica. There was a freeze in between that was considerably more severe than any Pleistocene event. Think glaciers in Houston. There have been at least two periods in the last billion years that were cold enough that there was sea ice at the equator, and glaciers on all land surfaces. The Pleistocene is comparatively mild!
Now it is true that the climate has warmed by about a degree Celsius (close to 2°F) in my lifetime. It is probably also true that the CO2-induced Greenhouse Effect has been responsible for a quarter to a half of that degree. Seasonal changes are thus occurring earlier in the Spring and later in the Fall than they were 50+ years ago (or even 20+ years ago). If we are paying attention, we are not "enjoying" the timelessness of nature, but the resiliency of nature, for which there is always change.