Autumnal Equinox

Today is the Autumnal Equinox, the first day of fall in the northern hemisphere. The new season officially arrives at 14:49 UTC (10:49 A.M. Eastern Daylight Time).

Seasons are the result of the tilt of the Earth’s axis, a 23.5° angle. Today, as autumn begins, the planet’s axis is tilted neither toward nor away from the sun. As a result, we receive approximately equal hours of day and night. In fact, the word “equinox” is Latin for “equal night”.

As a transitional season, autumn is a time when the heat of summer fades away and winter’s chill gradually returns. The largest drop in average temperature, however, usually lags the equinox by a few weeks.

Autumnal Equinox

Image Credit: Przemyslaw “Blueshade” Idzkiewicz

Late Arrival for Fall Foliage

Fall foliage is a traditional sign of the change of seasons.  In New York City, peak color is normally around the last week of October. This year, however, we did not hit the peak until somewhere between the second and third weeks of November.  Is this delay of fall color a sign of climate change?

Fall foliage displays are strongly influenced by the weather and therefore vary from year to year.  The late arrival of the fall colors this year could be an anomaly related to the exceptionally warm and wet conditions of the early autumn in the northeast.  Delayed color changes, however, are also happening elsewhere around the globe.  Climate change studies in both Europe and Asia have shown that leaves there have been changing color later in the season.

Scientists working with satellite data at the Seoul National University in South Korea have shown that the growing season in the northern hemisphere was extended by 6.5 days from 1982 to 2008.    Another study at the University of Southampton in England found that increased levels of carbon dioxide in the atmosphere is delaying the change of color in autumn leaves. Researchers there conducted an experiment with poplar trees and found that the those exposed to higher levels of carbon dioxide, a key component of photosynthesis, stayed green longer.

Climate change is a complex subject.  No one knows for certain how different aspects of the natural world, like the timing of fall foliage, will respond to changing environmental conditions.  This highly nuanced subject, in my opinion, warrants closer study.

The Science of Fall Foliage

The science behind the color of autumn leaves is a mixture of bio-chemistry and meteorology.  Atmospheric conditions trigger a biological process in deciduous trees that cause the leaves to change color.  This process allows a tree to protect itself from freezing and survive the winter.

Sunlight, groundwater, and temperature all play a part in the annual phenomenon of fall foliage.  The main variable that starts the ball rolling, however, is sunlight. As the days grow shorter in autumn, less sunlight is available to power photosynthesis, the chemical process that feeds the tree by converting carbon dioxide and water into glucose and oxygen.

The reduced amount of sunlight signals the tree to stop producing food and prepare stores for the winter. The tree turns off its food producers by slowly corking the connection between leaf-stems and its branches.  This blocks the movement of sugars from the leaves to the tree as well as the flow of water from the roots to the leaves.  As a result,  the leaves stop producing chlorophyll, the agent of photosynthesis and the reason for the green color of summer foliage.  As the green fades, other chemicals that have been present in the leaves all along begin to show.  These include xanthophyll and carotene that produce yellow and orange leaves, respectively.

The change of leaves happens every autumn, but the intensity and duration vary from year to year.  This is a result of weather conditions like temperature and rainfall.  With below average temperatures, sugars trapped in the leaves react with sunlight and the cooler air to produce anthocyanin.  This gives us more red and purplish leaves.  Warmer than normal temperatures tend to produce longer displays but with less intense colors.  Drought will cause leaves to turn brownish and break off early.

It should also be noted that the bio-chemical make-up of different species of trees react to seasonal and atmospheric conditions differently. Therefore, the more diverse the forest, the wider the range of colors in Autumn.

Recipe for Autumn Color

The general “weather recipe” for a colorful autumn includes:

  •  a wet spring
  •  no summer drought
  •  an autumn with warm sunny days and cool nights

This year in the northeast, we have collected all the ingredients needed for a beautiful fall foliage season.  Barring any major wind storms that blow the leaves off the trees or an early frost,  leaf peeping will be in full swing soon.

In New York City the leaves have just started to change, but the color is still a bit patchy.   We typically hit peak color sometime around the last week of October.

First Sign of Autumn in New York City, 2011

Photo Credit: MF @ WeatherGamut