Sunday, November 10, 2013

Climate Change & Fall Colors

The colors of fall are greatly anticipated in this part of the country.  Some plan vacations around a particular week trying to be in the peak of color.  Normally tourists plan for the middle two weeks of October to enjoy the color of western North Carolina.  However, I was surprised on a recent trip into that area to see how late the change of color was this year.

Near Lake Adger in western North Carolina (about 10 miles east of Hendersonville) on October 21, 2013.  Notice the lack of color.  There was still a lot of green.  Click on the image for a larger view.  Image Credit: Jim Gandy

It got me thinking about the set-up this year.  The summer had been very wet, but fall had turned out to be quite dry.  It should have been a good year for color in western North Carolina.  This is not to say that there wasn’t color, but you didn’t see the splash of all the colors at once.

I came across a blog post by a plant physiologist at Appalachian State University which put this in perspective.  Dr. Howie Neufeld goes into detail about how trees change color and what factors can affect fall colors.  It is quite informative and worth a read, but here is an excerpt on the two main factors:

“Trees cue in on two main environmental factors with respect to fall foliage color: daylength and temperature. As the days get shorter in August and September, trees sense this and begin anorderly process of leaf senescence, usually involving the loss of chlorophyll, the green pigment that plants use to capture light for photosynthesis, and the synthesis of anthocyanins, which give leaves their red color. Some leaves don’t make anthocyanins, and when the chlorophyll degrades, it reveals underlying pigments, such as carotenoids and xanthophylls which are responsible for our orange and yellow colored leaves. Trees use daylength as their main cue because it is a good proxy for coming cold weather. Year after year, daylength will come with great regularity at the same point in earth’s orbit, reliably signaling the inevitable onset of colder weather. But temperatures can vary greatly at any time of the year due to variations in the weather (like this year when temperatures were unusually cool all summer) and so are less reliable cues for the coming of fall. Trees use temperature only as a secondary cue; if the fall is cool they hasten the development of their fall color, while if it is warm, they procrastinate and delay it.

If global warming results in warmer fall temperatures, either during the day or night, this would tend to delay fall colors to later in the season. Will it mute the colors? It might, especially if the disconnect between daylength and temperature becomes extreme, thereby confusing trees and disrupting the synchrony of color development, such that some trees attain peak color later while others are less susceptible to the temperature cues….”

Turns out that fall has seen above normal temperatures in the Southeast this year.  In fact Columbia, SC did not see its first temperature below 50 degrees F until October 21st.  Plants and people were beginning to wonder if fall was ever going to arrive.  This might explain why so little color was present by that time of year.

The growing season ended for much of the Midlands on October 26th as a cold air mass pushed into the region dropping temperatures below freezing over much of the area.  This also hastened the change of colors, but the change was already a week or two behind.  Furthermore, some trees had already peaked while others were just getting started.  The result was that the trees producing red colors changed first followed by orange and then yellow.  It was more like a rolling change of color.  Unfortunately it was not the big splash of color from years past.

Scientists have been looking at the temperature trends across the country.  If you look at average fall temperatures across the nation, it’s clear that while there’s a lot of change from one year to the next, autumn got progressively warmer on average from 1982 through 2008.

The average fall temperatures (Sept.-Nov.) for the contiguous U.S. from 1982-2008.  Click on the image for a larger view.  Image Credit: Climate Central.

The strongest trend is noticeable west of the Mississippi, parts of which have warmed up to 1.3°F per decade. The Northeast is also one of the fastest warming regions of the country. Only small pockets of the Southeast have seen any form of cooling in the fall. That doesn't mean global warming is skipping over the region. Annual temperatures trends show that all states in the Southeast (and across the U.S. for that matter) have been warming since 1970, although at different rates. For example, Florida is the slowest warming state while Arizona is the fastest.

The temperature trends per decade for the climatological divisions of the U.S.  Click on the image for a larger view.  Image Credit: Climate Central.

On the precipitation side, it’s clear fall on the Eastern Seaboard and the Ohio River Valley has gotten wetter while its gotten dryer for many areas to the west. The trend in the Northeast is particularly striking and according to a NOAA report, it stretches back to the beginning of the 20th century.

The precipitation trends per decade for the climatological divisions of the U.S.  Click on the image for a larger view.  Image Credit: Climate Central.

Looking to the future, all areas of the country are expected to warm.  Some will warm faster than others and each year will see variability in the weather.  However, this year may be a sign of things to come.  Warmer temperatures, longer growing seasons, variability in rainfall may combine to mute fall colors.