13 October 2012

Fall Colors Are Peaking 10 Days Later Than the early 1980′s

Posted by Dan Satterfield

The fall colors in America are peaking over 10 days later now than 30 years ago. How can you determine such a subjective thing as when fall colors are peaking? Well, that’s just as interesting as the results!

Take a look at the graph below courtesy of Climate Central.

The NDVI is the Normalized Difference Vegetative Index and in this case it was used to graph the times when the leaves were changing colors the fastest over North America. This is a good proxy for peak colors and the results speak for themselves: Fall colors are peaking well over a week later than in 1980-1985. The USGS has a good explanation of the NDVI:

The Normalized Difference Vegetation Index (NDVI) has been in use for many years to measure and monitor plant growth (vigor), vegetation cover, and biomass production from multispectral satellite data.

NDVI = (Channel 2 – Channel 1) / (Channel 2 + Channel 1)

 (Channel 1; 0.58-0.68 micrometers) and near infrared (Channel 2; 0.725-1.10 micrometers):The principle behind NDVI is that Channel 1 is in the red-light region of the electromagnetic spectrum where chlorophyll causes considerable absorption of incoming sunlight, whereas Channel 2 is in the near-infrared region of the spectrum where a plant’s spongy mesophyll leaf structure creates considerable reflectance (Tucker 1979, Jackson et al.1983, Tucker et al. 1991). As a result, vigorously growing healthy vegetation has low red-light reflectance and high near-infrared reflectance, and hence, high NDVI values. This relatively simply algorithm produces output values in the range of -1.0 to 1.0. Increasing positive NDVI values, shown in increasing shades of green on the images, indicate increasing amounts of green vegetation. NDVI values near zero and decreasing negative values indicate non-vegetated features such as barren surfaces (rock and soil) and water, snow, ice, and clouds.

The graph above is based on the image below made from the raw data in the paper in Global Change Biology:

Notice here that not every spot is showing that fall is occurring later. In NW Illinois, Iowa and Eastern Nebraska it is actually happening earlier! Somehow I suspect some climate skeptic somewhere has cherry picked this piece of data and claimed that fall in Iowa is actually happening sooner than in the past and climate change is therefore a giant hoax.
It is interesting, and I am not sure why it is there, but I do have an educated guess though. This area of the country is the heart of the corn belt. I suspect that the exception to the rest of the country there is related to a change in agricultural practices. If there is less irrigation the dew-point will be low and night-time lows will drop. This could lead to fall colors peaking a bit sooner. It’s just a guess, it could be related to crop changes, cover crops, rainfall changes etc?
I’ll see what I can find out, but in the meantime, this is a good illustration of how science works. The scientific certainty that the planet is warming is not based on thermometers at airports. There are loads of indicators just like this that tell us we are getting warmer. Every cause other than increasing greenhouse gases has been ruled out. Yes, there are natural variations, but we now know that natural variations alone are not nearly enough to explain it.
That’s not my opinion alone. It’s shared by every major science body on the planet. Just a little tidbit of science straight from the pit of hell ;)

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