While autumn has officially begun in the northern hemisphere, its visual presence may vary by location. 

Tourists frequently visit the U.S. East Coast, from Maine and Vermont to New York, to witness maple and oak leaves change into vivid oranges and reds. This color transformation can occur from mid-September until early November.

However, closer observation reveals that this pattern isn’t always consistent annually. Studies indicate that climate change affects both the timing of leaf fall and the intensity of their colors. For instance, a tree typically bare by late October might retain leaves into November, while a drought-affected area could display muted, brown leaves that merely drop off.

Below is an explanation of why leaves change color in autumn and how this natural process is being influenced by climate change. 

Why do leaves change color in the fall?

As temperatures decrease and daylight hours shorten, trees receive less sunlight. This reduction causes chlorophyll, the pigment responsible for green colors and energy absorption, to decompose, thereby revealing other underlying color pigments. 

“They are always present,” explains Stephanie Spera, a University of Richmond professor researching changes in the fall foliage season. “They are simply concealed by the chlorophyll.”

Nevertheless, not all trees develop red hues in autumn. Certain species, such as Aspens—North America’s most common tree—turn yellow. “Aspen trees, along with some others, acquire additional nitrogen from root fungi, reducing their need to extensively recover residual nitrogen from their leaves,” states Susanne Renner, an honorary biology professor at Washington University in in St. Louis. 

As maples and oaks lack this ability, they must generate an energy-intensive “sunscreen” that enables their leaves to remain on branches—and absorb more nutrients—for an extended period.

Some trees do not change color at all. Evergreens, for instance, possess safeguards that prevent them from shedding their leaves entirely. “Their needles are exceptionally thin and coated in wax,” Spera notes. “And due to their slender nature, it’s not comparable to the substantial, energy-demanding process of a large leaf capturing all this light.”

How is climate change impacting fall foliage?

There isn’t a universal explanation for how climate change influences this natural phenomenon. Its effects differ based on tree species and their specific regions. 

“It is impossible to universally predict how climate warming will affect leaf coloration,” Renner states.

One certainty is that the autumnal color change stems from a chemical process, which is unlikely to be altered by climate change. 

“Maple trees will maintain their red color irrespective of temperature, due to the plant’s intrinsic chemical capacity to produce specific hues,” Renner explains.  

Nonetheless, trees require cold temperatures to develop the vivid colors we typically expect. If local leaves appear a dull brown, it might indicate that temperatures did not fall sufficiently to yield bright colors. 

“Autumn colors in certain areas and during particular years are quite muted because the nights lacked sufficient cold,” Renner observes. 

Elevated summer and autumn temperatures can also influence the onset of leaf fall. Without a more pronounced temperature drop, “trees lack the signal to cease chlorophyll production and photosynthesis,” Spera explains. In the Maine region Spera has researched, fall foliage has been postponed by nearly a day per decade, primarily attributed to warmer summer temperatures. 

Both rain and drought can affect leaves; excessive rain fosters fungus, while insufficient rain causes “drought scorch.” “The leaves quite literally crisp and detach from the trees,” Spera states. Following a summer of drought and inconsistent rainfall, specialists anticipate that New England’s renowned fall foliage, which typically attracts an estimated , will be “” this year.