5 Ways Climate Change Is Changing the Autumn Landscape

Falling Oak Leaves
Warmer autumns stress leaves out. Borut Trdina / Getty Images

The changing of leaf color in autumn may seem to happen like clockwork, but the process is much more delicate than you may know. It depends on a balance of moderate rainfall in spring followed by a dry autumn having sunny days and cool (but not freezing) nights—a recipe that climate change and global warming are endangering.

Here are some of the ways in which Earth's warming atmosphere and more frequent climate extremes (droughts, floods) may already be throwing the fall foliage you're used to seeing, off-kilter.

 

1. Droughts Lead to Earlier (But Duller) Color

As temperatures rise and more water evaporates from land and water bodies this contributes to unusual dryness. Drought, particularly if categorized as severe or extreme, will cause colors to appear sooner in the season. But before you grab binoculars, consider this: those colors will appear dull and muted and may be here today, and gone tomorrow. In response to the lack of water, trees prematurely create a sealing barrier between branch and leaf stem. While this stops the production of chlorophyll (the chemical that gives leaves their spring and summer green), it also triggers an early shutdown of leaves, which means leaves may drop before they've had enough time to fully change color. Hence, you get muted rather than brilliant yellows, oranges, reds, and browns.     

2. Warmer Autumns Delay & Shorten the Leaf Season

Happy to see temperatures in the 70s and 80s in October and November?

While you may be, nature is not. It is fall's less intense sunlight and cooler air that cue trees to "hibernate" for the coming winter—a chemical process that triggers the start of leaf color change. But if temperatures remain at their summertime highs and feel anything but fall-like, trees won't know that it is fall and their color change may be delayed later and later into the September, October, November months.

Not only this, but also the excessive heat during summer that extends into fall (known as "Indian summers") stresses trees and has a similar effect as drought, leading the leaves to an early shutdown and your leaf-peeping window, greatly shortened. 

Just how unseasonably warm have autumns become? Since 1970, autumn temperatures across the contiguous U.S. have risen by 0.46°F per decade. (That's nearly 2°F in the last 4 decades!) And that's just on average. When you look at these trends at a regional level, some regions' fall temperatures are showing warming trends of well over 1°F per decade. 

3. Heavy Precipitation Delays Color Change

Not only do warmer temps in autumn cause pushed-back color peaks, but soggy summers do too. And with global warming putting more heat into the atmosphere to fuel convection and rainfall, delays in the appearance of color could become the norm. While leaves need water, an excess of it is just as much of a stressor as not enough.  

What's more, if rainstorms are heavy enough, they can also knock leaves from their branches before their full-color potential is reached.       

4. Climate Extremes May Make Predicting Color Peak Impossible 

Yearly variations in temperature and rainfall mean that the timing of when leaves change color also varies slightly from year to year.

But when you add in the climate extremes that are predicted to become more frequent thanks to global warming, what you'll see is even more variability in peak season from one year to the next. 

5. The Current Long-Term Warming Trend Is Erasing Future Foliage

Another impact of global warming on fall foliage? The trees that thrive in cooler climates (including birch, sugar maple, aspen, and red oak) are essentially "migrating" northward. What happens is that trees become less populous in regions where heat and heat-loving invasive pests and diseases act as stressors. The U.S. Forest Service is already seeing evidence of these species (which are known for their vibrant fall color) having decreased population density and presence in some eastern U.S. forests. If this shift continues, you'll see less of your favorite vibrant pops of color as a result because there'll be less of these trees around to deliver them.