Dear Sara,

I would like to read your prediction of the effects of climate change on the traditional four weather seasons.

From a lifestyle preference, it has been nice for me to know that in the summer, there will be the warmth of the ocean. In the fall, we see the shedding of the leaves and the beauty of the trees when they’re bare. In the winter, there may be snow or outdoor sports. In the spring, we see the wonderful flower gardens that people have planted – and the beauty of that can be predicted.

I ask the question because I have some suspicion that that’s going to change in some way, shape, or form. And I don’t look forward to that.

– Claude in Durham, North Carolina

Dear Claude,

Here in North Carolina, the nights have turned chilly and the trees are changing color, so it’s time to return to your question. (For those who missed it, I’m breaking my answer to Claude’s question into four parts published over the course of a year. Click through to read about the impacts of climate change on spring and summer.)

My usual first step in answering a science-related question like yours, Claude, is to read a lot of peer-reviewed scientific articles about the subject. But when I dug into the research about the influence of climate change on autumn, what I found surprised me: Scientists still have a lot left to learn about how a warming climate will affect the season of fall.

Delayed fall foliage

One of the most obvious changes to occur in the fall, of course, is that the leaves of deciduous trees change color. For the trees, the purpose of this process is to retrieve nutrients from the leaves before they fall to the ground. For people, it’s big business, with foliage-related tourism worth billions of dollars in New England alone.

As the climate has warmed during the past few decades, the onset of fall colors across much of the Northern Hemisphere has been delayed. In the eastern United States, fall foliage arrives an average of two weeks late compared to the 1980s and 1990s, said Yingying Xie, an ecologist at Northwestern University, in an interview with my colleague Erika Street Hopman.

But other impacts, such as the influence of climate change on fruit ripening and amphibian dormancy, have received surprisingly little attention in the scientific community. Reading through papers and reports, I kept encountering passages like these: “the environmental control of autumn phenology remains significantly understudied” and “the impacts of warming on forests and ecosystems during the summer and autumn are less well understood.” I even read a paper titled “Autumn, the neglected season in climate change research.”

So fall, like a middle child, has been overlooked. It’s enough to make you feel sorry for the season.

A ‘human enchantment’

Why haven’t scientists devoted as much attention to fall as they have to, say, spring? Human nature itself may be partly to blame. In the aforementioned paper, authors Amanda Gallinat, Richard Primack, and David Wagner point to the “human enchantment with the sudden burst of spring flowers and wildlife following winter.”

They also note that the natural phenomena that occur in the fall can be challenging to study. Take fall foliage. If you want to monitor the impact of climate change on the timing of fall leaf coloring, you need a standard way to define an event that unfolds over the course of weeks. Do you define it as the moment when leaves first begin changing color? When 50% of the leaves have changed color? Or when all of the leaves have dropped to the ground?

In fact, researchers have used all of those definitions, complicating efforts to compare data gathered in different studies.

Complex cues

What’s more, the beauty of fall is controlled by many different factors, so teasing out how climate change will influence the season is not a simple task. In the case of fall foliage, for example, shorter days and falling temperatures are the primary cues for trees to start changing their leaves.

But extreme weather also plays a role. Droughts can cause abnormally early leaf drop and strong winds can cause leaves to die suddenly, cutting fall foliage season short. But abundant moisture can cause delayed leaf coloring. Even summer heat stress can influence when fall colors arrive.

The upshot is that researchers are still investigating the variables that influence how trees will respond to climate change during the fall, Xie said: “This is kind of still a black box.”

To add even more complication, scientists expect that as the climate warms, the ranges of tree species will change. Sugar maples, responsible for intense yellow, orange, and red foliage in the Midwest and northeastern U.S., could shift farther north and to higher elevations in coming decades. That could make fall colors in those regions less beautiful. But one study found that in many tree species, climate change could cause fall colors to grow more intense.

You can help

If you’re feeling whiplash from reading about all of these different factors, don’t worry. Through a variety of citizen science projects, you can help scientists gather more data about fall phenomena. A few projects to consider:

• If you vacationed in Acadia National Park before the advent of digital cameras around the year 2000, this scientist would like to see your fall foliage photos.
• If you live in the Southwestern U.S., you can report sightings of monarch butterflies, caterpillars, and eggs through this project. Those living east of the Rockies can participate in tagging monarchs with Monarch Watch.
• Anyone can contribute backyard observations about fall to Nature’s Notebook, Budburst, or Yale Climate Connections partner ISeeChange.

The bottom line

Scientists expect that the fall season will be altered in the future as a result of climate change, but they don’t yet have a thorough understanding of all the ways it could change. It’s a little like the experience of listening to this version of Antonio’ Vivaldi’s “Autumn” from “The Four Seasons,” which has been rewritten to include both the original melodies and jazz improvisations on those tunes. Like the future of fall, you don’t know exactly what’s coming next, but you can count on its being different.

Erika Street Hopman contributed reporting.

Tom Toro is a cartoonist and writer who has published over 200 cartoons in The New Yorker since 2010.

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Wondering how climate change could affect you or your loved ones? Send your questions to sara@yaleclimateconnections.org. Questions may be edited for length and clarity.

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