Climate Change: Predicting the Future of Fall Foliage in New England

In last year’s foliage forecast, I made reference to it being a “wet, cool summer,” which prompted this reader comment: It’s a sign of just how quickly things can change. When I wrote that forecast in early August, New England was coming off a very rainy July, and it was damp and humid but not particularly hot. Then August temperatures spiked — and held. When all was said and done, the official seasonal temperature average ranked it as the hottest summer on record.

The data showed that Greg was right. But when I dug into it a little deeper, it also showed something else: The New England nights are getting warmer. The seasonal temperature average is an average of the daily average temperature, which itself is an average of the day’s high and low temperatures. In the summer of 2021, Boston daytime highs throughout the summer averaged about 3°F above normal, which is significant, but the biggest factor in breaking the “hottest summer ever” record was the overnight lows, which were nearly 5°F above average — a major outlier. Nearly a third of the nights in Boston that summer never dropped below 70°F, so the daily average temperature had a high floor.

The Decline of “Good Sleeping Weather”

On all but a few summer nights of the year, no matter how hot it gets during the day, New Englanders can usually sleep comfortably with the windows open. Think of it as “good sleeping weather.” In the science community, the line between opening a window versus turning on air conditioning is measured in a very specific way, based on the assumption that when the outside temperature is 65°F or cooler, we don’t want AC. A cooling degree day is when the daily temperature mean (the high temperature plus the low temperature, divided by two) is above 65°F.

Portland, Maine, averages a few hundred cooling degree days a year. Berlin, New Hampshire, rarely breaks 200. (By comparison, a city like Atlanta may top 2,000 in a year.) But the trends are telling. In Boston, from 1950 to 2000 there was only one year when the number of cooling degree days exceeded 1,000; since 2010 there have been six years when that happened. In short: New Englanders are starting to need air conditioning more often to sleep comfortably at night, and these warmer nighttime temperatures lead to warmer seasonal temperature averages.

What Exactly Are Weather Averages?

When you hear on the news that the daily high was “seven degrees above average,” that average is based on 30 years of weather data, which are updated once a decade by the National Weather Service. In May 2021, the averages from 1981 to 2010 were retired, and those from 1991 to 2020 were put to work. In New England — along with nearly all of the rest of the U.S. — these new 30-year averages showed warmer temperatures. Concord, New Hampshire’s annual average temperatures rose by 0.3°F, Boston went up 0.5°F, and Burlington, Vermont, went up a whopping 1.6°F. However, these data can be misleading, since updating temperature averages every 10 years can minimize or mask any long-term impacts of a warming climate. It’s when you compare recent weather data to the 20th-century averages that you can see how stark the differences are. For example, the previous chart on Maine summer low temperatures shows that millennial residents (those born between 1981 and 1996) would have experienced just three summers with below-normal lows in their lifetime, compared with the 20th-century averages. Compared with the new 30-year averages (below), however, they would have experienced nearly 20 summers with below-normal lows — potentially leading people to believe that the ongoing climate warming is not as significant as it really is.

The National Oceanic and Atmospheric Association (NOAA) was quick to point this out in the press release for the new averages, and it included the graphic below for comparison. Its maps show each 30-year period used by the National Weather Service, and how those averages compare with the 20th-century averages. The maps show significant warming along both coasts and tell the story of changing land and water use in the center of the country.

There are even more straightforward comparisons to make, though. The U.S. has not had a single year with average temperatures below the 20th-century averages since 1996. Zooming out, the world has not had a single month with average temperatures below the 20th-century global averages since 1985. Yet every 10 years, the new National Weather Service averages come out, and we tend to accept them as the “new normal.”

The Story of New England’s Fall Temperatures

As I’ve often mentioned, New England’s beautiful fall foliage is best brought out by warm, sunny days and crisp, clear nights. Bright days make the colors really pop, and cool nights actually start the progression of colors. But our earlier deep dive into Boston’s 2021 summer temperatures leads to an important question: What is happening to New England’s autumn temperatures, the driver and main determinant of timing for our fall color?  The new data show that fall temperatures in New England are warmer compared with the 30-year averages, and much warmer compared with the 20th-century averages. Further, overnight lows are more atypical than their daytime counterparts.  This trend could already be seen before the latest National Weather Service averages came out. The New England Climate Center has a temperature difference map comparing average lows in September between the two most recent 30-year periods, 1971–2000 and 1981–2010. Much of the region saw warming of more than 1°F during overnights in those time periods.

The newest 30-year temperature averages, 1991–2020, show continued warming. September low temperatures warmed on average nearly 1°F in Boston and Concord, Hampshire, and 1.5°F in Portland, Maine. In Boston, if you look at average September temperatures back to the year 1900, 13 of the 20 warmest months have occurred since 2000.

So What Does It Mean for Fall Foliage?

Some suggest that in the short term, New England could be in for longer-lasting color, and a longer foliage season. The staff at Polly’s Pancake Parlor, a must-stop for breakfast when leaf peeping in the western White Mountains, have kept a long record of fall foliage data, spanning back to 1975. They are seeing the very first hints of colors earlier in August, but they’re noticing a longer time before peak arrives. This has added an extra weekend to their biggest business time. They, and I, are also seeing leaves falling more quickly afterward, ending the season soon after peak.

Part of this is due to the more extreme events that New England is experiencing in the fall. Over the past decade, September and October storms have brought huge amounts of rainfall, and yes, even snowfall, along with strong winds. A changing climate will likely continue to bring stronger storms in the seasonal battleground between summer and winter. Will New England’s forested landscapes themselves change, though? Certainly. They really always have, in response to a variety of factors. But trees and forests grow over decades and centuries. Change will take time.

The most prized fall foliage tree in New England is the sugar maple, and it turns out that good sleeping weather for people is also great growing weather for sugar maples. Today, New England’s hillsides and roadsides are draped in sugar maples — but with temperatures changing, how long will they be able to outcompete other trees, especially in young forests, to maintain this visual dominance? Oaks, especially white oaks, currently extend north only into southern and coastal New England. Will they spread farther north? Will this be at the expense of the sugar maple, which is at the southern end of its range? And while the big connected forests of the region may be more resilient, the human-impacted landscapes — from urban parks to suburban fragments to forest logging — may be more susceptible to changes on shorter timelines. Such disturbances restart the process of forest succession with young trees, and which trees grow in to replace the cut. In this process, though, climate change isn’t the only threat, nor is it the only disturbance in the forest. Invasive insects now threaten a number of key trees in our forests, and invasive plants can outcompete native ones to replace them as forests get re-established.

There are many questions that will need to be answered, and more research will need to be done. Overall, our fall foliage in New England is and will remain beautiful for some time. Each season is different based on the weather and other forces, and we are happy and proud to share our thoughts each season as the region welcomes visitors from all over the world to experience the show. But in the long term, change may be inevitable. And so, too, it may come to our fall traditions. FURTHER READING:New Climate Normals:https://www.ncei.noaa.gov/news/noaa-delivers-new-us-climate-normals/https://www.ncei.noaa.gov/access/us-climate-normals/Climate at a Glance Time Series:https://www.ncei.noaa.gov/access/monitoring/climate-at-a-glance/global/time-series/New England Climate Difference Maps:http://www.nrcc.cornell.edu/climate/difference/difference.html