Temperatures climbed well above normal a few days this week in Philadelphia, with Monday and Tuesday spiking eight degrees above the typical high of 80 for this time of year.
While temperatures often fluctuate, historical data over the last five decades suggest a trend toward warmer falls. Average temperatures have risen 2.7 degrees in the city from 1970 to 2020 for September, October, and November (astrological fall begins Sept. 22 with the equinox). That fits in with the pattern of warmer winters, springs, and summers suggestive of climate change.
There have been warm falls in the past, including in 1970, and temperatures do bounce up and down. But the overall trend is clear, according to data downloaded from the National Oceanic and Atmospheric Administration’s Northeast Regional Climate Center’s Applied Climate Information System.
Jen Brady, manager of analysis at Climate Central, a nonprofit comprised of scientists and journalists, calculates that Philadelphia experienced 15.3 more fall days above normal temperatures now than five decades ago.
Going back to 1874, Philadelphia has experienced 16 falls with average temperatures of 60 degrees or above. Nine of those years have come since 2001. The warmest fall ever recorded in Philly, with an average temperature of 63.6 degrees, occurred in 1931, a dust bowl year and an anomaly.
Nationally, the fall warming trend is fastest in Texas and the Southwest, with Reno, Nev., recording the highest temperatures, averaging 7.6 degrees warmer since 1970.
It is also worth noting that normal is actually slightly warmer than it was in the past. NOAA calculates normals from 30-year averages. So normals for fall have increased across most of the U.S. between the 30-year periods of 1981 to 2010 and 1991 to 2020. In some cases, the new normal has risen as much as 1 degree.
Warmer weather for more months means many people run air conditioners longer than in past decades. That not only contributes to more global warming from the burning of fossil fuels, but also increases energy costs for homeowners.
This week saw a flurry of activity on climate change.
On Thursday, Antonio Guterres, the head of the United Nations, called for “immediate, rapid, and large-scale” cuts in greenhouse gas emissions to curb global warming and avert climate disaster. He warned governments that climate change is proceeding faster than predicted.
On Friday, President Joe Biden hosted a virtual conference with a small group of global leaders at which he announced a new U.S.-European pledge to cut climate-wrecking methane leaks. The list of attendees included leaders of Argentina, Bangladesh, Indonesia, South Korea, Mexico, the United Kingdom, the European Council, European Union Commission, and United Nations.
China, India, and Russia, along with the United States, are the nations that emit the most climate-damaging gases from the production and burning of oil, natural gas, and coal, and there was no word on their leaders’ taking part.
Friday’s meeting followed a much bigger virtual White House climate summit in April that saw scores of heads of governments — representing allies and rivals, big economies and small — make sweeping speeches about the need for action against climate change.
In Pennsylvania, warming weather is already impacting local forests, and that could alter the state’s vibrant foliage season.
According to the Pennsylvania Forest Action Plan, some trees are already stressed, such as black cherry and sugar maple, and will likely continue to decline. Both hardwoods are commercially valuable for the state. It’s also possible that some tree species now dominant in the southern part of the state, such as oaks and hickories, may expand their range northward.
Amanda Gallinat, a researcher at the University of Wisconsin-Milwaukee with a background in ecology, wrote a paper calling autumn the neglected season when it comes to climate change research. She said the season remains hugely important, signaling the end of the growing and breeding season for many plant and animal species.
“There are a lot of ecological impacts of warming, and we’re learning more about them all the time,” Gallinat said.
For example, warmer, wetter years, such as Philly is currently experiencing, can push leaf senescence, or the aging that results in leaf-color change and leaf drop, to later in the fall. Forests cover 60% of the state and warmer conditions can delay the onset of peak colors and decrease the amount of time they’re visible. There are a lot of variables that affect the timing of leaf turning, and heat and wet are but two.
The years 2018, 2019, and 2020 have all been wetter than average, according to National Weather Service data. It’s unknown how 2021 will rank, but July and August were much wetter than average. And 2019 and 2020 were warmer than average, with August of this year 1.6 degrees above normal heading into September.
Gallinat said such warmer fall temperatures also allow invasive plants, such as honeysuckle and burning bush, to gain an advantage over native species by extending their growing season.
“If they have a longer growing season, it means they can put on more biomass and spread,” Gallinat said, “That can give them a sort of better fitness in following years.”
Long-term data, she noted, indicate that some birds are delaying autumn migrations, which might put them out of sync with cycles of food sources, such as native fruit, forcing them to feed on less nutritious invasive plants. The birds further disperse the seeds of the invasive plants.
Gallinat also said more mild temperatures allow pests such as mosquitoes and ticks to thrive.
“It can allow them to get another generation in, and so you see them persist later into the year,” Gallinat said, which increases the threat of West Nile Virus and Lyme disease.
This article contains information from the Associated Press.