by Dave Barr
The idea of an urban microclimate is probably not new to most of us. It’s been quite a while since we first heard a city described as a heat island, and it’s undoubtedly as true of Toronto as of any major North American or European city.
The whole study of the urban microclimate has proceeded apace, and it has attracted the interest of city planners and energy specialists in addition to naturalists. An engineering study based on precise meteorological data from Antwerp found that urban buildings required more energy for air conditioning than similar buildings in rural areas, just because the cities were warmer. But within the city, the demand for cooling energy was lower when a building was located beside the lawns and leafy trees of an urban park. A study at the University of Waterloo using satellite thermal imaging shows that planting native species in city back yards can cool local temperatures by as much as 4.5 degrees C.
Naturalists have been mainly interested by the ability of cities to host native species that need protection from the worst cold of winter. But recent research has added a lot of hard evidence to the heat island concept. And it highlighted one particular outcome that can be observed by any naturalist.
The study was done by Dr. Lin Meng and the final publication included nine other collaborators. This team was able to collect and study satellite and meteorological data from across 85 North American cities for the years from 2001 to 2014. That’s a huge amount of information to analyze. They were interested in the phenology of the urban forests in these cities. Phenology is the study, as Wikipedia states succinctly, “of periodic events in biological life cycles and how these are influenced by seasonal and interannual variations in climate, as well as habitat factors (such as elevation).”
All naturalists will be familiar with the phenology of leafing out of deciduous trees in their area. Some tree species sprout leaves first while others follow later, usually in a predictable order. The Spring phenology of trees is important because the life cycles of many other species, from moths to ants and squirrels are dependent on the status of the canopy.
The Lin Meng study focused on the variable they refer to as “start of season,” or SOS. This is the approximate date deciduous trees began to leaf out in the cities they studied. The study demonstrated for the first time based on hard science that the SOS of larger cities with a warmer urban microclimate was earlier in Spring than that of smaller, cooler cities. Leaf buds unfold six days earlier on average.
Simply put, Spring for the naturalist starts earlier in large cities than it does elsewhere. Is this a good thing… or maybe not so much?
Upsides and Downsides
There are a number of things about an early Spring that could well be favourable. Trees could have a longer growing season and therefore capture more carbon dioxide, produce more oxygen, grow faster and sequester more carbon, reducing the level of greenhouse gases. Early green-up is often tied to even earlier flowering, an important source of food for pollinating insects. The benefits of cool leafy urban parklands are available to the people in cities earlier, a welcome tonic to counter the short, frigid days of winter. And energy conservation starts sooner for those city buildings close to trees.
But it’s not all good. Pest species like the Emerald Ash Borer, may overwinter more successfully in the warmer urban microclimate. If pollinators are unable to synchronize their emergence with earlier tree flowering they may miss their critical annual feast. Leafing out too early could be just as dangerous as planting your vegetables too early. Both are in danger of damage from the occasional late frost. Early shading of the forest floor in urban woodlands could eliminate the prime growth period for sensitive, native plants that need to flower and set seed before being shaded by the canopy.
Should we try to do anything about this unnatural urban microclimate? The conditions in a ‘heat island’ clearly differ from those to which native species in a region have adapted naturally. In general it would seem obvious that any rapid, forced change in environment is likely to be more deleterious than beneficial. It all depends on the adaptability of the species involved. But in nature adaptation usually occurs over millennia rather than decades.
What can those of us who inhabit cities do to cool off the urban microclimate? Very little it would seem. The incessant demands of urban human populations are for more dwellings, more transportation and therefore more use of the energy that warms up the local environment. Governments seem powerless to resist that pressure. Green roofs and solar power are being tried, but with little impact so far.
The Light Effect
Lin Meng, however, also reveals an unexpected finding that may offer some hope. Her team found that although we assume the heat island is the main driver, there is another aspect of cities that is even more important to trees than temperature. A clue comes from satellite images of earth at night. NASA has been capturing these images for more than four decades. What they reveal is that major cities, like Toronto, are also light islands, glowing centres of radiant light energy that gleam in stark contrast to their rural and water surroundings. Unlike you and I in our homes, big cities leave the lights on all night.
Leafing out of deciduous trees, it emerges, is more influenced by changes in day length than temperature. The study demonstrated that the strongest correlation of early greening is with the size and intensity of the urban light island and not the heat island.
Did a light go on in your mind as it did in mine? Could light pollution be easier to bring under control than heat pollution? We know that downtown skyscrapers need to be heated or cooled at night. But do they have to have all their lights blazing continuously as well? What about plazas and parking lots? Do they need to be as bright as daylight all night long? A little serious thought should turn up plenty of other opportunities to control light pollution and perhaps even begin to reverse the early urban Spring phenomenon.
Suppose we can start to make some progress with light pollution. How will we know if it’s working? The likelihood of pulling together the resources to do another study like Meng’s is remote. Here’s where citizen science, led by naturalists, can come in. The work of Project Budburst is only one example. Citizen science observations on phenology are easy to do and inexpensive. All it should take is some organizing to communicate objectives, synchronize observations and analyze the data. What do you think?
Dig Deeper Into This Topic
- Lin Meng Green with phenology, 2021. https://www.science.org/doi/10.1126/science.abm8136#pill-R10
- Lin Meng et al., 2020. https://www.pnas.org/content/117/8/4228
- Project Budburst: https://budburst.org
- Y. Toparlar et. al., 2018. Impact of urban microclimate on summertime building cooling demand: A parametric analysis for Antwerp, Belgium: https://www.sciencedirect.com/science/article/pii/S0306261918309814
- University of Waterloo. Planting native trees, greenery can cool off your backyard: https://uwaterloo.ca/news/news/planting-native-trees-greenery-can-cool-your-backyard
- Wikipedia, Urban Heat Island: https://en.wikipedia.org/wiki/Urban_heat_island
- Wikipedia, Phenology: https://en.wikipedia.org/wiki/Phenology