Are a lack of trees driving away the Fraser Valley's summer rain?

Bill Hardy remembers when it used to frequently rain in the summer.

“I’m old enough now, and lived in Vancouver long enough, that I remember when it rained through the summer every couple of nights,” says Hardy, the president of the Canadian Nursery Landscape Association and a speaker at the Chilliwack Rotary Club’s Climate Fair this weekend.

The summer rain is much rarer now. In Abbotsford, the amount of rainfall each summer has plummeted in just two decades. Vancouver has also seen a sharp drop.

A changing global climate certainly has an impact on precipitation trends. But Hardy suggests disappearing trees and greenspaces is also likely affecting the Lower Mainland’s weather—and maybe even the amount of rain we get each year.

The urban heat island effect

The urban heat island effect is well-known and simple. The sun sends energy to Earth chiefly in the form of sunlight. Buildings and roads—especially dark-coloured ones—absorb the sunlight and convert it into heat (another form of energy). That energy gets baked into a city’s very bones. As roads and buildings warm, the surrounding air also begins to warm.

By contrast, when the sun hits the leaves of plants, it converts that energy into new roots, leaves, and branches. That energy is stored in material until it begins to decompose or is set alight. (So when a log burns and its substance erodes, it gives off heat.)

There’s a bonus as well: those plants also absorb water from the ground and transfer it to their leaves. Much of that water ends up exiting plants in the form of water vapour in a process known as transpiration. And as it enters the warm summer air, that ground-fed vapour produces a natural cooling effect.

So a forest or heavily treed park isn’t just cooler in summer because of the shade. It’s also cooler because all those trees and plants are pumping that ground-chilled water vapour into the air.

On the other hand, an area where large tracts of the ground are covered in buildings and pavement may have ground-level temperatures far above those in areas with more natural spaces. When such areas accumulate, they can combine to create a “heat island,” the excess heat from which can be stifling or even deadly, as the 2021 heat dome proved.

And there’s also increasing discussion about whether and how a heat island might affect the weather far overhead. It’s possible, some scientists suggest, that a heat island might have a knock-on effect that affects local rainfall patterns.

Research suggests the precise impact depends on the location and the season. In some places, heat islands may increase precipitation. On a prairie, for instance, tall buildings may cause passing air to rise to levels where the moisture cools and condenses.

But the precise geography and climate matters and in other places, a heat island could reduce precipitation overhead:

Hardy points to research from the University of Denver that looked at the impact on rainfall patterns in that mountainside town with notably dry summers.

Hardy suggests that our region’s growing heat islands could be tied to its markedly drier summers.

Clouds release rain when they move into cooler airspace that reduces their water-carrying capacity.

“When you create heat over a city, rain passes over it instead of falling on it,” says Hardy, summarizing the research.

Vancouver is naturally warmer than the cool ocean that surrounds it (one reason why its summers are usually relatively dry.) And the tie between heat islands and the weather hasn’t been studied in Vancouver in particular.

But we do know that the amount of summer rain has been declining in recent years.

Much of that change is likely tied to the way climate change is creating warmer weather and especially warmer nights. But the notion that Vancouver’s heat island—and diminishing number of trees—could also be impacting the weather isn’t random speculation. The idea echoes a major new study that found that tropical areas that endure deforestation may experience less rainfall as a result.

The idea is profound: “Rainforests don’t exist where there’s a lot of rain; there’s a lot of rain because of the forest,” Hardy says.

The relationship isn’t entirely that simple (you do need water before you have the first trees), but it does illustrate the way that our climate in the atmosphere and the conditions on the ground influence one another. And what we do know is that the Lower Mainland both has far fewer trees, and far less summer rain, than it did just a couple decades ago.

In Abbotsford, home to its own growing heat island, the months of July and August are now twice as dry as they were just 30 years ago (watch The Current for more on the dramatic changes to summer rainfall).

So if there’s not scientifically proven causation, there’s definitely correlation.

The potential solution is simple: trees. They also happen to come with an array of benefits that will still exist even if they can’t bring a bit of summer rain.

Hardy is the treasurer of the Green Cities Foundation, a non-profit aimed at building and promoting greenspaces in Canada’s cities. Its goals largely dovetail with Lower Mainland cities that want to preserve and rejuvenate their dwindling urban canopies.

It’s well-established that trees and greenspaces are important for human health, improve the local environment, can mitigate flooding and draining issues, and are generally more pleasing to be around in summer than a patch of scorched pavement.

But while it’s easy to gain consensus that trees are great, figuring out where to put them, and how to save the remaining ones, is much more difficult.

The dilemma of density and urban deforestation

Across the region, cities are maintaining, and sometimes increasing the number of trees in public spaces. But their urban forests are still suffering as trees are toppled on private lots.

Some of that is due to greenfield developments, whereby forested hillsides are bulldozed to make way for homes. But much of the loss of trees is the result of in-city densification that the municipalities—and the province—are promoting as a key way to address the affordable housing crunch.

Across BC, single-family lots are being subdivided into two or three new lots for two or three new homes. On some lots, apartments and townhome complexes are being built. That densification makes cities more efficient, reduces car-dependency and provides much-needed housing in a region with too little of it. But it inevitably shrinks yards and the amount of room for trees and greenery on each property. (Lawns have their issues, but they’re still much better at absorbing heat and releasing oxygen and water than your typical roof.)

Hardy, who sits on Maple Ridge’s environmental advisory committee, doesn’t suggest that densification be halted. But he says municipalities need to be cognizant of its effects when it occurs on a large scale.

“As things come to council, they go one development permit or one rezoning application at a time,” he said. “Quite often we don’t look at the impact to the overall neighbourhood. If every small developer is buying one or two lots and replacing it with a three-plex or a four-plex, that’s OK when you’re looking at it one lot by one lot. But when you stick 15 of those down each side of what used to be a single-family residential street with a lot of greenspace, all of a sudden you have no greenspace left.”

The solution, Hardy says, is for cities to deliberately and continuously look at how to increase greenspace on public property. That could include repurposing land used for parking or ensuring that developer contributions are spent on buying land for new parks. The public also has a role to play in advocating for more greenspace. (One large challenge is the opposition that can follow any reductions to parking spaces.)

Hardy points to a concept touted by urban forestry thinker Cecil Konijnendijk, who stresses that every person should have three trees visible from their home, 30% tree canopy coverage in their neighbourhood, and a small neighbourhood greenspace within 300 metres of their home.

“It puts a little bit of a different perspective on it,” Hardy says.

Most Fraser Valley neighbourhoods would meet the three-tree, 300-metre rules.

But many areas have far less than 30% tree canopy coverage.

A 2015 analysis of Abbotsford’s canopy coverage found that its urban tree cover declined from 47% in 2005 to 40% in 2017. But that figure factored in vast forests on Sumas Mountain. In the vast majority of central Abbotsford, the tree canopy coverage was below 30%. In Clearbrook, coverage was less than 15%.

In Chilliwack, a 2021 study found it had an urban tree canopy of just 27%.

Hardy notes that cities can incentivize property owners to retain trees on their properties. But he also stresses that cities need to rethink how public spaces are used, and how they create parks.

“Some of the science now is proving that smaller connected-by-greenways parks are more taken advantage of than larger parks,” he said.

Assembling the land for those parks and green spaces can be tricky—but still easier than developing larger ones that require huge blocks of land. In particular, they can be built or enhanced by repurposing little-used roadscapes, parking spaces, or underused laneways or alleys, as is being done in Maple Ridge.

Bringing more trees and greenery into cities could bolster their residents’ physical and mental health, reduce emissions and the local environment, and just make communities a better place to hang out and live.

If it helps it rain a little more, all the better.

Hardy is one of many speakers at the first Chilliwack Rotary Climate Fair this weekend at the Landing Sports Centre. The fair includes a trade show, job fair, and numerous speakers, including Dr Carin Bondar, Brian Minter, and Eddie Gardner. You can see the full list and schedule here.