Do trees really help clear the air in our cities – or are they trapping pollutants at street level?

Trees: friend or foe? Brooklyn's Prospect Park. Image: Spencer Platt/Getty.

It may sound like a no-brainer to say that trees improve air quality. After all, we know that trees absorb the greenhouse gas carbon dioxide (CO₂), and that their leaves can trap the toxic pollutants nitrogen dioxide (NO₂), ozone, and harmful microscopic particles produced by diesel vehicles, cooking and wood burning.

Yet some recent studies have suggested that trees may in fact worsen urban air quality by trapping pollutants at street level. A closer look at the evidence – and how it was collected – reveals the root of this dispute, and can help us come to a more nuanced understanding of the impacts of trees on our urban environment.

First things first; it is not trees that pollute the air of cities in the developed world. As car manufacturers are all too guiltily aware, it is mainly road vehicles that cause pollution – and their impacts are compounded by the choices we make about how and what we drive.

Many features of the urban landscape influence how air moves around a city. Impervious objects like buildings, and permeable ones like trees, deflect air from the path imposed by weather patterns, such as high and low pressure systems. The urban landscape turns freshening breezes into whorls of air, which can either contain pollution near its source – where it affects vulnerable hearts and lungs – or lift it away from ground level.

Whether the landscape traps or lifts air will depend very sensitively on the exact positioning of roads, buildings, gardens, street trees, intersections, even billboards and other street furniture.

Sticking points

Trees affect the urban environment in several subtle ways. From altering air flows, to collecting pollution deposits, to affecting the chemical make up of the atmosphere, their impacts are both pervasive and difficult to pinpoint.

As air swirls and twists past the urban fabric, microscopic pollutants can deposit on surfaces. Any surface will do, but trees are especially effective at trapping these particles, because of their large, porous surfaces.

We breathe air and – generally speaking – don’t lick leaves

One way we can tell whether trees are helping to reduce air pollution is by estimating the mass of pollutant deposited. Experiments to measure depositing pollutants are usually carried out in the middle of flat fields, where it is easier to interpret the measurements. But, of course, a city is a very different environment, and it’s not clear whether these results hold true in highly variable urban settings.

Experimental studies can certainly show that pollution ends up on leaves. But it is no easy task to convert such measurements into an estimate of how the concentrations – that is, the amount of pollutant per cubic metre of air – change. And it is this concentration change that really counts, since we breathe air and – generally speaking – don’t lick leaves.

Some pollutants, like NO₂, are both emitted by human fuel use and produced when chemical reactions take place in the atmosphere. Other pollutants, notably ozone, are only produced through reactions of nitrogen oxides with fumes from oil-based solvents, petrol and similar chemicals in the air.

The production of toxic ozone can happen purely as a result of our consumption of fossil fuels: particularly when hot, settled summertime conditions provide the light needed to kick-start the chemical reactions, while the stillness prevents dilution of the pollution into the global atmosphere.

That said, trees also release chemicals that react with nitrogen oxides to produce ozone, sometimes in sufficient quantity to make a difference, even in urban areas.

Trees also take up space. Parks and gardens are not usually sites with intense pollutant emissions, so they provide an important volume into which pollution can be diluted. This is evidenced by statistical studies, which show how concentrations of air pollutants vary according to the type of urban neighbourhood: the decrease of pollutant concentrations away from busy roads is modified by how tall the buildings are in the neighbourhood.

Seeing the wood for the trees

When assessing research on the effects of trees on urban air pollution, remember that no single study has yet put all the pieces of the puzzle together. With so many processes to consider, it’s little wonder that experiments based in a range of locations, using varying methods, yield vastly different results.

To produce a definitive study, it would take either many months of measurements before and after the planting of vegetation, or a shorter series of simultaneous measurements in two urban locations identical in all respects, except for the presence of trees or some other form of “green infrastructure” in one location. Both approaches are expensive and difficult to undertake in busy cities, which are constantly subject to all kinds of other changes.

So, we are left with piecing together the evidence as it is presented to us in reports. When doing so, look first at whether the study is, in fact, concerned with how air is dispersed in urban areas, and remember that such dispersion really depends on every part of the cityscape, not just on trees. Look to see if, and how, removal of pollutants by deposition is considered, and then check whether the study considers the effects of dilution or atmospheric chemistry.

Finally, consider the results of any single study in the light of the best available systematic approach to trees in the townscape before drawing conclusions.

Asking whether cities should have trees in it is a bit like asking whether a suit should have a person in it. There is every chance that urban trees could provide a “nature-based solution” to several pressing problems with the urban environment, but perhaps not in the way scientists and policy-makers seem currently to be thinking.

Rather than providing a technical fix that disguises our obsession with the diminishing returns of the internal combustion engine, increasing urban tree numbers could change our entire perspective on cities, facilitating the creation of liveable cities that value nature as an integral part of social, economic and environmental capital.The Conversation

Rob MacKenzie is professor of atmospheric science at the University of Birmingham.

This article was originally published on The Conversation. Read the original article.


The IPPC report on the melting ice caps makes for terrifying reading

A Greeland iceberg, 2007. Image: Getty.

Earlier this year, the Intergovernmental Panel on Climate Change (IPCC) – the UN body responsible for communicating the science of climate breakdown – released its long-awaited Special Report on the Ocean and Cryosphere in a Changing Climate.

Based on almost 7,000 peer-reviewed research articles, the report is a cutting-edge crash course in how human-caused climate breakdown is changing our ice and oceans and what it means for humanity and the living planet. In a nutshell, the news isn’t good.

Cryosphere in decline

Most of us rarely come into contact with the cryosphere, but it is a critical part of our climate system. The term refers to the frozen parts of our planet – the great ice sheets of Greenland and Antarctica, the icebergs that break off and drift in the oceans, the glaciers on our high mountain ranges, our winter snow, the ice on lakes and the polar oceans, and the frozen ground in much of the Arctic landscape called permafrost.

The cryosphere is shrinking. Snow cover is reducing, glaciers and ice sheets are melting and permafrost is thawing. We’ve known this for most of my 25-year career, but the report highlights that melting is accelerating, with potentially disastrous consequences for humanity and marine and high mountain ecosystems.

At the moment, we’re on track to lose more than half of all the permafrost by the end of the century. Thousands of roads and buildings sit on this frozen soil – and their foundations are slowly transitioning to mud. Permafrost also stores almost twice the amount of carbon as is present in the atmosphere. While increased plant growth may be able to offset some of the release of carbon from newly thawed soils, much will be released to the atmosphere, significantly accelerating the pace of global heating.

Sea ice is declining rapidly, and an ice-free Arctic ocean will become a regular summer occurrence as things stand. Indigenous peoples who live in the Arctic are already having to change how they hunt and travel, and some coastal communities are already planning for relocation. Populations of seals, walruses, polar bears, whales and other mammals and sea birds who depend on the ice may crash if sea ice is regularly absent. And as water in its bright-white solid form is much more effective at reflecting heat from the sun, its rapid loss is also accelerating global heating.

Glaciers are also melting. If emissions continue on their current trajectory, smaller glaciers will shrink by more than 80 per cent by the end of the century. This retreat will place increasing strain on the hundreds of millions of people globally who rely on glaciers for water, agriculture, and power. Dangerous landslides, avalanches, rockfalls and floods will become increasingly normal in mountain areas.

Rising oceans, rising problems

All this melting ice means that sea levels are rising. While seas rose globally by around 15cm during the 20th century, they’re now rising more than twice as fast –- and this rate is accelerating.

Thanks to research from myself and others, we now better understand how Antarctica and Greenland’s ice sheets interact with the oceans. As a result, the latest report has upgraded its long-term estimates for how much sea level is expected to rise. Uncertainties still remain, but we’re headed for a rise of between 60 and 110cm by 2100.

Of course, sea level isn’t static. Intense rainfall and cyclones – themselves exacerbated by climate breakdown – can cause water to surge metres above the normal level. The IPCC’s report is very clear: these extreme storm surges we used to expect once per century will now be expected every year by mid-century. In addition to rapidly curbing emissions, we must invest millions to protect at-risk coastal and low-lying areas from flooding and loss of life.

Ocean ecosystems

Up to now, the ocean has taken up more than 90 per cent of the excess heat in the global climate system. Warming to date has already reduced the mixing between water layers and, as a consequence, has reduced the supply of oxygen and nutrients for marine life. By 2100 the ocean will take up five to seven times more heat than it has done in the past 50 years if we don’t change our emissions trajectory. Marine heatwaves are also projected to be more intense, last longer and occur 50 times more often. To top it off, the ocean is becoming more acidic as it continues to absorb a proportion of the carbon dioxide we emit.

Collectively, these pressures place marine life across the globe under unprecedented threat. Some species may move to new waters, but others less able to adapt will decline or even die out. This could cause major problems for communities that depend on local seafood. As it stands, coral reefs – beautiful ecosystems that support thousands of species – will be nearly totally wiped out by the end of the century.

Between the lines

While the document makes some striking statements, it is actually relatively conservative with its conclusions – perhaps because it had to be approved by the 195 nations that ratify the IPCC’s reports. Right now, I would expect that sea level rise and ice melt will occur faster than the report predicts. Ten years ago, I might have said the opposite. But the latest science is painting an increasingly grave picture for the future of our oceans and cryosphere – particularly if we carry on with “business as usual”.

The difference between 1.5°C and 2°C of heating is especially important for the icy poles, which warm much faster than the global average. At 1.5°C of warming, the probability of an ice-free September in the Arctic ocean is one in 100. But at 2°C, we’d expect to see this happening about one-third of the time. Rising sea levels, ocean warming and acidification, melting glaciers, and permafrost also will also happen faster – and with it, the risks to humanity and the living planet increase. It’s up to us and the leaders we choose to stem the rising tide of climate and ecological breakdown.

Mark Brandon, Professor of Polar Oceanography, The Open University.

This article is republished from The Conversation under a Creative Commons license. Read the original article.