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Fixing air pollution is like walking: easy until the ground starts moving

Clean up your act, Shanghai. Just add some more trees at the right places. Corinna A. Carlson

There is something of a sensory dissonance in modern cities. Glamorous as they are, the breath of contemporary cities stinks. A recent study published in the journal Environmental Research Letters found that two million premature deaths worldwide are associated with outdoor air pollution.

In June, the Mayor of London published a report showing that air pollution, even in rich countries, is not a thing of the past. Using a different arithmetic to Silva and co-workers, the The UK government’s Committee on the Medical Effects of Air Pollution estimated in 2010 that reducing the average concentration of these microscopic particles by one unit (out of an atmospheric load of roughly 20 units) would save approximately four million life-years for those born in 2008.

The simplest and most effective way to improve the situation with urban air quality is to reduce emissions from fossil fuel burning. Emission controls have been effectively implemented for large industrial complexes but there is still much to do, particularly in those places with economies reliant on power from coal.

The most problematic pollution is that from traffic, especially traffic congestion in narrow streets. Improvements to engine design have led to fewer emissions per vehicle, but improvements in UK urban air pollution, for example, have stalled for almost a decade. Apparently, as a society, we have managed to undo the good work of the automotive engineers by driving bigger cars, more often.

Policymakers talk of the “linear roll-back” of pollution that should result from control of emissions. Collectively, we have been asked to follow that straight line, but nobody told us that the ground - the complicated social-economic-ecological system of cities - was moving under our feet.

One way to intervene to make city systems more sustainable is to use vegetation as “green infrastructure”. Vegetation in cities can provide many and varied benefits: decreased urban heat island effects, improve air quality, increase biodiversity, improve water quality, provide resilience to flooding and improve well-being.

Realising these benefits requires careful planning and proactive engineering: “the right tree in the right place”. Get it wrong and trees — street trees in particular — can even make things worse, by preventing air pollution from mixing away from the roadside, for instance. Get it right and cities could have better air quality and more biodiversity than the agricultural prairies surrounding them, leading to increased wellbeing, happiness and productivity. And best of all, much of this planning can be “bottom up”, with locals, rather than bureaucrats, deciding how they want their streets to be.

In all likelihood this planet will have to carry a few more billion people sometime this century. Cities are our best hope for accommodating so many people, but cities will fail ultimately if we don’t investigate how these super-systems work, if we don’t value the services provided to us by the non-human part of the living city, and if we don’t recognise the vulnerability of all human and non-human complex systems. Cities should not just exploit the potential that people and finite natural resources offer, but should enhance the experience of all those who live in them.

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