environment sustainability

Planetary boundaries 6 – Aerosols

brown cloudSix weeks into this series on the 9 planetary boundaries, we come to aerosols, or atmospheric aerosol loading to give it the proper title. Aerosols are airborne particles, of which there are lots of different types. Many airborne particles occur naturally in the environment, such as ash from volcanoes or dust from desert storms. Others, like smoke and pollution, are put there through human activity. Since the beginning of the industrial era, we have doubled the aerosol loading of the atmosphere.

They’re all bundled together in one category in the boundaries report, but different aerosols do different things. Since they are in the air that we breathe, there are often implications for human health – and it’s not just the industrial ones either. At this time of year, many people are acutely aware of the pollen count.

Aerosols also play a critical role in the climate, although we are really only beginning to understand it.  Some of them cool the planet, some warm it, and they interact with each other in unpredictable ways. It’s complex, but the aerosols boundary is an interesting one for a variety of reasons.

To illustrate why aerosols are a planetary boundary, let me give three examples of how they can affect our world.

  • Thanks to coal power stations, wood burning stoves and heavy industry, there is an ‘Asian Brown Cloud’ of pollution that hangs over India. This blocks around 10% of the sunlight hitting it, cooling the region and lowering evaporation rates and thus rainfall. This has disrupted the monsoon that brings rain to a billion people across Bangladesh, India, Burma and parts of China, exacerbating both floods and droughts.
  • Lest we get on our high horse about dirty Chinese industry, there is a theory that pollution from Europe and America was a factor in the long drought in the Sahel in the 70s and 80s. The darkening effect of Western pollution over the Atlantic reduced moisture take-up into the air circulating towards Africa. That reduced rainfall, leading to famine, millions of displaced people and death by starvation. As pollution controls have come in since the 80s and cleaned up the air over the Atlantic, the rains in the Sahel have recovered.  I suppose it’s hard to prove the connection in retrospect, but if it’s true, LiveAid is little compensation for the devastation our industry wreaked on the region.
  • Finally, soot from fires, especially from forest clearance (see the recent crisis in Singapore), rises into the atmosphere and accumulates at the poles. Here it falls and darkens the ice, making it less reflective. The less reflective it is, the faster it melts. It has been estimated that soot is speeding up Arctic melting by a fifth.

As these examples show, airborne pollution genuinely tinkers with the functioning of the planet, at a scale that can potentially affect millions of people. There is, however, quite a lot of good news.

  1. Aerosols don’t linger for decades in the atmosphere like CO2 and other greenhouse gases do. Aerosols wash out of the air with the rain, so the problem ends as soon as you stop producing the pollution.
  2. Air pollution is a major health problem, particularly affecting children through asthma and other respiratory conditions. Cleaning up the air would be a major benefit for human health.
  3. Sorting aerosol pollution would also be good for biodiversity and food security, since aerosols are the cause of the acid rain that harms crops and forests.
  4. There are few obvious and cheap techno-fixes for most of the planetary boundaries, but there are for aerosols. Higher standards for diesel engines is one, and clean-burning cookstoves is another.

Those cookstoves demonstrate an interesting point about the aerosol boundary – developing countries are more responsible than developed countries here. It’s usually the affluent who have the bigger ecological footprints, but the poorest are often a big source of aerosol pollution, through stoves burning coal or dung. Since these sorts of stoves are often inefficient and unhealthy, replacing them is something of a priority, and it would be a win for development, health and the climate all at once.

This all sounds good, but there is one complication. Some aerosol particles, like black carbon or dust, make climate change worse. But others mitigate it. Sulphur particles are bright and reflect sunlight back away from the surface, having a cooling effect. One recent study estimates that pollution has prevented around 0.07 degrees C of warming since 2000. There is a danger that in cleaning up aerosol pollution, we also remove something of an inadvertent safety net that will expose us to faster warming.

That, says Mark Lynas, may present us with an opportunity. If we can do this by accident, couldn’t we do it on purpose? By releasing sulphurous compounds in the upper atmosphere (above the clouds so it doesn’t cause acid rain), we may be able to geo-engineer ourselves a giant parasol of sorts. Initial estimates suggest it could cost $25-30 billion and it would be massively controversial, but it is theoretically possible. This is essentially what happens with a big volcanic eruption after all. The last big one was Pinatubo in 1991, and it cooled the planet for a whole year.

Of course, $25 billion is a lot of money and it ought to be spent on decarbonising the economy and fighting the cause rather than the symptoms of climate change. I don’t advocate those sorts of solutions myself, but I mention geoengineering because the whole point of the planetary boundaries is that human activity is already changing the planet. Whether we like it or not, we’re already tweaking the functioning of the planet. We’re just doing it through mismanagement, lurching from one unforeseen consequence to another. The more we understand what we’re doing, the more deliberate we can be in managing the planet properly. We did it with the ozone. We’re aware of the need to do it with climate change. And who knows? If we fail to stop runaway climate change, we may be pushed to more extreme solutions in future.

Because there are many different aerosols and they interact in such complex ways, the boundaries report does not attempt to identify a threshold limit. Although the big impacts of aerosols are visible in things like monsoon disruption, how it works exactly is still a bit of a mystery. A lot more research is needed on this planetary boundary.

Boundary: Atmospheric aerosol loading
Safe limit: unknown
Status:  unknown

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