Hello, I’m Sarah Cornell from the Stockholm Resilience Centre. And I’m going to explain why we’re concerned about the global human impact on atmospheric aerosols.
Aerosol is a rather technical term to describe the phenomenon of liquid droplets or particles, very small particles, that are held suspended in the atmosphere. Aerosols play many very important roles in the atmosphere and in the Earth system.
They can both absorb and reflect light, so they’re important in Earth’s heat balance, and in the climate system. They provide condensation nucleus points; water condenses on aerosol particles and affects where clouds are formed and where rainfall happens. And they also provide microsurfaces for chemical reactions in the atmosphere. So, they influence atmospheric chemistry. For example, the reactions leading to stratospheric ozone depletion, or the ozone hole, happened on polar clouds that formed on stratospheric aerosols in the upper atmosphere.
To understand atmospheric aerosols, and the additional loading that humans are creating, we take physical and chemical measurements of gases, of particles, and of rainfall. A lot of this work is done in particular locations in different ecosystems, especially in urban ecosystems where the particulate loading is highest. But we can also measure aerosols from space.
Aerosols can be emitted directly into the atmosphere and they can also be formed through chemical processes in the atmosphere. That’s what we call secondary aerosol. One of the main natural sources of aerosol is actually the world’s oceans, waves and bubble bursting eject saltwater directly into the air and the water evaporates and leaves little microcrystals of salt. Other natural primary sources include fire, volcanoes, and air-blown dust.
There are natural sources for secondary aerosol too. For example, plankton and land vegetation emit organic compounds that react in the atmosphere to make very small particles. These reactions that create aerosols that affect the distribution of clouds over forests and coastal zones.
We also see human impacts in both direct and secondary aerosols. Land use change and combustion processes change the global patterns of dust and smoke emission. And together with transport and industrial processes we are currently changing the emissions of a very large number of chemical precursor gases that become aerosols in the atmosphere.
You’re probably familiar with some aerosol systems already. Smoke is a suspension of carbon particles in the air; fog is a suspension of water droplets; and the gas emissions associated with human activities, interact with each other and with these natural systems, creating really much more complex aerosol systems like smog and photochemical haze.
In the last century killer smogs, or pea soupers, were a severe environmental problem, so industrial smoke emissions are very tightly controlled in many cities of the world. But however industrial and urban emissions are still the cause of many problem aerosols in other parts of the world.
Photochemical haze plagues cities like Shanghai and Los Angeles and many other mega-cities in the world.
These aerosol systems are complex. They involve both natural and anthropogenic sources, they involve many different kinds of chemical substances, they involve direct emissions and reactions that happen in the atmosphere, and these reactions involve solids, and liquids, and gases. In other words, the composition and the ultimate fate of aerosols depend on many different geographic and meteorological conditions.
This presents a major challenge to us when we’re trying to find a global measure for what is acceptable or not acceptable in terms of aerosol changes in the Earth system. On top of that the world’s ecosystems on land and in the oceans have evolved and adapted to the biogeochemical flows that aerosols provide.
The human impact isn’t just as simple as an increase in aerosol loading. In some instances human activities are removing or relocating aerosols. We risk setting off physical and ecological tipping points when we change atmospheric chemistry in this way. This animation shows the global patterns and total aerosol loading of the atmosphere. It’s clear that aerosols are a vital part of the Earth system, and a dynamic part, and also that there’s a huge amount of variability in their local and regional patterns.
This figure is from the Intergovernmental Panel on Climate Change, and it shows the complex effect of atmospheric precursor gases in the top of the red box, and aerosols in the lower part of the red box. It shows the effects of aerosols and gases on Earth’s radiative balance. Some have a positive effect on radiative forcing, leading to warming of the atmosphere; and others have a negative forcing, in other words they lead to cooling. The net global effect is a cooling, at the moment.
The climate planetary boundary already addresses the radiative forcing effect of aerosols. But there are good reasons to address anthropogenic aerosol directly in the planetary boundaries concept, in ways that address their physical and biogeochemical impacts, not just their effects on the global energy balance.
In this context it’s really important to recognize the regional complexity of aerosols. One example where humans are causing regime shifts that might affect the whole Earth system is the change to the Asian monsoon system caused by the intense brown cloud of atmospheric pollution over south Asia and the Indian Ocean.
Another is the change in tropical rainfall patterns caused when deforestation reduces the natural aerosol emissions from trees and its interaction with the water cycle. This change can trigger climatic and water cycle feedbacks that would accelerate regime shifts or tipping points from forest to grasslands.
And yet another example that can trigger climate and ecosystem feedbacks is the deposition of dark aerosol particles, soot, or black carbon, on ice sheets and glaciers, accelerating their melting.
These kinds of pollution changes affect the regional albedo, and result in shifts in weather patterns, and biogeochemical flows, and ecosystems and the biodiversity within them.
In summary, although there is no single value for a planetary boundary that incorporates all kinds of aerosols all around the world, there’s a very strong case for specific sub-boundaries to be defined for particular aerosol systems in order to maintain the functioning of global earth system processes.