Aerosols are tiny suspended liquid and solid particles found in the atmosphere. These particles degrade air quality and are active in the climate system. Over the past 20 years, man-made aerosols, those particles emitted hand-in-hand with greenhouse gases, have been the focus of intense scrutiny by the aerosol-climate community.
Let’s not forget natural aerosols. The bulk of aerosol particles are natural, not man-made, and these natural aerosols are just as important to the climate system. Natural aerosols travel long distances and affect exceptionally pristine areas of the Earth where small changes in the aerosol environment can produce relatively large responses. Most importantly the processes producing natural aerosols such as dust and volcanic emissions are not constant. Therefore, there is no background aerosol environment from which to quantify the perturbation made by human activity and man-made particles. How do we calculate the aerosol forcing imposed by human activity if we do not know what the aerosol effect was before people became involved?