For a long time scientific and public debates on global climate revolved around human-produced greenhouse gases. Only recently the role of aerosols, little solid particles or liquid droplets in the atomosphere, has been introduced to the discussion. However, aerosols quickly became a focal point of interest because neither their composition nor their effect on global climate had been thoroughly investigated.
It is clear that aerosols in general and stratospheric aerosols in particular have counterbalanced greenhouse gas warming, in other words the aerosols cause a negative climate forcing. In most current climate models the exact magnitude of aerosol forcing relies on uncertain estimations, though. Its exact quantication has turned out to be complicated because of the many factors that influence aerosol concentrations including a lot of indirect effects.
For example, air pollution control might lead to a vast decrease in aerosol numbers — and thus to global warming — because it decreases the availability of cloud condensation nuclei.
A year ago, Susan Solomon and co-workers made progress in quantifying the “background” aerosol concentration in the stratosphere, i.e., the concentration in absence of major volcanic eruptions. It was established that the background aerosol layer is variable and that it has notably increased over the past ten years thus counteracting climate change more than we were aware of. The finding has provoked numerous proposals to deliberately enhance this layer, e.g., by injection of minerals but it is unknown what effect such provisions would have on stratospheric chemistry, above all on ozone depletion pathways.
The new discoveries and discussions are captivating because they give us a better idea of the complexity of global climate. The experimental and theoretical investigations of aerosols are still in their infancy, yet it is already obvious that the effect of aerosols
on global climate is enormous.
 J. Hansen et al., Atmos. Chem. Phys. 11, 13421-13449 (2011)
 R. Makkonen et al., Atmos. Chem. Phys. 12, 1515-1524 (2012)
 S. Solomon et al., Science 333, 866-870 (2011)
 F. D. Pope et al., Nature Clim. Change, doi:10.1038/NCLIMATE1528 (2012)