Each summer, monsoon systems move vast columns of warm, humid air from the tropics to mid-latitudes, but it was unknown until now that these same ascents carry a significant portion of biogenic emissions – volatile organic compounds released by forests, oceans, and soils – up into the lower stratosphere. This continuous injection creates a thin veil of aerosols that, although fine, alters how Earth reflects and absorbs solar radiation.
What the study reveals: Persistent contribution of biogenic emissions to lower stratospheric aerosols
Researchers found that global monsoons, particularly those in South Asia, West Africa, and South America, act as climate elevators, transporting volatile organic compounds beyond the tropopause. Once in the stratosphere, these precursors undergo oxidation, generating sulfate and organic particles. The result is a persistent layer of aerosols, detectable throughout the year, even outside periods of strong monsoon activity.
This continuous presence contrasts with classical climate models, which only considered anthropogenic sources (industries, aviation) for stratospheric aerosols. Integrating the study's data shows an increase in average lower stratospheric aerosol concentration of several tens of percent compared to previous estimates, representing an additional global radiative cooling factor.
The authors note that while the individual effect of each monsoon is modest, the sum of all monsoon systems' contributions creates a global signal that can no longer be neglected in global energy balance evaluations. This discovery opens the door to revising radiative forcing budgets, especially long-term warming projections.