This essay presents the hypothesis that short-lived or evanescent microbial ecosystems exist in Earth’s lower troposphere (~ < 4 km). This hypothesis is supported by culture- and molecular-based studies that have shown diverse, viable, and metabolically active microbial communities within Earth’s atmospheric boundary layer. Surprisingly, microorganisms are routinely recovered in samples collected at extreme altitudes including those within the stratosphere (> 18 km). Volcanic eruptions, dust storms, fires, and sea spray are known to seed the atmosphere with microorganisms and to serve as potential nutrient sources while in the atmosphere and upon deposition. Recent research has demonstrated that microorganisms are metabolically active in clouds; for example, archaea capable of utilizing gases such as methane and hydrogen-nitrogen have been identified in clouds and in the atmosphere over natural and anthropogenic gas seeps. The only difference between this hypothesized ecosystem to more traditionally defined ecosystems is its evanescent characteristics where clouds or gas plumes eventually dissipate as they reside over and traverse Earth’s terrestrial and/or aquatic environments. The life cycle of these hypothesized evanescent airborne ecosystems would be short-lived relative to the classically defined biomes or ecosystems.