Biological soil crusts (BSCs) monitored over a 25-year period enhance the dissolution of the Ca-silicate plagioclase and the Mgsilicate olivine at Sonoran Desert and Colorado Plateau, USA, study sites. This first measured biological enhancement of weathering (BEW) for plagioclase is a mean of 2.3±0.4 and 3.0±0.4 for the Organic Pipe, Arizona and Moab, Utah study sites; and it is 4.9±0.8 and 3.9±0.3 for olivine at these respective sites. These BEWs are low compared to other biological agents such as lichens, tree roots, termites and especially ants. If these modern BEW for BCSs reflect the magnitude of BEW in Archean soil crusts, then the presence of abundant BSCs covering an Archean Earth surface would not contradict available evidence for no substantial atmospheric CO2 decline in Earth’s early atmosphere. The relatively low BEW value for BCSs indicates that BSCs would not be a useful geoengineering solution to high levels of atmospheric carbon dioxide. Keywords: carbon dioxide; drawdown; Earth’s habitability; faint sun hypothesis; in situ weathering.