In candidate formations for geological Carbon Capture and Storage (CCS), carbonate minerals (e.g., calcite) are ubiquitously presented. The dynamic process of chemically induced alteration on carbonate-rich reservoirs due to the injection of supercritical CO2 holds paramount importance for achieving an economic injectivity and structural integrity of the system. How carbonate rocks undergo deterioration and particularly how microcracks develop in the presence of carbon dioxide remain largely unknown. Here we employ a powerful tool of reactive force field (ReaxFF) molecular dynamics (MD) simulation, investigating into the impact of representative CO2 environments on Mode I tensile crack propagation in calcite at micro-scale. Our simulation results demonstrate that (1) both dry and wet CO2 environments favor the tensile crack propagation by lowering the fracture toughness of the pre-existing crack; (2) the wet CO2 environment promotes the growth velocity of the subcritical crack compared to the dry CO2 environment, under the same mechanical loading condition; (3) the interaction between the stressed crack and the CO2-water mixture diffusing into the crack opening leads to a small reduction of the system potential energy at an initial stage of subcritical growth; (4) The crack tip appears to be sharper in both dry CO2 and wet CO2 environments, albeit at a lower stress intensity factor than the vacuum case. The atomistic scale findings provide new insights on the process of subcritical calcite cracking induced by a reactive environment via CO2 injection, prior to the damage-enhanced dissolution phase.