Apparent Global Increase in Cloud Droplet Number Concentration After 2022 Attributed to MODIS Orbital Drift

A long-term, consistent satellite record of cloud droplet number concentration (N_d) is essential for understanding aerosol-cloud interactions and their climate effects. However, the Aqua MODIS-retrieved N_d exhibits an unexpected and substantial increase over the near-global oceans after 2022, contradicting the expected decline from continued emission reduction efforts. Here we demonstrate that this surge is not physical but largely an artifact of sensor orbital drift, which alters viewing geometry and solar illumination. By leveraging concurrent Suomi-NPP VIIRS observations unaffected by drift, we developed an empirical correction that removes this artificial signal and quantified global mean N_d artificial biases of +2.4 cm⁻³ in 2023 and +5.0 cm⁻³ in 2024. These biases substantially distort the N_d trends, reversing the previously decreasing global N_d trend into an apparent strong rise after 2022. These findings highlight the critical need to correct for such artifacts when constructing satellite-based climate data records.