B. B. Cael, Kelsey Bisson, Emmanuel Boss, Stephanie Dutkiewicz, Stephanie Henson

Climate change-driven trends in phytoplankton populations, as viewed by Earth-observing satellites, were thought to be masked by strong natural variability, so that >30 years of continuous data was needed to detect a climate trend. Here we show that climate change trends emerge more rapidly in ocean color (remote sensing reflectance, Rrs), as Rrs is multivariate and some wavebands have low interannual variability. We find significant trends in a twenty-year time-series of Rrs from the MODIS-Aqua satellite over 52% of the global surface ocean, primarily equatorward of 40º. The climate change signal in Rrs emerges after twenty years in similar regions covering a similar fraction of the ocean in a state-of-the-art ecosystem model, implying that the observed trends may indeed be driven by climate change. Ocean color may thus be a sentinel of climate change in surface ocean ecology and biogeochemistry.