Abstract
Strong natural variability has been thought to mask possible climate-change-driven trends in phytoplankton populations from Earth-observing satellites. More than 30 years of continuous data were thought to be needed to detect a trend driven by climate change(1). Here we show that climate-change trends emerge more rapidly in ocean colour (remote-sensing reflectance, R(rs)), because R(rs) is multivariate and some wavebands have low interannual variability. We analyse a 20-year R(rs) time series from the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite, and find significant trends in R(rs) for 56% of the global surface ocean, mainly equatorward of 40°. The climate-change signal in R(rs) emerges after 20 years in similar regions covering a similar fraction of the ocean in a state-of-the-art ecosystem model(2), which suggests that our observed trends indicate shifts in ocean colour-and, by extension, in surface-ocean ecosystems-that are driven by climate change. On the whole, low-latitude oceans have become greener in the past 20 years.