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.

Here we show that climate-change trends emerge more rapidly in ocean colour (remote-sensing reflectance, R) because R is multivariate and some wavebands have low interannual variability. We analyse a 20-year R time series from the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite, and find significant trends in R for 56% of the global surface ocean, mainly equatorward of 40°.

The climate-change signal in R emerges after 20 years in similar regions covering a similar fraction of the ocean in a state-of-the-art ecosystem model, 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.

Editor's Summary: An analysis of satellite data from July 2002–June 2022 shows that ocean colour, or remote-sensing reflectance, changed significantly during this period, and that this trend is likely to be driven by climate change.