Integrating global chlorophyll data from 1890 to 2010

Understanding large-scale phytoplankton dynamics requires accurate, multi-decadal measurements of abundance and distribution. Since 1890, marine phytoplankton abundance has been assessed using a diverse range of sensors and observational platforms, and inter-calibrating these data have been challenging. Consequently, syntheses of historical phytoplankton data have been rarely attempted, and the need for accurate, long-term assessments of phytoplankton abundance and distribution is commonly acknowledged. Here, we derive quantitative indices of phytoplankton abundance from measurements of upper ocean transparency and color-calibrated with direct measurements of surface chlorophyll. The strong correlation and linear scaling of the predicted data enabled the construction of a comprehensive, globally intercalibrated chlorophyll time series from 1890 to 2010. The calibrated chlorophyll data reproduced the well-established spatial features of phytoplankton surface biomass and were strongly correlated with chlorophyll concentration derived from two independent remote sensing platforms discontinuously available since 1978. These results suggest that with careful statistical treatment it is possible to generate a globally integrated chlorophyll time series extending 120 years into the past. This database, which is available in the web appendices of this paper, may enable new insights in the areas of climate science, biogeochemical cycling, and marine ecosystem structure and functioning over the past century.