The global ocean chlorophyll archive produced by the CZCS was revised using compatible algorithms with SeaWiFS. Both archives were then blended with in situ data to reduce residual errors. This methodology permitted a quantitative comparison of decadal changes in global ocean chlorophyll from the CZCS (1979–1986) and SeaWiFS (1997–2000) records. Global spatial distributions and seasonal variability of ocean chlorophyll were similar, but global means decreased over the two observational segments. Major changes were observed regionally: chlorophyll concentrations decreased in the northern high latitudes while chlorophyll in the low latitudes increased. Mid‐ocean gyres exhibited limited changes. The overall spatial and seasonal similarity of the two data records suggests that the changes are due to natural variability. These results provide evidence of how the Earth's climate may be changing and how ocean biota respond.