Phytoplankton Chlorophyll Trends in the Arctic at the Local, Regional, and Pan-Arctic Scales (1998-2022)

We analyzed the temporal trends (1998-2022) of surface phytoplankton Chlorophyll (Chl) concentration in the Arctic at the local, regional, and pan-Arctic scales. We used four empirically derived Chl satellite ocean color products: two global merged products and two MODIS products, one calibrated to the Arctic. At the local level, between 10% and 40% of the area with valid pixels showed statistically significant Chl trends, with similar to 2/3 ${\sim} 2/3$ of those pixels showing increases, and the other third indicating a decrease. At the regional level, only the Barents and Chukchi Seas had consistent Chl increases across products. At the pan-Arctic level, most products showed Chl increases in the months of July and September (0.3%-0.9% Chl year-1 ${\text{year}}<^>{-1}$), even after removing the effect of new open water pixels. Overall, Chl is changing in the Arctic, although trends vary threefold depending on the product and spatial-averaging assumptions used.