Phytoplankton have attracted increasing attention in climate science due to their impacts on climate systems. A new generation of climate models can now provide estimates of future climate change, considering the biological feedbacks through the development of the coupled physical-ecosystem model. Here we present the geophysical impact of phytoplankton, which is often overlooked in future climate projections. A suite of future warming experiments using a fully coupled ocean−atmosphere model that interacts with a marine ecosystem model reveals that the future phytoplankton change influenced by greenhouse warming can amplify Arctic surface warming considerably. The warming-induced sea ice melting and the corresponding increase in shortwave radiation penetrating into the ocean both result in a longer phytoplankton growing season in the Arctic. In turn, the increase in Arctic phytoplankton warms the ocean surface layer through direct biological heating, triggering additional positive feedbacks in the Arctic, and consequently intensifying the Arctic warming further. Our results establish the presence of marine phytoplankton as an important potential driver of the future Arctic climate changes.biogeophysical feedback | phytoplankton−climate interaction | Arctic climate changes P hytoplankton, aquatic photosynthetic microalgae, play a key role in marine ecology, forming the foundation of the marine food chain. Besides this ecological importance, the climatic importance of phytoplankton is also evident, given their role in carbon fixation, which potentially reduces human-induced carbon dioxide (CO 2 ) in the atmosphere (1-3). The great strides made in the field of biogeochemical modeling have improved climate models, enabling the investigation of carbon−climate feedback, such as diagnosing the strength of biogeochemical feedback and quantifying its importance in total carbon cycle responses. In fact, several modeling groups provide future climate projections including the biogeochemical process, as seen in a recent version of the Coupled Model Intercomparison Project (CMIP), i.e., CMIP5.In addition to their biogeochemical feedback, phytoplankton also modify physical properties of the ocean. Chlorophyll and related pigments in phytoplankton affect the radiant heating in the ocean by decreasing both the ocean surface albedo and shortwave penetration (4-6). Thus, higher phytoplankton biomass generally results in warmer ocean surface layer. This biogeophysical feedback is known to significantly impact the global climate (7-10) and large-scale climate variability, such as the El Niño-Southern Oscillation (11-13) and Indian Ocean dipole (14). Unlike biogeochemical feedback, however, biogeophysical feedback has been overlooked in many future climate projections simulated by state-of-the-art climate models, even in projections by so-called Earth System Models that include interactive marine ecosystem components.Greenhouse warming generally involves changes in physical fields that inevitably affect growth factors of phytoplankto...

Abstract. Many studies have shown a decrease in Arctic sea ice extent. It does not logically follow, however, that the extent of the marginal ice zone (MIZ), here defined as the area of the ocean with ice concentrations from 15 % to 80 %, is also changing. Changes in the MIZ extent has implications for the level of atmospheric and ocean heat and gas exchange in the area of partially ice-covered ocean and for the extent of habitat for organisms that rely on the MIZ, from primary producers like sea ice algae to seals and birds. Here, we present, for the first time, an analysis of satellite observations of pan-Arctic averaged MIZ extent. We find no trend in the MIZ extent over the last 40 years from observations. Our results indicate that the constancy of the MIZ extent is the result of an observed increase in width of the MIZ being compensated for by a decrease in the perimeter of the MIZ as it moves further north. We present simulations from a coupled sea ice–ocean mixed layer model using a prognostic floe size distribution, which we find is consistent with, but poorly constrained by, existing satellite observations of pan-Arctic MIZ extent. We provide seasonal upper and lower bounds on MIZ extent based on the four satellite-derived sea ice concentration datasets used. We find a large and significant increase (>50 %) in the August and September MIZ fraction (MIZ extent divided by sea ice extent) for the Bootstrap and OSI-450 observational datasets, which can be attributed to the reduction in total sea ice extent. Given the results of this study, we suggest that references to “rapid changes” in the MIZ should remain cautious and provide a specific and clear definition of both the MIZ itself and also the property of the MIZ that is changing.