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As a contribution to the Regional Carbon Cycle Assessment and Processes phase 2 (RECCAP2) project, we present synthesized estimates of Arctic Ocean sea‐air CO2 fluxes and their uncertainties from surface ocean pCO2‐observation products, ocean biogeochemical hindcast and data assimilation models, and atmospheric inversions. For the period of 1985–2018, the Arctic Ocean was a net sink of CO2 of 116 ± 4 TgC yr−1 in the pCO2 products, 92 ± 30 TgC yr−1 in the models, and 91 ± 21 TgC yr−1 in the atmospheric inversions. The CO2 uptake peaks in late summer and early autumn, and is low in winter when sea ice inhibits sea‐air fluxes. The long‐term mean CO2 uptake in the Arctic Ocean is primarily caused by steady‐state fluxes of natural carbon (70% ± 15%), and enhanced by the atmospheric CO2 increase (19% ± 5%) and climate change (11% ± 18%). The annual mean CO2 uptake increased from 1985 to 2018 at a rate of 31 ± 13 TgC yr−1 dec−1 in the pCO2 products, 10 ± 4 TgC yr−1 dec−1 in the models, and 32 ± 16 TgC yr−1 dec−1 in the atmospheric inversions. Moreover, 77% ± 38% of the trend in the net CO2 uptake over time is caused by climate change, primarily due to rapid sea ice loss in recent years. Furthermore, true uncertainties may be larger than the given ensemble standard deviations due to common structural biases across all individual estimates. Plain Language Summary: The Arctic Ocean is at present a net sink for atmospheric CO2 mainly due to the intense cooling of the inflowing waters from the Atlantic and the Pacific. Global warming is amplified in the Arctic Ocean and it experiences a rapid retreat of sea ice. Here, we present synthesized estimates of the Arctic Ocean CO2 uptake and their uncertainties from estimates obtained using different methods. Almost all estimates suggest that the Arctic Ocean was a net sink of CO2 from 1985 to 2018. The CO2 uptake is strong in late summer and early autumn and weak in winter, corresponding to the seasonal variation of sea ice. CO2 uptake has increased in recent years, especially in regions that have experienced sea ice loss. Compared to the global ocean, the Arctic Ocean is unique because climate change, in particular the change in sea ice cover, has enhanced the ocean CO2 uptake almost as much as the increase in atmospheric CO2 over the past 34 years. Moreover, this climate effect on the Arctic Ocean CO2 uptake has become more important in recent years and is the current main driver for the trend toward an increasing CO2 uptake in the Arctic Ocean. Key Points: The Arctic Ocean is estimated to be a net sink of CO2 of 116 ± 4 TgC yr−1 in pCO2 products and 92 ± 30 TgC yr−1 in ocean modelsThe Arctic Ocean CO2 uptake is explained by a steady‐state natural flux (70%) with atmospheric CO2 increase (19%) and climate change (11%)The CO2 uptake increased (31 ± 13 TgC yr−1 dec−1 in products and 10 ± 4 TgC yr−1 dec−1 in models) mostly by decreasing sea ice [ABSTRACT FROM AUTHOR]

Extreme events can severely impact marine organisms and ecosystems. Of particular concern are multivariate compound events, namely when conditions are simultaneously extreme for multiple ocean ecosystem stressors. In 2013–2015 for example, an extensive marine heatwave (MHW), known as the Blob, co-occurred locally with extremely low net primary productivity (NPPX) and negatively impacted marine life in the northeast Pacific. Yet, little is known about the characteristics and drivers of such multivariate compound MHW-NPPX events. Using five different satellite-derived NPP estimates and large ensemble simulation output of two widely-used and comprehensive Earth system models, GFDL-ESM2M-LE and CESM2-LE, we assess the present-day distribution of compound MHW-NPPX events and investigate their potential drivers on the global scale. The satellite-based estimates and both models reveal hotspots of frequent compound events in the center of the equatorial Pacific and in the subtropical Indian Ocean, where their occurrence is at least three times higher (more than 10 days per year) than if MHWs (temperature above the 90th percentile threshold) and NPPX events (NPP below the 10th percentile threshold) were to occur independently. However, the models show disparities in the northern high latitudes, where compound events are rare in the satellite-based estimates and GFDL-ESM2M-LE (less than 3 days per year), but relatively frequent in CESM2-LE. In the Southern Ocean south of 60° S, low agreement between the observation-based estimates makes it difficult to determine which of the two models better simulates MHW-NPPX events. The frequency patterns can be explained by the drivers of compound events, which vary among the two models and phytoplankton types. In the low latitudes, MHWs are associated with enhanced nutrient limitation on phytoplankton growth, which results in frequent compound MHW-NPPX events in both models. In the high latitudes, NPPX events in GFDL-ESM2M-LE are driven by enhanced light limitation, which rarely co-occurs with MHWs, resulting in rare compound events. In contrast, in CESM2-LE, NPPX events in the high latitudes are driven by reduced nutrient supply that often co-occurs with MHWs, moderates phytoplankton growth and causes biomass to decrease. Compound MHW-NPPX events are associated with a relative shift towards larger phytoplankton in most regions, except in the eastern equatorial Pacific in both models, as well as in the northern high latitudes and between 35° S and 50° S in CESM2-LE, where the models suggest a shift towards smaller phytoplankton, with potential repercussions on marine ecosystems. Overall, our analysis reveals that the likelihood of compound MHW-NPPX events is contingent on model representation of the factors limiting phytoplankton production. This identifies an important need for improved process understanding in Earth system models used for predicting and projecting compound MHW-NPPX events and their impacts. [ABSTRACT FROM AUTHOR]