Anomalous trends in global ocean carbon concentrations following the 2022 eruptions of Hunga Tonga-Hunga Ha'apai

The research addresses the unusual patterns observed in global satellite ocean color (OC) data following the January 2022 eruptions of the Hunga Tonga-Hunga Ha'apai submarine volcano. A previous analysis of NASA's MODIS data revealed significant anomalies in chlorophyll-a and phytoplankton carbon concentrations, particularly in the equatorial and southern hemisphere regions. These anomalies deviated substantially from the 25-year climatological record. The study's purpose is to investigate the cause of these anomalies, considering the unprecedented amount of water vapor and sulfate aerosols injected into the stratosphere by the eruption. The importance lies in understanding the impact of such events on satellite-based ocean monitoring and the potential for misinterpreting these artifacts as indicators of significant ecological shifts. Accurate monitoring of ocean biogeochemical properties is crucial for understanding climate change impacts on marine ecosystems, including phytoplankton populations, which are responsible for approximately half of global net primary production. Changes in phytoplankton populations can significantly impact ocean ecosystems and the services they provide, including food security and global biogeochemical cycles. Therefore, distinguishing between true biological responses and artifacts in satellite data is vital for accurate climate change impact assessments.

The study references previous research on the Hunga Tonga-Hunga Ha'apai eruption's atmospheric and radiative impacts. It builds upon prior work analyzing global satellite ocean color trends, which initially suggested a possible link between the observed phytoplankton anomalies and the volcanic eruption. The authors cite research detailing the eruption's unprecedented injection of water vapor and sulfate aerosols into the stratosphere, and the subsequent formation of aerosol layers. Existing literature on atmospheric correction (AC) processes in satellite ocean color retrieval is also reviewed, highlighting the assumptions made in these algorithms regarding aerosol distribution and optical properties. Finally, they consider previous studies documenting the impact of volcanic eruptions on ocean ecosystems, specifically regarding the transport of volcanic ash and nutrients to nutrient-depleted regions, and their effect on phytoplankton growth. This review provides a foundation for examining whether the observed ocean color anomalies are a result of a biological response or retrieval errors.

The study employed a multi-faceted approach. First, it analyzed ocean color data from six satellite missions (MODIS-Aqua, MODIS-Terra, VIIRS-SNPP, VIIRS-NOAA-20, OLCI-Sentinel-3A, OLCI-Sentinel-3B) processed by both NASA and ESA/EUMETSAT, using data products including remote sensing reflectance (Rrs), chlorophyll-a (Chla), and particulate backscattering coefficient (bbp). The data were analyzed for the Northern Hemisphere (NH), Southern Hemisphere (SH), and the South Pacific Subtropical Gyre (SPSG). These analyses involved comparing 2022 data to the 2002-2021 climatological record, utilizing standardized anomalies to account for inter-sensor variations and highlight deviations from the norm. Second, the study leveraged independent in situ measurements of bbp from BGC-Argo floats in the SPSG to assess the reality of the observed anomalies. Third, the authors utilized a vector radiative transfer model to simulate satellite observations under various atmospheric conditions—one with a typical tropospheric aerosol layer and another that included a stratospheric aerosol layer mimicking the post-eruption conditions. This involved incorporating stratospheric aerosol distribution data from NASA's Ozone Mapping and Profiler Suite (OMPS) and simulating the total reflectance reaching the satellite sensor. The simulated data were then processed using the standard NASA AC algorithm, enabling a comparison between simulated and observed anomalies in Rrs and derived biogeochemical products. The methodology incorporates multiple data sources and simulation techniques to establish a robust evaluation of the observed anomalies in satellite-derived ocean color data.

The analysis revealed consistently anomalous behavior in satellite-derived ocean color parameters across multiple sensors and processing algorithms during 2022, particularly in the Southern Hemisphere. The anomalies were most pronounced in the particulate backscattering coefficient (bbp), indicating a seemingly drastic reduction in phytoplankton biomass. Crucially, these anomalies were not observed in in situ measurements from BGC-Argo floats, strongly suggesting the anomalies were not due to actual changes in phytoplankton biomass. The temporal progression of these negative anomalies in ocean color strongly correlated with the increase in stratospheric aerosol optical thickness (AOT) observed following the Hunga Tonga-Hunga Ha'apai eruption, reaching levels 6-7 times the historical average in June-July 2022. Radiative transfer simulations demonstrated that the presence of stratospheric aerosols, particularly their interaction with ozone, introduced significant biases in the atmospheric correction process used to retrieve ocean color parameters from satellite measurements. The simulations effectively replicated the observed negative anomalies, demonstrating that the underestimation of ozone absorption caused by aerosols within the ozone layer leads to spectral biases and underestimation of water-leaving radiance, subsequently impacting the derived bbp values. Furthermore, the study found that while bbp was significantly affected, chlorophyll-a retrievals, using spectral band differences, showed less sensitivity. This highlights the wavelength-dependent impact of stratospheric aerosols on satellite ocean color data.

The findings directly address the initial research question by demonstrating that the observed anomalies in satellite ocean color data were primarily due to errors in the atmospheric correction (AC) process rather than actual changes in phytoplankton biomass. The strong correlation between the temporal evolution of stratospheric aerosol loading and the negative anomalies in ocean color, supported by the radiative transfer simulations, decisively points to retrieval errors. The discrepancy between satellite and in situ measurements provides compelling evidence against a significant biological response. The study's significance lies in its implications for interpreting satellite-based ocean color data. The results highlight the need for improved AC algorithms that can account for the presence of stratospheric aerosols. It emphasizes the potential for misinterpreting artifacts in satellite data as indicators of ecological change, especially in the context of future volcanic eruptions or potential geoengineering interventions.

This study conclusively demonstrates that the anomalous trends observed in global ocean carbon concentrations following the Hunga Tonga-Hunga Ha'apai eruption were artifacts resulting from biases in satellite ocean color retrieval algorithms. These biases were induced by the unusual stratospheric aerosol loading, invalidating key assumptions within the atmospheric correction process. The findings underscore the need for improved algorithms capable of handling stratospheric aerosol layers. Future research should focus on developing and validating such algorithms, requiring detailed knowledge of aerosol properties. Furthermore, the study warns of potential biases in satellite data from future volcanic eruptions or geoengineering projects involving stratospheric aerosol injection, emphasizing the need for careful interpretation of remotely sensed data in these contexts.

The study primarily focused on the South Pacific Subtropical Gyre (SPSG) region for detailed analysis, limiting the generalizability of some findings to other ocean regions. While the radiative transfer simulations effectively replicated the observed anomalies, the model's accuracy is dependent on the accuracy of input parameters, such as aerosol distribution and optical properties. Further, it is assumed that the aerosols are primarily scattering, and the effect of aerosol absorption was implicitly addressed in the modeling analysis. The study did not explore all possible aspects of the ocean's response to volcanic ash and aerosols beyond phytoplankton carbon, limiting a more extensive understanding of the complete ecosystem impact.