Quantifying spatiotemporal CO2 trends in surface fluxes over Iceland using OCO-2 XCO2 retrievals

Iceland represents one of the few emerged portions of the Mid-Atlantic-Ridge, a 16,000 km-long tectonic boundary separating the North American and Eurasian plates. This unique position gives rise to Iceland’s characteristic geothermal manifestations and volcanic complexes (Figure 1). Here, CO₂ from volcanic sources is released alongside other natural and anthropogenic emissions, as part of the global carbon cycle.

Figure 1. Map of Iceland with distribution of averaged NDVI from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) for June 2018. The red triangles show locations of volcanoes and fumaroles and the grey squares represent pixels where OCO-2 XCO₂ retrievals are available.

The quantification of these fluxes in relation to the plate tectonic system provides valuable insights into the nature of long-term CO₂ migration and retention, which, in turn, can help assess the potential for leakage and migration pathways in the context of geological storage of CO₂. However, volcanogenic sources of CO₂ remain poorly quantified, partly because such studies rely on ground-based measurements or airborne remote sensing, which can be challenging and hazardous during periods of volcanic unrest.

NASA’s Orbiting Carbon Observatory-2 (OCO-2), launched in 2014, has been proven to be capable of observing localised point source signals thanks to its unprecedented instrument precision and resolution. In this study, we explore the use of OCO-2’s column-averaged dry-air mole fractions of CO₂ (XCO₂) retrievals to assess the onshore CO₂ fluxes in Iceland.

Because volcanogenic sources of CO₂ typically exhibit small enhancements above background concentrations, a robust quantification of the influence of non-volcanic CO₂ contributors on OCO-2 retrievals is needed. Therefore, we analysed nearly a decade of OCO-2 data to reveal the long-term anthropogenic emissions trends and the seasonal variations due to biogenic fluxes. We observe a steady increase of ~2 ppm per year in average atmospheric CO₂ concentrations (Figure 2a), attributed to anthropogenic emissions, and an inverse trend between monthly XCO₂ averages and the Normalised Difference Vegetation Index (NDVI) (Figure 2b), reflecting seasonal vegetation growth as a carbon sink. However, due to severe weather conditions and prolonged winter darkness at high latitudes, no data was available from October to March, limiting our window of observation.

Figure 2. a) Evolution of OCO-2 XCO₂ observations over Iceland since 2015 with yearly means in red squares.
b) Evolution of mean OCO-2 XCO₂ observations and NDVI values over Iceland in 2018.

The observed trends provide the reference framework required for isolating volcanogenic CO₂ contributions and are informative for understanding the carbon cycle in this region. Despite observational challenges posed by Iceland’s location in the high North, our analysis of these OCO-2 retrievals brings important insights into resolving spatiotemporal CO₂ patterns from space over volcanically active regions. The ensuing step will be to quantify the relative contribution of known volcanogenic CO₂ sources (e.g., volcanos, fumaroles, and diffuse soil degassing, etc.) to OCO-2 retrievals.