The atmospheric CO2 growth rate (CGR) is characterised by large interannual variability, mainly due to variations in the land carbon uptake, the most uncertain component in the global carbon budget. We explore the relationships between CGR and global terrestrial water storage (TWS) from the GRACE satellites. A strong negative correlation (r = -0.68, p < 0.01) between these quantities over 2001-2023 indicates that drier years correspond to a higher CGR, suggesting reduced land uptake. We then show regional TWS-CGR correlations and use a new metric to assess their contributions to the global correlation. The tropics account for the entire global TWS-CGR correlation, with small cancelling contributions from the Northern and Southern Hemisphere extratropics. Tropical America explains the dominant contribution (69%) to the global TWS-CGR correlation, despite occupying < 12% of the land surface. Aggregating TWS by MODIS land cover type, tropical forests exhibit the strongest CGR correlations and contribute most to the global TWS-CGR correlation (39%), despite semi-arid and cropland/grassland regions both having more interannual TWS variability. An ensemble mean of four atmospheric CO2 flux inversion products also indicate a 74% tropical contribution to CGR variability, with tropical America and Africa each contributing 30% and 27%. Regarding land cover type, semi-arid/tropical forests contribute almost equally (37%/35%) to CGR variability, although tropical forests cover a smaller surface area (25%/10%). Timeseries of global and regional TWS and CO2 flux inversions through 2001-2023 also show changing regional contributions between global CGR events, which are discussed in relation to regional drought and ENSO events.

The long-term net sink of carbon (C), nitrogen (N) and greenhouse gases (GHGs) in the northern permafrost region is projected to weaken or shift under climate change. But large uncertainties remain, even on present-day GHG budgets. We compare bottom-up (data-driven upscaling, process-based models) and top-down budgets (atmospheric inversion models) of the main GHGs (CO2, CH4, and N2O) and lateral fluxes of C and N across the region over 2000-2020. Bottom-up approaches estimate higher land to atmosphere fluxes for all GHGs compared to top-down atmospheric inversions. Both bottom-up and top-down approaches respectively show a net sink of CO2 in natural ecosystems (-31 (-667, 559) and -587 (-862, -312), respectively) but sources of CH4 (38 (23, 53) and 15 (11, 18) Tg CH4-C yr-1) and N2O (0.6 (0.03, 1.2) and 0.09 (-0.19, 0.37) Tg N2O-N yr-1) in natural ecosystems. Assuming equal weight to bottom-up and top-down budgets and including anthropogenic emissions, the combined GHG budget is a source of 147 (-492, 759) Tg CO2-Ceq yr-1 (GWP100). A net CO2 sink in boreal forests and wetlands is offset by CO2 emissions from inland waters and CH4 emissions from wetlands and inland waters, with a smaller additional warming from N2O emissions. Priorities for future research include representation of inland waters in process-based models and compilation of process-model ensembles for CH4 and N2O. Discrepancies between bottom-up and top-down methods call for analyses of how prior flux ensembles impact inversion budgets, more in-situ flux observations and improved resolution in upscaling.

A document by Euan G. Nisbet. Click on the document to view its contents.

We develop an existing 1-D photochemistry model to include a comprehensive description of organic chemistry on Mars that includes the oxidation products of methane (CH$_4$) and ethane (C$_2$H$_6$), a longer-chain hydrocarbon that can be used to differentiate between abiotic and biotic surface releases of CH$_4$. We find that CH$_4$ is most volatile between 20–50 km during Mars’ northern summer, where the local atmospheric CH$_4$ lifetime lowers to 25–60 years. We study atmospheric formaldehyde (HCHO) and formic acid (HCOOH), as the two common oxidation products of CH$_4$ and C$_2$H$_6$, and acetaldehyde (CH$_3$CHO) and acetic acid (CH$_3$COOH) as unique products of C$_2$H$_6$. We focus our analysis of these gases at Mars’ aphelion and perihelion at latitudes between -30$^{\circ}$ and 30$^{\circ}$, altitudes from the surface to 70 km, and from a homogeonous initial condition of 50 pptv of CH$_4$ and C$_2$H$_6$. From this initial condition, CH$_4$ produces HCHO in a latitude-independent layered structure centred at 20–30 km at aphelion with column-averaged mixing ratios of 10$^{-4}$ pptv, and oxidation of C$_2$H$_6$ produces HCHO at 10$^{-2}$ pptv. Formic acid has an atmospheric lifetime spanning 1–10 sols below 10 km that shows little temporal or zonal variability, and is produced in comparable abundances (10$^{-5}$ pptv) by the oxidation of C$_2$H$_6$ and CH$_4$. We also find that oxidation of 50 pptv of C$_2$H$_6$ results in 10$^{-3}$ pptv of CH$_3$CHO and 10$^{-4}$ pptv of CH$_3$COOH. Subsequent UV photolysis of this CH$_3$CHO results in 10$^{-4}$ pptv of atmospheric CH$_4$, potentially representing a new atmospheric source of Martian CH$_4$.

Recent measurements collected by the Mars Curiosity Rover at the Gale Crater revealed an unexpectedly large seasonal cycle of molecular oxygen (O2). We use a 1-D photochemical model, including inorganic and organic chemistry, and its adjoint model to quantify the sensitivity of changes in O2 to changes in inorganic and organic compounds. We show that O2 changes are most sensitive to changes in organic compounds from the oxidation of methane. We find that an accelerated loss of atmospheric methane, achieved either by increasing the atmospheric loss or by imposing an additional surface loss, does not reconcile model and observed values of O2 but it helps to explain the O2 seasonal variation. The resulting changes in atmospheric composition are below the detection limits of orbiting instruments.