Reduction in Earth's carbon budget imbalance

Abstract

The Global Carbon Project (GCP) compiles an updated global carbon budget each year, synthesizing state‑of‑the‑art estimates of anthropogenic CO₂ emissions, land and ocean sinks, and the atmospheric CO₂ growth rate. The residual between these terms, referred to as the global carbon budget imbalance, reflects the aggregate inaccuracies of the individual component estimates. Growth rates derived from marine boundary layer (MBL) surface flask mixing ratio observations are assumed to be highly accurate. Hence, land and ocean sink estimates from process models are viewed as the primary source of the imbalance. Here we show that substantial discrepancies arise when marine boundary layer growth rate estimates are used to represent the whole atmosphere. Correcting for this discrepancy using atmospheric flux inversion estimates reduces the 0.76 petagrams of carbon per year (PgC yr^-1) root-mean-square (RMS) imbalance (from the 2023 GCP report) by up to 25%. Further investigation into the imbalance metric between the 2017 and 2023 GCP reports shows a reduction in imbalance resulting from updates to each carbon budget component, leading to a 16% overall reduction. These reductions provide quantitative evidence of improvements in process models and inventory emission estimates, driven by enhanced forcing data and the inclusion of new carbon cycle processes. Overall, we report a 37% reduction in the root-mean-square imbalance, from 0.91 to 0.57 PgC yr^-1, between the 2017 and 2023 GCP reports by combining process model and inventory improvements with atmospheric growth rate corrections. Our findings indicate that land and ocean process models are more accurate than previously believed and that the scientific understanding of Earth's carbon cycle is improving.

Fig. 1. Whole-atmosphere CO₂ growth rate estimates and their corresponding carbon budget imbalance.

Panel (A) displays the CO₂ growth rate estimates based on MBL observations from the GCP 2023 report (black bars) alongside the corrected values accounting for STE using the box model (red bars) and atmospheric flux inversions from CAMS (orange bars) and Jena CS (green bars). Panel (B) shows the corresponding carbon budget imbalance calculated using each CO₂ growth rate series from Panel (A). The RMS of imbalance is given in parentheses in the legends.

Fig. 2. Carbon budget component estimates from the 2017 (dashed line) and 2023 (solid line) GCP reports.

Negative values for ocean sink, land sink, cement carbonation sink, and MBL annual growth rate are plotted to reflect their role in the imbalance equation (Eq. 1). The dotted line in the fifth panel shows the whole-atmosphere growth rate from the CAMS inversion, and the dotted line in the bottom panel shows the resulting budget imbalance. The 2017 cement carbonation value is set to zero because cement carbonation sink was first included in the GCP 2020 report. Note that the y-axis scales of the panels differ. The values in parentheses for each panel label are the respective RMS of the difference between the GCP 2017 and 2023 time series, which gives an estimate of the magnitude of the changes.

Fig. 3. Improvements in the global carbon budget imbalance for the 1985–2016 period.

The y-axis represents the root mean square (RMS) of the annual imbalance time series. The black crosses represent the imbalance RMS from different GCP reports (x-axis). The STE box model (red triangle) annual growth rate (AGR) estimates accounts for an STE time of 2 years. The growth rate estimates from the CAMS (orange triangle) and Jena CS (green triangle) atmospheric flux inversions account for troposphere-sampling errors of the MBL sites in addition to the STE. The colored symbols (“+” and “x”) mark the imbalance when considering each component update between the 2017 and 2023 GCP reports, one by one, while keeping all other components unchanged from the 2017 report.