Climate Models Underestimate Pre-Industrial Fire Emissions
Abstract. An improved understanding of how fire regimes changed from the pre-industrial to the present-day is required to reduce uncertainty in anthropogenic aerosol radiative forcing. Direct observations of pre-industrial fires are scarce so here we evaluate assumptions about historical fire regime change using black carbon data from polar and alpine ice core records and global climate model simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) and the Large Ensemble Community Project (LENS). Black carbon is a tracer of both natural and anthropogenic combustion, enabling comparison of fire and industrial combustion emission changes from the pre-industrial (1850 CE) to the present day (1980 CE). Polar ice core records indicate relatively modest increases in mean and median 1980 CE/1850 CE black carbon ratios of 1.27 and 1.15, compared with 2.89 and 2.64 in the CMIP6 ensemble, respectively. Alpine ice cores record larger increases (mean 2.31; median 1.93), compared with even larger increases simulated in CMIP6 in alpine regions (mean 3.59; median 2.50). We tested three possibilities to explain the difference between black carbon simulated by models and recorded in observations: (1) pre-industrial fire emissions are too low in models; (2) changes in aerosol transport over the Industrial Era using data from LENS; and (3) differences in modelled aerosol properties and deposition parameterizations using CESM2 data. Analysis of these different drivers revealed that the most effective method to reconcile the model-observation disparity is to increase pre-industrial fire emissions in the models and that transport or aerosol properties are of secondary consideration.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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- RC1: 'Comment on egusphere-2026-2728', Anonymous Referee #1, 24 Aug 2026 reply
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RC2: 'Comment on egusphere-2026-2728', Anonymous Referee #4, 24 Aug 2026
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Summary
Previous work has shown a mismatch between pre-industrial black carbon (BC) in climate model simulations and BC inferred from ice core records, suggesting that pre-industrial fire emissions in commonly used inventories may be underestimated. Here, the authors test that idea using 37 ice core datasets and three Earth system model ensembles, and conclude that underestimated pre-industrial fire emissions are indeed the main cause of the disagreement, with changes in transport and deposition playing secondary roles.
Overall, the manuscript fits well within ACP’s scope and addresses a clearly defined scientific question. The analyses appear rigorous, the presentation is clear, and the study is well grounded in the existing literature. I do have a few logistical questions below, but I am in general supportive of publication once these points are addressed.
Major comments:
Firstly, if I understand it correctly, the authors argue "underestimated PI fire emissions are the main explanation for the model–ice core mismatch" through the following chain of reasoning (lines 400 to 410): Inventories show a large PI to PD increase in anthropogenic BC, but little change in fire BC. Since ice core BC reflects the sum of sources, the ice core PD/PI ratio should increase strongly, yet it instead resembles the weak change implied by fire emissions, so the authors infer that PI fire emissions must be higher than inventories indicate.
However, the potential weakness is the implicit assumption that anthropogenic and biomass burning BC are transported and deposited to ice core sites with similar efficiency. Anthropogenic BC may be removed near sources or remain within the boundary layer, while biomass burning plumes can be lofted and delivered more efficiently, and may also align seasonally with snowfall and preservation. If models at ~100 km resolution do not capture these source dependent “accessibility” differences, then the ice core ratio resembling the fire ratio does not uniquely imply underestimated PI fire emissions.
Another concern is that the manuscript does not directly quantify the relative roles of emissions, transport, and deposition in driving the model-ice core mismatch but instead infers their importance indirectly.
In particular, I do not feel fully convinced by the transport discussion in Section 3.4. First, the LENS ensemble mainly samples internal meteorological variability within a single model framework, so the resulting spread is not a strong test of whether forced PI-to-PD changes in circulation and removal, or structural model biases in these processes, contribute to the mismatch. Second, even if “PI versus PD transport differences” are found to be small in LENS, this does not rule out other transport-related explanations, particularly source-dependent delivery. Anthropogenic and biomass-burning BC can differ in injection height, seasonality, and removal efficiency, and models may represent these differently, which could affect how strongly each source contributes to deposition at alpine and polar ice-core sites.