Revised historical Northern Hemisphere black carbon emissions based on inverse modeling of ice core records

Abstract

Black carbon emitted by incomplete combustion of fossil fuels and biomass has a net warming effect in the atmosphere and reduces the albedo when deposited on ice and snow; accurate knowledge of past emissions is essential to quantify and model associated global climate forcing. Although bottom-up inventories provide historical Black Carbon emission estimates that are widely used in Earth System Models, they are poorly constrained by observations prior to the late 20th century. Here we use an objective inversion technique based on detailed atmospheric transport and deposition modeling to reconstruct 1850 to 2000 emissions from thirteen Northern Hemisphere ice-core records. We find substantial discrepancies between reconstructed Black Carbon emissions and existing bottom-up inventories which do not fully capture the complex spatial-temporal emission patterns. Our findings imply changes to existing historical Black Carbon radiative forcing estimates are necessary, with potential implications for observation-constrained climate sensitivity.

Fig. 1. Sensitivity of black carbon deposition at Northern Hemisphere ice core sites to source region emissions.

The footprint emission sensitivity (FES) in a grid cell is the simulated black carbon deposition (mg BC m⁻² month⁻¹) at the ice core site that a potential emission source of unit strength (1 kg s⁻¹) in that grid cell would produce. It combines wet and dry black carbon deposition, and accounts for black carbon source emissions below 100 m a.g.l., averaged over 100 years. The ice core sites are marked with white dots. For ice core sites close to each other, FES values are averaged. We combine ice cores in the region of northern Greenland (a - Summit, Tunu, NEEM, Humboldt), southern Greenland (b - ACT2, D4), three sites in the Eurasian Arctic (c - Holtedahlfonna, Lomonosovfonna and Akademii Nauk), and show separately a low-altitude Greenland site (d - Flade Isblink), one in the Canadian Arctic (e - Devon), and in the mid-latitudes Colle Gnifetti in the Alps (f) and Mt. Elbrus in the Caucasus (g). Notice that emission sensitivity maps are combined here for display purposes only but are treated separately in the analysis.

Fig. 2. Comparison of measured and modeled black carbon (BC) deposition fluxes for the thirteen ice cores.

Observed (decadal) mean annual black carbon deposition fluxes (gray) at all sites except for Holtedahlfonna, for which elemental carbon (EC) deposition is given with a sampling resolution of ca. 2–5 years. The a priori modeled values were obtained by using the biomass burning emissions and the black carbon inventories for CMIP5 (light blue) and CMIP6 (dark blue), combined with monthly emission sensitivities and scaled to the measured fluxes of the latest available decade. Modeled BC deposition fluxes using a posteriori emissions from the inversion are shown with black lines. Notice that the ordinate scales are different in each panel.

Fig. 3. Black carbon emission history obtained from the inversion.

A priori (thick black line) and a posteriori (shaded areas) emissions north of 30 °N for the different regions (a–d). CMIP5 and CMIP6 emissions (both including the same biomass burning emissions) are shown with dark and light gray lines, respectively. The regions for which the emissions are optimized are shown as an inset in (d) and reflect the colors used in (a–d).