Anthropogenic Decline of African Dust: Insights From the Holocene Records and Beyond

Abstract

African dust exhibits strong variability on a range of time scales. Here we show that the interhemispheric contrast in Atlantic SST (ICAS) drives African dust variability at decadal to millennial timescales, and the strong anthropogenic increase of the ICAS in the future will decrease African dust loading to a level never seen during the Holocene. We provide a physical framework to understand the relationship between the ICAS and African dust activity: positive ICAS anomalies push the Intertropical Convergence Zone (ITCZ) northward and decrease surface wind speed over African dust source regions, which reduces dust emission and transport. It provides a unified framework for and is consistent with relationships in the literature. We find strong observational and proxy-record support for the ICAS-ITCZ-dust relationship during the past 160 and 17,000 years. Model-projected anthropogenic increase of the ICAS will reduce African dust by as much as 60%, which has broad consequences.

Keywords: AMO; African dust; Atlantic SST; CMIP; Holocene; ITCZ.

Figure 1

(a) A schematic of the theoretical framework proposed to explain the relationships among the ICAS, the ITCZ position, and African dust. In current climate, the Atlantic ITCZ sits around 5°N in terms of annual mean with the main dust transport route to its north (about 12°N, see Figure S1). With increasing ICAS, the Northern Hemisphere (NH) Hadley cell weakens, and associated NH trade wind speed decreases, reducing dust emission and transport from Africa. Meanwhile, the ITCZ moves northward (light blue arrow) together with the dust transport route (light yellow arrow). (b) Regression of ICAS against precipitation and surface wind during JJA between 1979 and 2015. Color shaded areas and wind vectors depict, respectively, precipitation and surface wind anomalies associated with one standard deviation of ICAS. It indicates a northward displacement of the ITCZ and strengthening and weakening of the trade winds to the south and north of the ITCZ, respectively, because prevailing trade winds are northeasterly and southeasterly in the northern and southern hemispheres, respectively.

Figure 2

Time series of various dust activity proxies in the recent past, with vertical lines marking AMO phase changes. Both the original (gray lines) and low‐pass (0.1 year⁻¹ is the cutoff) filtered (bold solid) dust records are plotted. Each time series is detrended and then normalized by its standard deviation. Red (blue) shading indicates positive (negative) values. (a) Dust proxy based ERA‐interim wind; (b) dust time series from Barbados record; (c) dust proxy based on Cape Verde record; (d) Sahel precipitation index; (e) dust proxy based on CIRES‐20CR wind; (f) ICAS calculated based on the Hadley Centre Sea Ice and Sea Surface Temperature (HadISST) (Kennedy et al., 2011) data since 1850; and (g) AMO index since 1856. The correlation coefficients and associated p values are shown for each variable when regressed against the ICAS data using raw annual data.

Figure 3

(a) Titanium concentration from the Cariaco Basin, a proxy for ITCZ meridional position, for the last 14,000 years. Higher values indicate more northward position of the ITCZ. (b) Terrigenous flux at the Ocean Drilling Program Site 658C (20.75°N, 18.58°W), a proxy for dust deposition, for the last 17,000 years. (c) Dust fluxes at the two sites near the Bahamas, OCE205‐2 100GGC (26.0612°N, 78.0277°W) and 103GGC (26.0703°N, 78.05617°W), for the last 22,000 years. (d) Dust flux measured at the VM20 site (5.33°N, 33.03°W) for the last 20,000 years. (e) The Northern‐Southern Hemisphere temperature difference for the last 17,000 years. The M13²² uses temperature difference between the extratropics (90 and 30) of two hemispheres, and the S12²³ uses full hemisphere averages.

Figure 4

Scatter plot between ICAS proxy and (a) terrigenous flux at ODP658C and (b) the ITCZ position proxy for the last 11,300 years. Both correlations are statistically significant and suggest robust relationships for the last 11,300 years. Each point represents a terrigenous flux measurement and corresponding ICAS and Titanium records averaged over the same period that the dust record covers. Similar evidence is found for the period between 6,000 and 17,000 BP (SOM) (see Figure S3 for more discussions). The HadISST based ICAS splined with model simulated ICAS. The horizontal solid lines mark the mean ICAS during the last 167 years. The dashed lines are two standard deviations from the mean. Multimodel means for RCP4.5 (c) and RCP8.5 (d) are plotted using the solid blue lines. In c and d, only “good” models are used. The criteria for “good” models are given in Data and Method. The green shading marks the 25% to 75% range of the distribution among models and the pink the 10% to 90% range. Individual model results are plotted using solid gray lines.