Unraveling a Two-Decade Enigma: Sahara Dust and Its Role in Climate Change

Dust observable during sunset, off Peru’s shoreline. Image Courtesy: Jan-Berend Stuut (NIOZ)

Recent research contributes significantly to our comprehension of the global methane cycle.

The study introduces a new concept in which windblown mineral dust combines with sea spray, forming a Mineral Dust-Sea Spray Aerosol (MDSA).

The findings suggest that sunlight activates the MDSA, which then produces a considerable amount of chlorine atoms. These atoms oxidize atmospheric methane and tropospheric ozone via a mechanism called photocatalysis. With the majority of it comprising airborne dust from the Sahara Desert and sea-salt aerosol from the ocean, the research singles out MDSA as the main provider of atmospheric chlorine over the North Atlantic.

This study employs a blend of global modeling, lab testing, and field observations, which includes air samples from Barbados that show seasonal reduction of the stable isotope 13CO. This anomaly has baffled scientists for 20 years. They knew that the observed fluctuations in 13CO and C18O signaled the reaction of chlorine atoms with methane and that carbon monoxide is the initial stable product in atmospheric methane oxidation. However, the identified sources of atmospheric chlorine could not explain the level of 13CO reduction until now.

Utilizing a global 3-D chemistry-climate model (CAM-Chem), van Herpen and his team discovered that integrating the elevated chlorine from the MDSA process into the model aligned well with the data from Barbados and explained the 13CO reduction.

The research suggests that if the MDSA impact observed in the North Atlantic applies globally, and if it has comparable efficiency worldwide — both of which are not yet fully understood and need more investigation — global atmospheric chlorine concentrations could be about 40% higher than previously believed. Incorporating this into global methane modeling could potentially change our understanding of the relative proportions of methane emission sources.

Satellite image showing a Sahara dust storm moving over the Atlantic Ocean. Image Courtesy: NASA

Methane is a powerful greenhouse gas, with a Global Warming Potential (GWP) 83 times higher than carbon dioxide over 20 years and 30 times higher over 100 years, contributing to about one-third of modern warming. Atmospheric methane levels, now approximately 2.6 times higher than in pre-industrial times, are surging at an escalating pace, with the highest annual increases ever recorded in 2020 and 2021. Anthropogenic methane emissions are known to be the principal cause of the overall increase, with amplified natural emissions and atmospheric chemistry alterations due to anthropogenic emissions of various gases also playing a role.

While the cause for the recent acceleration remains unclear, the study by van Herpen et al. may have uncovered a crucial hint. Their conclusion that there’s more active chlorine impacting 13C than previously understood suggests a possible surge in methane from biological sources like agriculture and wetlands. This indicates that biological methane might have a slightly larger impact than previously estimated.

“Methane emissions from biological sources like wetlands and agriculture may be increasing as global temperatures rise,” said Maarten van Herpen, the lead author of the PNAS study. “However, recent surges in dust from North Africa might have boosted methane oxidation in the atmosphere, somewhat concealing the rise in biological methane emissions. Adjusting atmospheric modeling to consider this could reveal that methane emissions from biological sources are growing even faster than we thought.”

“The inclusion of these findings into methane budgets is likely to raise our assessment of how much methane originates from biological sources,” said Professor Matthew Johnson of the University of Copenhagen, a co-author of the study. “While methane oxidation from MDSA makes up a relatively small proportion of global methane, our data indicates that it causes significant changes in the 13C abundance in methane, which helps determine source contributions. The recent years have seen an unprecedented increase in atmospheric methane, and understanding the cause is essential. Models need to account for the revised chlorine isotope shift to get a clear picture of the increase in biological methane, identified as a critical tipping point.”

The way the MDSA process might operate in other parts of the world is not fully understood and needs more investigation, the study contends. Further research is currently in progress.

“Our ongoing research is centered on better understanding what specifically influences how much methane the MDSA particles remove from the atmosphere,” van Herpen explained, “To accomplish this, we are examining air samples from across the North Atlantic, provided by atmospheric observatories and commercial ships. Sailors are contributing to our research by collecting air samples as they navigate through the African dust cloud. We have gathered 500 flasks so far. Preliminary results are promising, but we need a full year of data before we can make any conclusions.”

Citation: “Photocatalytic chlorine atom production on mineral dust–sea spray aerosols over the North Atlantic” by Maarten M. J. W. van Herpen, Qinyi Li, Alfonso Saiz-Lopez, Jesper B. Liisberg, Thomas Röckmann, Carlos A. Cuevas, Rafael P. Fernandez, John E. Mak, Natalie M. Mahowald, Peter Hess, Daphne Meidan, Jan-Berend W. Stuut and Matthew S. Johnson, 24 July 2023, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2303974120

The NGO Spark Climate Solutions partially funded the study.

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