Methane is a highly potent greenhouse gas – with a Global Warming Potential (GWP) 83 times higher than carbon dioxide over two decades and 30 times higher over a century – accounting for nearly a third of modern warming. Atmospheric methane concentrations are increasing at an alarming rate, currently being almost 2.6 times higher than in pre-industrial times.
Anthropogenic methane emissions are the leading cause of this overall rise, while natural sources make a substantial contribution as well.
According to a recent study published in the journal Proceedings of the National Academy of Sciences (PNAS), changes in atmospheric chemistry resulting from emissions of different gases also play a significant part in the global methane budget.
The experts investigated a mechanism whereby blowing mineral dust combines with sea-spray to form Mineral Dust-Sea Spray Aerosol (MDSA), which is activated by sunlight to produce an abundance of chlorine atoms that oxidize atmospheric methane and tropospheric ozone through photocatalysis.
MDSA appears to be largely composed of blowing dust from the Sahara Desert mixed with sea salt aerosol from the ocean, and is the leading source of atmospheric chlorine over the North Atlantic.
The researchers combined global modeling with laboratory and field observations, including air samples from Barbados highlighting seasonal depletion of the stable isotope 13CO – an anomaly that puzzled scientists worldwide for over two decades.
Although it was known that changes in 13CO and C18O were caused by chlorine atoms reacting with methane, and that carbon monoxide is the first stable product in atmospheric methane oxidation, the known sources of atmospheric chlorine could not account for the observed degree of depletion in 13CO.
In the current study, by using a global 3-D chemistry-climate model (CAM-Chem), the experts discovered that, when increased chlorine from the MDSA mechanism was incorporated, it could explain the Barbados data and the 13CO depletion.
According to the scientists, if the MDSA from the North Atlantic is extrapolated globally, and its efficiency is similar in other regions – two aspects which are not yet well understood and require further research – global atmospheric chlorine concentrations are likely to be 40 percent higher than previously thought.
Factoring this into global methane estimations could change our understanding of the relative proportions of methane sources, suggesting possible increases in emissions from biological sources such as agriculture and wetlands.
“Methane emissions from biological sources such as wetlands and agriculture may be growing as global temperatures rise,” said lead author Maarten van Herpen, a managing director at the Acacia Impact Innovation BV.
“But recent increases in dust from North Africa have probably increased methane oxidation in the atmosphere, partly masking the growth in biological methane emissions. Adjusting atmospheric modeling to take this into account may show that methane emissions from biological sources are rising even faster than we thought.”
“When these findings are incorporated into methane budgets it is likely to increase our assessment of how much methane comes from biological sources,” added co-author Matthew Johnson, an atmospheric chemistry scientist at the University of Copenhagen.
“While methane oxidation from MDSA is relatively small in terms of global methane, our data shows it is causing large changes in the abundance of 13C in methane, which is used to determine source contributions. The past few years have seen atmospheric methane increase at an alarming rate, more than ever before, and it is important to understand the cause. Models need to take the revised chlorine isotope shift into account to get a clear picture of the increase in biological methane, which has been identified as a critical tipping point.”
Further research is needed to clarify how the MDSA mechanism operates in the North Atlantic as well as in other parts of the world. “Our current research is focused on getting a better understanding of what exactly influences how much methane MDSA particles are removing from the atmosphere.”
“To do that, we are analyzing air samples from across the North Atlantic, provided by atmospheric observatories and commercial ships. Seafarers are helping advance our research by filling flasks with air as they cross through the African dust cloud. We have collected 500 flasks so far. Early results are looking very encouraging, but we need a full year of data before we can draw conclusions,” van Herpen explained.