Wetlands are the dominant natural source of atmospheric methane, a powerful greenhouse gas that is second only to carbon dioxide in terms of its importance for climate change. It is expected that anthropogenic climate change will increase methane emissions from wetlands and lead to further warming. However, feedback on methane in wetlands has not been fully assessed in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report, challenging the achievement of the global greenhouse gas reduction targets set in the Paris Agreement.
In order to understand how the methane cycle in wetlands can develop in the future and promote climate feedback, scientists are increasingly looking at the earth's past.
"Records of ice cores show that atmospheric methane is very sensitive to climate, but we cannot measure atmospheric methane concentrations beyond that about 1 million years ago," said Dr. Gordon Inglis, lead author and Royal Society Dorothy Hodgkin Fellow at the University of Southampton.
“Instead, we need to rely on indirect 'proxies' that are preserved in the sediment record. Proxies are replacements for climate variables that cannot be measured directly, including geochemical data stored in fossils, minerals or organic compounds are."
The study that was published in geologyis the first to directly resolve the relationship between temperature and methane cycle in wetlands during the Paleocene-Eocene Thermal Maximum (PETM), an ancient warming event that could provide a glimpse into the future.
The authors used a geochemical tool developed at the University of Bristol to analyze organic compounds made by microbes living in ancient soils and types of peat. During PETM, they found that the ratio of two carbon isotopes in these compounds had changed – a change likely due to an increased amount of methane in the microbes' diet.
"We show that the PETM was linked to an increase in the methane cycle in wetlands. If some of this methane had been released into the atmosphere, this would have led to additional warming of the planet. Crucially, this could indicate changes that the methane cycle will have in the future is experienced due to anthropogenic emissions, "said Dr. Gordon Inglis.
"Our colleagues have previously shown that including methane emissions in climate model simulations is critical to interpreting past heat. However, until recently there were no tools to test these predictions. This study confirms that the methane cycle is during of PETM and possibly has increased during other warming events in the Earth's history, "said Professor Rich Pancost, director of the School of Earth Sciences at the University of Bristol.
Interestingly, temperature and methane cycle proxies are only paired at the start of this ancient warming event, with methane proxies quickly reverting to pre-event values, even though temperatures remain high for the duration of the PETM. This suggests that the onset of rapid global warming is particularly disruptive of the methane cycle in wetlands, which is particularly worrying given the rapid global warming we are currently witnessing.