Is Tropical Deforestation Contributing To The Rise In Atmospheric Methane? (DefMet)

Methane (CH4) is the most important anthropogenically enhanced greenhouse gas in the atmosphere after CO2. Growth in CH4 is increasing, and is currently tracking too high to meet Paris Agreement targets on climate change by 2100. As a result, in the 2021 Glasgow Climate Pact agreed at COP26, 100 countries pledged 30% cuts to their CH4 emissions by 2030, one of the key outcomes of the talks. Given this greater interest in understanding the global methane budget, it essential that all sources and sinks of methane are fully quantied to include the role of Earth's vast terrestrial ecosystems in mediating atmospheric exchange. This is because ecosystems are vulnerable to various agents of change, many of which can alter any methane- relevant ecosystem function with potential knock on eects for the global CH4 budget and Earth's climate. Globally, the atmospheric CH4 sinks dominate CH4 losses, with the far
smaller soil sink where CH4-consuming methanotrophs are considered the only terrestrial sink. However, data-driven global modelling eorts tend to over-estimate emission sources by ~151 Tg (million tonnes) each year when compared to smaller atmospheric 'top-down' derived estimates, possibly suggesting that a substantial terrestrial CH4 sink term is either poorly quantied or missing from the global CH4 budget.

This year, we have discovered only the second terrestrial CH4 sink in the Earth system (Gauci et al Nature under review) and the rst new major component of the methane budget to be identied for over 40 years: The woody surfaces of upland trees (we consider all trees on free draining, low water-table soils as 'upland' trees in contrast to wetland or oodplain trees which are far smaller in area and emit methane). Our conservative global estimate places this 'upland tree sink' (~50Tg each year) as potentially larger than the size of the global soil sink so there is a clear need to rene the high degree of uncertainty in our estimate with new data gathered from within the crowns and canopies of tropical trees. This will help us to see if trees are an important 'missing sink' in the Earth system. The crowns and canopies of trees are challenging to sample from and so are not well represented in our current estimates, however, our data
suggest that CH4 uptake should be larger on the woody surfaces of branches and twigs of tree crowns and canopies, than our current estimates assume. Further, this new discovery may be important from a global change perspective since, unlike the soil sink, which is not changing in global area, tropical upland forests (where the majority of this woody surface CH4 sink resides) are declining due to the eects of deforestation and land use change. Where natural forest is replaced with crops or pasture, the CH4 sink will be reduced. Deforestation is therefore a potential additional mechanism responsible for observed ongoing growth in atmospheric CH4 concentration. We propose to examine the full role of natural tropical upland forest in the methane cycle through a combination of eld measurements of uxes in West African trees and modelling. The result will be better understanding of the contribution of trees and
deforestation, the "new methane sink", to changing atmospheric methane concentrations over time so that we may address growth in CH4 concentration in accordance with the Glasgow Climate Pact.