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Air in karst underground as a sink of greenhouse gases

Principal investigator:

Asst. Prof. Dr. Matija Perne

Duration:

April 1, 2023 – March 31, 2026

Funding:

ARRS - Slovenian Research Agency, N2-0299






Abstract:

While methane accounts for around 15% of global greenhouse gas emissions, it is a primary contributor to climate change - over the first two decades after its release, it is more than 80 times more potent than carbon dioxide in terms of warming the climate system. Its atmospheric lifetime is 9.2 years. Its individual sinks are quantified less accurately.

A significant natural sink of methane is karst systems, with recent research showing that methane rapidly gets depleted in cave air; however, the scale and importance of this underground sink of greenhouse gases is only now beginning to be studied. Methane concentration in cave air is typically lower than in the outside atmosphere and methane has been shown to get depleted in a matter of hours. The proposed depletion mechanisms are microbial oxidation and oxidation by hydroxyl radicals produced by radon radiation, the latter strongly disputed.

The methane sink is constrained by the flow of air entering the underground, which is poorly known. According to current estimates, enough air visits underground karst systems that it is likely to be a major part of the total global methane sink, up to around 7% of the global total.

The importance of ground air for greenhouse gases is not limited to methane. Oxidation by hydroxyl radicals would deplete halocarbons too. Ground air has an elevated CO2 concentration and is the second largest CO2 reservoir on Earth. CO2 reacts with rocks, such as in carbonate dissolution (karst carbon sink), which partly happens underground, where it is controlled by ventilation. For nitrous oxide, no underground degradation mechanism is known, and water vapour and ozone are too short-lived for an underground sink to have an effect.

The project will seek to understand the role underground environments play in global climate response through innovative theory, modelling, measurement, and assessment. The fundamental source of information on underground air is instrumental monitoring. It is advancing with the technology, offering ever-increasing quantities of data but cannot cover all the space and time. The gaps will be filled in with modelling, both based on the known physical principles and data-driven. Model-based interpolation in the study sites will lead to the next step of extrapolation to the rest of the world.

The ultimate goal is to gain sufficient understanding so as to allow future generations the option of utilising karst systems for responding to climate change using nature-based solutions, such as passive optimisation of karst and cave air flow to maximise natural greenhouse gas sinks via geo- remediation. In the present, climate change, land use change, groundwater pumping, sea level rise, construction of dams and tunnels etc. are all affecting the ground air. The consequences for the greenhouse gases are hard to predict at the current state of knowledge. A better understanding of ground air will transfer to all the involved greenhouse gases, even though the mechanisms influencing each one of them are different.

Project partners: