Study says metal mining emissions may be much higher than estimated

New research published Wednesday in Environmental Science and Technology says the carbon footprint linked to metal mining could be much larger than previously estimated, with possible implications for climate goals as demand for metals grows.

The report said greener energy systems, including wind turbines, power grids and electric cars, will require a large increase in metal extraction. Conventional estimates of mining emissions mainly count energy used by heavy equipment on mine sites and diesel transport used to move ore.

The study examined acid mine drainage, a common mining side effect that the researchers said had not previously been formally quantified as a carbon dioxide source. Senior author Luke Bridgestock of the University of St Andrews told AFP that the team had found an emission source not well considered by the mining industry and that it could be very large in some settings.

The source explained that metal ores often contain sulfide minerals. When these minerals are exposed to oxygen-rich surface water, they can make the water acidic. When that acidic water reacts with certain rocks, including limestone, it can release carbon dioxide into the atmosphere. Acidic water can also flow into the ocean and release carbon dioxide when it reacts with seawater.

The researchers focused on the Iberian Pyrite Belt in southwest Spain, a copper-mining region with 82 active or historic mines. The source said the area generates more acid mine drainage than anywhere else in the world. The team measured the chemistry of rivers draining the mines, estimated how much acid mine drainage was neutralized through mineral reactions that produce carbon dioxide, and tracked changes as river water mixed with seawater.

The study found that acid mine drainage since industrial-scale mining began in the area in the 19th century had produced a similar amount of carbon dioxide to already known emissions. On that basis, the source said the area's true mining footprint to date is about double previous estimates.

The researchers also said emissions can continue long after a mine's reserves are depleted because metal sulfide minerals can take hundreds to thousands of years to react fully with surface water. Their calculations put the figure at seven to 165 tons of carbon dioxide per ton of copper produced, depending on sulfide content and local geology. The midpoint of that range is roughly ten times higher than conventional carbon footprint estimates.

The team is testing whether olivine, a fast-dissolving mineral, could neutralize acidic runoff without releasing carbon. Bridgestock said researchers next need to identify the mining regions most at risk so mitigation efforts can be targeted.