The Climate Cost of Metals Is Higher Than Expected, Even After Mine Closures

An abandoned mine appears still, almost returned to nature. Yet beneath the rocks and in the rivers, a chemical reaction can still release CO₂ for centuries. This forgotten pollution now disrupts the climate footprint attributed to metals.

In the silence of closed mines, an invisible chemistry continues to release carbon

When a mine closes, the machinery stops. Workers depart, leaving galleries that sometimes fill with water. Then the landscape seems to exhale, as if freed from an old pressure. But the fractured rocks remain exposed to water and oxygen, two elements capable of reigniting an invisible chemistry.

Some metallic ores contain sulfides, such as pyrite. Their oxidation produces sulfuric acid. This acid mixes with groundwater and rain. It creates the acid mine drainage, a flow capable of transporting iron, copper and other toxic metals.

This phenomenon has been known for a long time. It colors rivers orange and devastates aquatic ecosystems. Its climatic dimension, however, remained little studied. A study published in July 2026 in Environmental Science & Technology shifts this view. It shows that neutralizing these waters can also release CO₂.

In Spain, eighty-two mines studied reveal a phantom pollution that persists in the water

To analyze this phenomenon, researchers from the University of St Andrews studied 82 mines active or former. These sites lie in the Iberian pyrite belt, southwest of Spain, a region traversed by the Tinto and Odiel rivers and known for its extremely acidic waters.

The scientific team tracked the water chemistry from the old extraction sites to the estuaries. Analyses focused on reactions between acids and carbonate rocks, but also on alkaline treatments and interactions with seawater. At each step of neutralization, dissolved carbon can transform into carbon dioxide.

A climate accounting of mines could be amplified by as much as tenfold, according to the researchers

In this sulfide-rich region, emissions linked to acid mine drainage are substantial. They are comparable to the standard carbon footprint of copper production, which includes extraction, milling, and transport of ore, but often neglects post-closure chemical reactions.

The most striking finding concerns the long time horizon. Once all exposed sulfides have reacted, the cumulative emissions could reach very high levels. They could exceed by more than tenfold the footprint usually attributed to copper. CO₂ is thus not limited to the exploitation phase and can appear long after operations cease.

This discovery arrives in a context of strong metal demand. Copper, nickel and lithium are essential for batteries, wind turbines and electrical grids. The energy transition thus increases the need for extraction, but the true cost could be underestimated if these late emissions are not accounted for.

Acid mine drainage treatment also poses an environmental dilemma. It protects rivers and limits the dispersion of toxic metals, but some neutralization methods release CO₂. It becomes therefore necessary to develop greener solutions in order to reduce emissions without moving pollution into other forms.

A mine does not vanish after closure. It leaves behind galleries, waste heaps, and an active chemistry for centuries. As demand for metals rises, a question lingers in the background: how can this invisible legacy be integrated into the industrial and climate choices of tomorrow?