Study: Copper Mining and Climate Change

Advocacy, Art & Science

How would copper-sulfide mining contribute to global warming? How much carbon dioxide would PolyMet and Twin Metals emit?

First, it’s important to note that metal mining is a carbon-intensive process. According to research published by the prestigious journal Nature, emissions from primary mineral and metal production constituted approximately 10% of the total global energy-related greenhouse gas emissions.

Copper mining is particularly fuel intensive. From 2001 to 2017 in Chile, the world’s largest producer of copper, electricity consumption increased by 32% per unit of mined copper, largely due to decreasing ore grade.

And Twin Metals and PolyMet would be mining very low-grade ore.

To better understand how these two mines would impact the climate, we commissioned a study that draws from environmental reports, industrial standards, and 23 separate studies of energy use and greenhouse gas emissions from copper and nickel mines.

Watch the presentation on this report below.

Watch Dr. Emerman present on his findings

Key findings

Both PolyMet and Twin Metals seek to extract the poor, low-grade ore that would require them to crush a lot of rock to get a little copper. The ore body Twin Metals seeks to tap averages a mere 0.69% copper. For PolyMet it’s even less, just 0.26%. On average, these mines would need to pulverize 266 pounds of rock to produce one pound of copper.

The study shows that together, these two mines would emit between 807,000 to 1.9 million tons of CO2 each year.

The EPA’s Greenhouse Gas Equivalencies calculator helps put this number into perspective:

• Emissions from Twin Metals would emit the equivalent of 89,000 to 235,000 passenger cars each year.

• Emissions from PolyMet would emit the equivalent of 69,000 to 136,000 passenger cars each year.

• Together, the yearly emissions from these mines would be equivalent to adding as many as 372,000 passenger vehicles to the road, or the emissions produced by supplying energy to 200,000 homes for one year.


In order to get a picture of how much CO2 PolyMet and Twin Metals would emit, the study looked at estimates provided by the mining companies and state regulators, then compared these numbers with a study of industrial trends in sulfide mining.

This study then calculated such factors as the type of mine that would be built (underground or open pit), the grade of the ore, and plugged this data into the results of 23 separate studies of energy use in copper-nickel mines to estimate how much CO2 PolyMet and Twin Metals would release.

The following tables break down emissions by direct emissions (which accounts for blasting, the diesel needed to operate mines, loading and hauling ore, propane for heating), and indirect CO2 emissions (electricity purchased from outside sources, say the power company).

When looking at this chart, keep in mind that the reason that the industry standards are so much lower, is that it’s best practices for regulators and mining companies to double their estimates in order to account for a margin of error and provide a cushion for the variables that could affect greenhouse gas emission, such as how they source their power.

PolyMet’s deception

Of the two mines, PolyMet will be much larger. Whereas Twin Metals plans to process 20,000 tons of ore per day, PolyMet plans to process 32,000 tons per day of even lower quality ore. We also know that from documents submitted to financial regulatory agencies, that the Swiss-owned company also plans to expand the mine to be three to four times as large.

Without a detailed plan for this expanded mega-mine, it is not possible to accurately estimate how much CO2 would be emitted. However, it’s reasonable to conclude that, if this mine is processing three to four times the amount of ore, it will need three to four times the amount of energy for crushing (which consume the highest amount of electricity), blasting and hauling.

On the conservative end of this analysis, an expanded mine could produce over a 1.5 million tons of CO2 per year, perhaps much more. To put this into perspective, that is like adding 325,000 passage cars to the road each year.

Text on picture of canoe - Report: The impact of copper-sulfide mining on climate change.

Wetland destruction

In addition to emitting vast amounts of CO2 into the atmosphere, both these mines would destroy critical means of carbon sequestration, the most profound of which would be the destruction of wetlands.

According to a 2016 study, wetlands in the United States store a total of 11.52 petagrams (that’s 11.52 billion tons) of carbon, or roughly four years of US carbon emissions.

Researchers found that of the many varieties of wetlands, some of the most effective carbon sinks were freshwater wetlands in cooler climates — such as northern Minnesota.

Based on DNR’s figures on carbon sequestration in Minnesota peatland, Twin Metals would destroy 912.4 acres of peatland, and in turn, release 2.5 million tons of CO2.

This amount of greenhouse gas would be doubled by PolyMet, which would involve the largest permitted destruction of wetlands in Minnesota’s history, wiping out almost 1,000 acres of one of the most effective ways to capture CO2, further exasperating the damage this mine does to our planet.

From wetland and peatland destruction alone, Twin Metals and PolyMet would release around 5 million tons of CO2.

The dangers of mining in a warming world

In the past two decades, warmer winters, increased rainfalls, and less ice on lakes all point to the reality of Minnesota’s changing climate. There is growing alarm that PolyMet and Twin Metals are not designed to account for the reality of the changing climate. In particular, the likelihood that a mega rainstorm would trigger an environmental catastrophe.

In examining Twin Metals’ Maine Plan of Operations, mining experts found that each year, there is a 1% chance that a 100-year storm (defined as more than 5.49 inches in a 24-hour period) will result in an uncontrolled discharge of untreated wastewater into Birch Lake. This pollution would flow into the Boundary Waters, and would be nothing short of an environmental disaster.

One percent may seem low, but ask yourself: Does society accept a 1% risk on anything? If there were a 1% chance that the plane you were boarding would crash, would you board it? Probably not. That airplane would not even be allowed to take off. Over the life of the mine, increased rainfall from climate change would result in an 18% chance that toxic wastewater would spill over and contaminate the Boundary Waters.

In the past 20 years, there has been sharp uptick in mega rainstorms throughout Minnesota. 2.5 times as many mega rainstorms occurred in the 19-year period between 2000 to 2019 as occurred in the previous 26 years, from 1973 to 1999. Many will remember the 2012 storm that, in 24-hours, dropped 10.45 inches in Two Harbors. Seven years later, enough rain fell in a 6-hour period in Duluth to qualify as a “200-year flood.” Clearly, the reality of a changing climate, coupled with a toxic mine just miles away from the most visited Wilderness Area in the country, is a recipe for an environmental disaster.

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