Reserves, stocks and depletion: The stock above ground: materials in use and recycling

4.1 Society's material stock rose 23-fold in a century

A flow is measured per year: tonnes mined or consumed. A stock is measured at a date: tonnes in place. Reserves are a stock of ore below ground. Society also holds a stock above ground, the materials in buildings, infrastructure and durable goods, which Krausmann and colleagues estimated for the world from 1900 to 2010.

In-use stocks grew from 35 billion tonnes in 1900 to 792 billion in 2010, 23-fold, or 2.9 per cent a year, and 4 per cent a year in the three decades after 1945. GDP grew 27-fold and annual extraction tenfold, from 7 to 78 billion tonnes. The share of extraction going into stock-building materials, rather than fuel and food, rose from 18 to 55 per cent. Per person, stocks rose from 22 to 115 tonnes. Industrial countries averaged 335 tonnes per person in 2010. China went from 35 to 136 tonnes between 1990 and 2010, roughly where industrial countries stood in the early 1970s, while the rest of the world had 38. The stock is young: 82 per cent of it was 30 years old or less.

Most of the mass is sand, gravel and concrete, but the metals matter for the table. The 2010 stock contained about 25.6 billion tonnes of iron, 0.7 billion of copper and 0.4 billion of aluminium. The copper in use is of the same order as copper reserves today.

The stock drives future demand. If every country reached the industrial level by 2050, Krausmann's group calculates that global stocks would quadruple to about 3,137 billion tonnes, needing net additions of 59 billion tonnes a year against 26 billion in 2010.

• Krausmann et al. (2017), PNAS web ↗ · pdf pp. 1-5

4.2 New scrap, old scrap, and two recycling rates

Tilton and Guzmán separate two sources of secondary metal. New scrap arises in manufacturing: offcuts from stamping car bodies or drawing wire. Its composition is known and it is close to the mill, so it is cheap to remelt and nearly all of it is recycled. Old scrap comes from products at the end of their lives. It is scattered, mixed and contaminated with rubber, glass and plastic, so each extra tonne costs more to recover. Some uses are dissipative, such as lead in paint, and never come back at any sensible cost. A price rise also draws on old scrap that was not worth processing before, so secondary supply responds more in the short run than the long run.

US copper in 2025 shows the difference in scale. Old scrap yielded about 160,000 tonnes of copper; new scrap from fabricators about 760,000 tonnes. Scrap of both kinds met about 30 per cent of US supply. But new scrap only returns metal already counted as consumption, so Tilton and Guzmán leave it out when judging dependence on foreign suppliers.

Two rates are quoted and they answer different questions. The end-of-life recycling rate asks what share of the metal in discarded products is recovered. The recycling input rate, or recycled content, asks what share of metal entering use came from scrap. Krausmann's group shows the gap: in 2010, 77 per cent of end-of-life metal was recycled, yet secondary metal was only 27 per cent of metal inputs. The EU criticality study uses an input rate for end-of-life scrap: 83 per cent for lead, 55 for copper, 34 for zinc and 32 for aluminium, but 1 per cent for light rare earths and indium and zero for lithium, gallium and phosphate rock.

• Tilton & Guzmán (2016), Mineral Economics and Policy · pdf pp. 78-84
• USGS Mineral Commodity Summaries 2026, Copper web ↗ · pdf p. 76
• USGS Mineral Commodity Summaries 2026, Lead web ↗ · pdf p. 119
• Krausmann et al. (2017), PNAS web ↗ · pdf p. 3
• European Commission (2023), Study on the Critical Raw Materials for the EU web ↗ · pdf pp. 34-36, 122

4.3 Steel is the most recycled material and still mostly new

Smil calls steel the world's most recycled metal. It loses none of its properties when remelted and it is magnetic, so it separates easily from shredded waste. In the US the mass recovered dwarfs other materials: with steel at 100, paper is about 67, aluminium 6, glass 4 and plastics 3.

He divides steel scrap into three streams. Home scrap arises inside steelworks and returns to the furnace within days; in 2012 it was about 200 million tonnes, 35 per cent of all scrap. Prompt scrap comes from manufacturers, at 10 to 15 per cent of the steel they use. Obsolete scrap comes from discarded products after lives that average about 15 years for cars, 30 for machinery and 60 to 75 for structural steel. Obsolete scrap is the hard stream. It arrives mixed with copper, tin and other elements that cannot be removed from steel and must be diluted with clean scrap or pig iron, since structural steel generally needs less than 0.1 per cent of each.

Recovery of discarded steel is high. Recent US rates averaged about 98 per cent for structural beams and plates and about 90 per cent for appliances, and rebar reached 70 per cent in 2012, up from 40 per cent in the late 1990s. The World Steel Association put the global rate at 83 per cent in 2007. Each tonne made from scrap instead of pig iron saves about 1,100 kg of iron ore, 640 kg of coal and 2.9 MWh of energy.

Yet in 2012 electric arc furnaces, the main scrap users, made only 29 per cent of world steel. The bottleneck is the quantity of steel reaching the end of its life, not collection.

• Smil (2016), Still the Iron Age · pdf pp. 136-142

4.4 While stocks grow, recycling cannot close the loop

Recycling can only return what entered use years or decades earlier. When demand grows, today's scrap comes from a smaller past. A simple calculation shows the limit. Suppose use grows at 2.6 per cent a year, copper's average rate of primary output growth from 1975 to 2014 in Tilton and Guzmán's table, and products last 30 years. Metal reaching end of life today entered use when demand was 1.026 to the power of minus 30, or 46 per cent, of today's. Even with perfect collection, old scrap could meet less than half of current demand. With a 15-year life the ceiling is 68 per cent; at 5 per cent growth and a 30-year life it is 23 per cent.

This is Krausmann's result in miniature. In 2010 inputs to global stocks (about 41 billion tonnes) were nearly three times outflows (14.5 billion), so primary materials had to supply most inputs however good recycling became. Their aggregate recycling input rate across all materials was 12 per cent in 2010, after a U-shaped path from 23 per cent in 1900 down to 5 per cent around 1970.

The cap lifts as stocks saturate. Smil cites Pauliuk and colleagues, who find per-capita steel stocks in rich countries leveling off near 13 tonnes, and who project that primary steel demand may peak as early as 2025 and that electric furnaces, fed with scrap and direct-reduced iron, may match the blast-furnace route by 2050. In some regions scrap may then exceed steel demand.

So a circular economy is mostly a destination reached when stocks stop growing. Before then, Krausmann's group argues, more intensive use of existing stocks, longer service lives and lighter designs do more, because they cut the inflow needed for the same services.

• Tilton & Guzmán (2016), Mineral Economics and Policy · pdf p. 226
• Smil (2016), Still the Iron Age · pdf pp. 137, 144, 227-230
• Krausmann et al. (2017), PNAS web ↗ · pdf pp. 3-5

See it in the table

Check your understanding

Krausmann and colleagues find that 77 per cent of end-of-life metal was recycled in 2010, yet secondary metal was only 27 per cent of metal inputs. What explains the gap?

Demand for a metal grows 2.6 per cent a year and its products last 30 years. Even with 100 per cent collection, what share of today's demand could old scrap cover at most?

In 2025, US new scrap yielded about 760,000 tonnes of copper and old scrap about 160,000 tonnes. Why do analysts of import dependence often exclude new scrap?