Vaclav Smil: "Four materials rank highest on the scale of necessity, forming what I have called the four pillars of modern civilization: cement, steel, plastics, and ammonia are needed in larger quantities than are other essential inputs.
"And these four materials, so unlike in their properties and qualities, share three common traits: they are not readily replaceable by other materials (certainly not in the near future or on a global scale); we will need much more of them in the future; and their mass-scale production depends heavily on the combustion of fossil fuels, making them major sources of greenhouse gas emissions."
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