One weather system, six countries.

Russia was, in 2010, among the largest wheat exporters in the world. Egypt was the largest importer on earth, and roughly a fifth of what an Egyptian household spent went on food. Cairo had subsidised bread for decades, because every Egyptian government since the 1970s had understood what happens when it stops.

That summer a heatwave settled over western Russia and did not lift. By August the crop was ruined, and the government faced a choice between feeding its own population and honouring the contracts it had signed.

It banned exports. This was a reasonable thing for a government to do for its own people, and it was announced as exactly that.

The price of wheat in Chicago rose by roughly seventy per cent in six weeks. Chicago is eight thousand kilometres from the fields that burned and not one grain of Russian wheat is traded there. It moved because Chicago is where the world agrees what wheat is worth. Take a large exporter out of the market and you have not altered a single contract — you have repriced the entire curve, and every importer on earth is now buying against it.

Egypt was buying against it. The subsidy bill, already among the largest lines in the budget, became impossible, and the price at the counter moved.

In January 2011 the crowds came into Tahrir Square. They came for many reasons, most of them older and deeper than the price of bread, and it would be crude to say that a heatwave brought down a government. But the chant in the street was bread, freedom, dignity — bread was first, and bread was the one thing that had changed that year.

Within eighteen months the government had fallen, the canal had a new set of managers, shipping insurers had repriced the route, and a European refinery that had never heard of a Russian wheat field was paying more to move crude through a waterway it had used without incident for forty years.

The shock had crossed six countries. Nobody along that chain was doing anything unusual; each was responding correctly to what was in front of them, and the sum of correct local decisions was a government on the pavement and a refining margin gone.

This section follows those chains — what moved, why it moved, and where it will be felt next. The mine is always somewhere else. The bill is always local.