The clock is ticking. Every second, it seems, someone in the world takes on more debt. The idea of a debt clock for an individual nation is familiar to anyone who has been to Times Square in New York, where the American public shortfall is revealed. Our clock (updated September 2012) shows the global figure for almost all government debts in dollar terms.

Does it matter? After all, world governments owe the money to their own citizens, not to the Martians. But the rising total is important for two reasons. First, when debt rises faster than economic output (as it has been doing in recent years), higher government debt implies more state interference in the economy and higher taxes in the future. Second, debt must be rolled over at regular intervals. This creates a recurring popularity test for individual governments, rather as reality TV show contestants face a public phone vote every week. Fail that vote, as various euro-zone governments have done, and the country (and its neighbours) can be plunged into crisis.