When the markets blew up in 2008, policymakers rushed to make comparisons with the 1930s. True, that was a terrible depression, but one that was over quite quickly. The Great Depression was caused by a sudden collapse of demand and shrinking money supply. Now, as then, policymakers assumed that if the government expanded its deficits and central banks printed lots of money, that would fix the problem. It hasn?t, and it should be clear by now that it isn?t going to.

Why not? Because what we are really dealing with is a structural depression. In reality, the global economy is facing not one crisis, but three.

There is a debt crisis. The developed world has been building up debts on a spectacular scale for three decades. According to McKinsey data, global debt now stands at $158 trillion; that is up from $77 trillion in 2000. Put another way, global debt amounts to 266 percent of global GDP now, compared with 216 percent a decade ago. While economists used to think that debt was largely neutral ? on the grounds that once person?s borrowing is another person?s loan ? we are now discovering that borrowing on that scale is unsustainable.

Then there is a currency crisis. For most of the post-WWII period, the dollar was the anchor of the global economic system. That worked when the U.S. was the overwhelmingly dominant economy. It doesn?t work anymore. The dollar is now down to 60 percent of reserves, as central banks diversify away from a currency falling in value. At some point we will come up with a new core currency ? perhaps the Chinese renminbi, perhaps gold. But until we do, there will be more chaos ahead.

And finally, there is the euro, perhaps the most dysfunctional monetary system ever created. Welding together the currencies of 17 very different economies, without any kind of fiscal union to compensate for the differences between them, was always a high-risk experiment. By now we can surely agree that it has failed. The euro was meant to promote faster growth and greater stability. It has become instead a cause of depression and volatility. Until it is dismembered, there is little chance of the global economy?s returning to stability.