inflation
i've made the prediction that our oil consumption per capita will halve in 15 years. unless we can double our energy efficiency we're going to do half as much. we'll travel half as often. eat out half as often. build half as many new cars, houses, planes, phones, computers, etc. if we consume half as much our economy will be halved in size. but our debt will still be there. in absolute dollars. if we don't inflate our (now decreased) income we will succumb to the cost of servicing the debt. that rate of inflation is 5%. lower bound. assuming we don't take on any more debt.