Negative interest rates are even more of a brain-twister than zero interest rates. Here is what Herman Daly has to say about that:

Suppose for a moment that GDP growth, economic growth as we gratuitously call it, entails uneconomic growth by a more comprehensive measure of costs and benefits — that GDP growth has now begun to increase counted plus uncounted costs by more than counted plus uncounted benefits, making us inclusively and collectively poorer, not richer. If that is the case, and there are good reasons to believe that it is, would it not then be reasonable to expect, along with Summers, that the natural rate of interest is negative, and that maybe the monetary rate should be too? This is hard to imagine, but it means that savers would have to pay investors (and banks) to use the funds that they have saved, rather than investors and banks paying savers for the use of their money. To keep the GDP growing sufficiently to avoid unemployment we would need a growing monetary circular flow, which would require more investment, which, in turn, would only be forthcoming if the monetary interest rate were negative (i.e., if you lost less by investing your money than by holding it). A negative interest rate “makes sense” if the goal is to keep on increasing GDP even after it has begun to make us poorer at the margin — that is after growth has already pushed us beyond the optimal scale of the macro-economy relative to the containing ecosphere, and thereby become uneconomic.

A negative monetary interest rate means that citizens will spend rather than save, so savings will not be available to finance the investments that produce the GDP growth needed for full employment. The new money for investment comes from the Fed. Quantitative easing (money printing) is the source of the new money. The faith is that an ever-expanding monetary circulation will pull the real economy along behind it, providing growth in real income and jobs as previously idle resources are employed. But the resulting GDP growth is now uneconomic because in the full world the “idle” resources are not really idle — they are providing vital ecosystem services. Redeploying these resources to GDP growth has environmental and social opportunity costs that are greater than production benefits. Although hyper-Keynesian macroeconomists do not believe this, the micro actors in the real economy experience the constraints of the full world, and consequently find it difficult to follow the unlimited growth recipe…

These painful choices could be avoided if only we were richer. So let’s just focus on getting richer. How? By growing the aggregate GDP, of course! What? You repeat that GDP growth is now uneconomic? That cannot possibly be right, they say. OK, that is an empirical question. Let’s separate costs from benefits in the existing GDP accounts, and develop more inclusive measures of each, and then see which grows more as GDP grows. This has been done (ISEW, GPI, Ecological Footprint), and results support the uneconomic growth view. If growth economists think these studies were done badly they should do them better rather than ignore the issue.