HOMELAND security is a strange beast. Governments will happily spend billions of dollars fighting foreign wars and making the lives of travellers miserable with layer upon layer of security at airports. Yet, as Britain’s farmers have recently discovered, those same governments will also happily skimp on basic flood defence. What, it is worth considering, might be done if military-sized budgets were to be deployed against natural, as well as human threats?

If an odd couple of trillion dollars were hanging around in some Treasury official’s back pocket, Mark Jacobson of Stanford University has a suggestion about how to spend them. He would use them to build a specially designed wind farm off the coast of Louisiana, to protect New Orleans and its neighbours from hurricanes. Katrina, after all, killed 1,833 people. That is more than 60% of the number who died in the attacks of September 11th 2001. More trillions would bring more defence; all along the Gulf and Atlantic coasts, if required.

Dr Jacobson’s calculations, which he describes in Nature Climate Change, depend on a clear understanding of how hurricanes work. Turbines would steal energy from them, of course, which would make them somewhat less destructive. But that would not be enough to have a big effect. However, by extracting this energy from the winds in a storm’s leading edge, serried rows of turbines hundreds of kilometres long would also calm the water over which the hurricane’s eye—its driving force—subsequently passed.

This turns out to be crucial. Rough water feeds a hurricane, paradoxically, by creating friction between air and sea which slows down the winds circulating around the storm’s eye. This lets the air in those winds ascend the eyewall more easily, rather than just going around in circles. It is this ascent, which sucks yet more air into the cyclone, that powers the storm.

Calming the waters before a hurricane with windmills could thus, according to Dr Jacobson’s calculations, lower its maximum wind speed by 50-80%. It would also reduce the amount of water surging onto the land, which is the principal cause of destruction, by as much as 80%. A beast so tamed would do far less harm. And, as a bonus, when the turbines were not calming hurricanes, they could pay part of their way by generating electricity.