"To understand the true nature of warnings, it helps to see them not as single events, like an air-raid siren, but rather as swarms of messages racing through overlapping social networks, like the buzz of gossip. Residents of New Orleans didn't just need to know a hurricane was coming. They also needed to be informed that floodwaters were threatening to breach the levees, that not all neighborhoods would be inundated, that certain roads would become impassible while alternative evacuation routes would remain open, that buses were available for transport, and that the Superdome was full.

"No central authority possessed this information. Knowledge was fragmentary, parceled out among tens of thousands of people on the ground. There was no way to gather all these observations and deliver them to where they were needed. During Hurricane Katrina, public officials from top to bottom found themselves locked within conventional channels, unable to receive, analyze, or redistribute news from outside. In the most egregious example, Homeland Security secretary Michael Chertoff said in a radio interview that he had not heard that people at the New Orleans convention center were without food or water. At that point they'd been stranded two days.