The best part of book tours are the questions. After spending years with the same ideas and sentences - they become old friends - it's invigorating to see how people react, to keep track of which concepts spark their curiosity. It's also fun to consider questions that never occured to me while writing the book. For instance, I was recently stumped by a seemingly obvious query that I hadn't really considered. It was asked by a 4th grader: "What," he wanted to know, "is the downside of creativity? Isn't it possible that humans are too creative?"

I muttered something incoherent about nuclear weapons and human ingenuity creating the seeds of its own destruction. I'm pretty sure I quoted Einstein. But I could tell he wasn't satisfied, that my answer struck him as facile and trite, which it was. So here's my attempt to give him a better answer, because I think the absurd success of human creativity comes with a real cost.

One of the scientists I spend a lot of time with in Imagine is Geoffrey West, a brilliant theoretical physicist at the Sante Fe Insititute. (He has done a lot of intriguing work on cities, trying to figure out why cities are "the most important invention in the history of human civilization" and why some cities are so much more innovative than others, at least measured by per capita production of patents.) Although West celebrates the inventiveness of cities - all those knowledge spillovers leads to new knowledge - he is quick to point out that our creativity has its disadvantages. New ideas, after all, have a disturbing tendency to become new things, and things aren't free.

West illustrates the problem by translating the modern human lifestyle - and we live surrounded by our own inventions - into watts. “A human being at rest runs on 90 watts,” he told me. “That’s how much power you need just to lie down. And if you’re a hunter-gatherer and you live in the Amazon, you’ll need about 250 watts. That’s how much energy it takes to run about and find food. So how much energy does our lifestyle [in America] require? Well, when you add up all our calories and then you add up the energy needed to run the computer and the air-conditioner, you get an incredibly large number, somewhere around 11,000 watts. Now you can ask yourself: What kind of animal requires 11,000 watts to live? And what you find is that we have created a lifestyle where we need more watts than a blue whale. We require more energy than the biggest animal that has ever existed. That is why our lifestyle is unsustainable. We can’t have seven billion blue whales on this planet. It’s not even clear that we can afford to have 300 million blue whales.”

The historian Lewis Mumford described the rise of the megalopolis as “the last stage in the classical cycle of civilization,” which would end with “complete disruption and downfall.” In his more pessimistic moods, West seems to agree: he knows that nothing can trend upward forever, that eventually our creativity will make life utterly unsustainable. In fact, West sees human history as defined by this constant tension between expansion and scarcity, between the relentless growth made possible by our creativity and the limited resources that hold our growth back.

Of course, the only solution to the problem of human innovation is more innovation. After a resource is exhausted, we are forced to exploit a new resource, if only to sustain our craving for growth. West cites a long list of breakthroughs to illustrate this historical pattern, from the discovery of the steam engine to the invention of the Internet. “These major innovations completely changed the way society operates,” West says. “It’s like we’re on the edge of a cliff, about to run out of something, and then we find a new way of creating wealth. That means we can start to climb again.”

But the escape is only temporary, as every innovation eventually leads to new shortages. We clear-cut forests, and so we turn to oil; once we exhaust our fossil-fuel reserves, we’ll start driving electric cars, at least until we run out of lithium. Although human creativity has generated a seemingly impossible amount of economic growth, it has also inspired the innovations that allow the growth to continue. So here's the paradox: creativity is the only solution to the very real problem of creativity.

There is a serious complication to this triumphant narrative of cliff edges and innovation, however. Because our lifestyle has become so expensive to maintain, every new resource now becomes exhausted at a faster rate. This means that the cycle of innovations has to constantly accelerate, with each breakthrough providing a shorter reprieve. The end result is that our creativity isn’t just increasing the pace of life; it is also increasing the pace at which life changes. “It’s like being on a treadmill that keeps on getting faster,” West says. “We used to get a big revolution every few thousand years. And then it took us a century to go from the steam engine to the internal-­combustion engine. Now we’re down to about 15 years between big innovations. What this means is that, for the first time ever, people are living through multiple revolutions.”

Needless to say, such revolutions aren't fun. They're unsettling and disruptive. But they appear to be the inevitable downside of our ceaseless ingenuity, for creativity comes with a multiplier effect: new ideas beget more new ideas. The treadmill is going fast. And it's getting faster.