What's the difference? Scientists still argue — though not as much as deniers would have you believe — about the extent to which climate change is the result of human activity. And they still argue — quite a lot, actually — about how quickly the climate shifts in response to new conditions. As I understand Ruddiman, we humans may have been screwing up the climate for far longer than anyone thought. But that's good — because if we could change things then, we should certainly be able to change them now.
The gist of Ruddiman's argument is that 8,000 years ago, carbon dioxide levels in the atmosphere began their sharpest increase in 350,000 years — a CO2 spike that correlates with the origins of agriculture. Then, 5,000 years ago, methane levels jumped — at roughly the same moment humans started growing rice in paddies (organic matter decomposing in water emits methane). By 2,000 years ago, agriculture and forest-clearing had added as much as 140 billion tons of CO2 to the air, enough to stave off what would likely have been another ice age.
Since then, the climate has wiggled back and forth between warm and cold. Around AD 800, things got weirdly hot; Antarctic ice cores show atmospheric CO2 peaking then at 285 parts per million. Around 1300, CO2 levels started dropping, and by 1600 that number had decreased to as low as 275 ppm. According to Ruddiman, humans caused that nosedive, too — by dying in large numbers: In the 14th century, about one-third of Europe's population died in the Black Plague, and around the same time, some 50 million Native Americans were being wiped out by European germs. The much-reduced surviving population burned less wood and coal, grew less food, and even allowed wooded areas to grow back.
Today things are heating up again. Atmospheric CO2 concentrations have been on the steep incline of an exponential growth curve since 1800. Today they're at roughly 380 ppm. How high will temperatures rise as a result of so much carbon? We don't know. But the more you mess up the climate, the more feedback effects there are and the more chaotic it gets, until eventually we reach tipping points, when various important climatic systems — Arctic summer sea ice, for example, or the Indian summer monsoon — suddenly disappear or change dramatically.
But I see hope in Ruddiman's conclusions. If humans have been changing the climate for eight millennia, that means we can keep right on doing it. We can steer the climate back on course. (I should add that Ruddiman's hypothesis is controversial, and he himself interprets his findings much more conservatively.)
So what do we do? You already know the drill: Make machines more energy efficient. Use less fossil fuel. Sequester CO2. Protect rain forests. Develop alternative energy sources like wind and solar power. Build more nuclear plants. Maybe even change the planet through geoengineering, the once far-out idea that the greenhouse effect can be reversed by, say, releasing fleets of mirrors or sulfur particles into the atmosphere. Ruddiman suggests we focus on reducing the concentrations of methane, a more potent greenhouse gas than CO2, by trapping emissions from landfills and changing the fertilizer used by rice farmers.
The point is, exhorting corporations and governments to act now isn't just ringing a rhetorical bell: We can rescue the globe's climatic system as abruptly as we can push it over the edge. And no matter what we call our current epoch, it would be nice if we kept the planet healthy enough to let us live to see the next one.
Peter Schwartz (email@example.com) is a cofounder and chair of the Global Business Network.