With Speed and Violence: Why Scientists Fear Tipping Points in Climate Change by Fred Pearce Read Free Book Online Page A

moved in lockstep permits no other interpretation. It also probably involves changes to ocean currents and the temperature feedbacks from growing and melting ice.
    We will return to this conundrum later. What matters here is that a minor change in the planet's heating-much less, indeed, than we are currently inflicting through greenhouse gases-could cause such massive changes worldwide. The planet seems primed to leap into and out of glaciations and, perhaps, other states too.

    Some see this hair trigger as rather precisely organized. Will Steffen says that for a couple of million years, Earth's climate seems to have had just two "stable states": glacial and interglacial. There was no smooth transition between them. The planet simply jumped, at a signal from the orbital wobble, from the glacial to the interglacial state, and made the jump back again with a little, but not much, more decorum. "The planet jumps straight into the frying pan and makes a bumpy and erratic slide into the freezer," Steffen says. The glacial state seems to have been anchored at carbon dioxide levels of around 19o ppm, while the interglacial state, which the modern world occupied until the Industrial Revolution, was anchored at about 280 ppm. The rapid flip between the two states must have involved a reallocation of about 22o billion tons of carbon between the oceans, land, and the atmosphere. Carbon was buried in the oceans during the glaciations and reappeared afterward. Nobody knows quite how or why. But the operation of the hair-trigger jump to a much warmer state raises critical questions for the Anthropocene.
    In the past two centuries, humanity has injected about another 22o billion tons of carbon into the atmosphere, pushing carbon dioxide levels up by a third, from the stable interglacial level of 28o ppm to the present 380 ppm. The figure continues to rise by about 20 ppm a decade. So the big question is how Earth will respond. Conventional thinking among climate scientists from Arrhenius on predicts that rising emissions of carbon dioxide will produce a steady rise in atmospheric concentrations and an equally steady rise in temperatures. That's still the IPCC story. But Steffen takes a different view: "If the ice age seemed to gravitate between two steady states, maybe in future we will gravitate to a third steady state." Nature might, he concedes, fulfill the expectations of climate skeptics and push back down toward 28o ppm; but if it was going to do that, we would already see evidence of it. And we don't.
    Other scientists, including Alley, are not convinced by Steffen's sense of order in the system. Sitting in his departmental office, Alley likens the climate system to "a drunk-generally quiet when left alone, but unpredictable when roused." When he is writing scientific papers or committee reports, his language is not so vivid. He talks of a "chaotic system" vulnerable to "forcings" from changes in solar radiation or greenhouse gases. "Abrupt climate change always could occur," he says. But "the existence of forcings greatly increases the number of possible mechanisms [for] abrupt change"; and "the more rapid the forcings, the more likely it is that the resulting change will be abrupt on the timescale of human economies or global ecosystems." Drunks, in other words, may be unpredictable, but if you shout at them louder or push them harder, they will react more vehemently. Right now, moreover, we are offering our drunk one more for the road.

    The past io,ooo years, since the end of the last ice age, have not been without climate change. The Asian monsoon has switched on and off; deserts have come and gone; Europe and North America have flipped from medieval warm period to little ice age. None of these events has been as dramatic as the waxing and waning of the ice ages themselves. But most were equally abrupt, and civilizations have come and gone in their wake. Even so, human society in general has prospered, learning to plant crops,