William H. Calvin, "Computers as Modelers of Climate," in The Greatest Inventions of the Past 2,000 Years (John Brockman, editor), Simon & Schuster (2000), pp. 86-88. See also

Webbed Reprint Collection
This 'tree' is really a pyramidal neuron of cerebral cortex.  The axon exiting at bottom goes long distances, eventually splitting up into 10,000 small branchlets to make synapses with other brain cells.
William H. Calvin

University of Washington
Seattle WA 98195-1800 USA

Q:  The Greatest Invention of the Past 2,000 Years?

A:  Computers – not for all the obvious reasons but because they’re the essential tool for preventing a climate-triggered collapse of civilization in the future.

Computer simulations may allow us to understand the earth’s fickle climate and how it is affected by detours of the great ocean currents. These detours cause abrupt coolings -- the average global temperature can drop dramatically in just a few years, with droughts that set up El-Niño-like forest fires even in the tropics. While volcanic eruptions and Antarctic ice shelf collapses can also abruptly cool things, what we’re talking about here is a flip-flop: a few centuries later, there’s an equally abrupt rewarming. This cycle has repeated every few thousand years (though it has been 12,000 years since the most recent one).

If such events happened more gradually, taking centuries to ramp down, we could likely cope via technofixes. The big problem is not the temperature change per se, but the less-than-a-decade speed of the transition. The suddenness of the next cooling and drying is sure to set off massive warfare over remaining resources.

While these worldwide coolings have been commonplace in the past, they are not any more inevitable than local floods are -- we may be able to stabilize things if we learn enough about the nonlinear triggers. Everything we know about the geophysical mechanisms suggests that another abrupt cooling could easily happen – indeed, that our greenhouse-effect warming could trigger an abrupt cooling in several different ways.

The best understood part of the flip-flop tendencies involves what happens to the warm Gulf Stream waters off Ireland, once they split into the two major branches of the North Atlantic Current. They go on to sink to the depths of the Greenland-Norwegian Sea and the Labrador Sea. That’s because the cold dry winds from Canada evaporate so much warm water that the surface waters become cold and extra salty, tending to sink through the deeper waters. At some sinking sites, giant whirlpools 15 km in diameter can be found, carrying surface waters down into the depths. Routinely flushing the cold waters in this manner makes room for more warm waters to flow far north -- and that makes Europe much more agriculturally productive than is Canada or Siberia.

But this flushing mechanism can fail if fresh water accumulates on the surface, diluting the dense waters. The increased rainfall that occurs with global warming causes more rain to fall into the oceans at the high latitudes. Ordinarily, rain falling into the ocean is not considered a problem -- but at these sites in the Labrador and Greenland-Norwegian Seas, it can be catastrophic. So can meltwater from the nearby Greenland ice cap, especially when it comes out in surges (which happen when an ice surge blocks a fjord, backing up a year of meltwater -- and then the ice dam collapses in a day). By shutting down the high-latitude parts of this "Nordic Heat Pump," such consequences of global warming can abruptly cool Europe’s climate. If Europe’s agriculture reverted to the productivity of Canada’s (at the same latitudes but their winds off the Pacific Ocean are not heated in the Atlantic manner), 22 out of 23 Europeans could starve.

The surprise is that it isn’t just Europe that gets hit hard. Most of the habitable parts of the world have similarly cooled during past episodes, probably via changes in water vapor, the major greenhouse gas. Another failure of the Nordic heat pump would cause a population crash that would take much of civilization with it, all within a decade.

Ways to postpone such a climatic shift are conceivable, however — cloud-seeding to create rain shadows in critical ocean locations is just one possibility. Although we can't yet do much about everyday weather or about greenhouse warming, we may nonetheless be able to stabilize the climate enough to prevent an abrupt cooling.

Devising a long-term scheme for stabilizing the North Atlantic’s flushing mechanism has now become one of the major tasks of our civilization, essential to prevent a human population crash whose wars over food would leave a balkanized world where everyone hated their neighbors for good reasons.

But it remains to be seen whether humans are capable of passing this intelligence test that the climate sets for us. We are likely to understand the flip-flops only via computer models. And computer simulations of possible interventions are the key to safely intervening. But if we succeed, we may be able to keep our civilization from unraveling in another episode of cool, crash, and burn.


WILLIAM H. CALVIN is a theoretical neurophysiologist on the faculty of the University of Washington School of Medicine. He has written about the brain in such books as Conversations with Neil's Brain (with the neurosurgeon George A. Ojemann), The Cerebral Code, and How Brains Think. His most recent book (with the linguist Derek Bickerton) is Lingua ex machina: Reconciling Darwin and Chomsky with the Human Brain. His cover story for The Atlantic Monthly, "The great climate flip-flop," is being expanded into a book, Cool, Crash, and Burn: The Once and Future Climate of Human Evolution. || Home Page || Calvin publication list || The Bookshelf ||