If climate change were easy to understand, the debate would have been over years ago. But because climate effects don't often follow in direct proportion to their causes, it is inevitable that model predictions are hedged by uncertainties, and that the consequences of the greenhouse effect often seem contradictory and counterintuitive. What now are we to make of the suggestion that, as mean temperatures continue to climb, northern Europe has colder winters in store?
It seems clear such a thing can happen. As the last ice age thawed 12,000 years ago, the north Atlantic was plunged abruptly into, and then out of, a cold snap called the Younger Dryas that lasted over a thousand years. Those switches took just a few decades.
Modern global warming could trigger the same thing, and for the same reason: shutting down of north Atlantic ocean circulation. The tropics are hotter than high latitudes because they receive more sunlight, but this difference would be even greater were it not for the wind-driven currents of warm ocean water that carry heat towards the poles. In the north Atlantic this current is called the gulf stream, crossing the ocean from southwest to northeast.
Water brought from the south has to get back down there if the ocean is not to become lopsided. As the poleward current cools, the water becomes denser and sinks, producing a return flow of deep, cold water heading towards the equator along the western edge of the Atlantic. But according to the latest analysis by researchers in Southampton of ocean water along a cross-section of the sub-tropical north Atlantic, this conveyor-like circulation is slowing. Between 1957 and 1992 there was scarcely any sign of change, but the measurements in 2004 show a 30 per cent decrease in circulation relative to 50 years ago.
The cause of the change is probably a "freshening" of surface ocean water. There is more fresh water being injected into the north Atlantic from rain, rivers and melting ice, probably owing in turn to global warming. Fresh water is less dense than salty water, and so it sinks less readily at the northern extremity of the conveyor. The same thing is thought to have caused the Younger Dryas event: as the ice sheets melted, fresh water shut down the north Atlantic circulation and cut off the heat supply from the tropics.
Admittedly, the Southampton results provide just a series of snapshots along one section of the ocean. They could reflect a short-term fluctuation, not a trend. But they are supported by observations in 2003 of a freshening of the poleward end of the conveyor and a corresponding build-up of saline water in the tropical Atlantic. It's most unlikely that the gulf stream will shut down altogether in the near future, however, and it's not clear how climate cooling in the Atlantic will balance against direct warming from the greenhouse effect.
Of course, what most of us worry about is simply whether our heating bills will go up or down. But perhaps this latest fear will help bring home the message that the planet's climate is not a simple matter of receiving warmth from the sun. It is controlled by complex natural machinery, and the parts sometimes break down.
Quantum computing
Oxford physicist David Deutsch has won the $100,000 Edge of Computation prize for more or less inventing, alongside IBM's Charles Bennett, the concept of quantum computing. A quantum computer harnesses the quantum world's parallel realities to create computing power that dwarfs anything available from "classical" technology. Deutsch is legendary for his otherworldliness, but when he speaks, it is worth listening. So the fact that he now believes real quantum computers to be just years, rather than decades, away has to be taken seriously. Thus far, however, quantum computers have struggled with pitifully small capacities. Adding more quantum bits is like a juggler adding more balls: one or two are feasible, but tens or hundreds look impossible. Quantum bits have to be kept in a state called a "superposition," which is prone to collapse in a phenomenon known as "decoherence": the very process that produces our familiar, dependable world from the indeterminate quantum soup. Decoherence is considered to be the reason why Schrödinger's hypothetical cat, killed or spared according to the outcome of a quantum event and thus potentially in a superposition of live and dead states, could never be made flesh. But researchers in Colorado have now made a six-atom "Schrödinger cat"—a collection of six atoms in a superposition of two states that, like "live" and "dead," are as different as can be.
Having watched the progress of quantum computing over the past ten years or so—which has been ingenious, but painfully slow—I'm sceptical that a breakthrough like this will put these devices on our desks any time soon. I am not David Deutsch, however.
Caffeine is good for you
It's heartening that sometimes science tells you exactly what you want to hear: brain-scanning studies have now shown that caffeine improves short-term memory. I seem to recall that a study last year suggested it could also have the opposite effect. But then I haven't had my morning coffee yet.
