University of East Anglia, UK

An oceanographer describes a missing piece of the climate puzzle.

Most school students know that increasing atmospheric carbon dioxide raises global temperatures. But I've always been fascinated by the other half of the climate–CO2 connection: why, in the past, have increasing temperatures driven up atmospheric CO2?

That CO2 and temperature are locked in a powerful, positive-feedback embrace is obvious from ice-core evidence. But if we add up all the mechanisms that we know about, we fall short of explaining the rise in CO2 levels seen at the end of glaciations.

Upwelling in the Southern Ocean may be the missing piece. Today, this process brings deep CO2-rich water rapidly to the surface, where it vents carbon to the atmosphere. If upwelling was shut down during glaciations, we could fit the data better.

It has been suggested that sea ice might have blocked the air–sea transfer of CO2 during times of glacial maxima (B. B Stephens & R. F. Keeling Nature 404, 171–174; 2000).

There is good evidence that sea ice was extensive in the region, but any upwelling would have melted that ice, because the rising water has a temperature above freezing point. So the sea ice is evidence that the upwelling itself was absent. What stopped it?

One recent paper (J. R. Toggweiler et al. Paleoceanography 21, PA2005; 2006) argues that the critical factor was a shift to the north of the westerly wind belts that drive the upwelling. I and a colleague propose a subtler connection, a change in the balance of surface heat flux, that would also reduce the upwelling to near-zero (A. J. Watson & A. C. N. Garabato Tellus B 58, 73–87; 2006).

The theories are convergent in many respects, but make distinct predictions that we can test against new proxy evidence. This problem will be solved pretty soon, I think.

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University of British Columbia, Vancouver, Canada

A marine biologist dives into the history of the Gulf of California.

A decade ago, I coined the term 'shifting baselines' to describe how society perceives environmental change. The concept has caught on: there's even a website, at http://www.shiftingbaseline.com, featuring short, explanatory films.

The films push the idea that the standards by which society assesses change are themselves changing. We tend to use the state of affairs that prevailed when we first became aware of an issue as our reference point for evaluating future change — a baseline that shifts with each generation.

A set of three brilliant papers illustrates how this can shape our understanding of ecosystems.

The most recent paper (A. Sáenz-Arroyo et al. Fish Fish. 7, 128–146; 2006) reconstructs from historical sources, such as pirates' logs, details of the Gulf of California's ecosystem stretching back to the sixteenth century. The researchers argue that the past abundance of creatures such as marine mammals, turtles and oysters recounted in these sources should be considered when setting conservation targets today.

Their previous work examined records of Gulf groupers, fish that once dominated the area's reefs (A. Sáenz-Arroyo et al. Fish Fish. 6, 121–133; 2005), concluding that fishery statistics didn't go back far enough to accurately map the species' decline.

Further, they quizzed three generations of artisanal fishers (A. Sáenz-Arroyo et al. Proc. R. Soc. Lond. B 272, 1957-1962; 2005), and found that fishers' knowledge of the location or habits of species disappeared within one generation, if the species became rare.

We are all affected by this kind of collective amnesia. It allows us to handle change. But it is also the reason why we accept losses that would be intolerable, were we aware of them.

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Johannes Gutenberg University, Mainz, Germany

A cold-matter physicist is amazed by atoms' ability to divide themselves up equally.

Imagine having a box containing an even number of objects, N. You want to divide them into two boxes, each of which contains exactly N/2 objects. Sounds easy, right?

But let's complicate things a bit. Let's suppose you can't count the objects, nor look at them. Will you still be able to make the split fairly?

A collaboration of researchers from the Massachusetts Institute of Technology and Harvard University, both in Cambridge, recently showed that it's possible to do so for atoms. They divided into two equal halves a Bose–Einstein condensate of 1 million sodium atoms (G.-B. Jo et al. Phys. Rev. Lett., in the press; preprint here).

Bose–Einstein condensates are a novel state of matter that forms at a temperature close to absolute zero. They behave like quantum entities with pronounced wave-like properties. These are properties that I exploit in my own work with condensates, and they also underpin the atom division.

The Cambridge team stored their matter waves in microfabricated magnetic traps, made out of thin wires. The researchers changed the currents in the wires to split slowly the one potential well that was holding the atoms into two.

In a non-interacting gas, this splitting process would probably give a skewed distribution of atoms, and the distribution would be different every time. In this case, the quantum interactions favour a system in which each well contains exactly N/2 atoms.

In fact, the evidence suggests that the splitting is accurate to within 50 atoms. I find that truly remarkable from a fundamental point of view. More practically, this dividing of atoms could also be useful in building novel atom interferometers and atomic clocks.

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University of Oklahoma, Norman, Oklahoma, USA

An astronomer invites you to contemplate the history of some of the oldest stars in the Universe.

Much of my work is an attempt to determine what kinds of stars formed and what types of element synthesis occurred when our Galaxy was very young. This means that I am particularly interested in a class of stars referred to as 'carbon-enhanced metal-poor'.

The composition of a star reflects the properties of the interstellar medium at the time it formed, which evolves as generations of stars come and go. Metal-poor stars were born early in the history of our Galaxy, before dying stars enriched the interstellar medium with heavy elements. The fact that some of these metal-poor stars are carbon-enhanced provides insight into the types of stars that came before them.

Recently, one group reported that around 20% of metal-poor stars are carbon-enhanced (S. Lucatello et al. Astrophys. J. 652, L37–L40; 2006). A previous study had produced a lower figure (J. Cohen et al. Astrophys. J. 633, L109–L112; 2005), prompting a battle between the competing groups. But both papers agree that more metal-poor stars are carbon-enhanced than are younger, high-metallicity stars.

To me, this is one of the most interesting and compelling results to come from the study of such stars. Massive stars — with at least ten times the mass of the Sun — produce carbon efficiently, so this gives us a clear indication that massive stars, although very rare today, were much more common early in the history of the Universe.

The differences between the studies' numbers may lie in how the authors define a carbon-enhanced star, or could be a matter of statistics. I look forward to future papers that address these issues — and perhaps continuing the controversy.

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Rensselaer Polytechnic Institute, Troy, New York

Childhood memories cause a nanotechnologist to go nuts for plant-derived nanomaterials.

As a child growing up in Kerala, southern India, I marvelled at the unusual cashew fruit, with its kidney-shaped nut dangling from a swollen apple.

Since then, nanotechnology has become my passion. So it was with a curious mix of scientific interest and childhood memories that I read a recent paper describing how nanomaterials could be derived from plant sources such as the cashew nut.

I had never thought of a cashew nut as anything more than a food item. However, a little research reveals that cashew-nut-shell liquid, rich in natural long-chain phenols, already has applications ranging from hydrophobic coatings to anti-ageing creams.

George John and Praveen Kumar Vemula at the City College of New York, in their recent article (G. John & P. K. Vemula Soft Matter 2, 909–914; 2006), show how cashew-nut-shell liquid can also serve as a starting material for a variety of nanostructures.

The oil contains molecules that have phenol groups for heads, and long hydrocarbon tails. These can form structures such as lipid nanotubes and twisted nanofibres.

To make this happen, the molecules' structure is first modified by attaching water-loving sugar groups to the phenols. The cooperative effect of head groups hydrogen bonding and the hydrophobic interactions of the tails leads the molecules to self assemble into bilayers. These then further organize into the fibres and tubes.

Using a similar strategy, it should be possible to develop a wide range of novel soft nanomaterials from other plant resources. The breadth of precursors available in our plants and crops should inspire all nanotechnologists — not just those fond of cashew nuts.

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