Nagoya University, Japan

A theoretical chemist compares love to hydrogen bonds.

Water molecules assemble into ice "palm to palm", like Romeo and Juliet on their first encounter. Each molecule reaches out to four neighbours, forming hydrogen bonds that lock the molecules into a tetrahedral network. And like the love of Shakespeare's pair, water's hydrogen bonds are resilient. Ice contrives to keep its network, even in the tightest of spaces.

Researchers recently predicted that ice constrained by a carbon nanotube's wall will form either tubular structures or intricate arrangements of double- and quadruple-stranded helices, depending on temperature, pressure and nanotube diameter (J. Bai et al. Proc. Natl Acad. Sci. USA 103, 19664–19667; 2006).

I have spent many years studying the structure and dynamics of water, but am still amazed by these luxuriant ice structures. Had computer simulations not shown how strenuously ice's network can adapt for its molecules to keep their four hands touching, we could hardly have imagined such structures would be possible.

Simulations have also predicted that confined ice can have two symmetrically different phases, which become deformed and indistinguishable when put under pressure (K. Koga et al. Nature 412, 802–805; 2001). So we expect that one type of ice will easily transform into the other through collective motion of its hydrogen bonds.

My prediction is that confined liquid water, which has a disordered network of hydrogen bonds, will undergo similar structural rearrangements. Molecular mechanisms may cause large changes to the network structure of water trapped in proteins or at membrane surfaces, for example. These studies could therefore help us begin to understand another intimate relationship — the relationship between water and life.

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University of East Anglia, Norwich, UK

A biogeochemist is keen to find out whether oceanic plankton can help to keep our planet cool.

For years I have been fascinated by the idea that oceanic plankton can play a significant part in controlling climate. This concept is, of course, at the heart of gaian ideas of the Earth as a self-regulating system, proposed by James Lovelock.

It was given expression through the CLAW hypothesis (published two decades ago by R. Charlson, J. Lovelock, M. Andreae & S. Warren), which supposes that the gas dimethyl sulphide produced by marine plankton influences cloud formation and hence albedo and climate.

However, direct evidence for a link between plankton and clouds has been slow to emerge. A recent paper (N. Meskhidze & A. Nenes Science 314, 1419–1423; 2006) shows a tantalizing seasonal and spatial association between sea-surface chlorophyll (an indicator of biological activity) and atmospheric properties for a six-year period over a substantial area of the Southern Ocean.

Over high-chlorophyll areas, the number of cloud droplets doubled whereas the droplets' size decreased by 30% compared with other regions, leading to an atmospheric cooling comparable to that over highly polluted regions.

Meskhidze and Nenes attribute these changes to plankton emitting the gas isoprene. I am sceptical whether the sea-to-air flux of this compound is sufficient to produce the observed effects, but finding out what does give rise to the apparent association will keep me and other scientists involved in projects such as the Surface Ocean – Lower Atmosphere Study (http://www.solas-int.org) busy for many years.

It is vital to understand what is happening in order to be able to predict how future changes in biological activity in the oceans may mitigate or enhance climate change.

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British Antarctic Survey, Cambridge, UK

An ice-core scientist wonders what makes the Earth run hot and cold.

In the past 800,000 years, Earth has seen long, cold phases punctuated every 100,000 years by short, warm interglacials. If I claim to understand climate, then I should know why these cycles occur and why we are in a warm phase today.

The most obvious external controls on our climate are small changes in Earth's orbit. These affect the variation of incoming sunlight (insolation) with season and latitude. 'Milankovitch theory' says that this in turn controls the occurrence of glaciations.

There is one obvious problem: although 100,000 years is the period of eccentricity of Earth's orbit, insolation shows much stronger effects at shorter periods, such as 41,000 and 23,000 years.

A recent paper (E. Tziperman et al. Paleoceanography 21, PA4206; 2006) suggests a way around this. It uses a model in which climate varies with an average period controlled by internal features — such as the time needed for ice-sheet growth — on a 100,000-year timescale.

However, the exact timing of climate changes is paced by orbital cycles at shorter periods. The result is that a wide range of plausible internal controls on climate can give similar predictions of how climate has evolved with time, all of them with a 'Milankovitch imprint'.

This frees us from the apparent misconception that we need an external forcing with a period of 100,000 years, but it does not identify the internal mechanisms responsible.

I used to think this was a problem for others to solve, but as part of the team that extended the ice-core record back 800,000 years, I have the tantalizing hope that the clues we need might be locked in our cold room.

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Southern Methodist University, Dallas, Texas, USA

Fossils from ancient forests in Africa provide a palaeobotanist with insight into past climates.

I have spent many years collecting and studying fossil plants from regions in or near eastern Africa's rift valley, which runs southwards from Ethiopia to Kenya, and beyond.

These fossils provide evidence of ancient forests that once linked their living counterparts, the forests that today lie to the east and west of the rift. They also highlight past shifts in the region's climate, thought to be a driver of human evolution in the area, as grasslands became more common.

But were regional climatic changes mainly the result of changes in global climate? Or were they more to do with the development of the rift itself?

From Kenya's arid rift, I have studied 12.6-million-year-old fossils of Cola and Dioscorea (wild yam), plants that today grow side-by-side in much wetter African environments. The rift is an obvious culprit for drying here: the valley lies in the rain shadow of the rift's elevated margins.

More recently, my students and I have found much older examples of the same plant genera on the northwestern Ethiopian plateau, which has a long dry season.

The plateau is not in a rain shadow, but a recent modelling study (P. Sepulchre et al. Science 313, 1419–1423; 2006) surprised me by demonstrating that even moderate elevational changes could account for today's drier climate here, too.

It suggests that the high Ethiopian plateau acts as a barrier to incoming moist air masses, and need only have been 400–1,000 metres lower than today for the plants we found fossilized there to have flourished.

Other factors would surely have played an important part, but this work highlights palaeoaltitude as a significant driver of the region's climate.

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Imperial College London, UK

A planetary scientist learns how comet dust gets from the inner to the outer Solar System.

I was lucky enough to be part of a team studying the grains of comet dust collected last year by NASA's Stardust mission. Comets are primitive, pristine objects, and the Stardust samples are changing the way we think about how our Solar System formed.

Among many surprising findings, perhaps the most significant is that a large fraction of the dust grains are minerals formed at high temperatures — temperatures expected only in the inner Solar System. How did this stuff get out to where the comet began its life, in the cold, outer regions of the Solar System?

At the recent Lunar and Planetary Science Conference in Houston, Texas, I learned about a numerical simulation that potentially offers a neat solution (F. J. Ciesla and J. N. Cuzzi, abstract here).

Observations of dusty disks around young stars show an inward flow towards the central star. Ciesla and Cuzzi's simulation suggests that this inward transport is confined to the top and bottom of the disk. It predicts that there is a narrow region near the disk's midplane where dust flows outwards — a flow sufficient to account for the Stardust results.

So now we know that comets contain a mixture of stuff from the inner Solar System, and we have a physical model that can explain how it got there. But we're still left with one question.

Virtually everything in the inner Solar System — Earth, Mars, the Moon, almost all meteorites — is depleted in volatile elements, which can't condense at high temperatures. But the cometary dust grains don't show this depletion signature. Why not? It'll be fun finding out.

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