University of Texas, Austin, USA

A geophysicist ponders the mysteries of intraplate earthquakes.

During my first semester at college, I attended a lecture describing plate tectonics, and immediately knew that geophysics would be my major and hopefully my career. Subsequent lectures, textbooks, journal articles and, later, my own research educated me about how elegantly plate tectonics explains the processes that control the locations of most earthquakes.

However, some of the largest-known North American earthquakes — including those of 1811–12 famed for ringing church bells in Boston and changing the course of the Mississippi River — are associated with the New Madrid Seismic Zone (NMSZ) in the Southern and Midwestern United States, far from known plate boundaries. So what causes these events? Eric Calais of Purdue University in Indiana and Seth Stein at Northwestern University in Illinois present some surprising results from an examination of Global Positioning System (GPS) data from the region (E. Calais and S. Stein Science 323, 1442; 2009).

Previous studies found that the NMSZ was moving at a different rate and in a different direction from the North American Plate, implying that strain would steadily accumulate until released by a large-magnitude earthquake. But, incorporating three years' worth of extra GPS data, Calais and Stein found motions indistinguishable from those of the North American Plate, corresponding to extremely low strain rates. It is not clear what the underlying processes causing the NMSZ earthquakes are. Is strain accumulation variable over time in intraplate settings? What are the implications for hazard prediction?

The results leave me perplexed, but oddly comforted — there are plenty of mysteries left for the next generation of geophysicists. And perhaps one day a theory will elegantly explain intraplate seismicity, just as plate tectonics did for interplate seismicity.

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Old Dominion University, Norfolk, Virginia

An astrobiologist considers life's oldest oxygen.

The presence of atmospheric oxygen would have been necessary for the evolution of eukaryotes — organisms that group their genetic material into a membrane-bounded nucleus — so the question of when oxygen first became available is important in dating their rise. The availability of such oxygen is linked to the evolution of cyanobacteria, oxygen-producing microbes that appeared early in Earth's history and exist to this day.

Fossil microbial mats preserved in the Pongola Supergroup, a rock succession in South Africa, suggest that cyanobacteria were already highly diverse 2.9 billion years ago. But conclusive proof of their presence can be provided only by the presence of hydrocarbon biomarkers — stable chemical compounds found in the walls of single-celled organisms.

Work by Jacob Waldbauer at the Woods Hole Oceanographic Institution in Massachusetts and his colleagues focuses on biomarkers from shallow-marine deposits in the younger, 2.6-billion-year-old sedimentary rocks preserved in South Africa's Transvaal Supergroup. Detailed laboratory analyses extracted biomarkers called hopanes, possibly attributable to cyanobacteria, as well as steranes, biomolecules typically found in eukaryotes (J. R. Waldbauer et al. Precamb. Res.10.1016/j.precamres.2008.10.011; 2008). The biosynthesis of steranes requires free oxygen; therefore, the fossil steranes imply that oxygen was readily available 2.6 billion years ago. This is at least 200 million years before a persistent oxygen-containing atmosphere is thought to have arisen.

Waldbauer et al. show that cyanobacteria had colonized the floor of Earth's ancient oceans by 2.6 billion years ago at the latest. Free oxygen has been available in the atmosphere ever since, and set the stage for the evolution of more complex organisms.

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US Geological Survey, Pasadena, California

A seismologist considers a new method of earthquake prediction.

I am acutely aware that numerous methods of earthquake prediction at one time held great promise, but fell apart under proper scrutiny. In recent years, I have heard about many studies purporting to uncover evidence of electromagnetic precursors, almost all of which involved weak or non-existent statistical analysis.

But occasionally I come across research that is not so easy to dismiss. For example, data from the French micro-satellite DEMETER, which was launched in 2003 to investigate electromagnetic perturbations in the ionosphere, have been analysed by a team of French and Czech researchers (F. Nmec et al. Geophys. Res. Lett. doi:10.1029/2007GRL032517; 2008). These authors find that there are very-low-frequency electromagnetic fluctuations in the ionosphere above the epicentres of moderate and large earthquakes that occur a day or two before the ground starts to shake.

Nmec and colleagues' results could be fatally flawed. If electromagnetic disturbances are generated when earthquakes occur, what are apparently true signals of one earthquake could actually be signals related to a preceding shock. Or the analysis might go awry because of subtle data-selection biases. But if there are fatal flaws, they are not obvious.

In any case, as the authors themselves emphasize, the significance of the DEMETER results can be demonstrated only when data from many earthquakes are averaged. This highlights a key point: it is entirely possible for precursors to be real but of no use for prediction. If earthquake scientists can separate consideration of earthquake precursors from the highly charged debates about earthquake prediction, the research community might just learn something about earthquake processes.

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Queen's University, Belfast, Northern Ireland

A microbiologist learns that all marine creatures must suffer for the greed of a few.

Phosphate is an essential nutrient for all forms of life. Demand for it tends to outstrip supply to such an extent that it limits the overall productivity of many ecosystems, including vast tracts of the seas. I study the curious strategies by which creatures obtain sufficient phosphate for life as they know it.

Some microorganisms, for instance, keep a phosphate store for when times are hard. They scavenge for the nutrient in their surroundings with high-affinity uptake systems and then produce polyphosphate, an insoluble polymer that packs hundreds of phosphate subunits into a single strand. Strands of polyphosphate then form intracellular granules that can be broken down by cellular enzymes when they are needed.

This kind of 'luxury' uptake was recently the focus of a study by Ellery Ingall of the Georgia Institute of Technology in Atlanta and his colleagues. Diatoms — unicellular, silica-walled algae — accumulate phosphate during summer blooms to levels far beyond their immediate needs. Indeed, polyphosphate produced by plankton accounted for 7–11% of the total phosphate in the surface waters of Effingham Inlet, a fjord on Vancouver Island, Canada (J. Diaz et al. Science 320, 652–655; 2008).

This self-indulgent behaviour seems to have far-reaching consequences. Decaying plankton eventually sink to the ocean floor, where they spill unused polyphosphate onto the sediment surface. Notably, Ingall and his team found that soluble phosphate was not released at this point. Instead, polyphosphate molecules seeded the precipitation of minerals called apatites, a process that took only a few years. So diatom greed may ultimately lower the ceiling on marine productivity by locking away the oceans' most hard-to-come-by nutrient. That is important as well as curious.

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University of Sheffield

A palaeobiologist calls for greater biological realism in climate models.

The world's most sophisticated climate models fail to adequately replicate climate at high latitudes and over continents' interiors during ancient periods of greenhouse-gas-induced warming: the wintertime predictions are consistently too cold. This makes me worry that the field is missing fundamental feedback processes that amplify warming. If so, climate models might be underestimating how much anthropogenic warming will happen in the future.

What might these mysterious processes be? Lee Kump and David Pollard of Pennsylvania State University in University Park think they have found one. They propose that marine phytoplankton that emit dimethylsulphide — already recognized as a major source of cloud-seeding particles far out to sea — became thermally stressed during the Cretaceous period (100 million years ago). As a result, the phytoplankton grew more slowly and reduced their emissions. Fewer biologically derived aerosol particles meant fewer nuclei for cloud condensation, which, in turn, led to less extensive cloud cover and more transparent clouds. Solar radiation was thus reflected less, and polar temperatures rose by 10–15 °C (L. R. Kump and D. Pollard, Science 320, 195; 2008).

Kump and Pollard's work is exciting for its dramatic result. Nevertheless, the duo's findings are ultimately unsatisfactory; the effects of heat on biological aerosol emissions need to be better described in their model for it to generate really solid conclusions. Although some recent field and laboratory experiments do suggest that marine algae produce less dimethylsulphide when carbon dioxide concentrations approach those of the Cretaceous, much more research is needed. If such results agree with Kump and Pollard's assumptions, I might worry less about climate models — but maybe even more about global warming.

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Australian Weather and Climate Research, Tasmania, Australia

An oceanographer ponders the difficulty of accurately estimating abyssal-ocean warming.

Estimating how much oceans are warming and where within them heat is stored is a fascinating challenge for me and my fellow oceanographers. So far, most studies comparing observations and models of changing ocean temperatures have focused on the upper 1 kilometre of water. But what about the abyssal depths, from about 3,000 metres to the bottom? Are changes in those waters really so slow as to be essentially irrelevant to atmospheric warming?

The most comprehensive surface-to-bottom measurements of ocean temperature were collected by research ships over many months during the World Ocean Circulation Experiment in the 1990s. By comparing these observations with more recent ones from the World Climate Research Programme's CLIVAR Project, Greg Johnson and his colleagues have shown that the Pacific Ocean's abyssal waters have warmed during the past two decades (G. C. Johnson et al. J. Clim. 20, 5365–5375; 2007).

Although the temperature increase is small — up to about 0.01 °C — compared with the much larger changes in the upper 1,000 metres of the ocean, it has occurred over a thickness of several kilometres, implying a huge quantity of heat storage. The deep warming is strongest in the south-west Pacific, where newly ventilated abyssal waters enter from the south.

The Pacific warming, and abyssal warming elsewhere, means that we should start considering abyssal waters when estimating sea-level rise and the climate's sensitivity to increasing greenhouse-gas concentrations. There is plenty to find out: how does the heat reach abyssal waters? Is the warming human-induced? Designing and implementing an adequate abyssal-water-observing system is a high priority.

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Tyndall Centre for Climate Change Research, NOC, Southampton, UK

An oceanographer sees potential in accelerating rock weathering to soak up carbon dioxide from the air.

With CO2 emissions increasing by more than 2% per year, rather than decreasing by the 3% or so needed to effectively mitigate climate change, I am not surprised that many scientists are seeking alternative solutions to simply cutting greenhouse-gas outputs.

Various geoengineering schemes have been proposed — such as fertilizing the oceans with iron, a limiting resource for planktonic algae that take CO2 from the atmosphere — but these are unlikely to sequester large amounts of carbon in the long-term and may have serious ecological side effects. The thermodynamics of enhancing geochemical weathering look feasible, but the reactions are too slow to be really practicable.

Geochemists and engineers at Harvard University in Massachusetts and Pennsylvania State University recently suggested a kinetically preferable idea. They propose using the electrolysis of sea water to produce sodium hydroxide and hydrochloric acid, in a variant of the well-known industrial 'chloralkali' process, (K. Z. House et al. Environ. Sci. Technol. 41, 8464–8470; doi:10.1021/es0701816 2007).

Sodium hydroxide could either be used to scrub CO2 directly from the air, producing sodium bicarbonate, which is neutral and could be discharged into the sea, or be pumped directly into the ocean, increasing sea water's alkalinity and so its ability to absorb CO2. The hydrochloric acid could be neutralized fairly easily, because it reacts rapidly with both carbonate and silicate rocks.

The scheme House et al. outline looks promising if it were operated using a solar or geothermal electricity source near a supply of basic rocks. A mid-ocean volcanic island would be good. And the environmental consequences of the scheme's discharges should be less severe than those of the ocean acidification that humans are already causing.

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California Institute of Technology, USA

A planetary scientist foresees a shift in the debate about Earth's heat flow.

Measurements of the heat coming out of Earth's interior have long posed a puzzle for understanding the planet's history.

Earth's heat output is estimated to be around 44 terawatts, about twice that expected from radioactive decay. The difference can be attributed to cooling of the deep Earth, implying a present-day cooling rate of 100 kelvin per billion years. But simple models with this much cooling 'blow up' when they are run back in time, predicting ridiculous temperatures for the early Earth. Acceptable models rely on unconventional deviations from the usual simple scaling laws for mantle convection. This is an attractive but untested idea.

I and many others have wondered whether an alternative explanation is that today's heat flow is higher than the average for the past half a billion years. Such fluctuations could arise as a result of the dispersal and accumulation of continental land masses.

A recent paper (J. Korenaga Earth Planet. Sci. Lett. 257, 350–358; 2007) assessed this possibility by taking advantage of a long-known connection between sea level and the heat flow from sea-floor spreading. It finds little room for more than a few percent fluctuation in heat flow around its long–term decline.

I think this pushes the problem back into the realm of models, focusing attention on plate tectonics, the deep water cycle (because water affects how rocks flow), and perhaps even the long-standing question of whether Earth's mantle is well mixed from top to bottom.

On a decadal timescale, we can hope that better measurements of heat generation and flow will be combined with more realistic theory. Like many central Earth science questions, the heat-flow problem resists quick resolution.

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Swiss Federal Institute of Technology, Zürich, Switzerland

A climate scientist worries that attempts to curb atmospheric carbon dioxide levels are challenged on two fronts.

What is your carbon footprint? I must admit that, as someone who frequently travels across continents, mine is well above the Swiss average. Even worse, my footprint has grown over the past few years despite the fact that I am well aware of the consequences of my actions.

Now, imagine that everyone else on this planet has increased their carbon footprint as well. This is not hypothetical. A recent paper tells us that global carbon emissions have grown at the unexpectedly high rate of more than 3% per year since 2000 (M. Raupach et al. Proc. Natl Acad. Sci. USA doi:10.1073/pnas.0700609104; 2007).

In particular, the rapidly increasing appetite for energy of the emerging markets in Asia has led to a dramatic increase in fossil-fuel burning. As a result, global CO2 emissions now exceed the worst-case scenarios of just a few years ago. This is far from the direction that we ought to be taking to achieve a stabilization of greenhouse gases that "prevents dangerous interference with the climate system", as the Climate Convention in Rio set out to achieve in 1992.

Unfortunately, the situation may become even more difficult. Earth's biosphere has so far helped to mitigate the carbon problem by removing a substantial fraction of the emitted CO2, but this 'sink' function may diminish.

There is some evidence that sinks are already weakening (C. Le Quéré et al. Science doi:10.1126/science.1136188; 2007), and coupled climate–carbon-cycle models tend to support the view that the trend will persist. If so, we are challenged at both ends — by unexpectedly rapidly increasing emissions and by diminishing sink strengths — making climate stabilization a truly grand challenge.

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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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University of Wisconsin-Madison, USA

A geophysicist wonders how and why faults behave in so many different ways.

I'm involved, with colleagues, in a project of the Integrated Ocean Drilling Program (IODP) to drill deep into the Nankai Trough subduction zone off southwestern Japan — a site of numerous great earthquakes and tsunamis.

Major unknowns in the generation of tsunamis include how far earthquake fault slip can propagate up towards the sea bed and what factors control how that slip stops in accretionary wedges — the submarine mountain ranges created as sediment and rock are scraped off the sinking plate.

My research focus is on faults in such wedges, which are generally thought of as aseismic, or incapable of earthquakes. By drilling into the wedge faults at Nankai Trough, we hope to learn how aseismic faults give way with depth to the seismic faulting associated with tsunamis.

Recently, a new kind of slow-motion earthquake was observed in this wedge (Y. Ito & K. Obara Geophys. Res. Lett. 33, L02311; 2006). Suddenly, wedges don't seem so aseismic after all.

These 'very-low-frequency' earthquakes, some as large as magnitude 4.4, have previously gone unrecognized because their seismic waves don't show up in the frequency range in which earthquakes are normally detected.

By chance, the quakes were detected exactly where my IODP team plans to start drilling later this year. We hope to install sensors deep in the subsurface to record the earthquakes up close and to measure pore fluid pressure and strain in the rock. We'll also collect samples for laboratory studies of the frictional properties of the rock.

Taken together, the in situ and sample data should yield insight into the processes responsible for these slow-motion quakes. This might help us to understand the aseismic–seismic transition.

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Max-Planck Institute for Biogeochemistry, Jena, Germany

A biogeochemist finds inspiration for life on the ocean floor.

My research interests lie in understanding the interplay between the physical and chemical conditions that constrain life, and the feedback processes by which life shapes the Earth's environment.

I want to understand these interactions in terms of a thermodynamic hypothesis that states that systems dissipate as much energy as possible. Can life be seen as an emergent outcome of this tendency for the whole Earth system? To test this, one would need to show that it is possible to predict the emergence of life from the hypothesis, as well as its impact on Earth's early environment.

Two articles (M. J. Russell & A. J. Hall GSA Memoir 198, 1–32; 2006, and M. J. Russell Am. Sci. 94, 32–39; 2006) could provide a starting point. The authors give a detailed picture of the thermodynamics of life emerging at hydrothermal mounds on the ocean floor.

One of the earliest metabolic reactions would have involved the conversion of hydrogen, carbon dioxide and sulphur compounds into organic carbon, acetate and water. This would have happened in the hot, mineral-rich spring water seeping into the hollow mound.

But its influence would have been felt more widely. Removing sulphur from the environment would have changed atmospheric composition and cloud cover, affecting the amount of sunlight reaching the ground. And acetate may have served as fuel for methanogens, methane-producing organisms known to live in vents. Increased methane production would have raised its levels in the atmosphere, resulting in higher surface temperatures on Earth.

Quantifying these interactions should help us to understand whether the evolution of our planet emerged from general thermodynamic trends.

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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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