Ohio State University, Columbus

A glaciologist ponders iceberg calving from a safe distance.

If the sea level rises catastrophically, it will be due to rapid retreat of Earth’s ice sheets. At the perimeter of these sheets, where warm, saline waters meet flowing ice, complex processes occur, including the fracturing of ice to form icebergs — a process known as calving. Calving is poorly understood owing to a lack of detailed observations: researchers willing to install instruments in frigid water beneath a continually collapsing wall of ice that is prone to frequent floods of meltwater have been scarce.

To better understand calving, Jason Amundsen at the University of Alaska Fairbanks and his colleagues took a clever, and much safer, approach. They deployed an impressive array of instruments several kilometres from the calving front on and near Jakobshavn Isbræ, one of Greenland’s largest glaciers. They then ‘listened’ to the sounds of calving using sophisticated audio equipment, ‘watched’ the motion of the ice with time-lapse photography and ‘felt’ the rumble of icebergs using seismometers and tide gauges.

By combining these remote observations with straightforward theory, they found that the ice front behaves similarly to road traffic, with dense packs of icebergs and sea ice forming a jam. Once this icy mélange weakens, large bergs capsize, pushing others out of the way, and the calving wall retreats. Calving continues until the front migrates far enough inland that the ice is too thick to fracture all the way through, putting on the brakes (J. M. Amundson et al. J. Geophys. Res. doi:10.1029/2009JF001405; 2010).

The results are encouraging to those interested in modelling ice-sheet behaviour because they provide a mechanism to explain relationships between ice thickness, fracturing and retreat. They also provide a great example of how a diverse arsenal of observational tools can solve the most formidable problems in Earth science.

Joseph Fourier University, Grenoble, France

A geoscientist ponders possible links between erosion and Earth’s climate.

Mountain ranges have been eroding at an increasing rate over the past 60 million years — seemingly in response to a cooling climate. Some researchers have proposed that this higher rate of erosion has increased the rate at which tectonic plates move at Earth’s surface, suggesting that there is a link between Earth’s climate and its tectonics.

As a member of the Earth science community who studies the relationship between these factors, I was interested in findings by Anthony Dosseto of Macquarie University in Sydney, Australia, and his co-authors. They measured the change in the sediment erosion rate over the past 100,000 years, a period that includes the last glacial cycle (A. Dosseto et al. Geology 38, 395–398; 2010). By dating sediments from several locations in the Murrumbidgee River catchment of southeastern Australia, they discovered that the residence time — the length of time for which sediments remain on the landscape before they are eroded away — varied over geological time. The residence time was longer during warmer periods (such as around 100,000 years ago and today) and shorter during colder periods (such as around 15,000 years ago).

The authors interpret this change in residence time as a consequence of variations in vegetation type. The absence of trees in the higher parts of the catchment during cold periods resulted in an increased erosion rate, whereas the eucalyptus forest that was present during the warm periods slowed the erosion rate.

These results provide a rare quantitative estimate of the influence of vegetation and climate on erosion. This link might also be relevant to estimating how the current anthropogenic changes to Earth’s climate and vegetation affect soil erosion.

University of Oxford, UK

A biogeochemist weighs up the climatic influence of carbon dioxide.

Carbon dioxide constitutes a vanishingly small fraction of our atmosphere, but punches well above its weight in terms of greenhouse warming. So just how potent is it?

The geological record provides clues because, over time, Earth has oscillated between greenhouse and icehouse climates. But reconstructing coincident atmospheric CO2 concentrations is notoriously difficult. Modelling and proxy calculations are starting to converge on a single picture of atmospheric CO2 during greenhouse episodes, except for one fly in the ointment: estimates derived from the ratio of carbon isotopes in soil-precipitated carbonates are always higher than those derived from any other source.

Daniel Breecker, now at the University of Texas at Austin, and two co-workers confirm that these estimates are too large (D. O.Breecker et al. Proc. Natl Acad. Sci. USA 107, 576–580; 2010). The numbers relied on measurements of CO2 in soil pores, thought to reflect the growing-season mean. But the creation of soil carbonates is more likely during the driest and warmest parts of the growing season, when the release of CO2 from plant respiration is at a minimum.

To understand the implications of this, think of a gin and tonic. If you have less gin than you thought, you must lower the amount of tonic to get the same tasty ratio. Likewise, because there is less carbon than we thought from plant respiration, we lower our estimate of atmospheric carbon to accord with the observed ratio. The newly calculated values align beautifully with the emerging consensus. A mere 1,000 parts per million by volume (just two and a half times current atmospheric levels and similar to those predicted for AD 2100) is sufficient to induce the hottest greenhouse conditions — such as those of the Mesozoic period 251 million to 65 million years ago. CO2 truly is a heavyweight greenhouse gas.

Max Planck Institute for Biogeochemistry, Jena, Germany

A biogeochemist looks at where all the emitted carbon dioxide is going.

Humanity is currently performing a huge global experiment, emitting increasing amounts of CO2 into the atmosphere by burning fossil fuels. I find it astonishing that although we scientifically explore other planets, we still don’t understand Earth’s important carbon cycle.

Corinne Le Quéré at the University of East Anglia in Norwich, UK, and her team have put together the pieces of the contemporary global carbon cycle. They analysed observations and modelling results on fossil-fuel emissions and the terrestrial and ocean carbon cycle, which are the major contributors to the atmospheric carbon budget (C. Le Quéré et al. Nature Geosci. 2, 831–836; 2009).

The bottom line is that humans are emitting more CO2 than projected in the pessimistic scenarios outlined by the United Nations Intergovernmental Panel on Climate Change. The researchers find that only 40–45% of this CO2 remains in the atmosphere; the rest is ‘cleaned up’ by the ocean and land — the ‘carbon sink’. It would be interesting to know whether the fraction taken up by oceans and land remains constant, because any alterations will change the global climate–carbon-cycle feedback.

The study also indicates that we are moving towards saturation of the carbon sink, but the uncertainties are large. Many carbon pools and processes, particularly those below ground in the soil, are not well understood and are hardly accounted for in carbon-cycle models (P. Ciais Nature 462, 393; 2009).

The message from Le Quéré et al. is that more observations are needed, that data should be fully integrated with models, and that these efforts must be more targeted and coordinated if we are to understand what is going on with the Earth system in our huge experiment.

Pennsylvania State University

A biogeochemist ponders muddy molecules and past climates.

I am amazed by how humble fossil lipids in muddy sediments can yield insight into Earth’s history. The structures and relative abundances of these marine biomarkers, which originate from cellular membranes, provide records of physiological and ecological responses to changing ocean chemistry and temperature. They help to quantify ancient climates, and may, for example, offer a peek at the future by providing clues to ocean temperatures when the poles were free of ice.

Shifting abundance ratios of membrane lipids from marine Archaea — a proxy called TEX86 — faithfully indicate modern sea-surface temperatures. Yet ancient temperatures signalled by TEX86 can be significantly higher than those indicated by other proxies, making TEX86 hard to interpret.

Julius Lipp and Kai-Uwe Hinrichs at the Center for Marine Environmental Sciences in Bremen, Germany, show that the constituent compounds in TEX86 may be a mixture derived from ancient microbes and those living in muddy sediments today (J. S. Lipp and K.-U. Hinrichs Geochim. Cosmochim. Acta 73, 6816–6833; 2009).

The authors identified the mud-dwellers’ lipids from their polar functional groups; ancient lipids lack these groups because they are quickly lost after burial. The core hydrocarbons waving the polar flags probably account for the proxy’s overestimation of temperature. By identifying contributions from organisms living in sediments, the researchers provide a powerful means to discern which environments preserve the primary TEX86 signature and thus under which conditions we can reliably use this important proxy.

Climate scholars should take note and take heart, because this work will ultimately strengthen our interpretations of these muddy molecules to help us better understand Earth’s past and future climate.

Laboratory of Climate and Environmental Sciences, Gif sur Yvette, France

A geoscientist is astounded by Earth’s huge frozen carbon deposits.

I believe that the vulnerability of soil carbon to warming is one of the largest sources of uncertainty in the projection of future climate change. If, in a warmer world, bacteria decompose organic soil matter faster, releasing carbon dioxide, this will set up a positive feedback loop, speeding up global warming.

I was stunned to learn, from an article by Charles Tarnocai of Agriculture and Agri-Food Canada in Ottawa and his colleagues, that the global mass of soil carbon needs to be revised upwards by a frightening amount: from the 2,500 billion tonnes of carbon previously accounted for to more than 4,000 billion tonnes (C. Tarnocai et al. Glob. Biogeochem. Cycles doi:10.1029/2008GB003327; 2009). This is a result of the previously overlooked presence of vast amounts of peat, Siberian yedoma deposits (organic-rich permafrost) and other frozen carbon stores at high latitudes.

These massive stores deserve special attention because the boreal and arctic regions that house many of them are expected to warm more rapidly than average in the coming decades. Even a small leakage from these stores could cause an explosion in the growth rate of atmospheric CO2 as well as methane, a potent greenhouse gas emitted by flooded thawed soils.

So what do these findings mean for the role of high latitudes in the Earth system? We need more extensive field observations to monitor the stability of frozen carbon, and studies to measure the decomposition rates of such stores. And we should incorporate these processes into climate models such as those used by the United Nations Intergovernmental Panel on Climate Change. If I had to pick just one new PhD subject right now, exploring this terra incognita of frozen carbon and its impact on climate change would be the one.

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.

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.

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 (https://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.

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.