Authors: Appy Sluijs, Richard E. Zeebe, Peter K. Bijl & Steven M. Bohaty

Authors: C. M. Moore, M. M. Mills, K. R. Arrigo, I. Berman-Frank, L. Bopp, P. W. Boyd, E. D. Galbraith, R. J. Geider, C. Guieu, S. L. Jaccard, T. D. Jickells, J. La Roche, T. M. Lenton, N. M. Mahowald, E. Marañón, I. Marinov, J. K. Moore, T. Nakatsuka, A. Oschlies, M. A. Saito, T. F. Thingstad, A. Tsuda & O. Ulloa

Authors: Nicholas D. Ward, Richard G. Keil, Patricia M. Medeiros, Daimio C. Brito, Alan C. Cunha, Thorsten Dittmar, Patricia L. Yager, Alex V. Krusche & Jeffrey E. Richey

Temperate and tropical rivers serve as a significant source of carbon dioxide to the atmosphere. However, the source of the organic matter that fuels these globally relevant emissions is uncertain. Lignin and cellulose are the most abundant macromolecules in the terrestrial biosphere, but are assumed to resist degradation on release from soils to aquatic settings. Here, we present evidence for the degradation of lignin and associated macromolecules in the Amazon River. We monitored the degradation of a vast suite of terrestrially derived macromolecules and their breakdown products in water sampled from the mouth of the river throughout the course of a year, using gas chromatography time-of-flight mass spectrometry. We identified a number of lignin phenols, together with 95 phenolic compounds, largely derived from terrestrial macromolecules. Lignin, together with numerous phenolic compounds, disappeared from our analytical window following several days of incubation at ambient river temperatures, indicative of biological degradation. We estimate that the net rate of degradation observed corresponds to 30–50% of bulk river respiration. Assuming that a significant fraction of these compounds is eventually remineralized to carbon dioxide, we suggest that lignin and other terrestrially derived macromolecules contribute significantly to carbon dioxide outgassing from inland waters.

Authors: A. E. Gleason & W. L. Mao

The strength of iron at extreme conditions is crucial information for interpreting geophysical observations of the Earth’s core and understanding how the solid inner core deforms. However, the strength of iron, on which deformation depends, is challenging to measure and accurately predict at high pressure. Here we present shear strength measurements of iron up to pressures experienced in the Earth’s core. Hydrostatic X-ray spectroscopy and non-hydrostatic radial X-ray diffraction measurements of the deviatoric strain in hexagonally close-packed iron uniquely determine its shear strength to pressures above 200 GPa at room temperature. Applying numerical modelling of the rheologic behaviour of iron under pressure, we extrapolate our experimental results to inner-core pressures and temperatures, and find that the bulk shear strength of hexagonally close-packed iron is only ∼ 1 GPa at the conditions of the Earth’s centre, 364 GPa and 5,500 K. This suggests that the inner core is rheologically weak, which supports dislocation creep as the dominant creep mechanism influencing deformation.

Authors: Matt J. Ikari, Chris Marone, Demian M. Saffer & Achim J. Kopf

Slow slip forms part of the spectrum of fault behaviour between stable creep and destructive earthquakes. Slow slip occurs near the boundaries of large earthquake rupture zones and may sometimes trigger fast earthquakes. It is thought to occur in faults comprised of rocks that strengthen under fast slip rates, preventing rupture as a normal earthquake, or on faults that have elevated pore-fluid pressures. However, the processes that control slow rupture and the relationship between slow and normal earthquakes are enigmatic. Here we use laboratory experiments to simulate faulting in natural rock samples taken from shallow parts of the Nankai subduction zone, Japan, where very low-frequency earthquakes—a form of slow slip—have been observed. We find that the fault rocks exhibit decreasing strength over millimetre-scale slip distances rather than weakening due to increasing velocity. However, the sizes of the slip nucleation patches in our laboratory simulations are similar to those expected for the very low-frequency earthquakes observed in Nankai. We therefore suggest that this type of fault-weakening behaviour may generate slow earthquakes. Owing to the similarity between the expected behaviour of slow earthquakes based on our data, and that of normal earthquakes during nucleation, we suggest that some types of slow slip may represent prematurely arrested earthquakes.

Author: Charles D. Koven

Climate change can be thought of in terms of geographical shifts in climate properties. Examples include assessments of shifts in habitat distributions, of the movement needed to maintain constant temperature or precipitation, and of the emergence and disappearance of climate zones. Here I track the movement of analogue climates within climate models. From the model simulations, I define a set of vectors that link a historical reference climate for each location to the location in a changed climate whose seasonal temperature and precipitation cycles best match the reference climate. I use these vectors to calculate the change in vegetation carbon storage with climate change due to ecosystems following climate analogues. Comparing the derived carbon content change to direct carbon projections by coupled carbon–climate models reveals two regions of divergence. In the tropical forests, vector projections are fundamentally uncertain because of a lack of close climatic analogues. In the southern boreal forest, carbon losses are projected in the vector perspective because low-carbon ecosystems shift polewards. However, the majority of carbon–climate models—typically without explicit simulation of the disturbance and mortality processes behind such shifts—instead project vegetation carbon gains throughout the boreal region. Southern boreal carbon loss as a result of ecosystem shift is likely to offset carbon gains from northern boreal forest expansion.

Authors: David Topping, Paul Connolly & Gordon McFiggans

Clouds profoundly influence weather and climate. The brightness and lifetime of clouds is determined by cloud droplet number concentration, in turn dictated by the number of available seed particles. The formation of cloud droplets on non-volatile atmospheric particles is well understood. However, fine particulate matter in the atmosphere ranges widely in volatility. Co-condensation of semi-volatile compounds with water increases a particle’s propensity for cloud droplet formation, with potential consequences for feedbacks between the terrestrial biosphere and climate. Here we systematically study cloud droplet formation, using a cloud parcel model extended to include co-condensation of semi-volatile organic compounds under a broad variety of realistic conditions. As an air parcel rises and cools, the concentration of organic vapour that it can hold declines. Thus, the simulated organic vapours become increasingly saturated as they ascend, and so condense on growing particles as they swell into cloud droplets. We show that condensation of increasingly volatile material adds to the soluble mass of these droplets and facilitates the uptake of additional water, which leads, in turn, to a substantial increase in the number of viable cloud droplets. We suggest that the co-condensation of semi-volatile organic compounds with water vapour has a substantial impact on the radiative properties of clouds.

Authors: Marta Béjar-Pizarro, Anne Socquet, Rolando Armijo, Daniel Carrizo, Jeff Genrich & Mark Simons

Segmentation can influence the extent of earthquake rupture and event magnitude: large megathrust earthquakes result from total rupture of relatively continuous segments of the subduction interface. Segmentation is attributed to variations in the frictional properties of the seismogenic zone or to topographic features on the down-going plate. Structures in the overriding plate may also influence segmentation, but their importance has been dismissed. Here, we investigate the links between interface segmentation at the North Chile seismic gap and a crustal-scale fault structure in the overriding plate that forms a coastal scarp of about 1 km in height. We use satellite interferometric synthetic aperture radar (InSAR) and Global Positioning System (GPS) data to measure interseismic surface deformation between 2003 and 2009 and compare the deformation with rupture extent during well-documented earthquakes. From these data we infer the degree of coupling and segmentation at depth. We find that along a 500-km-long segment, the base of the strongly coupled seismogenic zone correlates with the line of the surface coastal scarp and follows the outline of the Mejillones Peninsula. This correlation implies that large-scale structures in the overriding plate can influence the frictional properties of the seismogenic zone at depth. We therefore suggest that the occurrence of megathrust earthquakes in northern Chile is controlled by the surface structures that build Andean topography.

Authors: Pauli Paasonen, Ari Asmi, Tuukka Petäjä, Maija K. Kajos, Mikko Äijälä, Heikki Junninen, Thomas Holst, Jonathan P. D. Abbatt, Almut Arneth, Wolfram Birmili, Hugo Denier van der Gon, Amar Hamed, András Hoffer, Lauri Laakso, Ari Laaksonen, W. Richard Leaitch, Christian Plass-Dülmer, Sara C. Pryor, Petri Räisänen, Erik Swietlicki, Alfred Wiedensohler, Douglas R. Worsnop, Veli-Matti Kerminen & Markku Kulmala

Atmospheric aerosol particles influence the climate system directly by scattering and absorbing solar radiation, and indirectly by acting as cloud condensation nuclei. Apart from black carbon aerosol, aerosols cause a negative radiative forcing at the top of the atmosphere and substantially mitigate the warming caused by greenhouse gases. In the future, tightening of controls on anthropogenic aerosol and precursor vapour emissions to achieve higher air quality may weaken this beneficial effect. Natural aerosols, too, might affect future warming. Here we analyse long-term observations of concentrations and compositions of aerosol particles and their biogenic precursor vapours in continental mid- and high-latitude environments. We use measurements of particle number size distribution together with boundary layer heights derived from reanalysis data to show that the boundary layer burden of cloud condensation nuclei increases exponentially with temperature. Our results confirm a negative feedback mechanism between the continental biosphere, aerosols and climate: aerosol cooling effects are strengthened by rising biogenic organic vapour emissions in response to warming, which in turn enhance condensation on particles and their growth to the size of cloud condensation nuclei. This natural growth mechanism produces roughly 50% of particles at the size of cloud condensation nuclei across Europe. We conclude that biosphere–atmosphere interactions are crucial for aerosol climate effects and can significantly influence the effects of anthropogenic aerosol emission controls, both on climate and air quality.

Authors: Sandrine Bony, Gilles Bellon, Daniel Klocke, Steven Sherwood, Solange Fermepin & Sébastien Denvil

Predicting the response of tropical rainfall to climate change remains a challenge. Rising concentrations of carbon dioxide are expected to affect the hydrological cycle through increases in global mean temperature and the water vapour content of the atmosphere. However, regional precipitation changes also closely depend on the atmospheric circulation, which is expected to weaken in a warmer world. Here, we assess the effect of a rise in atmospheric carbon dioxide concentrations on tropical circulation and precipitation by analysing results from a suite of simulations from multiple state-of-the-art climate models, and an operational numerical weather prediction model. In a scenario in which humans continue to use fossil fuels unabated, about half the tropical circulation change projected by the end of the twenty-first century, and consequently a large fraction of the regional precipitation change, is independent of global surface warming. Instead, these robust circulation and precipitation changes are a consequence of the weaker net radiative cooling of the atmosphere associated with higher atmospheric carbon dioxide levels, which affects the strength of atmospheric vertical motions. This implies that geo-engineering schemes aimed at reducing global warming without removing carbon dioxide from the atmosphere would fail to fully mitigate precipitation changes in the tropics. Strategies that may help constrain rainfall projections are suggested.

Authors: Martin Ziegler, Paula Diz, Ian R. Hall & Rainer Zahn

The rise in atmospheric CO2 concentrations observed at the end of glacial periods has, at least in part, been attributed to the upwelling of carbon-rich deep water in the Southern Ocean. The magnitude of outgassing of dissolved CO2, however, is influenced by the biological fixation of upwelled inorganic carbon and its transfer back to the deep sea as organic carbon. The efficiency of this biological pump is controlled by the extent of nutrient utilization, which can be stimulated by the delivery of iron by atmospheric dust particles. Changes in nutrient utilization should be reflected in the δ13C gradient between intermediate and deep waters. Here we use the δ13C values of intermediate- and bottom-dwelling foraminifera to reconstruct the carbon isotope gradient between thermocline and abyssal water in the subantarctic zone of the South Atlantic Ocean over the past 360,000 years. We find millennial-scale oscillations of the carbon isotope gradient that correspond to changes in dust flux and atmospheric CO2 concentrations as reported from Antarctic ice cores. We interpret this correlation as a relationship between the efficiency of the biological pump and fertilization by dust-borne iron. As the correlation is exponential, we suggest that the sensitivity of the biological pump to dust-borne iron fertilization may be increased when the background dust flux is low.

Authors: Pedro N. DiNezio & Jessica E. Tierney

Authors: Hrvoje Tkalčić, Mallory Young, Thomas Bodin, Silvie Ngo & Malcolm Sambridge

Authors: A. Baker, R. W. Allmendinger, L. A. Owen & J. A. Rech