Letters in 2013

Slow earthquakes form part of a spectrum of fault behaviour between steady creep and fast rupture during a normal earthquake. Laboratory simulations of slow slip in rock samples taken from the Nankai subduction zone, Japan, reveal similar characteristics to fast earthquakes, implying that some slow slip events could be prematurely arrested earthquakes.

Climate change can be thought of in terms of geographical shifts in climate properties. Tracking the geographical movement of analogous climate conditions between historical and future climate model simulations, and calculating the impact of such shifts on vegetation carbon storage, suggests that boreal forests will lose carbon as low-carbon ecosystems shift in.

The brightness and lifetime of clouds is determined by cloud droplet number concentration, which is in turn dictated by the number of available seed particles. Model simulations suggest that condensation of semi-volatile organic compounds enhances the formation of cloud droplets, with consequences for cloud dynamics.

Atmospheric aerosol particles can significantly influence the climate system. Analyses of observations and observation-based modelling data reveal that biogenic aerosol emissions soar in response to warming, exerting a cooling effect in a negative feedback loop.

Ridges on the down-going plate in a subduction zone can segment the seismogenic zone and influence earthquake occurrence, but the role of the overriding plate is unclear. InSAR and GPS satellite measurements indicate that segmentation of the subduction zone in northern Chile correlates with a 1-km-high coastal scarp, implying that overriding plate structure can influence seismicity.

Predicting the response of tropical rainfall to climate change remains a challenge. An analysis of climate model simulations suggests that in an emission scenario without mitigation, a large fraction of tropical precipitation change will be independent of global surface warming over the twenty-first century.

The growth of ice on Antarctica about 34 million years ago affected sea level. A combination of modelling and marine sediment analyses shows that sea level near the developing ice sheet first fell and then rose as a result of crustal deformation imposed by the ice growth.

Saturn is brighter than expected for a gas giant of its age. Calculations of Saturn’s thermal evolution show that the presence of layered convection in Saturn’s interior—much like that observed in the Earth’s oceans—would have slowed the planet’s cooling and may explain Saturn’s anomalous luminosity.

Whether changes in atmospheric circulation over West Antarctica during the past few decades are part of a longer-term trend is unclear. Ice cores reveal a significant increase in the oxygen isotopes from precipitation over the past 50 years, but the anomaly cannot be distinguished from natural climate variability.

The response of tropical precipitation to global warming varies spatially and the factors controlling the spatial patterns of precipitation changes are unclear. An analysis of climate model simulations shows that warm regions are projected to become wetter in annual mean, whereas seasonally high rainfall anomalies are expected in regions that are currently wet.

Following the spring-time polar sunrise, ozone concentrations in the lower troposphere episodically decline to near-zero levels. Measurements on Alaskan snow and sea ice suggest that photochemical reactions in surface snow serve as a major source of reactive bromine to the overlying atmosphere, contributing to episodic ozone depletion.

In the northwestern United States, two neighbouring volcanic chains—the Snake River Plain and High Lava Plains—decrease in age in opposing directions. Laboratory experiments show that diverging volcanic chains can form where vigorous mantle circulation beneath a subduction zone causes an upwelling mantle plume to split into two.

Multi-decadal variability in coral growth rates has been documented throughout the Caribbean over the past 150–200 years. Analyses of observational and model data suggest that anthropogenic aerosols were a key driver of variations in coral growth in the western Caribbean in the second half of the twentieth century.

In the Southern Ocean, the biological cycling of dissolved CO₂ is thought to be influenced by the delivery of iron by dust particles. Reconstructions of nutrient utilization from the South Atlantic Ocean show millennial-scale links between dust flux and the efficiency of the biological pump.

Lavas erupted from individual volcanic centres often have one of two distinct compositions. High pressure and temperature experiments on lava samples collected from St Vincent Volcano in the Caribbean, combined with thermal modelling, show that this compositional bimodality is generated by volcanic systems with low heat and water content.

In the Antarctic interior, assessments of surface mass balance may overestimate accumulation because high winds remove some of the annual snowfall. Geophysical observations reveal localized zones of persistent wind scour (where little or no snow accumulates) that are predicted to occur across approximately 5% of the Antarctic surface.

Subducting slabs of oceanic lithosphere often stagnate before reaching the lower mantle. Laboratory experiments under high pressures and temperatures show that pyroxene, a common mineral in such slabs, is transformed to its dense, high-pressure counterpart, majorite garnet, at a very slow rate, temporarily keeping the slabs buoyant compared to the surrounding mantle.

In sharp contrast to events in the Arctic region, sea ice surrounding Antarctica has expanded slightly in the past few years. A combination of observations and climate model simulations suggests that cooling of the surface ocean by meltwater from the Antarctic ice shelves has contributed significantly to this sea ice expansion.

A whirling vortex has been observed in the atmosphere at the south pole of Venus. Cloud motions tracked by the Venus Express spacecraft suggest that the south polar vortex is long-lived, erratic and baroclinic in character.

Foreshocks precede some—but not all—earthquakes. Analysis of all earthquakes larger than magnitude 6.5 that occurred between 1999 and 2011 shows that earthquakes at plate boundaries are often preceded by increasing foreshock activity in the days leading up to the quake, whereas earthquakes in plate interiors often are not.