Letters in 2010

Ice clouds in the tropical tropopause layer have a key role in dehydrating air that is entering the stratosphere. Cloud-chamber measurements suggest that their high humidity can be explained if heterogeneous ice nucleation on glassy aerosols is a significant nucleation mechanism in this region.

The evolution of Cordilleran-type orogens is controlled by a balance between forces that result in uplift and forces that destabilize the crustal growth. Numerical modelling indicates that the buoyancy of the underlying mantle wedge is an important factor in the collapse and fragmentation of Cordilleran orogens.

Mineral dust and marine sediment resuspension are generally considered the primary sources of the nutrient iron to the oceans. Numerical model results suggest that iron released by hydrothermal activity is also an important source of dissolved iron, particularly in the Southern Ocean.

The five interglacial periods that occurred after the Mid-Brunhes Event 430,000 years ago were longer and warmer than the preceding interglacials. Climate modelling indicates that a change in the seasonality of insolation following the event, in conjunction with higher greenhouse-gas concentrations, could have caused the relative warmth.

Landslide erosion is a primary control of landscape relief. A wide-ranging analysis of landslide geometry shows that soil-based landslides are generally less voluminous than landslides that involve the failure of bedrock, and provides refined metrics for estimating the volume of a landslide from the area of the failure

Nanoparticles are a key component of atmospheric aerosols, growing rapidly under ambient conditions. Exposure of nanoparticles to organic vapours shows that various organic species can enhance the growth of nanoparticles.

There is evidence for the existence of differentiated crust early in Earth’s history, but little is known about the timing and nature of the crust and its formation. New samarium–neodymium data from the Dresser Formation in Western Australia point to differentiation of the early crust from the mantle more than 4.3 billion years ago.

The onset of fluvial erosion in an area of tectonic uplift is thought to reflect the timing of the uplift. Geomorphological data from the Yellow River in Tibet, indicate that the rapid incision of this river channel occurred as a result of climate change, at least six million years after the onset of plateau uplift.

Anomalies of the El Niño/Southern Oscillation in the Pacific Ocean and the Indian Ocean Dipole often occur simultaneously. An analysis of observations and models suggests that, in addition, the negative phase of the Indian Ocean Dipole tends to occur 14 months before an El Niño event, with implications for El Niño predictability.

Increasing concentrations of carbon dioxide in sea water are driving a progressive acidification of the ocean, with as yet unclear impacts on marine calcifying organisms. Simulations with an Earth system model suggest that future changes in the marine environment could be more severe than those experienced during the Palaeocene–Eocene thermal maximum, both in the deep ocean and near the surface.

Widespread glacier acceleration has been observed in Greenland in the past few years. Oceanographic observations taken in summer 2008 show that ocean waters melted a substantial fraction of ice along the calving fronts of three West Greenland glaciers, indicating that submarine melting has a profound influence on grounding-line stability.

The recent rapid increase in mass loss from the Greenland ice sheet is primarily attributed to an acceleration of outlet glaciers. Oceanographic data obtained in summer 2008 show that subtropical waters that reside year-round in the shelf ocean off Greenland continuously enter a large glacial fjord in East Greenland and contribute to melting at the glacier terminus.

Ninety-four million years ago, during Ocean Anoxic Event 2, there was a marked increase in the burial of organic carbon in marine sediments. Measurements of stomata in fossil leaves show that the two main pulses of carbon burial were associated with a decline in atmospheric CO₂ levels of up to 26%.

Reducing tropical deforestation is at present considered a cost-effective option for mitigating climate change. Satellite-based estimates of forest loss suggest that urban population growth and urban and international demand for agricultural products are key drivers of deforestation in the tropics.

In the Arctic spring, sunlight-induced reactions convert gaseous elemental mercury into compounds that are rapidly deposited on the snowpack. Analysis of the isotopic composition of mercury in snow samples collected during an atmospheric mercury depletion event suggests that sunlight triggers the re-emission of mercury from the snowpack.

Several periods of global ocean anoxia punctuated the Cretaceous period. Marine-sediment chemistry indicates that extensive volcanism at the beginning of Oceanic Anoxic Event 2 released sulphur to the oceans, triggering a biogeochemical cascade that led to enhanced surface productivity and depletion of oxygen in the underlying waters.

The rise in atmospheric carbon dioxide at the end of the Last Glacial Maximum has been attributed to the release of carbon dioxide from the deep Southern Ocean. However, reconstructions of the radiocarbon signature of Chilean margin intermediate waters during the glacial termination do not reflect the influence of such a release.

Jupiter’s large moons Ganymede and Callisto are similar in size and composition, but different in surface and interior characteristics. Simulations with geophysical models of core formation indicate that the difference in impact energy received by the two satellites during the period of late heavy bombardment can explain the dichotomy.

The last glacial period was punctuated by several periods of massive iceberg discharge from the Northern Hemisphere ice sheets. Numerical simulations indicate that these discharge events are linked to an interplay between ice-sheet accumulation, marine ice-shelf stability and periodically oscillating surface ocean temperature.

The last glacial period was characterized by large, rapid climate fluctuations. An analysis of a speleothem from New Mexico shows that the coldest conditions over Greenland coincide with increased winter precipitation in the southwestern United States, which can be attributed to a southward displacement of the polar jet stream and the North American storm track.