GEOPHYSICS: Atmosphere Drives Earth's Tipsiness

For more than a century, geophysicists who track Earth's rotation have sensed a rhythmic unsteadiness about the planet, an ever-so-slight wobbling whose source remained frustratingly mysterious. But researchers have been homing in on the roots of the so-called Chandler wobble, and now a report in the 1 August issue of Geophysical Research Letters fingers the shifting pressures of the deep sea and ultimately the fickle winds of the atmosphere.     

GEOPHYSICS: Enhanced: A Strained Earth, Past and Present

Plate tectonics leads to deformation within converging or transforming plates, but the rates at which this happens are difficult to evaluate. In this Perspective, Ramsay highlights a new technique reported by Müller et al. for overcoming this problem in past and present mountain-building regions, which have particularly complex deformation patterns.     

GEOPHYSICS: Earth Under Strain

Large strains can build up in the slowly convecting mantle when it encounters less mobile regions, for example, at the bottom of rigid plates. In their Perspective, Mackwell and Rubie discuss new insights into the effect of this deformation on mantle rocks gained by Bystricky et al. Olivine, the major upper mantle material, is shown to decrease in strength without a change in deformation mechanism. Such a change was previously proposed to explain mantle processes.     

Earth's Viscosity

Seismic methods are now being used to determine not only Earth's elastic properties, but also by how much it departs from a perfectlyelastic body. The seismic anelasticity (Q) varies by several orders of magnitude throughout the mantle, the main feature being an extremely dissipative zone in the upper mantle above 400 kilometers. Recent determinations of viscosity by McConnell show a similar trend. The two sets of data indicate that the ratio of viscosity to Q is roughly a constant, at least in the upper mantle of Earth. On the assumption that this relation is valid for the rest of Earth, viscosities are estimated in regions that are inaccessible for direct measurement. The implied presence of a low-viscosity zone in the upper mantle, overlying a more viscous, less deformable, lower mantle, reconciles viscosites calculated from the shape of Earth and from postglacial uplift. The mismatch of the deformational characteristics at various levels in Earth, coupled with the changing rate of rotation, may be pertinent to the rate of release of seismic energy as a function of depth.     

Enigmatic biodiversity correlations: ant diversity responds to diverse resources

A pattern noted in ecology is that diversity at one level begets diversity at other levels. In the case of consumers competing for similar resources, the diversity of those resources is thought to provide some degree of niche diversification in which a diverse set of consumer species can coexist. If, however, the diverse resources are not sufficiently distinct from one another, from the standpoint of the consumer species, such niche diversification will not exist. We experimentally show that a diverse array of twigs attracted 80% more species of twig-nesting ants than a monospecific collection of twigs. The specific tree species from which the twigs were derived did not explain the pattern. It appears that diversity per se at one level (twigs) creates conditions that promote diversity at another level (nesting ants).     

GEOPHYSICS: Core Takes a Page From the Sea Floor

On page 1338, a group of geophysicists suggests that the mysterious boundary between Earth's molten iron core and its rocky mantle most resembles an inverted sea floor, with liquid-iron-laced sediments collecting on the roof of the core. They argue that a slow, inverted rain of precipitates rising to the core-mantle boundary and settling into a kilometers-thick layer might explain a variety of observations, from a subtle nodding of Earth's axis to seismic speed zones at the boundary. Their story will be difficult to verify, however, because painting a portrait of the core-mantle boundary depends on very indirect geophysical evidence.     

ERTS: Surveying Earth's Resources from Space

Several errors occurred in the Research News story "ERTS: Surveying earth's resources from space" (6 Apr. 1973) by Thomas H. Maugh II. In column 3, page 50, the statement, "It thus maps an area of about 6.5 km(2) every day," should read "6.5 x 10(6) km(2)." In Fig. 3, the land masses are identified as Manhattan and Staten Island; actually, the spit at the left of the figure is Sandy Hook, New Jersey, and the barrier beaches further north are Rockaway, Long Beach, and Jones Beach, from left to right. The negative for Fig. 4 was inadvertently flipped over so that the illustration is a mirror image of the actual picture. It has also since been brought to the author's attention that " the previously unsuspected difference in salinity" in the Great Salt Lake described in the caption of Fig. 2 was, in fact, well known before the ERTS picture was taken.     

Earth's energy imbalance: confirmation and implications

Our climate model, driven mainly by increasing human-made greenhouse gases and aerosols, among other forcings, calculates that Earth is now absorbing 0.85 +/- 0.15 watts per square meter more energy from the Sun than it is emitting to space. This imbalance is confirmed by precise measurements of increasing ocean heat content over the past 10 years. Implications include (i) the expectation of additional global warming of about 0.6 degrees C without further change of atmospheric composition; (ii) the confirmation of the climate system's lag in responding to forcings, implying the need for anticipatory actions to avoid any specified level of climate change; and (iii) the likelihood of acceleration of ice sheet disintegration and sea level rise.