Reviews of Geophysics

Geophysical applications of radar interferometry to measure changes in the Earth's surface have exploded in the early 1990s. This new geodetic technique calculates the interference pattern caused by the difference in phase between two images acquired by a spaceborne synthetic aperture radar at two distinct times. The resulting interferogram is a contour map of the change in distance between the ground and the radar instrument. These maps provide an unsurpassed spatial sampling density (∼100 pixels km⁻²), a competitive precision (∼1 cm), and a useful observation cadence (1 pass month⁻¹). They record movements in the crust, perturbations in the atmosphere, dielectric modifications in the soil, and relief in the topography. They are also sensitive to technical effects, such as relative variations in the radar's trajectory or variations in its frequency standard. We describe how all these phenomena contribute to an interferogram. Then a practical summary explains the techniques for calculating and manipulating interferograms from various radar instruments, including the four satellites currently in orbit: ERS‐1, ERS‐2, JERS‐1, and RADARSAT. The next chapter suggests some guidelines for interpreting an interferogram as a geophysical measurement: respecting the limits of the technique, assessing its uncertainty, recognizing artifacts, and discriminating different types of signal. We then review the geophysical applications published to date, most of which study deformation related to earthquakes, volcanoes, and glaciers using ERS‐1 data. We also show examples of monitoring natural hazards and environmental alterations related to landslides, subsidence, and agriculture. In addition, we consider subtler geophysical signals such as postseismic relaxation, tidal loading of coastal areas, and interseismic strain accumulation. We conclude with our perspectives on the future of radar interferometry. The objective of the review is for the reader to develop the physical understanding necessary to calculate an interferogram and the geophysical intuition necessary to interpret it.

Details of Earth's shallow subsurface—a key component of the critical zone (CZ)—are largely obscured because making direct observations with sufficient density to capture natural characteristic spatial variability in physical properties is difficult. Yet this inaccessible region of the CZ is fundamental to processes that support ecosystems, society, and the environment. Geophysical methods provide a means for remotely examining CZ form and function over length scales that span centimeters to kilometers. Here we present a review highlighting the application of geophysical methods to CZ science research questions. In particular, we consider the application of geophysical methods to map the geometry of structural features such as regolith thickness, lithological boundaries, permafrost extent, snow thickness, or shallow root zones. Combined with knowledge of structure, we discuss how geophysical observations are used to understand CZ processes. Fluxes between snow, surface water, and groundwater affect weathering, groundwater resources, and chemical and nutrient exports to rivers. The exchange of gas between soil and the atmosphere have been studied using geophysical methods in wetland areas. Indirect geophysical methods are a natural and necessary complement to direct observations obtained by drilling or field mapping. Direct measurements should be used to calibrate geophysical estimates, which can then be used to extrapolate interpretations over larger areas or to monitor changing processes over time. Advances in geophysical instrumentation and computational approaches for integrating different types of data have great potential to fill gaps in our understanding of the shallow subsurface portion of the CZ and should be integrated where possible in future CZ research.

Studies of the last 125 million years of oceanographic and climatic history have benefited greatly from the impetus provided by the Deep Sea Drilling Project. Knowledge of the sedimentary and paleontologic record of the major ocean basins, in conjunction with study of pelagic marine sections exposed on land, has permitted both the testing of old and the development of new hypotheses to explain local and global ocean chemical, sedimentologic and biotic events. Some of the more striking and topical problems in paleoceanography are the oceanic “anoxic events” of early to middle Cretaceous age, the biotic crisis at the Cretaceous/Tertiary boundary, the Eocene/Oligocene extinctions and climatic and circulation events, the Messinian “salinity crisis” (late Miocene) and its effects on the world ocean, and Pleistocene glacial cycles and paleoceanography. Possible explanations of these events, which have been proposed over the last five years, are reviewed in this paper.

Application of new concepts and techniques, especially in micropaleontology and geochemistry, has led to refinements in stratigraphic resolution and in recognition of paleoenvironmental signals. Among the most powerful tools now in use is stable isotope geochemistry. Paleomagnetic studies and statistical techniques for processing micropaleontological data have contributed greatly to stratigraphic resolution.

Although we are beginning to unravel the complex interactions of global sea level changes, climate and ocean chemistry and their influence on life, there are innumerable challenging problems still remaining. As stratigraphic resolution and coverage improve so does our perception of the ocean system.

A new, speculative, and, we hope, provocative summary of the North Atlantic circulation is described, including both horizontal currents (wind‐driven) and the primarily (thermohaline) meridional flows that involve the transformation of warm to cold water at high latitudes. Our picture is based on a synthesis of a variety of independent investigations that are contained in the literature as opposed to a presentation of the results of one technique or the point of view of one author. We describe a thermohaline cell (the so‐called thermohaline conveyor belt) that is concentrated within the Atlantic and Southern oceans (rather than essentially global), with the most important upwelling sites being in the circumpolar and the equatorial current regimes. We concentrate on deep water formation and its replacement relative to intermediate‐water formation. It has been pointed out recently that the formation of 13 Sv (1 Sv = 10⁶m³ s⁻¹) of southward flowing North Atlantic Deep Water is compensated for in the upper ocean by northward cross‐equatorial transport. We suggest that this thermocline layer flow passes through the Straits of Florida, transits the Gulf Stream system on its inshore side, and exits through the North Atlantic Current system after recirculation and modification. There is now a clear observational basis for the structure of recirculating gyres on the southern and northern sides of the Gulf Stream. We suggest a recirculation for the North Atlantic Current as well. We also describe a C‐shaped component to the southern Gulf Stream recirculation and identify a roughly 10‐Sv circulation in the eastern North Atlantic associated with the Azores Current. Recirculations play an important role in deep boundary current regimes and in water mass formation and modification. The transport of the deep western and northern boundary currents in the North Atlantic Ocean may be boosted (roughly doubled or tripled) by counterclockwise recirculating gyres and by additions of modified bottom or intermediate water. While the North Atlantic is the most completely observed ocean, there are still significant gaps in our knowledge of its circulation.

Our understanding of the global dust cycle is limited by a dearth of information about dust sources, especially small‐scale features which could account for a large fraction of global emissions. Here we present a global‐scale high‐resolution (0.1°) mapping of sources based on Moderate Resolution Imaging Spectroradiometer (MODIS) Deep Blue estimates of dust optical depth in conjunction with other data sets including land use. We ascribe dust sources to natural and anthropogenic (primarily agricultural) origins, calculate their respective contributions to emissions, and extensively compare these products against literature. Natural dust sources globally account for 75% of emissions; anthropogenic sources account for 25%. North Africa accounts for 55% of global dust emissions with only 8% being anthropogenic, mostly from the Sahel. Elsewhere, anthropogenic dust emissions can be much higher (75% in Australia). Hydrologic dust sources (e.g., ephemeral water bodies) account for 31% worldwide; 15% of them are natural while 85% are anthropogenic. Globally, 20% of emissions are from vegetated surfaces, primarily desert shrublands and agricultural lands. Since anthropogenic dust sources are associated with land use and ephemeral water bodies, both in turn linked to the hydrological cycle, their emissions are affected by climate variability. Such changes in dust emissions can impact climate, air quality, and human health. Improved dust emission estimates will require a better mapping of threshold wind velocities, vegetation dynamics, and surface conditions (soil moisture and land use) especially in the sensitive regions identified here, as well as improved ability to address small‐scale convective processes producing dust via cold pool (haboob) events frequent in monsoon regimes.

We use the Total Ozone Mapping Spectrometer (TOMS) sensor on the Nimbus 7 satellite to map the global distribution of major atmospheric dust sources with the goal of identifying common environmental characteristics. The largest and most persistent sources are located in the Northern Hemisphere, mainly in a broad “dust belt” that extends from the west coast of North Africa, over the Middle East, Central and South Asia, to China. There is remarkably little large‐scale dust activity outside this region. In particular, the Southern Hemisphere is devoid of major dust activity. Dust sources, regardless of size or strength, can usually be associated with topographical lows located in arid regions with annual rainfall under 200–250 mm. Although the source regions themselves are arid or hyperarid, the action of water is evident from the presence of ephemeral streams, rivers, lakes, and playas. Most major sources have been intermittently flooded through the Quaternary as evidenced by deep alluvial deposits. Many sources are associated with areas where human impacts are well documented, e.g., the Caspian and Aral Seas, Tigris‐Euphrates River Basin, southwestern North America, and the loess lands in China. Nonetheless, the largest and most active sources are located in truly remote areas where there is little or no human activity. Thus, on a global scale, dust mobilization appears to be dominated by natural sources. Dust activity is extremely sensitive to many environmental parameters. The identification of major sources will enable us to focus on critical regions and to characterize emission rates in response to environmental conditions. With such knowledge we will be better able to improve global dust models and to assess the effects of climate change on emissions in the future. It will also facilitate the interpretation of the paleoclimate record based on dust contained in ocean sediments and ice cores.

The Shuttle Radar Topography Mission produced the most complete, highest‐resolution digital elevation model of the Earth. The project was a joint endeavor of NASA, the National Geospatial‐Intelligence Agency, and the German and Italian Space Agencies and flew in February 2000. It used dual radar antennas to acquire interferometric radar data, processed to digital topographic data at 1 arc sec resolution. Details of the development, flight operations, data processing, and products are provided for users of this revolutionary data set.

In 1995, two groups [Kageyama et al., 1995; Glatzmaier and Roberts, 1995a, 1995b] reported results of numerical integrations of fully three‐dimensional, fully nonlinear dynamos. Their papers were precursors of a stream of such models that have focused particularly on the geodynamo. They provide us, in unprecedented detail, with spectacular realizations of interesting geomagnetic field behaviors, such as secular variation and even polarity reversals. The proliferation of models has, however, created some confusion and apparently conflicting results. This can be partly attributed to the different ways in which different groups have modeled the core, normalized their equations, defined their dimensionless parameters, chosen their boundary conditions, and selected their energy sources. This has made it difficult to compare the results of different simulations directly. In this paper, we first try, as far as possible, to overcome this difficulty, so that all reported results can be compared on common ground. We then review the results, emphasizing three major topics: (1) onset and evolution of convection, (2) character of the magnetic field generated, and (3) comparison with the observed geomagnetic field. Although there are large differences in the way that the simulations are defined, the magnetic fields that they generate have some surprising similarities. The fields are dominated by the axial dipole. In some models they are most strongly generated in shear layers near the upper and lower boundaries and near the tangent cylinder, an imaginary surface touching the inner core on its equator. Convection rolls occur within which a type of the α effect distorts the toroidal field lines to create poloidal magnetic field. Some features of the models are found to strongly affect the fields that they produce. In particular, the boundary conditions defining the energy flow (e.g., an inhomogeneous heat flux or distribution of buoyancy sources) are very influential and have been extensively studied. They change the frequency and the mode of magnetic polarity reversals as well as the ratio in strengths of the dipole and nondipole moments. As the ultimate goal of geodynamo simulations is to explain the features of the real geomagnetic field, it is essential that proper comparisons be made between simulation results and observations. It is remarkable that polarity reversals reminiscent of the paleomagnetically observed field reversals have already been simulated by some of the models. Other features such as drift of the field, its secular variation, and statistical properties of Gauss coefficients are discussed in this paper and are compared with observations. These comparisons are rather primitive, not only because self‐consistent dynamo models are still too new and too few but also because many of the observations (and especially the paleomagnetic data) are themselves not yet reliable or decisive enough. The aim of the third part of this paper is therefore more to demonstrate the potential use of simulations than to elucidate the nature of geomagnetic field generation.

The soil water density is defined as the ratio of soil water mass to soil water volume. It is a cornerstone in defining thermodynamic states of either saturated or unsaturated soils for quantifying water storage and movement in the subsurface and for mechanical stability of landscape. So far, it has been widely treated as identical to the free water density, that is, a constant of 0.997 g/cm³, but can be remarkably different from this value as it is subject to a wide range of variation in energy levels. Some experimental and theoretical evidence indicate that it can be as high as 1.680 g/cm³ and as low as 0.752 g/cm³. However, to date, there is no unanimous agreement upon a reliable experimental method to measure the soil water density or a unified theory to explain why and how the soil water density can deviate remarkably from the free water density. Consequently, the understanding of the soil water density is controversial and elusive, or some theories are contradictory to each other. In this review, the authors will (1) conduct critical reviews on the experimental and theoretical methodologies to identify their limitations, flaws, and uncertainties, (2) synthesize some recent findings on intermolecular forces, interfacial interactions, and soil water retention mechanisms to clarify molecular‐scale physicochemical mechanisms governing the soil water density, and (3) propose a unified model to quantify soil water density variation. It is found that capillarity associated with surface tension tends to generate tensile stress in soil water and thereby decreases the soil water density, whereas adsorption stemmed from cation hydration, surface hydration, and interlamellar cation hydration tends to produce compressive stress thus increases the soil water density. Furthermore, the abnormally high water density greater than 1.15 g/cm³ is a result of cation and surface hydration that involves significant water structure change around exchangeable cations and mineral surface hydroxyls. The unified soil water density model, explicitly quantifying adsorptive and capillary water, could potentially reconcile the unresolved controversies. The critical reviews and the unified model also would allow us to further confine the upper and lower bounds of the soil water density. The upper bound is theoretically inferred to be around 1.872 g/cm³, whereas the lower bound is around 0.995 g/cm³; both are higher than that reported in the literature. With the unified model and measured soil water retention curves, it is demonstrated quantitatively that the soil water density significantly impacts the magnitude of various fundamental soil properties such as matric potential, specific surface area, and volumetric water content. The abnormally high soil water density has significant implications to the conventional concepts of matric potential and pore water pressure in soils and other earthen porous materials.

A review of theoretical and observational results describing atmospheric gravity wave (AGW)/traveling ionospheric disturbance (TID) phenomena at high latitudes is presented. Some recent experimental studies of AGW's using the Chatanika incoherent scatter radar and other geophysical sensors are reported. Specifically, the following features are described in detail: (1) cause/effect relations between aurorally generated AGW's and TID's detected at mid‐latitudes, including probable ‘source signature’ identification, (2) AGW source phenomenology, particularly a semiquantitative assessment of the relative importance of Joule heating, Lorentz forces, intense particle precipitation, and other mechanisms in generating AGW's, and (3) detection of TID's in the auroral ionosphere. Several instances of F region electron density, temperature, and plasma periodicities accompanied by horizontal plasma velocities which were consistent with theoretical AGW/TID models are documented.