Glacial isostatic adjustment: physical models and observational constraints

By far the most prescient insights into the interior structure of the planet have been provided on the basis of elastic wave seismology. Analysis of the travel times of shear or compression wave phases excited by individual earthquakes, or through analysis of the elastic gravitational free oscillations that individual earthquakes of sufficiently large magnitude may excite, has been the central focus of Earth physics research for more than a century. Unfortunately, data provide no information that is directly relevant to understanding the solid state 'flow' of the polycrystalline outer 'mantle' shell of the planet that is involved in the thermally driven convective circulation that is responsible for powering the 'drift' of the continents and which controls the rate of planetary cooling on long timescales. For this reason, there has been an increasing focus on the understanding of physical phenomenology that is unambiguously associated with mantle flow processes that are distinct from those directly associated with the convective circulation itself. This paper reviews the past many decades of work that has been invested in understanding the most important of such processes, namely that which has come to be referred to as 'glacial isostatic adjustment' (GIA). This process concerns the response of the planet to the loading and unloading of the high latitude continents by the massive accumulations of glacial ice that have occurred with almost metronomic regularity over the most recent million years of Earth history. Forced by the impact of gravitationaln-body effects on the geometry of Earth's orbit around the Sun through the impact upon the terrestrial regime of received solar insolation, these surface mass loads on the continents have left indelible records of their occurrence in the 'Earth system' consisting of the oceans, continents, and the great polar ice sheets on Greenland and Antarctica themselves. Although this ice-age phenomenology has been clearly recognized since early in the last century, it was for over 50 years considered to be no more than an interesting curiosity, the understanding of which remained on the periphery of the theoretical physics of the Earth. This was the case in part because no globally applicable theory was available that could be applied to rigorously interpret the observations. Equally important to understanding the scientific lethargy that held back the understanding of this phenomenon involving mantle flow processes was the lack of appreciation of the wide range of observations that were in fact related to GIA physics. This paper is devoted to a review of the global theories of the GIA process that have since been developed as a means of interpreting the extensive variety of observations that are now recognized as being involved in the response of the planet to the loading and unloading of its surface by glacial ice. The paper will also provide examples of the further analyses of Earth physics and climate related processes that applications of the modern theoretical structures have enabled.

GRACE, time-varying gravity, Earth system dynamics and climate change

Continuous observations of temporal variations in the Earth's gravity field have recently become available at an unprecedented resolution of a few hundreds of kilometers. The gravity field is a product of the Earth's mass distribution, and these data-provided by the satellites of the Gravity Recovery And Climate Experiment (GRACE)-can be used to study the exchange of mass both within the Earth and at its surface. Since the launch of the mission in 2002, GRACE data has evolved from being an experimental measurement needing validation from ground truth, to a respected tool for Earth scientists representing a fixed bound on the total change and is now an important tool to help unravel the complex dynamics of the Earth system and climate change. In this review, we present the mission concept and its theoretical background, discuss the data and give an overview of the major advances GRACE has provided in Earth science, with a focus on hydrology, solid Earth sciences, glaciology and oceanography.

Sea level: measuring the bounding surfaces of the ocean

The practical need to understand sea level along the coasts, such as for safe navigation given the spatially variable tides, has resulted in tide gauge observations having the distinction of being some of the longest instrumental ocean records. Archives of these records, along with geological constraints, have allowed us to identify the century-scale rise in global sea level. Additional data sources, particularly satellite altimetry missions, have helped us to better identify the rates and causes of sea-level rise and the mechanisms leading to spatial variability in the observed rates. Analysis of all of the data reveals the need for long-term and stable observation systems to assess accurately the regional changes as well as to improve our ability to estimate future changes in sea level. While information from many scientific disciplines is needed to understand sea-level change, this review focuses on contributions from geodesy and the role of the ocean's bounding surfaces: the sea surface and the Earth's crust.