Glossary

The development of modern society has already triggered a global temperature rise of one degree Celsius compared to the pre-industrial age. This is a man-made, i.e. "anthropogenic" effect caused by the release of greenhouse gases such as CO₂. If the temperature continues to rise, global warming is likely to reach 1.5 degrees between 2030 and 2052. This increase alone poses considerable risks to natural and social systems. If the earth warms by an average of two degrees, the consequences will be much more serious, so that the need for adaptation and the social conflicts caused by climate change will be less severe in most cases if global warming reaches 1.5 degrees. In the Paris Agreement to reduce greenhouse gas emissions, which entered into force as a binding international treaty on 4 November 2016, the signatory states set themselves the ambitious goal of limiting global warming to 1.5 degrees in Article 2.1a. The Paris Agreement will replace the Kyoto Protocol in 2020.

Method for measuring the height of water surfaces: With the help of a radar or laser, the height of the sea surface or a lake level, for example, can be measured from a satellite. For this purpose, short-wave radio or laser pulses are emitted vertically downwards, reflected at the (sea) surface and the signal is received back at the satellite. These methods are known as radar or laser altimetry.

Water-bearing rock layer. A layer of rock or sediment that contains groundwater. Important: in German, the terminology is slightly different, please refer to the entry in German language for details.

Related terms: Aquitard = aquifer with low conductivity. Aquiclude (or aquifuge) = non-permissive rock layer.

The accuracy of high-precision atomic clocks depends, among other things, on the gravitational potential value at the place of observation. The effect responsible for this, known as relativistic red shift, is based on Einstein's general theory of relativity. By comparing the frequencies of two atomic clocks with a relative accuracy of 10^-18, differences in height can be derived with an accuracy of 1 cm, as long as it can be assumed that both clocks at the same location would provide a matching result.

In contrast to weather, which refers to daily or very short-term events, climate refers to an average condition in the atmosphere over a longer period of 30 to 40 years. All processes such as average temperature, precipitation, wind direction, wind strength and solar radiation are observed. The sum of the measured data can be used to determine typical seasonal fluctuations, as well as deviations from the usual average values. However, the climate is also influenced by other factors such as the altitude of a location or the proximity of the sea (marine climate versus continental climate).

Climate models are computer-aided tools that simulate the climate system by solving the basic physical equations describing the climate system on supercomputers. The resulting computer models are usually used to analyse data, understand climate processes and make predictions. Over time, these models have evolved from simple atmosphere-ocean applications to complex simulators of the Earth system, known as global climate models (Reklim.de).

Climate models often work with scenarios in order to calculate climate changes and predict the effects of currently observable developments on the future - for example through a constant release, increase or reduction of greenhouse gases. Climate models are important for numerous areas of the economy and society. For example, the insurance industry uses regional and global climate models to develop loss scenarios for the future. One question, for example, is: How does the annual loss expectation for floods change under a changing climate?

Numerical modelling experiments on the long-term development of climatic conditions on Earth. While the weather conditions are primarily determined by the initial conditions (air pressure systems over the North Atlantic determine the weather in Europe a few days later), the climatic conditions are primarily dependent on the boundary conditions (the concentration of greenhouse gases in the atmosphere determines the planet's energy balance and thus the long-term temperature development). Predictions of future developments based on initial conditions are referred to as forecasts in meteorology, while long-term forecasts based on changed boundary conditions are referred to as climate projections for better differentiation.

The cryosphere is all parts of a climate system that are covered with ice. The Earth's cryosphere includes glaciers, polar ice, ice sheets, sea ice and snow. The cryosphere is an important indicator of climate change and contributes significantly to the Earth's energy balance. For example, if the Antarctic ice sheet recedes, the sea level or the salt content of the sea changes. The Earth's ability to reflect the sun's rays also decreases when ice and snow melt. As a result, air, water and soil warm up more quickly.

The Earth's gravitational field is made up of the Earth's gravitational pull and the latitude-dependent centrifugal acceleration caused by the Earth's rotation. The matter in and on the Earth is not evenly distributed. Water, loose sediments (such as sand dunes) or magmatic rocks (such as rock formations) have very different densities and therefore generate stronger or weaker gravitational forces. In addition, mass redistributions take place on the Earth's surface, caused for example by the global water cycle or the melting of ice masses. The gravitational field therefore varies both spatially and temporally. This variable gravitational pull means that the physical shape of the Earth is not a perfect rotating sphere, nor a "flattened sphere" (rotational ellipsoid), but a "potato-like structure" (geoid) with indentations and bulges that change over time. Minimal changes in the geoid can be precisely measured from space using satellite gravimetry.

see also Satellite gravimetry

El Nino is a recurring weather phenomenon in the Pacific. It is an interplay between the atmosphere and the ocean. El Nino is regarded as an example of large-scale climate fluctuations. In the case of El Nino, it is an oscillation between two distinct states in which the circulation in the atmosphere changes fundamentally for a certain period of time. However, the climate does not tilt, as is often assumed. Instead, the air pressure and wind pattern between South America and Indonesia changes at irregular intervals of three to eight years. El Nino in 1982/83 led to flooding in the deserts of Peru and at the same time to enormous drought in Australia. La Nina is the counterpart with increased drought on the coasts of South America and above-average rainfall in eastern Australia and south-east Asia.

An "Essential Climate Variable" (ECV) is a physical, chemical or biological variable or a group of related variables that significantly contribute to the characterization of the Earth's climate system. With the ECVs the World Meteorological Organisation defines environmental variables that are of particular importance for monitoring global change and which should be observed with higher priority. Total water storage (TWS) has been essential climate variable since 2022.

Floods are recurring natural events caused by a complex interplay of meteorological and hydrological processes. Floods are defined as the state of a body of water in which the water level or discharge (or both) is significantly higher than the normal water level. This does not necessarily result in flooding. In waters with ebb and flow (tides), high tide refers to the highest water level of a tide at the transition from high tide to low tide. A distinction is made between regularly recurring floods (tides, spring floods) and irregular or one-off events (tsunami, storm surges, "flood of the century" on a river).

The geoid is an irregularly shaped reference surface in the Earth's gravitational field, at which all points experience the same gravitational potential (equipotential surface). It is therefore a theoretical reference surface that is represented to a good approximation by the mean sea level of the world's oceans and is therefore directly accessible outside the land masses. The geoid is used to define heights and to measure and describe the shape of the earth. As the geoid is highly irregularly deformed, a model of the geoid calculated at the GFZ is also known as the "Potsdam potato".

Relief movement of the earth's crust and upper mantle in regions of the earth formerly covered by continental ice sheets (e.g. Scandinavia and North America) that continues to this day. This is due to the viscosity of the material in the upper mantle, which is only slowly adapting to the greatly changed pressure conditions following the strong melting since the last glacial maximum 20,000 years ago.

Glaciology is the science of the glaciers. However, it also deals with other forms of ice that occur in the environment, such as snow, permafrost or the inland ice and ice shelves of the Arctic and Antarctic regions.

Glaciology is closely related to hydrology, geography, meteorology and polar research.

Glaciology is particularly relevant for the research of climate change, (ant-)arctic ecosystems and historical weather conditions (palaeoclimatology).

Generic term for a class of satellite positioning systems that are operated by different countries or groups of countries. These include GPS of the USA, Galileo of the European Union, the Russian system GLONASS, the Chinese solution BEIDOU, as well as other regional satellite positioning services of other countries.

Satellite positioning system of the US military that is also available to civilian users. With the help of nominally 24 satellites in three different orbital planes, the 3D position of a receiver can be determined with centimetre accuracy at any point on the Earth by measuring the time of flight to at least four satellites. Similar positioning accuracies are also achieved in low-Earth orbits with special satellite receivers, which are installed on the GRACE satellites, among others.

see also Global Navigation Satellite Systems (GNSS)

Gravity is also known as the force of gravity or mass attraction. Gravity is the force that two or more bodies exert on each other due to their mass. The best-known gravitational force is the Earth's gravitational pull. It causes bodies on earth to fall "downwards", i.e. towards the centre of the Earth. In the solar system, the gravitational force meets the centrifugal force of planets. These forces cancel each other out at the centre of mass of the celestial bodies. As a result, the planets move in stable orbits around the sun.

A gravity field model describes the spatial distribution of the Earth's gravitational pull in a mathematically compact form for a specific time epoch. The representation of the gravitational potential for a point in the outer space of the Earth (outside the Earth's masses including the atmosphere) in spherical coordinates (longitude, latitude, altitude) is described by an infinite series of (completely normalised) spherical surface functions of degree l and order m. The gravitational potential is a direct measure of how much the gravitational potential of a point in the outer space of the Earth (outside the Earth's masses including the atmosphere) is proportional to the gravitational potential of the Earth.

The gravitational potential is a direct measure of how much energy a body can gain in free fall due to the gravitational influence of the surrounding masses. If a body of mass m moves in free fall from A to B, the gain in kinetic energy is equal to its mass multiplied by the difference in gravitational potentials in A and B.

German-American satellite missions to monitor temporal changes in mass distribution in the Earth system. Each mission consists of two identical satellites whose relative positions in space are continuously measured. The first GRACE mission was launched in 2002 and provided data until 2017. Observations have been continued with GRACE-FO since 2018. After the nominal mission duration of GRACE-FO expired in 2023, the mission is currently in the "Extended Mission Phase", which could allow observations for several more years under optimal conditions. For the long-term continuation of the observation programme, the GRACE-C mission is currently being prepared for a launch in 2028.

Ground track refers to the vertical projection of a satellite's trajectory onto the surface of the orbiting celestial body such as the Earth, i.e. the imaginary track that the satellite draws on the ground beneath it. The GRACE satellites fly almost exactly over the two poles (the inclination of their orbital plane relative to the equator is 89°) in approx. 94 minutes on an almost circular path at an altitude of around 500 km. Due to the Earth's rotation, this results in 15–16 ground tracks over the course of a day, with a distance of around 1300 km at the equator and only a few kilometres at the poles.

see aquifere

The science of the water cycle. Hydrology analyses the temporal and spatial distribution of water, its properties and its interactions with the environment. The following sciences are related disciplines or sub-disciplines of hydrology, depending on the definition: hydrogeology, hydrometeorology, hydrometry, hydrography, limnology, hydrology, oceanology, glaciology. Hydrology is in close dialogue with the disciplines of geology, soil science (pedology), meteorology, geography and many other geosciences.

Hydrology is relevant for water management, water supply, energy production, forecasting and the management of hydrological extremes such as floods, heavy rainfall and droughts.

Continental ice masses with a large horizontal extent (several thousand kilometres) and thickness (several thousand metres). There are currently ice sheets on Earth in Greenland and Antarctica. At the time of the last glacial maximum around 20,000 years ago, large parts of North America and Scandinavia were also covered by ice sheets.

The Intergovernmental Panel on Climate Change - IPCC for short - is an intergovernmental committee on climate change. The IPCC is also known as the Intergovernmental Panel on Climate Change. It is a scientific body that collates the current status of climate change and thus provides political decision-makers with clear guidance for their decisions. By scientifically analysing the opportunities and risks arising from the results of climate research, strategies for reducing climate change or greenhouse gas emissions can be derived.

In Germany, the German IPCC Coordination Centre was set up at the Project Management Agency of the German Aerospace Center (DLR Project Management Agency) in 1998 on behalf of the Federal Ministry for the Environment, Nature Conservation, Building and Nuclear Safety (BMUB) and the Federal Ministry of Education and Research (BMBF). One of its tasks is to facilitate the transfer of knowledge between climate research and climate policy and to strengthen the contribution of German climate science to the IPCC.

The K-Band Ranging System (KBR) system, also known as the Microwave Instrument (MWI), is the key science instrument of GRACE and GRACE-FO. It provides precise 1 micron (about the diameter of a blood cell) measurements of the distance change between the two satellites – and, in turn, fluctuations in Earth’s gravity – by measuring microwave signals sent between the two satellites. Each satellite transmits signals to the other at two frequencies – 24 gigahertz (K-band) and 32 gigahertz – (Ka-band), allowing for ionospheric corrections.

The "Laser Ranging Interferometer" (LRI) measures changes in distance between the two GRACE-FO satellites. Laser interferometry refers to all measurement methods that utilise the superposition or interference of laser waves to perform measurements in the accuracy range of fractions of the wavelength of visible light.

As a result, LRI achieves an accuracy in the nanometre range and is several orders of magnitude more precise than conventional microwave-based distance measurement methods. Following the successful technology demonstration on GRACE-FO, the LRI is to be further developed for future missions to become the primary distance measurement instrument for future gravity field missions such as GRACE-I.

The current satellite laser radar station Potsdam (SLR - Satellite Laser Ranging) has been operating continuously within the worldwide ILRS network (ILRS - International Laser Ranging Service) since January 2003. The SLR method was introduced in 1964 and is still one of the most accurate space-based geodetic techniques, whereby the measurement accuracy has been continuously improved. The observations are made between the ground stations and laser retroreflectors on the satellites. They are used to determine the satellite orbit (satellite positions and velocities) as well as to monitor Earth rotation parameters (polar motion and day length) and the three-dimensional deformations of the solid Earth (station coordinates and velocities). SLR observations also help to determine the long-wave coefficients of the Earth's gravitational field.

A laser retroreflector is a passive optical mirror system that reflects incident laser beams back to the light source. The reflections from laser reflectors are primarily used for time-of-flight and distance measurement, especially to satellites. A network of globally distributed ground stations is used for this "Satellite Laser Ranging (SLR)", one of which is located at the GFZ in Potsdam. The laser reflectors are usually made up of several small triple mirrors - optical prisms that reflect each incident beam into themselves. They have a similar principle to the cat's eye reflectors used in traffic engineering, but are ground much more precisely (to an accuracy of a few arc seconds or better) in order to maintain the tight focussing of the laser beam.

Globally defined grid data in two dimensions (longitude and latitude) of the mass anomalies determined from the data of the GRACE missions. For this purpose, all individual sensor data streams are first processed separately with high temporal resolution (Level-1), then all data of one month are merged into a global gravity field model (Level-2) and this is then broken down into individual grid data sets of mass anomalies (Level-3). The GFZ's Gravis portal offers special grid data sets for the continental terrestrial water storage (TWS), ice mass changes in Greenland and Antarctica as well as barystatic sea level changes.

A gravity anomaly is the local deviation of the gravitational acceleration from the theoretical normal value on a reference surface. In the case of the Earth, this is usually a reference ellipsoid. The gravitational acceleration is a vectorial quantity and is calculated from the scalar gravitational potential by gradient formation in all three spatial directions. Measured gravity anomalies can be used to infer the underlying mass anomalies.

See also gravity field modelling

In geography, a meridian is a semicircle on the Earth's surface running from the North Pole to the South Pole. 

New types of satellite missions are regularly developed with the help of purely virtual and highly modularised test environments. Taking detailed satellite models into account, a wide range of interactions between various sensors, satellite systems and the space environment in different orbits can be realistically simulated in order to optimise the quality of scientific data from future gravity field missions.

The ocean bottom pressure is the combined pressure caused by the hydrostatic effect of the seawater column and the overlying atmosphere. Temporal changes in ocean bottom pressure and their spatial gradients are indicators of numerous dynamic processes in the ocean (such as tides, wind-driven circulation and, in narrowly defined regions on the continental slope, meridional overturning circulation).

The German meteorologist and polar researcher Alfred Wegener formulated his theory of continental drift in 1912. Plate tectonics, i.e. the movement of different earth plates that either move towards each other (collision or subduction), away from each other (divergence) or past each other (transform displacement), could only be explained years later.

The rock material of the earth's crust (the outermost layer of the earth) is heated as it sinks deeper into the earth's interior in the upper mantle (the layer of earth that underlies the crust). This reduces the rock density and hot, (viscous) liquid rock rises to the surface, where it then cools down again and sinks. This process of heating, rising, cooling and sinking is known as convection. The movement of continental plates is caused by convection processes. Earthquakes, tsunamis and volcanic activity occur repeatedly along plate boundaries.

Method for measuring the Earth's gravitational field using satellites. Over the past 20 years, various modern variants have been realised: (1) Time-of-flight measurements between high-flying GPS satellites and a low-flying satellite (high-low satellite-to-satellite tracking). First successfully realised in 2000 with GFZ's CHAMP mission. (2) Time-of-flight measurements between two low-flying satellites (low-low satellite-to-satellite tracking). First successfully realised with the German-American GRACE mission in 2002. (3) With a triaxial gradiometer in the centre of mass of a low-flying satellite (satellite gradiometry) with active compensation of non-gravitational disturbance forces. Realised for the first time in 2009 by the ESA mission GOCE with the participation of the GFZ as part of the scientific analysis of the mission (High-Level Processing Facility). Before the launch of these modern gravity field satellite missions CHAMP, GRACE and GOCE, SLR measurements to passive satellites such as LAGEOS (since 1976) were primarily used to determine the Earth´s gravity field. The first experimental SLR observations on Telegraphenberg took place in 1974.

Since 2001, the GFZ has been operating a satellite receiving station on Spitsbergen (78° 55' North, 11° 56' East) to receive data from research satellites in polar orbits. The station is located about one kilometre outside the town of Ny-Ålesund and is only about 1,200 km from the North Pole.

Sea ice is the ice that forms when seawater freezes. Sea ice does not include ice from glaciers and ice sheets that is permanently on land, nor ice masses that break off from it, i.e. icebergs. The observation of sea ice in the Arctic is important for researchers because sea ice is considered a critical element in climate change and an early warning system for global warming.

Consideration of temporal and spatial uncertainties in various information components required for the calculation of gravity field models from GRACE satellite data. These uncertainties include systematic errors of individual sensors (such as temperature dependencies), effects of the satellite platform on the sensors (occasional activity of manoeuvring thrusters), external influences such as solar activity (increased non-gravitational disturbance forces) and errors in geophysical background models (oceanic tides in the Antarctic coastal regions are e.g. much less well known than in the open ocean). The mathematical description of temporally and spatially variable errors, including their correlations via error covariance matrices, allows the optimal combination of different data sources for the calculation of gravity fields and mass distributions.

The total amount of water present on the Earth's land masses is referred to as Terrestrial Water Storage (TWS, often also: Total Water Storage). TWS mainly comprises water storage in snow, ice (in glaciers and ice sheets), surface water (in rivers, lakes, wetlands and man-made reservoirs), soil water in the saturated and unsaturated zones, and groundwater.

Strictly speaking, water on the surfaces of plants or short-term surface storage in depressions etc. also contributes to TWS. However, these are negligible and typically very short-lived (a few hours to a few days after a precipitation event) and are usually ignored.

The total amount of water in the Earth’s system is almost constant over time. However, the water stored in the oceans and the atmosphere is not included in TWS. Accordingly, long-term changes in the water cycle can be estimated by the temporal variations in the size of TWS, which in turn can be monitored with the help of the GRACE satellites.

The water cycle describes the transport of water from the oceans, through the atmosphere, onto land and from there back into the oceans. During this journey, the water changes its physical state several times, i.e. it moves as liquid water, as water vapour or as ice. The water cycle is a closed system, i.e. the earth does not lose any water.

The main processes involved in the water cycle are evaporation (also known as evapotranspiration), precipitation, surface runoff in streams and rivers, seepage, underground transport through soil and groundwater, and storage, e.g. in glaciers, snow, lakes or aquifers.

To a certain extent, water is also exchanged with the earth's interior, where it is transported by the subduction of continental plates and extracted again by volcanic activity.