The Earth’s cryosphere, encompassing its glaciers, ice sheets, and sea ice, is a fundamental component of the global climate system. Accurately monitoring this frozen water is a core objective of modern environmental science and hydrology. Measurements of ice are systematically collected to track changes in the planet’s energy balance and predict future sea level rise. Monitoring involves determining the vertical dimension (thickness), the horizontal spread (area), and the total quantity of ice (mass).
Measuring Ice Thickness and Depth
Determining the vertical dimension of large ice formations requires a combination of direct, local sampling and advanced remote sensing technology. For local measurements, scientists utilize Ground-Penetrating Radar (GPR), which is towed across the ice surface, often by snowmobiles or aircraft. The GPR system emits a radio wave pulse into the ice and records the time it takes for the signal to reflect back from the boundary between the ice and the underlying bedrock or ocean water.
This non-destructive method provides continuous, high-resolution profiles of the ice thickness along a specific transect. Traditional ice cores, which involve drilling and extracting a physical cylinder of ice, remain a necessary tool for ground-truthing GPR data and calibrating the radar’s signal velocity within the ice.
For wide-scale monitoring of ice elevation, satellites carrying altimeters provide regular measurements of the ice surface height. Missions such as ICESat-2 and CryoSat-2 use laser and radar altimetry to send pulses toward the surface and calculate the distance based on the return time. Laser altimeters offer high precision, while radar altimeters can penetrate clouds and cover a wider area, although with less precision. By tracking changes in the ice surface height, scientists can infer corresponding changes in ice thickness over large regions like the Greenland and Antarctic ice sheets.
Determining Ice Extent and Area
The horizontal spread of ice, known as its extent or area, is primarily measured through satellite-based remote sensing. For sea ice, passive microwave sensors are the workhorse technology, continuously monitoring the polar regions since 1979. These sensors detect the natural microwave energy emitted from the Earth’s surface, which allows them to distinguish between open water and ice.
A significant advantage of passive microwave sensors is their ability to see through cloud cover and measure ice coverage during the months of polar darkness. Scientists typically report sea ice extent, which is defined as the total area of ocean where the ice concentration is at least 15 percent. Sea ice area, a different metric, refers to the actual portion of the ocean surface covered by ice, which is a lower value than the extent.
Optical satellite imagery also contributes to area determination, providing visual confirmation of ice boundaries and melt ponds. Additionally, Synthetic Aperture Radar (SAR) is used, generating high-resolution images by transmitting its own microwave energy and measuring the reflected signal. The combination of these remote sensing methods allows researchers to track the geographical footprint of ice bodies across the globe with high temporal resolution.
Calculating Total Ice Volume and Mass
The goal of these measurements is to determine the total ice volume and mass. Volume is calculated by combining the horizontal extent and the vertical thickness measurements, often summarized by the basic formula: Volume equals Area multiplied by Thickness. In practice, this calculation is highly complex, as ice thickness varies significantly across the ice body, requiring advanced interpolation techniques to map the spatial distribution of the thickness data.
Once the total ice volume is established, scientists must convert this figure into mass, which is the most environmentally relevant metric for climate studies. This conversion is achieved by multiplying the volume by the density of the ice. The density is not uniform; it varies depending on whether the ice is fresh ice, multiyear sea ice, or includes layers of snow and firn (partially compacted snow).
Tracking changes in ice mass is important because it directly relates to global sea level change. Scientists estimate that the addition of approximately 360 gigatons of meltwater from ice sheets and glaciers raises the global mean sea level by one millimeter. Therefore, the calculated mass change provides a measure of the cryosphere’s contribution to rising oceans.