Satellite images of Earth’s surface are the primary data source for remote sensing, which collects information about our planet without physical contact. This technology provides scientists with an unparalleled, continuous, and comprehensive view of global processes. Satellites house sophisticated sensors, including optical, infrared, and radar instruments, that measure reflected and emitted radiation from the land, oceans, and atmosphere. These orbital platforms capture changes across vast regions, ensuring consistent data collection over decades for establishing long-term trends.
Monitoring Earth’s Dynamic Climate Systems
Scientists rely on satellite data to track climate systems. Measurements of sea surface temperature (SST) are acquired using infrared and microwave radiometers. Infrared sensors provide high spatial resolution but cannot penetrate clouds, capturing only the ocean surface temperature. Microwave sensors can penetrate most cloud cover to measure temperature slightly below the surface, though their spatial resolution is coarser.
SST measurements track large-scale phenomena, such as ocean warming and severe weather events. Satellite altimeters precisely measure the height of the ocean surface, which maps the circulation of ocean currents. Variations in sea surface height indicate major current systems, like the Gulf Stream, which redistribute heat globally. This data monitors the rate of global mean sea level rise, a consequence of thermal expansion and melting ice.
In the polar regions, satellites track the extent and thickness of ice sheets covering Greenland and Antarctica, as well as the annual cycle of sea ice. Instruments like ICESat-2 use lasers to measure ice elevation with high precision, revealing the rate of mass loss from glaciers and ice caps. Consistent, long-term records forecast the impact of melting ice on global sea levels. Geostationary satellites provide continuous monitoring of atmospheric conditions, allowing for the real-time forecasting and tracking of severe weather phenomena, including hurricanes and typhoons.
Tracking Changes in Terrestrial Ecosystems and Land Use
Satellite imagery monitors the health of terrestrial ecosystems and human impact on the land surface. Vegetation health is quantified using spectral indices like the Normalized Difference Vegetation Index (NDVI), which measures the difference between near-infrared and red light reflected by plants. Higher NDVI indicates denser, healthier vegetation, allowing scientists to track seasonal vitality, drought stress, or landscape recovery. The Landsat program, with decades of continuous data, tracks these slow changes in vegetation cover.
The images reveal the extent of deforestation and reforestation efforts across continents. Scientists map the expansion of agriculture into former forest lands, a significant factor in the global carbon cycle. Monitoring agricultural fields allows for precision farming applications, such as assessing crop yield potential and managing water resources. This information helps ensure food security by providing early warnings of crop failure.
Urbanization patterns document the global trend of cities expanding into surrounding areas. High-resolution imagery maps the boundaries of urban sprawl, providing data on population density and infrastructure development. Scientists use this data to understand how city growth affects local climate, often leading to urban heat islands. Comparing historical and current data analyzes the long-term interaction between human activity and the environment.
Assessing Geological Processes and Hazards
Scientists monitor the Earth’s crust and surface using specialized satellite techniques that often precede or follow geological events. Interferometric Synthetic Aperture Radar (InSAR) uses radar signals from orbiting satellites, such as Sentinel-1, to detect ground deformation. By comparing radar images taken at different times, InSAR measures changes in surface elevation with millimeter-level accuracy. This tracks the slow inflation or deflation of a volcano’s magma chamber, which can signal an impending eruption.
InSAR also measures ground movement caused by tectonic plate activity and earthquakes. Post-seismic deformation maps reveal how strain is released along fault lines, offering insights into rupture dynamics and regional seismic risk. Satellites monitor infrastructure stability, detecting subsidence or uplift caused by the extraction of groundwater, oil, or gas. They also monitor movement caused by large civil engineering projects like dams and bridges.
Following a natural disaster, satellite imagery provides an assessment of the damage. Optical and radar images map the extent of floodwaters, identify structural collapse after an earthquake or tsunami, and determine wildfire boundaries. This rapid mapping capability is crucial for directing emergency response teams and allocating resources for disaster relief and recovery efforts.