This page illustrates a subset of the 50 Essential Climate Variables identified by the Global Climate Observing System (GCOS) for worldwide monitoring. GCOS is an international network of people and observing systems that gather data for monitoring global climate. Panels of experts helped identify which climate observations should be made on an ongoing basis, and agreed upon principles and guidelines for the best ways to make them. You can read more about how essential climate variables are used to support climate research and policy and find links to vetted data sets on the Global Observing Systems Information Center (GOSIC) site »
River Discharge
River discharge is the volume of water that flows past a point of a river per unit time; it is commonly expressed in cubic feet per second, or sometimes, in gallons per day. River discharge, also called flow rate, is calculated by multiplying the average velocity of the water by the cross-sectional area of the river channel. Discharge is measured by stream gauges or weirs. River discharge forecasts are based on historical flows, topography, and current conditions of soil and weather.
More information: Streamflow
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Snow Cover
Snow cover is defined as the amount of snow on the ground, usually expressed as a percentage of areal (spatial) coverage. In the Northern Hemisphere, snow cover has a significant influence on climate due to the fact that it reflects much more incoming sunlight energy than land or ocean. Snow is four to six times more reflective than bare ground, so once the snow melts in the spring, the ground warms rapidly. Satellites are used to measure large expanses of snow cover. Instruments on the ground measure snow depth and water content.
More information: Snow Extent
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Glaciers and Ice Caps
Glaciers and Ice caps are "permanent" snow and ice features. They are made of fallen snow that has been compressed by its own weight for many years, becoming ice. Glaciers flow downhill like giant frozen rivers. Ice caps, sometimes called ice fields, are giant domes of frozen ice on land; they are generally on flat surfaces and can flow outward in all directions. Scientists monitor glaciers, ice caps, and ice sheets using instruments on satellites, ground-based monitoring, and aerial surveys.
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Ice Sheets
Ice caps that are larger than 50,000 square kilometers are called Ice sheets. Ice sheets are found in Greenland and Antarctica. They are made of frozen snow and ice. Instruments on aircraft and satellites measure the areal extent ice sheets. Scientists also monitor ice sheet thickness using gravity measurements and other techniques at the surface.
More information: Ice Sheets
Links to data: Atlas of the Cyrosphere
Permafrost
Permafrost is soil, water, and rock that has been frozen continuously for at least two years. It exists in regions where the summer warmth is not strong enough to reach the base layer of the frozen ground. Permafrost and seasonally frozen ground cover 24% and 60%, respectively, of the Northern Hemisphere. Scientists use probes and core samples, as well as instruments on satellites to monitor permafrost.
More information: Frozen Ground
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Albedo
Albedo is a measure of how much solar energy a surface reflects; it is calculated as the ratio of incoming to outgoing solar energy, and is therefore a unitless quantity. An albedo value of one would be a perfect reflector, where 100 percent of the energy is reflected. Conversely, a value of zero describes a surface that absorbs all solar energy it receives. Freshly fallen snow is extremely reflective; it has an albedo of 0.95 or 95%. Forests, crops, and bare ground are not very reflective, so their albedo values are much lower. Albedo is measured by instruments on satellites.
More information: Albedo
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Land Cover
Land cover is defined as the physical material covering land at Earth's surface. Land cover includes both human-made and natural surfaces such as soil, water, crops, roads and buildings. By monitoring land cover over time, scientists can detect plant health and changes in vegetative cover. Land cover changes also signal changes in land use, such as the change from a farm field to a shopping mall. Scientists monitor plant health in order to forecast food supplies or the onset of drought. Satellite instruments that make images of land at various wavelengths are used to monitor land cover. Ground-based surveys are also used to calibrate the images from satellites.
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Soil Carbon
Soil carbon is the amount of carbon held in the soil. It is a small, but vital, part of productive soil that is primarily made from decaying plant material. The amount of carbon held in the soil depends on several factors, including the type of soil and climate. Scientists measure soil carbon with automated instruments that monitor the carbon emissions from the soil. They also sample the soil and use chemical analysis to measure the amount of stored carbon.
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Fire Disturbance
The impact of fire can dramatically alter vegetative cover and local air quality. Images from satellites and airplanes are used to monitor fire activity. When fighting fires, smaller fire-spotting aircraft are also deployed to help track fire behavior.
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Soil Moisture
Soil moisture is the amount of moisture (water) above and within the soil. Monitoring soil moisture is important for agriculture and for anticipating the potential for flooding. Soil moisture probes are relatively rare, so soil moisture is generally calculated based on recent precipitation and temperature conditions. The NASA Gravity Recovery and Climate Experiment (GRACE) satellite mission uses observations and sophisticated mathematical calculations to estimate changes in soil moisture from space. Instruments on board the Aqua and Terra satellites also measure soil moisture.
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