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The NCAR-based Community Earth System Model (CESM) is one of the world’s most sophisticated models of global climate. Created by scientists at NCAR, the Department of Energy, and collaborators, this powerful model simulates the many processes in our climate system, ranging from clouds and atmospheric chemicals to ice to marine ecosystems. (©UCAR. This illustration is freely available for media and nonprofit use)
Cirrus: These high, thin, icy clouds act as a warming influence on climate overall, because they allow sunlight in but trap long-wave radiation rising from Earth’s heated surface. The CCSM depicts these and other clouds through parameterization—tracking the conditions that form such clouds and then specifying how much of a given rectangle of land in the model grid is covered by each cloud type.
Stratus:These low, dense, very reflective clouds act as a cooling influence on climate overall, because they reflect a great deal of sunlight. Subtropical oceans often feature huge areas of marine stratocumulus. The CCSM depicts these and other clouds through parameterization—tracking the conditions that form such clouds and then specifying how much of a given rectangle of land in the model grid is covered by each cloud type.
Cumulus:These puffy clouds, which sometimes build in tower-like formations, are linked to strong updrafts and the showers and thunderstorms that result. Cumulus are difficult to represent in models because each cloud covers only a small part of Earth’s surface, but when taken together, cumulus clouds have a large influence on global circulation. The CCSM depicts these and other clouds through parameterization—tracking the conditions that form such clouds and then specifying how much of a given rectangle of land in the model grid is covered by each cloud type.
Precipitation and evaporation
The process of converting water vapor to water droplets or snowflakes releases heat into the atmosphere. The CCSM simulates this process, as well as the evaporation of water from soil, wetlands, lakes, and oceans and the amount of rain or snow reaching the surface.
Sea ice helps keep polar regions cold, because it reflects most of the sunlight that hits it. When sea ice melts, it does not raise sea level directly (because the ice is already afloat, like a melting ice cube in a glass of water). However, the dark surface ocean exposed by melting sea ice absorbs most of the sunlight it receives, which leads to further warming. Research using the CCSM indicates that Arctic summertime sea ice may diminish greatly, as soon as the 2030s.
One of the main elements of a climate model is its depiction of winds. The global circulation transports warm air poleward and cold air toward the equator. This flow operates through several persistent loops that produce trade winds in the tropics, westerly winds at midlatitudes, and easterlies across the poles. The actual winds at any one spot are influenced by day-to-day weather as well as climate cycles such as El Niño. Where winds converge, rising motion and rain or snow may develop.
Heat and salinity exchange
Because water has a higher heat capacity than soil, it takes longer for sea surfaces to warm up or cool down relative to the continents. Climate models must account for the transfer of heat between ocean and atmosphere. They also need to reflect changes in salinity (salt content) that occur as ocean water evaporates, or as fresh water enters the oceans through increased rainfall or increased glacial melting.
Atmospheric model layers
Contrasts in temperature and wind are much stronger vertically than horizontally. Only a few miles above ground, temperatures are usually frigid (even in summer), and winds may howl at 200 miles per hour (320 kilometers per hour) or more. The CCSM divides the atmosphere into 26 layers, tracking each layer as well as exchanges of energy and moisture between layers. Near the ground, where vertical contrasts are strongest, the layers (which are defined by atmospheric pressure) can be as thin as 1,000 feet (about 300 meters) or less.
Ocean currents, temperature, and salinity
The behavior of the ocean is much more difficult to observe than the atmosphere, and many ocean processes are still poorly understood. As recently as the 1990s, most climate models used a “slab” ocean—one that behaves as a single unit. Today, the CCSM and other sophisticated models include a much more dynamic depiction of the ocean that tracks changes in ocean currents, temperature, and salinity. These control such phenomena as the North Atlantic’s overturning circulation, which helps keep Europe warm for its latitude but which may be sensitive to climate change.
Ocean model layers
Much like the atmosphere, the ocean needs to be divided into several layers in order for a model to accurately depict the three-dimensional flow and other qualities. The CCSM includes 40 ocean layers, ranging in thickness from about 33 feet (10 meters) near the sea surface to about 800 feet (250 meters) in the deep ocean.
Ocean bottom topography
In order to accurately depict the ocean circulation in three dimensions, the CCSM includes undersea ridges, valleys, and other topographic features of the ocean bottom.
Most of the world’s oceans outside the Arctic feature a relatively warm sea surface and a thin region called a thermocline that separates the warm surface layer from colder, deeper waters. In some parts of the world, colder and deeper water regularly crosses the thermocline to mix with warmer surface waters, or vice versa. The CCSM can simulate these overturning processes.
Large mountain chains exert a major influence on temperature, precipitation, and wind patterns for miles around. Even when mountains are relatively modest in size or extent, they play an important role in local climate. Early climate models tracked the atmosphere at points separated by hundreds of miles, so mountain ranges appeared in highly smoothed form. The CCSM and other contemporary models operate at higher resolution so the topography is much less smoothed.
Land surface processes
The atmosphere is strongly affected by what lies beneath it—forests, deserts, ice sheets, mountains, and grasslands. The CCSM includes each of these elements, tracking the exchange of energy and moisture between them and the atmosphere. Urban areas are not yet depicted in the standard version of the CCSM or other global climate models, although work is under way to add them.
Moisture stored near and just below ground level affects how much rain and snow can be absorbed by the soil and how quickly a region dries out if precipitation slackens. The CCSM includes a depiction of moisture in 10 soil layers.
Outgoing heat energy
Virtually all of the energy that reaches our planet from the Sun leaves the Earth system in one form or another. In order to keep their simulation of Earth’s climate in proper balance, the CCSM and other climate models account for this outgoing radiation.
Incoming solar energy
To create an accurate portrayal of Earth’s climate, the CCSM calculates the amount of incoming solar radiation by location, time of day, and time of year. When reproducing past climates, the model can also include estimates of solar variability based on sunspot counts, carbon dating of organic material, and other indirect evidence.
Transition from solid to vapor
Besides melting, snowpacks can erode through a process called sublimation, in which moisture goes directly from ice crystals to water vapor in the atmosphere. Sublimation is common in dry mountain climates, where a snowpack may leave little or no water behind as it shrinks (even in temperatures below freezing). The CCSM depicts sublimation as well as snowmelt.
Evaporative and heat energy exchanges
When water evaporates, it draws heat from the lakes, rivers, or oceans from which it came and stores that heat within its molecular bonds. Heat can also leave the land and ocean surface directly through contact between the surface and molecules in the atmosphere (a process called conduction). The CCSM depicts these and other processes that move heat energy around the Earth system.
Large areas of snow have a major effect on weather. Because of their light color, they reflect large amounts of sunlight and help keep temperatures colder than they would otherwise be, especially close to the ground. The CCSM tracks the seasonal waxing and waning of snowfall across mountainous and high-latitude areas. The large ice sheets in Greenland and Antarctica are also included. Some of the processes that control ice sheets are still being studied by scientists and are not yet part of the CCSM and other global models.
A key part of the global water cycle is the flow of water from rivers, lakes, and land areas down toward the sea. The CCSM depicts runoff more precisely than earlier models, which enhances its treatment of the water cycle overall.