Ageneral circulation model (GCM) is a type ofclimate model. It employs amathematical model of the general circulation of a planetaryatmosphere or ocean. It uses theNavier–Stokes equations on a rotating sphere withthermodynamic terms for various energy sources (radiation,latent heat). These equations are the basis for computer programs used tosimulate the Earth's atmosphere or oceans. Atmospheric and oceanic GCMs (AGCM andOGCM) are key components along withsea ice andland-surface components.
GCMs and global climate models are used forweather forecasting, understanding theclimate, and forecastingclimate change.
Atmospheric GCMs (AGCMs) model the atmosphere and imposesea surface temperatures as boundary conditions. Coupled atmosphere-ocean GCMs (AOGCMs, e.g.HadCM3,EdGCM,GFDL CM2.X, ARPEGE-Climat)[2] combine the two models. The first general circulation climate model that combined both oceanic and atmospheric processes was developed in the late 1960s at theNOAAGeophysical Fluid Dynamics Laboratory[3] AOGCMs represent the pinnacle of complexity in climate models and internalise as many processes as possible. However, they are still under development and uncertainties remain. They may be coupled to models of other processes, such as thecarbon cycle, so as to better model feedback effects. Such integrated multi-system models are sometimes referred to as either "earth system models" or "global climate models."
Versions designed for decade to century time scale climate applications were created bySyukuro Manabe andKirk Bryan at theGeophysical Fluid Dynamics Laboratory (GFDL) inPrinceton, New Jersey.[1] These models are based on the integration of a variety of fluid dynamical, chemical and sometimes biological equations.
The acronymGCM originally stood forGeneral Circulation Model. Recently, a second meaning came into use, namelyGlobal Climate Model. While these do not refer to the same thing, General Circulation Models are typically the tools used formodeling climate, and hence the two terms are sometimes used interchangeably. However, the term "global climate model" is ambiguous and may refer to an integrated framework that incorporates multiple components including a general circulation model, or may refer to the general class of climate models that use a variety of means to represent the climate mathematically.
Atmospheric (AGCMs) and oceanic GCMs (OGCMs) can be coupled to form an atmosphere-ocean coupled general circulation model (CGCM or AOGCM). With the addition of submodels such as a sea ice model or a model forevapotranspiration over land, AOGCMs become the basis for a full climate model.[4]
General Circulation Models (GCMs) discretise the equations for fluid motion and energy transfer and integrate these over time. Unlike simpler models, GCMs divide the atmosphere and/or oceans into grids of discrete "cells", which represent computational units. Unlike simpler models which make mixing assumptions, processes internal to a cell—such as convection—that occur on scales too small to be resolved directly are parameterised at the cell level, while other functions govern the interface between cells.
Three-dimensional (more properly four-dimensional) GCMs apply discrete equations for fluid motion and integrate these forward in time. They contain parameterisations for processes such asconvection that occur on scales too small to be resolved directly.
A simple general circulation model (SGCM) consists of a dynamic core that relates properties such as temperature to others such as pressure and velocity. Examples are programs that solve theprimitive equations, given energy input and energydissipation in the form of scale-dependentfriction, so thatatmospheric waves with the highestwavenumbers are most attenuated. Such models may be used to study atmospheric processes, but are not suitable for climate projections.
Atmospheric GCMs (AGCMs) model the atmosphere (and typically contain a land-surface model as well) using imposedsea surface temperatures (SSTs).[5] They may include atmospheric chemistry.
AGCMs consist of a dynamical core that integrates the equations of fluid motion, typically for:
A GCM containsprognostic equations that are a function of time (typically winds, temperature, moisture, and surface pressure) together withdiagnostic equations that are evaluated from them for a specific time period. As an example, pressure at any height can be diagnosed by applying thehydrostatic equation to the predicted surface pressure and the predicted values of temperature between the surface and the height of interest. Pressure is used to compute the pressure gradient force in the time-dependent equation for the winds.
OGCMs model the ocean (with fluxes from the atmosphere imposed) and may contain asea ice model. For example, the standard resolution ofHadOM3 is 1.25 degrees in latitude and longitude, with 20 vertical levels, leading to approximately 1,500,000 variables.
AOGCMs (e.g.HadCM3,GFDL CM2.X) combine the two submodels. They remove the need to specify fluxes across the interface of the ocean surface. These models are the basis for model predictions of future climate, such as are discussed by theIPCC. AOGCMs internalise as many processes as possible. They have been used to provide predictions at a regional scale. While the simpler models are generally susceptible to analysis and their results are easier to understand, AOGCMs may be nearly as hard to analyse as the climate itself.
The fluid equations for AGCMs are made discrete using either thefinite difference method or thespectral method. For finite differences, a grid is imposed on the atmosphere. The simplest grid uses constant angular grid spacing (i.e., a latitude/longitude grid). However, non-rectangular grids (e.g., icosahedral) and grids of variable resolution [6] are more often used.[7] The LMDz model can be arranged to give high resolution over any given section of the planet.HadGEM1 (and other ocean models) use an ocean grid with higher resolution in the tropics to help resolve processes believed to be important for theEl Niño Southern Oscillation (ENSO). Spectral models generally use aGaussian grid, because of the mathematics of transformation between spectral and grid-point space. Typical AGCM resolutions are between 1 and 5 degrees in latitude or longitude: HadCM3, for example, uses 3.75 in longitude and 2.5 degrees in latitude, giving a grid of 96 by 73 points (96 x 72 for some variables); and has 19 vertical levels. This results in approximately 500,000 "basic" variables, since each grid point has four variables (u,v,T,Q), though a full count would give more (clouds; soil levels). HadGEM1 uses a grid of 1.875 degrees in longitude and 1.25 in latitude in the atmosphere; HiGEM, a high-resolution variant, uses 1.25 x 0.83 degrees respectively.[8] These resolutions are lower than is typically used for weather forecasting.[9] Ocean resolutions tend to be higher, for example, HadCM3 has 6 ocean grid points per atmospheric grid point in the horizontal.
For a standard finite difference model, uniform gridlines converge towards the poles. This would lead to computational instabilities (seeCFL condition) and so the model variables must be filtered along lines of latitude close to the poles. Ocean models suffer from this problem too, unless a rotated grid is used in which the North Pole is shifted onto a nearby landmass. Spectral models do not suffer from this problem. Some experiments usegeodesic grids[10] and icosahedral grids, which (being more uniform) do not have pole-problems. Another approach to solving the grid spacing problem is to deform aCartesiancube such that it covers the surface of a sphere.[11]
Some early versions of AOGCMs required anad hoc process of "flux correction" to achieve a stable climate. This resulted from separately prepared ocean and atmospheric models that each used an implicit flux from the other component different than that component could produce. Such a model failed to match observations. However, if the fluxes were 'corrected', the factors that led to these unrealistic fluxes might be unrecognised, which could affect model sensitivity. As a result, the vast majority of models used in the current round of IPCC reports do not use them. The model improvements that now make flux corrections unnecessary include improved ocean physics, improved resolution in both atmosphere and ocean, and more physically consistent coupling between the atmosphere and ocean submodels. Improved models now maintain stable, multi-century simulations of surface climate that are considered to be of sufficient quality to allow their use for climate projections.[12]
Moist convection releases latent heat and is important to the Earth's energy budget. Convection occurs on too small a scale to be resolved by climate models, and hence it must be handled via parameters. This has been done since the 1950s. Akio Arakawa did much of the early work, and variants of his scheme are still used,[13] although a variety of different schemes are now in use.[14][15][16] Clouds are also typically handled with a parameter, for a similar lack of scale. Limited understanding of clouds has limited the success of this strategy, but not due to some inherent shortcomings of the method.[17]
Most models include software to diagnose a wide range of variables for comparison with observations orstudy of atmospheric processes. An example is the 2-metre temperature, which is the standard height for near-surface observations of air temperature. This temperature is not directly predicted from the model but is deduced from surface and lowest-model-layer temperatures. Other software is used for creating plots and animations.
Coupled AOGCMs usetransient climate simulations to project/predict climate changes under various scenarios. These can be idealised scenarios (most commonly, CO2 emissions increasing at 1%/yr) or based on recent history (usually the "IS92a" or more recently theSRES scenarios). Which scenarios are most realistic remains uncertain.
The 2001IPCC Third Assessment ReportFigure 9.3 shows the global mean response of 19 different coupled models to an idealised experiment in which emissions increased at 1% per year.[19]Figure 9.5 shows the response of a smaller number of models to more recent trends. For the 7 climate models shown there, the temperature change to 2100 varies from 2 to 4.5 °C with a median of about 3 °C.
Future scenarios do not include unknown events – for example, volcanic eruptions or changes in solar forcing. These effects are believed to be small in comparison togreenhouse gas (GHG) forcing in the long term, but large volcanic eruptions, for example, can exert a substantial temporary cooling effect.
Human GHG emissions are a model input, although it is possible to include an economic/technological submodel to provide these as well. Atmospheric GHG levels are usually supplied as an input, though it is possible to include a carbon cycle model that reflects vegetation and oceanic processes to calculate such levels.
For the six SRES marker scenarios, IPCC (2007:7–8) gave a "best estimate" of global mean temperature increase (2090–2099 relative to the period 1980–1999) of 1.8 °C to 4.0 °C.[20] Over the same time period, the "likely" range (greater than 66% probability, based on expert judgement) for these scenarios was for a global mean temperature increase of 1.1 to 6.4 °C.[20]
In 2008 a study made climate projections using several emission scenarios.[21] In a scenario where global emissions start to decrease by 2010 and then decline at a sustained rate of 3% per year, the likely global average temperature increase was predicted to be 1.7 °C above pre-industrial levels by 2050, rising to around 2 °C by 2100. In a projection designed to simulate a future where no efforts are made to reduce global emissions, the likely rise in global average temperature was predicted to be 5.5 °C by 2100. A rise as high as 7 °C was thought possible, although less likely.
Another no-reduction scenario resulted in a median warming over land (2090–99 relative to the period 1980–99) of 5.1 °C. Under the same emissions scenario but with a different model, the predicted median warming was 4.1 °C.[22]
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AOGCMs internalise as many processes as are sufficiently understood. However, they are still under development and significant uncertainties remain. They may be coupled to models of other processes inEarth system models, such as thecarbon cycle, so as to better model feedback. Most recent simulations show "plausible" agreement with the measured temperature anomalies over the past 150 years, when driven by observed changes in greenhouse gases and aerosols. Agreement improves by including both natural and anthropogenic forcings.[23][24][25]
Imperfect models may nevertheless produce useful results. GCMs are capable of reproducing the general features of the observed global temperature over the past century.[23]
A debate over how to reconcile climate model predictions that upper air (tropospheric) warming should be greater than observed surface warming, some of which appeared to show otherwise,[26] was resolved in favour of the models, following data revisions.
Cloud effects are a significant area of uncertainty in climate models. Clouds have competing effects on climate. They cool the surface by reflecting sunlight into space; they warm it by increasing the amount of infrared radiation transmitted from the atmosphere to the surface.[27] In the 2001 IPCC report possible changes in cloud cover were highlighted as a major uncertainty in predicting climate.[28][29]
Climate researchers around the world use climate models to understand the climate system. Thousands of papers have been published about model-based studies. Part of this research is to improve the models.
In 2000, a comparison between measurements and dozens of GCM simulations ofENSO-driven tropical precipitation, water vapor, temperature, and outgoing longwave radiation found similarity between measurements and simulation of most factors. However, the simulated change in precipitation was about one-fourth less than what was observed. Errors in simulated precipitation imply errors in other processes, such as errors in the evaporation rate that provides moisture to create precipitation. The other possibility is that the satellite-based measurements are in error. Either indicates progress is required in order to monitor and predict such changes.[30]
The precise magnitude of future changes in climate is still uncertain;[31] for the end of the 21st century (2071 to 2100), for SRES scenario A2, the change of global average SAT change from AOGCMs compared with 1961 to 1990 is +3.0 °C (5.4 °F) and the range is +1.3 to +4.5 °C (+2.3 to 8.1 °F).
The IPCC'sFifth Assessment Report asserted "very high confidence that models reproduce the general features of the global-scale annual mean surface temperature increase over the historical period". However, the report also observed that the rate of warming over the period 1998–2012 was lower than that predicted by 111 out of 114Coupled Model Intercomparison Project climate models.[32]
The global climate models used for climate projections are similar in structure to (and often share computer code with)numerical models for weather prediction, but are nonetheless logically distinct.
Mostweather forecasting is done on the basis of interpreting numerical model results. Since forecasts are typically a few days or a week and sea surface temperatures change relatively slowly, such models do not usually contain an ocean model but rely on imposed SSTs. They also require accurate initial conditions to begin the forecast – typically these are taken from the output of a previous forecast, blended with observations. Weather predictions are required at higher temporal resolutions than climate projections, often sub-hourly compared to monthly or yearly averages for climate. However, because weather forecasts only cover around 10 days the models can also be run at higher vertical and horizontal resolutions than climate mode. Currently theECMWF runs at 9 km (5.6 mi) resolution[33] as opposed to the 100-to-200 km (62-to-124 mi) scale used by typical climate model runs. Often local models are run using global model results for boundary conditions, to achieve higher local resolution: for example, theMet Office runs a mesoscale model with an 11 km (6.8 mi) resolution[34] covering the UK, and various agencies in the US employ models such as the NGM and NAM models. Like most global numerical weather prediction models such as theGFS, global climate models are often spectral models[35] instead of grid models. Spectral models are often used for global models because some computations in modeling can be performed faster, thus reducing run times.
Climate models usequantitative methods to simulate the interactions of theatmosphere, oceans,land surface andice.
All climate models take account of incoming energy as short waveelectromagnetic radiation, chieflyvisible and short-wave (near)infrared, as well as outgoing energy as long wave (far) infrared electromagnetic radiation from the earth. Any imbalance results in achange in temperature.
The most talked-about models of recent years relate temperature toemissions ofgreenhouse gases. These models project an upward trend in thesurface temperature record, as well as a more rapid increase in temperature at higher altitudes.[36]
Three (or more properly, four since time is also considered) dimensional GCM's discretise the equations for fluid motion and energy transfer and integrate these over time. They also contain parametrisations for processes such as convection that occur on scales too small to be resolved directly.
Atmospheric GCMs (AGCMs) model the atmosphere and impose sea surface temperatures as boundary conditions. Coupled atmosphere-ocean GCMs (AOGCMs, e.g.HadCM3,EdGCM, GFDL CM2.X, ARPEGE-Climat[37]) combine the two models.
Models range in complexity:
Other submodels can be interlinked, such asland use, allowing researchers to predict the interaction between climate and ecosystems.
The Climber-3 model uses a 2.5-dimensional statistical-dynamical model with 7.5° × 22.5° resolution and time step of 1/2 a day. An oceanic submodel is MOM-3 (Modular Ocean Model) with a 3.75° × 3.75° grid and 24 vertical levels.[38]
One-dimensional, radiative-convective models were used to verify basic climate assumptions in the 1980s and 1990s.[39]
GCMs can form part ofEarth system models, e.g. by couplingice sheet models for the dynamics of theGreenland andAntarctic ice sheets, and one or morechemical transport models (CTMs) forspecies important to climate. Thus a carbon chemistry transport model may allow a GCM to better predictanthropogenic changes incarbon dioxide concentrations. In addition, this approach allows accounting for inter-system feedback: e.g. chemistry-climate models allow the effects of climate change on theozone hole to be studied.[40]
In 1956,Norman Phillips developed a mathematical model that could realistically depict monthly and seasonal patterns in thetroposphere. It became the first successful climate model.[41][42] Following Phillips's work, several groups began working to create GCMs.[43] The first to combine both oceanic and atmospheric processes was developed in the late 1960s at theNOAAGeophysical Fluid Dynamics Laboratory.[1] By the early 1980s, the United States'National Center for Atmospheric Research had developed the Community Atmosphere Model; this model has been continuously refined.[44] In 1996, efforts began to model soil and vegetation types.[45] Later theHadley Centre for Climate Prediction and Research'sHadCM3 model coupled ocean-atmosphere elements.[43] The role ofgravity waves was added in the mid-1980s. Gravity waves are required to simulate regional and global scale circulations accurately.[46]