GCM's (General Circulation Models) or sometimes mistakenly referred to as Global Climate Model, Typically refers to a three-dimensional model of the global atmosphere used in climate modeling (often erroneously called “Global Climate Model”). This term often requires additional qualification (e.g., as to whether or not the atmosphere is fully coupled to an ocean as in AOGCM, which stands for Atmosphere-Ocean General Circulation Model.
NASA/GISS Climate Model Scenarios
The length scales that are resolved in these models is typically on the order of 100s of kilometers (i.e. features that size or smaller are not directly resolved). The timestep for the models (how often the fields are updated) is usually 20 minutes to an hour. Thus in any day there would be 24 to 72 loops of the main calculations.
The basic variables are the temperature, humidity, liquid/ice water content and atmospheric mass. The physics usually consists of advection, radiation calculations, surface fluxes (latent, sensible heat etc.), convection, turbulence and clouds. More elaborate Earth System models often contain tracers related to atmospheric chemistry and aerosols (including dust and sea salt).
Climate modeling is actually only 'one' of the ways we come to understand climate on earth. That's because we don't have extra planets that are just like earth to experiment on. Some say that models can be wrong. It's a common mistake people make... the point is not that models 'can' be wrong. The point is models are 'always' wrong, that's because models are not real, they are models. But that does not mean that they are not relevant and useful for understanding how things work. The truth is that if you only use measurements and raw data, you can't even get close to the truth about climate.
In science, it is often preferable to do an experiment. But to do an experiment with global climate, we would need more planets just like earth to test. Then we could try different mixes of greenhouse gases in the atmosphere and see what happens. However, since we don't have any extra planets, we need to model the climate: Experiment: Webster, Wiki
The reality is that raw data is terribly misleading when measuring the earths temperature. Many aberrations need to be compensated for. That means that if you want to get a more accurate picture, you need to model the measurements and correct for aberrations. You make a model and test it. And others test it. And you refine it. And you refine it some more. It will never be perfect, and it can always be improved, especially when dealing with complex systems like climate: Model: Webster, Wiki
An Important Point
Many people falsely assume that models are not perfect therefore we don't understand climate. This is a misnomer. Models only help us understand climate in a more reasonable way, they will never be perfect. But they do reasonably show us what is happening in the climate of earth. The actual observations are clear enough at this point, that modeling is not required to understand that the earth is warming. We see accelerating ice melt and sea level rise as well as other affects, such as seasonal shift, prolonged forest fire seasons, regional drought trends, stronger storms and also increased flood events due to more moisture in the atmosphere due to warming oceans. All of these lines of evidence confirm the basic physics along with the observation, and these physical lines of evidence match the models.
We know the amount of GHG's mankind has produced quantitatively.
We know that should produce a relatively certain amount of forcing.
In other words, the anthropogenic signal is strong enough that we are certain this global warming is:
Attributable to the forcings caused by human output of greenhouse gases.
Some are still "under the mistaken impression that concern about global warming is based on climate models, which in reality play little role in our understanding -- our understanding is based mainly on how the Earth responded to changes of boundary conditions in the past and on how it is responding to on-going changes."
By feeding data into the model simulations, scientists are able to test potentials and evaluate the various influences of the earth climate system.
Just because they are simulations does not mean that they are not useful or accurate. In fact modeling climate is the only way to estimate climate functions with any degree of accuracy.
Modern climate models are more accurate than the instruments in some cases. The models actually help identify measurement problems in the real world. And that 'is very' important, because measuring the global climate is a lot of measuring. The models are so good, they can actually help identify instrumental aberrations that can then be examined and fixed.
What has been learned?
Using the above processes, mistakes and aberrations from measurements are corrected for with models which are tested against the measurements and then refined.
Absolutely. Continued investigation and testing of the measurements and the models will help refine the models and the measurements as well. Science is about discovery through testing. The scientific method is very careful and methodical. It is the best and only way to understand our climate.
...it may have taken much longer to realize there was a problem in our atmospheric composition that may be dramatically affecting climate. The models allowed us to see that the temperature was rising when it should have been falling.
The models used are called Global Circulation Models (GCM's). The major forces are fairly well known. So when the global temperature started doing something other than the model, the lead hypothesis was either something is wrong with the model, or something changed in the climate system.
Upon further examination...
...it was found that indeed, human GHG output and aerosols were changing the way our climate system operated. Some effects had a cooling impact, and some had a warming impact. Both combined however still lead to further warming.
"Let’s review the successful predictions of the models."
That the globe would warm, and about how fast, and about how much.
That the troposphere would warm and the stratosphere would cool.
That nighttime temperatures would increase more than daytime temperatures.
That winter temperatures would increase more than summer temperatures.
Polar amplification (greater temperature increase as you move toward the poles).
That the Arctic would warm faster than the Antarctic.
The magnitude (0.3 K) and duration (two years) of the cooling from the Mt. Pinatubo eruption.
They made a retrodiction for Last Glacial Maximum sea surface temperatures which was inconsistent with the paleo evidence, and better paleo evidence showed the models were right.
They predicted a trend significantly different and differently signed from UAH satellite temperatures, and then a bug was found in the satellite data.
The amount of water vapor feedback due to ENSO.
The response of southern ocean winds to the ozone hole.
The expansion of the Hadley cells.
The poleward movement of storm tracks.
The rising of the tropopause and the effective radiating altitude.
The clear sky super greenhouse effect from increased water vapor in the tropics.
The near constancy of relative humidity on global average.
The expanded range of hurricanes and cyclones--a year before Cyclone Catarina showed up off the coast of Brazil, something which had never happened before.
"Looks like a pretty good track record to me. Are there problems with the models, and areas where they haven't gotten it right yet? Sure there are. The double Inter-Tropical Convergence Zone which shows up in some coupled models, ENSO variability, insufficiently sensitive sea ice, diurnal cycles of moist convection, and the exact response of climate to clouds are all areas of ongoing research. But the models are still the best thing we have for climate prediction under different scenarios, and there is no reason at all to think they're getting the overall picture wrong."