Climate models
Climate models are a tool for calculating the future and historic climate. The models use input data from observations and experiments. Scientists often use climate models to study how the climate may change when the composition of the atmosphere changes with, for example, changed levels of greenhouse gases and aerosols.
The climate models are three-dimensional mathematical descriptions of the climate system: the atmosphere, land surface, oceans, lakes and ice. In a climate model, the atmosphere is divided into so-called grids along the earth’s surface and up into the air, as visualized in the figure. The movements of the atmosphere and the preservation of energy, water and mass follow well-known physical laws that can be described by mathematical formulas. For each grid various meteorological, hydrological and climatological variables such as temperature, precipitation and wind are calculated. Since the climate is global, the calculations for a global climate model (GCM) must consider global processes in the atmosphere and be above clouds and winds.
It requires a lot of computer power to run a climate model, and even though the computing capacity is constantly increasing, calculations in the global climate models are still made with a rather sparse grid, ranging from 100 to 300 kilometers. This means that the level of detail on a local or regional scale is low in the global model. However, if you want to study a smaller part of the earth in more detail, you can use so called regional climate models (RCM). In a regional model, the grid is laid over a smaller area, which means that you can get a denser grid (and more detail) without requiring too much computing power; you can for example have a grid cell that is 10 by 10 kilometers. What happens outside the calculation area in a regional climate model is governed by the results of a global climate model. In this way changes that take place outside the regional model area are considered. This way of using the results from a global model in a regional model is called regional downscaling.
What is the difference between climate and weather?
A weather forecast is expected to provide answers about the timing and location of weather events the coming hours up to 10 days. When calculating the climate, the models are expected to provide answers to how the weather behaves over significantly longer periods of time. The responses of the climate models concern the general behavior in the form of averages and extremes and are not tied to a specific time or a specific place during the period. In a weather forecast it is important to know the initial state, i.e. what temperature it is in each grid in the model when it starts.