Scientists do not have a limitless supply of computing power at their disposal, and so it is necessary for models to divide up the Earth into grid cells to make the calculations more manageable (read more inWhat is an Earth System Model (ESM)?).
This means that at every step of the model through time, it calculates the average climate of each grid cell. However, there are many processes in the climate system and on the Earth’s surface that occur on scales within a single cell.
To solve this problem, these variables are “parameterised”, meaning their values are defined in the computer code rather than being calculated by the model itself.
In many cases, it is not possible to narrow down parameterised variables into a single value, so the model needs to include an estimation. Scientists run tests with the model to find the value – or range of values – that allows the model to give the best representation of the climate.
This complex process is known variously as model “tuning” or “calibration”. While it is a necessary part of climate modelling, it is not a process that is specific to it.
Parameterisations may also be used as a simplification where a climate process isn’t well understood. Parameterisations are one of the main sources of uncertainty in climate models.
The graphic below shows some of the processes that are typically parameterised in models.
A list of 20 climate processes and properties that typically need to be parameterised within global climate models. Image courtesy of MetEd, The COMET Program, UCAR.
Credit: Carbon Brief (https://www.carbonbrief.org/qa-how-do-climate-models-work)