A climate model is a simplified, mathematical representation of the earth's climate system. They are based on the physical, chemical and biological properties of its components, their interactions and feedbacks between them. Climate models are used to run simulations of future climates based on a range of assumptions. Large numbers of these simulations taken from different climate models have been used to produce the UKCP09 climate change projections.
The climate system can be represented by computer models comprised of differential equations based on the basic laws of physics, fluid motion and chemistry. The degree to which the model can simulate the response of the climate system depends, to a large degree, on the level of understanding of the physical, geophysical, chemical and biological processes that govern the climate system. They are of varying complexity depending on the intended use and capabilities and include:
1. Simple Climate Models: Due to computational expense, simple climate models are calibrated to yield an equivalent response in temperature and sea level to more complex climate models. Their simplification is typically in the form of a reduction in dimensionality to two or even zero. They can be used to explore the potential sensitivity of the climate to a particular process over a range of parameters. Simple climate models are also used within larger integrated assessment models to analyse the costs of emission reduction and impacts of climate change.
2. Earth system Models of Intermediate Complexity (EMICs): These models are designed to bridge the gap between the three-dimensional comprehensive models and simple models. The main characteristic of EMIOCs is that they describe most of the processes implicit in comprehensive models, albeit in a more reduced form. They also explicitly simulate the interactions among several components of the climate system including biogeochemical cycles. They are computationally efficient enough to allow for long-term climate simulations over several tens of thousands of years or a broad range of sensitivity experiments over several millennia.
3. Global Climate Models or General Circulation Models (GCMs): These are three-dimensional climate models that solve the equations for fluid motion and energy transfer around the globe and integrate these forward in time. They solve the equations at intervals in time (typically 30 minutes) at a number of points forming a grid over the globe at different levels in the atmosphere. Horizontally this results in grid spacing of around 275-300 km over the UK.
4. Coupled Atmosphere-Ocean Global Climate Models (AOGCM):
Complex climate model involving coupling comprehensive three-dimensional atmospheric GCMs within ocean general circulation models, with sea-ice models and with models of land-surface processes. Information about the state of the atmosphere and the ocean adjacent to, or at the sea surface is used to compute exchanges of heat, moisture and momentum between the two components. Computational limitations mean that the majority of sub-grid scale processes are parameterised.
5. Regional Climate Models (RCMs): Use of high-resolution global models is computationally very demanding which poses limits to the increase in resolution obtainable. To provide information at the regional scale, regional climate models (RCMs) have been developed. They model the climate at a higher resolution for a finite area (limited at approximately 25-50 km resolution) and are driven by the boundary conditions of the GCM. With the higher resolution, including a higher resolution of the underlying geography, RCMs are more able to simulate climate processes and feedbacks operating at the regional scale.
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- A number of countries have developed GCMs. In the UK the Met Office Hadley Centre has developed its own models (e.g. HadCM3). Due to computing power it is not possible to model each point in space and time so each of these global climate models breaks the globe into a grid, usually at a resolution of 265 by 300 km over the UK.
- There are different types of GCM's depending on whether they incorporate dynamics from the atmosphere, the ocean or both. Two types of GCMs have been used in UKCP09, the GCM and the AOGCM.
- The Met Office has developed a regional climate model (HadRM3) that was used in UKCP09.
- More details about the climate models used in UKCP09 are given in Box 2.1 of the UKCP09 Climate Change Projections report. A commentary about the strengths and weaknesses of climate models is given in Annex 3.
- IPCC Fifth Assessment Report, Working Group 1 report The Physical Science Basis, Chapter 9