Climate Model Evolution

Climate Model Evolution

Loop of schematic images showing the Evolution of Climate Models

With the development of digital computers in the 1950s, it became theoretically possible to develop weather and climate models. Forecasting short-range weather with NWP models got attention first. Scientists only began to develop computerized climate models (also known as general circulation models or GCMs) in the late 1960s to early 1970s, but used the same equations as in NWP models.

Many details in early climate models (for example, sea surface temperatures and land/sea ice) had to be set to fixed or seasonal values because they could not be calculated: computational resources were insufficient and/or the processes involved were not well understood.

As our physical understanding of ocean, land, and ice processes advanced and computing power increased, scientists were able to add more climate processes and improve existing ones. For instance, models that realistically simulate overturning ocean circulation and its interaction with the atmosphere replaced prescribed ocean surfaces. Realistic land surface models replaced simple "bucket" models to better simulate vegetation effects on moisture, momentum, and heat transfer between the land and the atmosphere. The effect of aerosols, both natural and those generated by human activities, was also added. The inclusion of aerosols resulted in climate models capturing the temporary halt to increasing average global temperature during the mid-20th century, an effect that was not previously simulated.

More recently, scientists have added processes with long time scales or complicated physics and chemistry. These include the carbon cycle, atmospheric chemistry, and a biosphere capable of responding to model climate change (for example, vegetation cover changing from tundra to forest in response to predicted warming). Scientists have also added interactions between land and sea ice and the rest of the climate model.

Some of the processes included in climate models are relatively straightforward, while the impact of others needs to be more crudely estimated because of their characteristic time or space scales. In the next section, we will talk about how these processes are included in climate models.