Increased Model Complexity

Increased Model Complexity

Conceptual Model of Earth System Processes Operating on Timescales of Decades to Centuries

About 30 years ago, Francis Bretherton developed this system flow chart of the Earth's climate and biospheric cycles, which has forever become known as the Bretherton diagram. When seeing this diagram for the first time, the first reaction of many people is laughter: the diagram is so very complex. It's got everything. It's got human impacts, it's got volcanism, it's got space physics, it's got deep sea sediment cores, and the solar system. Bretherton put everything in. This diagram could be seen as a model roadmap; Bretherton's grand view of where models would need to evolve. Indeed, they have been evolving in this direction over the last 20 years.

One of the more significant additions from early efforts has been the inclusion of ecosystem models. This includes a terrestrial ecosystem component in the land model along with a marine ecosystem component in the ocean model.

Why include ecosystems?

Most of the climate models of the last 20 years have specified the amount of carbon dioxide in the atmosphere, not predicted it. It was prescribed based on very good measurements of carbon dioxide, but there has always been a goal to make carbon dioxide a predicted quantity. To do that, models need to simulate the carbon cycle. Now many models are being run that do not prescribe the amount of carbon dioxide in the atmosphere. Instead, only emissions of carbon dioxide due to the burning of fossil fuel are set. Then the model calculates how much carbon dioxide remains in the atmosphere.

In addition to simulating the processes that make up the carbon cycle, models now include chemistry that impacts atmospheric gas composition, as well as aerosols and clouds. Adding complexity to models, like atmospheric chemistry, ecosystems, and the carbon cycle, lets the model do what the real climate system is doing: exchanging not only moisture and energy between the surface and the atmosphere, but also carbon dioxide.