Carbon cycle models

It is clear from the description of the carbon cycle in the ocean that representing the system in a computer model is a complex task. The chemical reactions of carbon dioxide in the surface waters need to be represented to provide a good estimate of the air–sea flux of carbon dioxide. There are also several mechanisms for the carbon dioxide entering at the surface to reach deep ocean waters. The physics of the ocean needs to be well represented in the model to capture the solubility pump. Ocean biology needs to be included in the model to capture the biological pump, including the carbonate pump.

To represent the physics of the ocean, a version of the Met Office Hadley Centre climate model, HadCM3, is used. The ocean component is presently run at a horizontal resolution of 3.75 degrees longitude by 2.5 degrees latitude, compared to the version used in climate prediction which has a 1.25 degree resolution. The reduced resolution means the model represents the ocean physics less well, but it does require less computing resources and hence greater testing of the carbon cycle in the model can be performed.

To represent the ocean biology in the model, the biological system is greatly simplified and is represented by a single class of phytoplankton, a single class of zooplankton, detritus (the particulate waste products from the biology) and (nitrogenous) nutrient. A schematic of how these components fit together can be seen to the right. Phytoplankton require sunlight and nutrients to grow. Zooplankton prey on phytoplankton and in turn are prey themselves, which is represented by a mortality loss for the zooplankton. Waste products and dead material form detritus, which breaks down back to nutrient. This type of model for the ocean biology is known as a NPZD (Nutrient, Phytoplankton, Zooplankton, Detritus) model.

Further to the four biological components are two components representing carbon in the ocean. These are dissolved inorganic carbon and alkalinity. The dissolved inorganic carbon is taken up by phytoplankton growth and returned with biological breakdown. The amount of carbon in the forms of dissolved inorganic carbon and in the four biological components is kept track of in the model. Alkalinity is required to calculate the proportion of dissolved inorganic carbon that is in the form of carbon dioxide in the surface waters, in order to calculate the air–sea flux of carbon dioxide. Alkalinity is treated in a similar fashion to dissolved inorganic carbon with its concentration changed by biological processes. As well as interacting in the biological model, each of the six components making up the ocean carbon cycle are also moved around the ocean by the ocean physics.

Although the model makes many simplifications of what actually occurs in the ocean, the results are encouraging. Below, the zonally integrated air–sea flux of carbon dioxide from the model is compared to the zonal integral from a global ocean climatology compiled from available observations [Takahashi et al., 1997]. In both the Atlantic and the Pacific the model generally compares well with the observations. The major difference between the model and observations occurs around the equator in the Pacific. The physics of the model brings too much deep carbon-rich water to the surface in the equatorial Pacific, producing a large flux of carbon dioxide to the atmosphere. 

In representing the biology in the model an important quantity to reproduce is the primary production. This is the amount of carbon taken up during phytoplankton growth. If the model can reproduce this quantity, it is a step towards correctly representing the biological and carbonate pumps in the ocean. Global fields of primary production have recently been estimated using satellite data of the ocean colour. Zonal integrals of one such global field [Antoine at al., 1996], are compared to the model results below. The overall comparison is good. The main differences occur in the subtropics (20–40 degrees latitude north and south) in the Pacific and from 0–30 degrees north in the Atlantic. It is known that the biology in the subtropics behaves somewhat differently to elsewhere in the ocean and the present ecosystem model is unable to capture this. The feature in the Atlantic at 0–30 degrees north seen in the observations is a result of a coastal region of high primary production. As the model physics operates at a coarse resolution, it is unable to represent these coastal features well.

The overall results from the model are encouraging. A list of the ongoing research topics in the ocean carbon cycle modelling group includes both work using the HadOCC model to gain a greater understanding of the role of the ocean carbon cycle for climate and ongoing development of the model itself.