From Plants to People, Carbon Cycle Modeling Takes on New Realism

Carbon in all its various forms—carbon dioxide, methane, organic matter, calcium carbonate—cycles through the atmosphere, biosphere, and hydrosphere. Understanding the lifecycle of carbon and its fluxes between Earth’s various “spheres” is important to comprehending, planning for, and modeling current and future global climate. This is particularly true because carbon’s lifecycle is long—methane and carbon dioxide, for example, remain in the atmosphere for one or more decades—which means that carbon dynamics significantly influence environmental change.

This long lifetime also means that ecosystem and policy planners need to develop climate and carbon-related adaptation and mitigation strategies decades in advance. Understanding the lifecycle of carbon and its fluxes between Earth’s various “spheres” is important to comprehending, planning for, and modeling current and future global climate. To do this, tools are needed that can assess how climate will be affected, for example, in the event that large swathes of tropical rain forest disappear. Lacking the right information, policies will fail.

Biogeochemical models help scientists refine their understanding of the carbon cycle, and climate models shed light on the effects of global climate change. Until recently, however, carbon cycle modeling and climate system modeling were generally considered to be separate efforts. The Community Climate System Model (CCSM), one of the few climate models that has incorporated a carbon cycle component for quite a while, is designed to address exactly these questions. And, in the model’s newest version, CCSM4, the carbon cycle component will be even more robust.

For CCSM4, scientists in the CCSM Biogeochemistry Working Group wanted to create a model that considers human carbon emissions in its scenarios, as opposed to atmospheric trajectory of carbon dioxide, the way most climate models have done in the past. This model output, says Scott Doney, a senior scientist at Woods Hole Oceanographic Institution and part of the Biogeochemistry Working Group, will help guide economic and environmental policies, as well as technology decisions related to release of anthropogenic carbon. The information will answer questions such as how climate will change in future, how much carbon will be removed from or added to the atmosphere, and whether there any potential surprises that planners should take into consideration⎯critical questions for ensuring a healthy global society.

In 2005, prior to the current CCSM4 effort, scientists ran a preliminary set of experiments coupling simple carbon models with CCSM’s land and ocean components. Based on the lessons learned, the carbon cycle team wanted to add more sophisticated biogeochemical components as a standard part of CCSM.

The team plans to wrap up work on the development of CCSM4 soon, so that its model runs can be generated in time for the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). It’s currently unclear what carbon cycle components will be included in the frozen model version, Doney says. The carbon cycle group wants to add the updated carbon cycle component, but it needs to ensure that any new functionality doesn’t impede how well the model currently works.

In the realm of CCSM4 and carbon cycle capabilities, what is guaranteed so far is that the new CCSM ocean model will have fully dynamic plankton capabilities, and the new CCSM land model will have a better treatment of plants, trees, soils, and other components, ensuring that carbon cycle dynamics pertaining to ecosystem function are accurate. By spring 2009, the team will have decided what to include. Regardless, it is guaranteed to set a new benchmark for the carbon cycle work to come.