Researchers at the Max Planck Institute of Meteorology have developed a coupled climate–carbon cycle model that specifies the maximum volumes of carbon dioxide that humans may emit to remain below the critical threshold for climate warming of 2 °C, based on stabilization at 450 ppm in the 22nd century. The scientists incorporated into their calculations data relating to the carbon cycle, namely the volume of carbon dioxide absorbed and released by the oceans and forests.
According to the model, admissible carbon dioxide emissions will increase from approximately seven billion tonnes of carbon in the year 2000 to a maximum value of around ten billion tonnes in 2015. In order to achieve the long-term stabilization of the atmospheric carbon dioxide concentration, the emissions will then have to be reduced by 56% by the year 2050 and approach zero towards the end of this century.
Although, based on these calculations, global warming would remain under the two-degree threshold until 2100, further warming may be expected in the long term: “It will take centuries for the global climate system to stabilize,” says Erich Roeckner of the Max Planck Institute. A paper on the work was published in the journal Climatic Change.
The scientists used a new method with which they reconstructed historical emission pathways on the basis of already-calculated carbon dioxide concentrations. To do this, Erich Roeckner and his team adopted the methodology proposed by the International Panel on Climate Change (IPCC) for simulations being carried out for the future Fifth IPCC Assessment Report: earth system models that incorporate the carbon cycle were used to estimate the anthropogenic carbon dioxide emissions that are compatible with a prescribed concentration pathway. In this case, the emissions depend solely on the proportion of the anthropogenic carbon in the model that is absorbed by the land surface and the oceans. Repetition of the experiments using different pre-industrial starting dates enabled the scientists to distinguish between anthropogenic climate change and internal climate variability.
The model used for this study is based on a low-resolution spatial grid with a grid spacing of around 400 kilometers, which takes the atmosphere, plus the land surface, the ocean, including sea ice, and the marine and terrestrial carbon cycle into account.
The overall aim of the study is to simulate future changes in the climate and carbon dioxide emissions in a single scenario in which the carbon dioxide equivalent concentrations in the atmosphere are stabilized in the long term at 450 parts per million (ppm), so that global warming increases to a maximum of two degrees above the pre-industrial level.
The authors also noted that their results suggest that the 450 ppm stabilization scenario may not be sufficient to fulfill the European Union climate policy goal of limiting the global temperature increase to a maximum of 2 °C compared to pre-industrial levels. The data are currently being evaluated by other European climate centers.