New climate model for the IPCC

Researchers from the Alfred Wegener Institute are introducing for the first time the results of their global models directly into the Intergovernmental Panel on Climate Change database. The data is particularly interesting because the underlying model, developed at AWI, describes sea ice and oceans with a much broader definition than conventional methods. Climatologists and stakeholders around the world are using the results to determine the effects of climate change on humans and the environment.

How much will the Earth warm up in the coming decades as a result of climate change? How is it going to change our world? These are some of the most pressing issues of our time, and researchers around the world are using climate models to try to find answers. But the Earth's climate is extremely complex and modeling with super computers is difficult. Each climate model has its strengths and weaknesses. In order to better estimate the future climate evolution, the results of different climate models of the world are compared, because the comparisons make it possible to more clearly determine the most probable climatic trend and the degree of uncertainty of the forecasts. About 50 research institutes around the world are participating in this large-scale international project, known as the Coupled Models Comparison Project (CMIP). This is extremely important as the results are integrated into an international database and form the basis of the next report of the Intergovernmental Panel on Climate Change (IPCC), to be published in 2021 – the sixth evaluation report, RA6.

Only three institutions from Germany

For the first time, the Alfred Wegener Institute, the Helmholtz Center for Polar and Marine Research (AWI), with its own model, is part of this great project of international comparison. Just a few days ago, AWI researchers captured their first detailed results of climate models in the international CMIP database. "This is a peculiarity peculiar to one of the institutions that make a significant contribution to the data on which the evaluation report will be based," comments meteorologist Dr. Tido Semmler, who coordinates the work of the the AWI for the CMIP international comparison project. "Only three German research institutions are involved: the Max Planck Institute of Meteorology in Hamburg, the German Aerospace Center and us." In addition, the German Center for Climate Informatics (DKRZ) in Hamburg plays a key role, as it offers the three institutions computer time and storage space, as well as support for the conducting simulations and providing data.

There is a particular reason why AWI is now involved: the experts have used a new method, still little used, to model the climate: an "unstructured grid" which, in terms of climate research, is a mini revolution. Until now, almost all research groups around the world have worked with what we call "structured grids". The principle of these grids is simple: since global climate modeling is much too complex, researchers divide Earth and atmosphere into mesh boxes, cubes whose ridges are typically 100 km long. In these boxes, supercomputers are used to model the biological, chemical and physical processes that affect the climate. But the fact that a 100-kilometer length is far too rudimentary to take directly into account important processes – such as small eddies of just a few kilometers in the Gulf Stream and other ocean currents that result in an increased exchange of heat and heat. humidity between the sea and the atmosphere. Many climate models do not accurately describe the course of the Gulf Stream, which originates in the Gulf of Mexico and heads north along the Florida coast before heading east toward the Gulf of Mexico. from Europe. In many models, the current moves far too far in the north, small swirls not being included.

An adjustable grid

Ideally, we would have a more finely meshed global network with squares not exceeding ten kilometers. But that would increase the number of individual calculations. Even for climate simulations covering only a few years, a central computer would need several weeks. But AWI's experts have come up with an alternative "unstructured grid". This reduces the individual grid elements (approximately 10 kilometers) for selected regions. While the global climate can be modeled with the help of a grid with normal size squares, the flexibly adjustable mesh allows for some zooming in on regions. specific, such as the Gulf Stream. The unstructured grid has added an interesting new dimension to climate modeling, which is very important for the CMIP process.

A single direct comparison

"Generally, in climate modeling, we combine different models that simulate different things, for example, a model describing the ocean in detail and a second model representing processes in the atmosphere," says Tido. Semmler. "The FESOM ocean model, which we have developed, uses an unstructured network, but for the atmosphere we use a conventionally structured model developed at the Max Planck Institute of Meteorology." This makes comparisons in the context of the CMIP particularly interesting: the Max Planck Institute of Meteorology couples the atmospheric model to its oceanic model, which is based on the traditional grid. But colleagues at AWI associate the atmospheric model with their own ocean model, which uses the unstructured grid. "We and CMIP partners look forward to seeing the direct comparison of these results," said Tido Semmler.

Important data for research on the climate impact

The results of some 50 climate models, which AWI and other CMIP partners are currently feeding into the international database, will be used by many more researchers over the next two years. Especially by the experts who study the impact of climate change on the populations and habitats of our planet. These research findings will in turn form the basis of the IPCC reports; the 6th Assessment Report and the summary of the IPCC report, the synthesis report, which will include mainly policy recommendations.

Examples of modeling results (see online chart): Mean global temperature trend from 1850 to 2100 according to AWI's new climate model.

The AWI climate model includes the main natural factors of Earth's temperature, such as solar radiation, natural concentrations of greenhouse gases and aerosols, as well as volcanic aerosols. The gray line represents the series of controls with natural factors and greenhouse gas concentrations of 284 ppm CO.₂ for the year 1850. The black line indicates the average historical trend in global temperature with increasing greenhouse gas concentrations from 1850 to 400 ppm CO.₂ today, which led to a net global warming of about 1 ° C. The colored lines indicate the possible evolution of the average global temperature depending on the emission scenario.

Fluctuations in the lines show the natural variations in average global temperature without greenhouse gas emissions. For historical trend (black line) and moderate scenarios (warming of about 4 ° C with 871 ppm CO₂ in 2100) for the future (yellow line), several simulations were carried out to estimate the degree of uncertainty of the results. The AWI models have a fluctuation range of about half a degree Celsius.

For the low emissions scenario (445 ppm CO₂ in 2100), concerted efforts will have to be made to reduce greenhouse gas emissions in order to limit the rise in average global temperature to less than 2 ° C; with the high emission scenario (1142 ppm of CO₂ in 2100), it is assumed that no action has been taken to reduce greenhouse gas emissions, so that, according to the current model, the average temperature of the planet will increase. about 5 ° C.