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New Climate Model for the Intergovernmental Panel on Climate Change

How much could the Earth heat up in the coming decades as a result of climate change? How would this change our world? These are some of the most pressing questions of our time, and researchers around the globe are using numerical climate models in an attempt to find answers to them. But Earth’s climate is extremely complex and difficult to model on supercomputers. Each climate model has its specific strengths and weaknesses. In order to realistically estimate the bandwidth of possible future climate developments, the results of all major climate models worldwide are conjointly evaluated and compared. In this way, scientists estimate the uncertainty in the climate projections and derive the most likely answers of the climate system to the given scenarios.

About 50 research institutions worldwide are taking part in this extensive international project, known as the Coupled Model Intercomparison Project (CMIP). It is extremely important because the results contribute to an international database and form the basis for the next Assessment Report AR6 of the Intergovernmental Panel on Climate Change (IPCC), which will be published in 2021.

Over the last decade, the Alfred Wegener Institute (AWI) in Bremerhaven has developed the new sea ice ocean model FESOM. It is characterized by a flexible mesh that can be configured to highly resolve dynamically active regions of the ocean such as the Gulf Stream, the North Atlantic Current, the Kuroshio Current, the Southern Ocean, or coastal regions. In dynamically less active regions such as the subtropics, a coarser mesh resolution is used to save computing time. FESOM has been coupled to the atmospheric climate model ECHAM from the Max Planck Institute for Meteorology in Hamburg, which is well-established and has been used in various previous international model intercomparison projects. The new coupled climate model is called AWI-CM (Alfred Wegener Institute Climate Model). Between 2017 and 2019, the model simulations with AWI-CM for the Coupled Model Intercomparison Project 6 (CMIP6) were carried out at DKRZ. The flexible horizontal grid point spacing for both ocean and sea ice ranges between approximately 8 km in dynamically active regions and approximately 80 km in the subtropics (Figure 1). The atmosphere model is run with a constant horizontal resolution of approximately 100 km. 

 

Figure 1: Horizontal grid resolution of the ocean and sea ice component of the AWI-CM model used for the CMIP6 model simulations.

The CMIP6 model simulations encompass a 500 year spin-up run (stabilization of the model climate), a 500 year control run (investigation of the pre-industrial climate, constant 1850 conditions), historical runs for 1850 to 2014, idealized climate change simulations with 1% per year CO2 increase or abrupt change to 4*CO2, as well as eight scenario runs covering the period from 2015 to 2100. The computations on DKRZ’s supercomputer Mistral consumed nearly 2 million node hours and produced 2.7 petabytes of data (2.7 million gigabytes) being stored on disk space and archived on tape using the high performance storage system.

The most important model data, i.e. the key data of the historical simulation and the future scenario simulations, have already been published through DKRZ’s ESGF node (of the international distributed climate data base Earth System Grid Federation) and are therefore available to scientists worldwide. DKRZ has made according tools available and helped with hands-on-training to convert the data into the worldwide standardized format needed for publication.

Video 1:  Change in the 2m-Temperature relative to the period 1995-2014 for scenario SSP370 simulated with AWI-CM.

The global mean temperature change simulated with AWI-CM for the different scenarios up to the year 2100 (Figure 2) is of the order of the according results of the previous international model intercomparison CMIP5, while the Arctic sea ice extent is declining faster compared with the CMIP5 average. Areas that are wet in present-day climate become mostly wetter and areas that are dry in present-day climate become mostly drier in the future. This is consistent with previous climate model simulations. Ocean currents remain comparatively stable in the AWI climate simulations which leads to a continued warm Gulf Stream and North Atlantic Current and therefore a comparatively pronounced warming of the North Atlantic and parts of Europe.

Figure 2: Development of the historical and possible future deviations of the global mean temperature from the climatological 30-year average (1951-1980) in °C from computations with the model AWI-CM and from observations from the Goddard Institute for Space Studies (GISS, purple line).

To illustrate the different experiments, the graph in Figure 2 shows the global mean 2m air temperature anomaly relative to the simulated 30-years mean temperature of the period 1951-1980 for the different experiments carried out with AWI-CM within the framework of CMIP6. The main natural drivers of the Earth’s temperature, such as solar radiation, natural greenhouse gases and aerosol concentrations, as well as volcanic aerosols have been considered. The grey line represents the control run (piControl) with natural drivers and greenhouse gas concentrations on pre-industrial level (284 ppm CO2). The black lines show the simulated historical mean development of the global temperature with increasing greenhouse gas concentrations from 1850 up to 400ppm CO2 today, which have led to a net global warming of circa 1°C. The colored lines show the calculated possible future developments of the mean global temperature for the different emission scenarios (low emissions: SSP126, medium emissions: SSP245, medium high emissions: SSP370, high emissions: SSP585). In the cases of the historical development (black lines) and of the medium high emission scenario SSP370 (around 4°C warming at 871 ppm CO2 in 2100 compared to 1850) for the future (yellow lines) several calculations have been performed, respectively, to estimate the uncertainty in the results. It becomes clear that the different simulations yield very similar results except for natural year-to-year fluctuations of up to 0.5°C. The results are therefore statistically robust.

The low emission scenario (445 ppm CO2 in 2100, SSP126) requires concentrated efforts to reduce greenhouse gas emissions in order to keep the 2-degree goal i.e. to limit the rise in the global mean temperature to 2°C compared to 1850. The high emission scenario (1142 ppm CO2 in 2100, SSP585) assumes that no measures are taken to reduce greenhouse gas emissions. The present calculations show that, as a consequence, the global mean temperature could rise by about 5° C compared to 1850.

Author/Contact:


Dr. Tido Semmler, Alfred Wegener Institute, Helmholtz Center for Polar- and Marine Research (Email:  , Phone: +49 471 4831 2287)

 

Publication:
Semmler, T., Danilov, S., Gierz, P., Goessling, H., Hegewald, J., Hinrichs, C., Koldunov, N.V., Khosravi, N., Mu, L., Rackow, T., Sein, D., Sidorenko, D., Wang, Q., Jung, T. (submitted 2019): Simulations for CMIP6 with the AWI climate model AWI-CM-1-1, submitted to Journal of Advances in Modeling Earth Systems (JAMES), Preprint on Earth and Space Science Open Archive (ESSOAr) under https://www.essoar.org/doi/10.1002/essoar.10501538.1

Semmler, T.; Danilov, S.; Rackow, T.; Sidorenko, D.; Barbi, D.; Hegewald, J.; Sein, D.; Wang, Q.; Jung, T. (2018). AWI AWI-CM1.1MR model output prepared for CMIP6 CMIP. Version 20191219. Earth System Grid Federation. https://doi.org/10.22033/ESGF/CMIP6.359

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