Arctic warming, associated with climate change, is the leading cause of the rapid melting of Arctic sea ice and Greenland ice sheet. According to Dániel Topál, junior research fellow at the Institute for Geological and Geochemical Research of the ELKH Research Centre for Astronomy and Earth Sciences (CSFK), a breakthrough is close in understanding the uncertainties surrounding model projections of the first ice-free Arctic summer. The results suggest that climate models overestimate Arctic ice melt caused by anthropogenic greenhouse gases, which overestimation is a sort-of compensation for the lack of a comprehensive representation of planetary-scale atmospheric wave motions generated by tropical ocean-atmosphere interactions in climate models. Correcting for this effect, the researchers paint a much more optimistic picture of the future. According to the results, the probability of the first ice-free summer could be extended by at least a decade. The paper presenting the study was published in a leading scientific journal, Nature Climate Change.
Increases in atmospheric concentrations of so-called greenhouse gases released into the atmosphere by industry, agriculture and other human activities are clearly warming the planet. Perhaps the most tangible sign of this is the rapid melting of Arctic sea ice and the Greenland ice sheet. Reducing uncertainties around the future of the Arctic is of paramount importance because the Arctic is warming at a much higher rate per unit of carbon dioxide emitted than the global average. This is known as climate sensitivity, the accurate quantification of which is one of the greatest challenges and unresolved problems facing climate scientists today. This is the task undertaken by Dániel Topál, a PhD student at CSFK at the beginning of the research, who has now obtained his doctorate.
"A common practice to evaluate the 'accuracy' of climate models is to directly compare the polar ice melt simulated by the models with the ice melt observed from satellite measurements. However, this approach is missing a step that has been overlooked in research to date. The main problem is that computer models of the climate system give much less weight to ice melting caused by planetary-scale atmospheric circulation compared to real observations. Therefore, we need to consider the role of large-scale atmospheric circulation changes when assessing the climate sensitivity of the Arctic."
Using a proprietary modelling approach, already published recently by Dániel Topál et al., the authors have accurately quantified atmospheric circulation-induced melting in a complex coupled ice-ocean-atmosphere-land climate model (Community Earth System Model v.2), which explicitly incorporates the physics of the melting of the Greenland ice sheet and Arctic sea ice. In this experiment, the modelled wind field was replaced by a wind field based on real observations from a so-called climate reanalysis, while the atmospheric concentration of carbon dioxide in the model was kept constant. This allowed the researchers to accurately separate wind-induced ice melting from the direct melting caused by carbon dioxide emissions. Arctic winds are responsible for about 50 percent of the melting seen over the last four decades. The article estimates that the same figure is just over two percent in the climate models used in the latest report of the Intergovernmental Panel on Climate Change (IPCC). The extent to which carbon dioxide forcing is responsible for wind changes in the models is irrelevant to the methodology, making the model evaluation procedure proposed by the researchers more general and more informative than current best practice.
"If ice melt simulated in a climate model is due to different physical causes than in reality, we cannot directly compare the model output with our measurements. In each available model, I calculated the difference between the melting caused by the modelled wind field and the 'real' wind field. It was striking to see that the error was systematic across all models. It was then that the real scale of the problem became clear. I felt it was important to develop a simple procedure to correct this error, which we could then present to the climate community as an alternative route."
The result of the procedure is that correcting the error slows down the rate of greenhouse gas forcing-induced ice melt predicted by the models. This is a direct consequence of the fact that the models predict ice melt at a rate similar to satellite measurements for each unit increase in carbon dioxide concentration over the past four decades, but without simulated wind effects. The authors' conclusions could fundamentally shape the methods used to evaluate the next generation of climate models for climate sensitivity.
"Knowing that the effects of the polar atmospheric circulation are of paramount importance for understanding the physical mechanism behind the melting, the models compensate for the missing wind effect by 'over-emphasizing' the greenhouse warming. It is important to note that this in no way implies that the role of humans in modifying the climate is in question, but rather that our article draws attention to the real dangers of overtuning models to replicate past climate change. We still are unsure of whether what portion of the observed winds might be caused by anthropogenic forcing."
Fig. 1. For three different emissions scenarios (Community Earth System Model 2, red; Max Plank Institute Grand Ensemble, blue; and 21 models from the Coupled Model Intercomparison Project CMIP6, black), cumulative probability density functions for the Arctic to become sea-ice-free are shown (solid lines). If atmospheric-circulation observations are accounted for, the functions shift (dashed lines; uncertainties indicated by thin dashed lines): all three ensembles signal a delay of roughly ten years for the Arctic to become sea-ice-free (dates below the horizontal axis).