First successful model of climate change simulation over the past 3 million years RealClimate

Guest article by Matteo Willeit, Potsdam Institute for Climate Impact Research

A new study published in Progress of science shows that the main characteristics of the natural variability of the climate during the last 3 million years can be reproduced with an effective model of the terrestrial system.

the
The Quaternary is the most recent geological period, covering the past ~ 2.6 million
years. It is defined by the presence of associated glacial and interglacial cycles
with the cyclical growth and disintegration of the continental ice sheets in the north
Hemisphere. Climatic variations
during the Quaternary are best seen in the isotopes of oxygen measured in the deep-sea sediment cores, which represent the variations of ice in the world
volume and temperature of the ocean. These data clearly show that there is a general trend towards
larger ice caps and cooler temperatures over the last 3 million years,
accompanied by an increase in the magnitude of glacial-interglacial variations
and a transition of mostly symmetry cycles with a periodicity of 40,000 years
highly asymmetrical cycles of 100,000 years to about 1 million years ago. However, the ultimate causes of these
transitions in the dynamics of the ice cycle are still discussed.

Among
others, the role of CO2 changes in the formation of Quaternary climate dynamics is not essential.
still largely understood, largely because of the low observational
CO2 concentrations in the atmosphere for the period prior to 800 000 BP years, beyond
period covered by high quality data on ice cores.

In an article published today in Science Advances (Williet et al., 2019)we have been able to reproduce the natural variability of the Quaternary climate with a terrestrial model of intermediate complexity. In addition to the ocean and the atmosphere, the model includes interactive ice caps for the northern hemisphere and a fully coupled global carbon cycle. It is solely motivated by changes in the orbital configuration and different scenarios for slowly varying boundary conditions, namely CO.₂ degassing of volcanoes as a geological source of CO₂, and changes in sediment distribution on continents.

Model simulations provide self-consistent reconstruction of CO₂, climate and ice sheets limited by available observations, ie isotopes of oxygen and reconstructions of sea surface temperature. The fact that the model can reproduce the main characteristics of the history The observed climate gives us confidence in our general understanding of the functioning of the climate system and provides some constraints on the contribution of external forcings and internal feedbacks to climate variability.

Our results imply a high sensitivity of the Earth's system to relatively small variations in CO₂. A gradual decrease of CO₂ values ​​lower than ~ 350 ppm caused the beginning of ice sheet growth in Greenland and more generally on the NH at the end of the Pliocene at the beginning of the Pleistocene. Subsequently, the waxing and weakening of the ice sheets gradually removed the thick layer of unconsolidated sediments that had previously formed on continents as a result of weathering. weather over millions of years. The erosion of this layer of sediment – essentially destroyed by bulldozing by glacier movement – has affected the evolution of glacial cycles in several ways. First, ice sheets sitting on soft sediments are generally more mobile than those on hard bedrock, as ice slides more easily over sediments than bedrock. In addition, the transport of glacial sediments to ice sheet margins generates significant amounts of dust, which, when deposited on the surface of the icecap, increases the melting of ice caps by lowering surface albedo. Our results show that the gradual increase in the exposed bedrock surface gradually led to the formation of more stable ice sheets less sensitive to orbital forcing, which finally paved the way for the transition to 100,000-year cycles. about 1 million years ago.

The simulations further suggest that the global temperature never exceeded the pre-industrial value of more than 2 ° C in the Quaternary. The evolution of the ice cap is very sensitive to temperature and the initiation of NH glaciation about 3 million years ago would not have been possible in the model if the global temperature had been higher than 2 ° C compared to the preindustrial period at the beginning of the Quaternary. As the model has been shown to accurately reproduce sea level changes over the last 400,000 years as well as the spatial distribution of the ice cap at the last glacial maximum (Ganopolski & Brovkin 2017)we are convinced that the sensitivity of ice sheets to climate is well represented in the model.

Similarly, our results indicate that current CO₂ a concentration of about 410 ppm is unprecedented for 3 million years. The model's climate sensitivity is around 3 ° C for a doubling of CO₂ concentration, which is at the center of the range of current best estimates of climate sensitivity, between 1.5 and 4.5 ° C. It is possible that the actual climate sensitivity is less than 3 ° C. In this case, the modeled value of CO₂ The concentration needed to match the oxygen isotope registration at the start of the Quaternary would be higher than in the simulations of the current model, but it would still be unlikely that it would exceed the current value. In the context of future climate change, our results imply that a failure to significantly reduce CO₂ Emissions in line with the Paris Agreement's goal of limiting global warming to well below 2 ° C will not only drive Earth's climate away from Holocene-like conditions, but will also push it beyond the observed climatic conditions throughout the current geological period.

References

1. M. Willeit, A. Ganopolski, R. Calov and V. Brovkin, "The Mid-Pleistocene transition in glacial cycles explained by the reduction of CO2 and the elimination of the regolith", Progress of science, flight. 5, pp. Eaav7337, 2019. http://dx.doi.org/10.1126/sciadv.aav7337

2. A. Ganopolski and V. Brovkin, "Simulation of the climate, ice sheets and CO₂ evolution over the last four glacial cycles with a model of terrestrial system of intermediate complexity ", Climate of the past, flight. 13, pp. 1695-1716, 2017. http://dx.doi.org/10.5194/cp-13-1695-2017