Back to the future? How lessons from the climate of the past can prepare us for the time ahead

• Main Author: Stepanek, Christian
• Format: Conference Object
• Language: unknown
• Published: Elite Netzwerk Bayern 2020
• Subjects: Sea ice
• Online Access: https://epic.awi.de/id/eprint/51244/; https://hdl.handle.net/10013/epic.32c77ca0-1de3-4d44-841f-d47428c4a11a

Climate patterns are influenced by internal variability and forcing. A major forcing is carbon dioxide that influences, together with other greenhouse gases, the equilibrium temperature of the Earth system. Over millions of years, carbon dioxide concentrations in the atmosphere have been regulated by a fine balance between outgassing from the Earth’s mantle – via volcanic activity – and removal and sequestration – via chemical weathering. Small disturbances of this equilibrium have been amplified by climate system feedbacks and caused, over millions of years, a transition of the Earth system from a nearly ice free “hothouse” state to the modern glaciated “icehouse”, with major ice sheets at high latitudes. Over the last two million years, orbital forcing, i.e. the astronomical configuration of the Earth-Sun-system – that is related to quasi-periodical climate transitions at multi-millennial time scale – had strongest control on climate. Variations of the Earth’s orbital elements create pronounced oscillations between more and less extensive glaciation in particular of the Northern Hemisphere. These are known as the glacial-interglacial cycles of the Pleistocene. Since the dawn of the industrial era climate has been deteriorated at a rate that is unique during the recent geologic history of the last 50-60 million years. Current rise in carbon dioxide occurs at a rate that is unprecedented, bringing us from the stage of Pleistocene glacial cycles again closer to a hothouse climate. Current levels of carbon dioxide excite the climate/earth system to a state far beyond its natural equilibrium. Climate system components with large thermal inertia, including ice sheets and oceans, cause delayed reaction of the climate and earth system to anthropogenic activity, and cause continued warming even if current levels of anthropogenic greenhouse forcing are stabilized. Recent research highlights that the climate system’s reaction to anthropogenic forcing intensifies. Related to distortion of the probability density function of climate variables, e.g. temperature, formerly extreme weather conditions become more abundant. Record-breaking ocean heat content, accelerated retreat of smaller ice masses like alpine glaciers, and minimum sea ice extent show that climate is changing at a pace so far not experienced by modern societies. We will approach the characteristics of a warmer than modern world, as it is projected for the future, from the perspective of the geological past. Earth history contains examples of climate states both colder and warmer than the current one. We will learn about potential future climate conditions by means of combining inference from geological and glaciological records with climate modelling. Records, or archives, provide detailed, but very localized, information on past environmental conditions. These can be translated into information on the climate of the past. Climate models, on the other hand, enable the direct study of dynamics of past, present and future climates. Marrying models and archives enables us to learn about both characteristics and dynamics of the climate of the past. Furthermore, we may identify those characteristics of past climates that may be expected again in the future. Last but not least, we will learn that the past provides a test-bed where we can evaluate climate models with respect to their applicability for future warmer-than-present climate states.