Description: Thermodynamic arguments imply that global mean rainfall increases in a warmer atmosphere; however, dynamical effects may result in more significant diversity of regional precipitation change. Here we investigate rainfall changes in the mid-Pliocene Warm Period (~ 3 Ma), a time when temperatures were 2–3ºC warmer than the pre-industrial era, using output from the Pliocene Model Intercomparison Projects phases 1 and 2 and sensitivity climate model experiments. In the Mid-Pliocene simulations, the higher rates of warming in the northern hemisphere create an interhemispheric temperature gradient that enhances the southward cross-equatorial energy flux by up to 48%. This intensified energy flux reorganizes the atmospheric circulation leading to a northward shift of the Inter-Tropical Convergence Zone and a weakened and poleward displaced Southern Hemisphere Subtropical Convergences Zones. These changes result in drier-than-normal Southern Hemisphere tropics and subtropics. The evaluation of the mid-Pliocene adds a constraint to possible future warmer scenarios associated with differing rates of warming between hemispheres.

Description: 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. In this talk we will review recent climate change and survey methods of climate research. This will be put into context of future climate projections.

Description: 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.

Description: Deglacial transitions of the middle to late Pleistocene (terminations) are linked to gradual changes in insolation accompanied by abrupt shifts in ocean circulation. However, the reason these deglacial abrupt events are so special compared with their sub-glacial-maximum analogues, in particular with respect to the exaggerated warming observed across Antarctica, remains unclear. Here we show that an increase in the relative importance of salt versus temperature stratification in the glacial deep South Atlantic decreases the potential cooling effect of waters that may be upwelled in response to abrupt perturbations in ocean circulation, as compared with sub-glacial-maximum conditions. Using a comprehensive coupled atmosphere–ocean gen-eral circulation model, we then demonstrate that an increase in deep-ocean salinity stratification stabilizes relatively warm waters in the glacial deep ocean, which amplifies the high southern latitude surface ocean temperature response to an abrupt weakening of the Atlantic meridional overturning circulation during deglaciation. The mechanism can produce a doubling in the net rate of warming across Antarctica on a multicentennial timescale when starting from full glacial conditions (as compared with interglacial or subglacial conditions) and therefore helps to explain the large magnitude and rapidity of glacial termina-tions during the late Quaternary.