Description: The Pliocene Model Intercomparison Project is the first coordinated climate model comparison for a warmer palaeoclimate with atmospheric CO2 significantly higher than pre-industrial concentrations. The simulations of the mid-Pliocene warm period show global warming of between 1.8 and 3.6 °C above pre-industrial surface air temperatures, with significant polar amplification. Here we perform energy balance calculations on all eight of the coupled ocean–atmosphere simulations within PlioMIP Experiment 2 to evaluate the causes of the increased temperatures and differences between the models. In the tropics simulated warming is dominated by greenhouse gas increases, with cloud albedo feedbacks enhancing the warming in most of the models, but by widely varying amounts. The responses to mid-Pliocene climate forcing in the Northern Hemisphere mid-latitudes are substantially different between the climate models, with the only consistent response being a warming due to increased greenhouse gases. In the high latitudes all the energy balance components become important, but the dominant warming influence comes from the clear sky albedo. This demonstrates the importance of specified ice sheet and high latitude vegetation boundary conditions and simulated sea ice and snow albedo feedbacks. The largest components in the overall uncertainty are associated with cloud albedo feedbacks in the tropics and polar clear sky albedo, particularly in sea ice regions. These simulations show that high latitude albedo feedbacks provide the most significant enhancements to Pliocene greenhouse warming.

Description: Based on simulations with 15 climate models in the Pliocene Model Intercomparison Project (PlioMIP), the regional climate of East Asia (focusing on China) during the mid-Pliocene is investigated in this study. Compared to the pre-industrial, the multi-model ensemble mean (MMM) of all models shows the East Asian summer winds (EASWs) largely strengthen in monsoon China, and the East Asian winter winds (EAWWs) strengthen in south monsoon China but slightly weaken in north monsoon China in the mid-Pliocene. The MMM of all models also illustrates a warmer and wetter mid-Pliocene climate in China. The simulated weakened mid-Pliocene EAWWs in north monsoon China and intensified EASWs in monsoon China agree well with geological reconstructions. However, there is a large model–model discrepancy in simulating mid-Pliocene EAWW, which should be further addressed in the future work of PlioMIP.

Description: In the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2), coupled climate models have been used to simulate an interglacial climate during the mid-Piacenzian warm period (mPWP; 3.264 to 3.025 Ma). Here, we compare the Atlantic Meridional Overturning Circulation (AMOC), poleward ocean heat transport and sea surface warming in the Atlantic simulated with these models. In PlioMIP2, all models simulate an intensified mid-Pliocene AMOC. However, there is no consistent response in the simulated Atlantic ocean heat transport nor in the depth of the Atlantic overturning cell. The models show a large spread in the simulated AMOC maximum, the Atlantic ocean heat transport and the surface warming in the North Atlantic. Although a few models simulate a surface warming of ∼ 8–12 ∘C in the North Atlantic, similar to the reconstruction from Pliocene Research, Interpretation and Synoptic Mapping (PRISM) version 4, most models appear to underestimate this warming. The large model spread and model–data discrepancies in the PlioMIP2 ensemble do not support the hypothesis that an intensification of the AMOC, together with an increase in northward ocean heat transport, is the dominant mechanism for the mid-Pliocene warm climate over the North Atlantic.

Description: There is an increasing need to understand the pre-Quaternary warm climates, how climate–vegetation interactions functioned in the past, and how we can use this information to understand the present. Here we report vegetation modelling results for the Late Miocene (11–7 Ma) to study the mechanisms of vegetation dynamics and the role of different forcing factors that influence the spatial patterns of vegetation coverage. One of the key uncertainties is the atmospheric concentration of CO2 during past climates. Estimates for the last 20 million years range from 280 to 500 ppm. We simulated Late Miocene vegetation using two plausible CO2 concentrations, 280 ppm CO2 and 450 ppm CO2, with a dynamic global vegetation model (LPJ-GUESS) driven by climate input from a coupled AOGCM (Atmosphere-Ocean General Circulation Model). The simulated vegetation was compared to existing plant fossil data for the whole Northern Hemisphere. For the comparison we developed a novel approach that uses information of the relative dominance of different plant functional types (PFTs) in the palaeobotanical data to provide a quantitative estimate of the agreement between the simulated and reconstructed vegetation. Based on this quantitative assessment we find that pre-industrial CO2 levels are largely consistent with the presence of seasonal temperate forests in Europe (suggested by fossil data) and open vegetation in North America (suggested by multiple lines of evidence). This suggests that during the Late Miocene the CO2 levels have been relatively low, or that other factors that are not included in the models maintained the seasonal temperate forests and open vegetation.

Description: Eight general circulation models have simulated the mid-Pliocene warm period (mid-Pliocene, 3.264 to 3.025 Ma) as part of the Pliocene Modelling Intercomparison Project (PlioMIP). Here, we analyse and compare their simulation of Arctic sea ice for both the pre-industrial period and the mid-Pliocene. Mid-Pliocene sea ice thickness and extent is reduced, and the model spread of extent is more than twice the pre-industrial spread in some summer months. Half of the PlioMIP models simulate ice-free conditions in the mid-Pliocene. This spread amongst the ensemble is in line with the uncertainties amongst proxy reconstructions for mid-Pliocene sea ice extent. Correlations between mid-Pliocene Arctic temperatures and sea ice extents are almost twice as strong as the equivalent correlations for the pre-industrial simulations. The need for more comprehensive sea ice proxy data is highlighted, in order to better compare model performances.

Description: During an interval of the Late Pliocene, referred to here as the mid-Pliocene Warm Period (mPWP; 3.264 to 3.025 million years ago), global mean temperature was similar to that predicted for the end of this century, and atmospheric carbon dioxide concentrations were higher than pre-industrial levels. Sea level was also higher than today, implying a significant reduction in the extent of the ice sheets. Thus, the mPWP provides a natural laboratory in which to investigate the long-term response of the Earth's ice sheets and sea level in a warmer-than-present-day world. At present, our understanding of the Greenland ice sheet during the mPWP is generally based upon predictions using single climate and ice sheet models. Therefore, it is essential that the model dependency of these results is assessed. The Pliocene Model Intercomparison Project (PlioMIP) has brought together nine international modelling groups to simulate the warm climate of the Pliocene. Here we use the climatological fields derived from the results of the 15 PlioMIP climate models to force an offline ice sheet model. We show that mPWP ice sheet reconstructions are highly dependent upon the forcing climatology used, with Greenland reconstructions ranging from an ice-free state to a near-modern ice sheet. An analysis of the surface albedo variability between the climate models over Greenland offers insights into the drivers of inter-model differences. As we demonstrate that the climate model dependency of our results is high, we highlight the necessity of data-based constraints of ice extent in developing our understanding of the mPWP Greenland ice sheet.

Description: In the Pliocene Model Intercomparison Project (PlioMIP), eight state-of-the-art coupled climate models have simulated the mid-Pliocene warm period (mPWP, 3.264 to 3.025 Ma). Here, we compare the Atlantic Meridional Overturning Circulation (AMOC), northward ocean heat transport and ocean stratification simulated with these models. None of the models participating in PlioMIP simulates a strong mid-Pliocene AMOC as suggested by earlier proxy studies. Rather, there is no consistent increase in AMOC maximum among the PlioMIP models. The only consistent change in AMOC is a shoaling of the overturning cell in the Atlantic, and a reduced influence of North Atlantic DeepWater (NADW) at depth in the basin. Furthermore, the simulated mid-Pliocene Atlantic northward heat transport is similar to the pre-industrial. These simulations demonstrate that the reconstructed high-latitude mid-Pliocene warming can not be explained as a direct response to an intensification of AMOC and concomitant increase in northward ocean heat transport by the Atlantic.

Description: The Pliocene Model Intercomparison Project (PlioMIP) is the first coordinated climate model comparison for a warmer palaeoclimate with atmospheric CO2 significantly higher than pre-industrial concentrations. The simulations of the mid-Pliocene warm period show global warming of between 1.8 and 3.6 °C above pre-industrial surface air temperatures, with significant polar amplification. Here we perform energy balance calculations on all eight of the coupled ocean–atmosphere simulations within PlioMIP Experiment 2 to evaluate the causes of the increased temperatures and differences between the models. In the tropics simulated warming is dominated by greenhouse gas increases, with the cloud component of planetary albedo enhancing the warming in most of the models, but by widely varying amounts. The responses to mid-Pliocene climate forcing in the Northern Hemisphere midlatitudes are substantially different between the climate models, with the only consistent response being a warming due to increased greenhouse gases. In the high latitudes all the energy balance components become important, but the dominant warming influence comes from the clear sky albedo, only partially offset by the increases in the cooling impact of cloud albedo. This demonstrates the importance of specified ice sheet and high latitude vegetation boundary conditions and simulated sea ice and snow albedo feedbacks. The largest components in the overall uncertainty are associated with clouds in the tropics and polar clear sky albedo, particularly in sea ice regions. These simulations show that albedo feedbacks, particularly those of sea ice and ice sheets, provide the most significant enhancements to high latitude warming in the Pliocene.

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.