Climate research is of immediate relevance to society because we need to understand how the complex Earth System reacts to human-caused changes in the composition of the atmosphere, in particular greenhouse gases. Comprehensive climate research rests on three pillars: Observation, theoretical understanding and model development, and climate model simulations.In a large collaborative effort we combine the three pillars. For the first pillar, we develop and apply high-precision methods to measure concentrations of gases and their isotopes, and chemical substances on polar ice cores. We focus on abrupt climate change found in high-resolution records, their effects on the various components of the climate system, and their regional expression. In addition we measure very low concentrations of some unique radio-isotopes to estimate groundwater storage and renewal in reservoirs around the world. An essential element is the development of climate models including biogeochemical components which are tested by modern observations. We employ a hierarchy of climate models ranging from process models, models of reduced complexity, to comprehensive climate models. Model development and testing constitutes the second pillar of this project. The third pillar is extensive model simulations of many paleoclimatic variables and a direct comparison of the results with the paleoclimatic records. In many cases we aim at simulating changes in the past which have not yet been measured in paleoclimatic archives. This involves simulations over complete ice age cycles, in particular before 800,000 years before the present. These same models are then used to project, in a probabilistic framework, anthropogenic climate change with the goal to learn more about the global responses and sensitivity of the climate system to this unprecedented perturbation.