ICE&LASERS propose to make a breakthrough in two challenges of paleoclimate science:(1) Extending the Antarctic ice core records to 1.5 million years ago is critical to understand the unexplained climate shift from 40,000-year periodicities to 100,000-year ones, calling for a different climate sensitivity to orbital forcing. We propose to revolutionize ice core science by building an innovative probe making its own way into the ice sheet within a single field season, to measure in situ the depth profile of H2O isotopes in ice as well as greenhouse gas concentration in trapped gases, down to bedrock. This high gain/high risk project will allow us to rapidly qualify different “oldest ice” sites, and to immediately obtain the main climatic signals of interest;(2) Why the atmospheric CO2 and CH4 concentrations varied by up to 40 and 100%, respectively, during glacial-interglacial cycles is still highly debated. We will combine revolutionary detectors with new extraction techniques to measure with unsurpassed accuracy and resolution the concentrations of CH4, CO2 and CO (a tracer related to the CH4 cycle), and the isotopic ratios of CO2 and CO in polar ice. We will constrain theories of past changes in the carbon cycle and of climate feedbacks, and will provide more insight into possible natural feedbacks under a warming future.ICE&LASERS tackles both scientific challenges, thanks to an analytical revolution for measuring trace gases and their stable isotopes: Optical-Feedback Cavity-Enhanced Absorption Spectroscopy (OFCEAS), recently patented by one of the four CNRS research units involved in the project. ICE&LASERS will contribute to maintain European ice core science at its current leading position, and to optimize the transfer of innovative laser physics to important environmental problems.