Precise timing and characterization of abrupt climate change 8200 years ago from air trapped in polar ice

International audienceHow fast and how much climate can change has significant implications for concerns about future climate changes and their potential impacts on society. An abrupt climate change 8200 years ago (8.2 ka event) provides a test case to understand possible future climatic variability. Here, methane concentration (taken as an indicator for terrestrial hydrology) and nitrogen isotopes (Greenland temperature) in trapped air in a Greenland ice core (GISP2) are employed to scrutinize the evolution of the 8.2 ka event. The synchronous change in methane and nitrogen implies that the 8.2 ka event was a synchronous event (within ±4 years) at a hemispheric scale, as indicated by recent climate model results [Legrande, A. N., Schmidt, G. A., Shindell, D. T., Field, C. V., Miller, R. L., Koch, D. M., Faluvegi, G., Hoffmann, G., 2006. Consistent simulations of multiple proxy responses to an abrupt climate change event. Proceedings of the National Academy of Sciences 103, 837–842]. The event began with a large-scale general cooling and drying around 8175±30 years BP (Before Present, where Present is 1950 AD). Greenland temperature cooled by 3.3±1.1 °C (decadal average) in less than 20 years, and atmospheric methane concentration decreased by 80±25 ppb over 40 years, corresponding to a 15±5% emission reduction. Hemispheric scale cooling and drying, inferred from many paleoclimate proxies, likely contributed to this emission reduction. In central Greenland, the coldest period lasted for 60 years, interrupted by a milder interval of a few decades, and temperature subsequently warmed in several steps over 70 years. The total duration of the 8.2 ka event was roughly 150 years