Paleoclimate proxy evidence suggests that the climatic impacts of Heinrich events extended far beyond the North Atlantic basin (1). Enduring questions include whether Heinrich events initiated climate change or occurred in response to it, and how they may be linked to abrupt millennial-scale climate oscillations, typified by Dansgaard-Oeschger (DO) cycles in Greenland ice cores (2). Early correlation between DO cycle variability and sea surface temperature (SST) proxies found that Heinrich events occurred within the coldest, longest stadials of the Greenland δ18Oice record (3), referred to as Heinrich stadials (HSs) 1 to 6. This correlation could suggest causation: The addition of fresh water causes a shutdown of the Atlantic meridional overturning circulation (AMOC) and prolonged cold conditions (4). However, proxy evidence suggests that AMOC had already slowed and that North Atlantic SSTs were already cold before ice-rafted debris (IRD) deposition, challenging this view (2). Reconstructing event phasing across different proxy records is hampered by the difficulty in dating Heinrich sediment deposits (1). As of yet, the intrastadial timings of individual Heinrich events have not been well determined, and estimates of individual event duration vary by an order of magnitude (1). The sequence and phasing of millennial-scale climate phenomena appear to be intrinsically linked to the initiation of deglaciation (5), providing additional impetus for investigation.

Here we present a precise, highly resolved record of atmospheric methane (CH4) concentrations, determined by a recently developed continuous measurement technique (6), for the Last Glacial Period and the deglacial transition from the West Antarctic Ice Sheet (WAIS) Divide (WD) ice core (Fig. 1). Ice core CH4 is a globally integrated signal, primarily reflecting the response of the terrestrial biosphere, predominantly wetlands, to hydroclimatic change (7). The WD trace gas record is especially appropriate for paleoreconstruction because rapid occlusion of air bubbles in the firn column results in minimal smoothing of atmospheric signals (estimated gas age distribution width 20 to 50 years, fig. S1). The sampling resolution of our CH4 record varies between 0.5 and 13 years, meaning that all climate-driven CH4 signals preserved in the ice are captured (8). Continuous CH4 analysis produces measurements with excellent internal precision [≤ ± 0.5 parts per billion (ppb), 2σ] that are reproducible to within ±3 ppb (2σ) over intervals of days to weeks。