Atmospheric methane (CH 4) is a potent greenhouse gas, and its mole fraction has more than doubled since the preindustrial era 1. Fossil fuel extraction and use are among the largest anthropogenic sources of CH 4 emissions, but the precise magnitude of these contributions is a subject of debate 2,3. Carbon-14 in CH 4 (14 CH 4) can be used to distinguish between fossil (14 C-free) CH 4 emissions and contemporaneous biogenic sources; however, poorly constrained direct 14 CH 4 emissions from nuclear reactors have complicated this approach since the middle of the 20th century 4,5. Moreover, the partitioning of total fossil CH 4 emissions (presently 172 to 195 teragrams CH 4 per year) 2,3 between anthropogenic and natural geological sources (such as seeps and mud volcanoes) is under debate; emission inventories suggest that the latter account for about 40 to 60 teragrams CH 4 per year 6,7. Geological emissions were less than 15.4 teragrams CH 4 per year at the end of the Pleistocene, about 11,600 years ago 8 , but that period is an imperfect analogue for present-day emissions owing to the large terrestrial ice sheet cover, lower sea level and extensive permafrost. Here we use preindustrial-era ice core 14 CH 4 measurements to show that natural geological CH 4 emissions to the atmosphere were about 1.6 teragrams CH 4 per year, with a maximum of 5.4 teragrams CH 4 per year (95 per cent confidence limit)-an order of magnitude lower than the currently used estimates. This result indicates that anthropogenic fossil CH 4 emissions are underestimated by about 38 to 58 teragrams CH 4 per year, or about 25 to 40 per cent of recent estimates. Our record highlights the human impact on the atmosphere and climate, provides a firm target for inventories of the global CH 4 budget, and will help to inform strategies for targeted emission reductions 9,10. 14 CH 4 emissions from nuclear power plants 4,5. By contrast, palaeoatmospheric 14 CH 4 measurements from ice cores offer a direct constraint on natural geological CH 4 emissions without these complications. Whereas geological CH 4 emissions have the potential to change on tectonic-and glacial-cycle timescales 14 , they have very probably been constant over the past few centuries. The preindustrial-era emission estimates can therefore be applied to the modern CH 4 budget with confidence. Ice core 14 CH 4 analysis is challenging owing to both the very large sample requirement (~1,

The Last Interglacial (129-116 ka) represents one of the warmest climate intervals of the last 800,000 years and the most recent time when sea level was meters higher than today. However, the timing and magnitude of peak warmth varies between reconstructions, and the relative importance of individual sources contributing to elevated sea level (mass gain versus seawater expansion) during the Last Interglacial remains uncertain. Here we present the first mean ocean temperature record for this interval from noble gas measurements in ice cores and constrain the thermal expansion component of sea level. Mean ocean temperature reaches its maximum value of 1.1±0.3°C warmer-than-modern at the end of the penultimate deglaciation at 129 ka, resulting in 0.7±0.3m of elevated sea level, relative to present. However, this maximum in ocean heat content is a transient feature; mean ocean temperature decreases in the first several thousand years of the interglacial and achieves a stable, comparable-tomodern value by ~127 ka. The synchroneity of the peak in mean ocean temperature with proxy records of abrupt transitions in oceanic and atmospheric circulation suggests that the mean ocean temperature maximum is related to the accumulation of heat in the ocean interior during the preceding period of reduced overturning circulation.

Permafrost and methane hydrates are large, climate-sensitive old carbon reservoirs that have the potential to emit large quantities of methane, a potent greenhouse gas, as the Earth continues to warm. We present ice core isotopic measurements of methane (Δ14C, δ13C, and δD) from the last deglaciation, which is a partial analog for modern warming. Our results show that methane emissions from old carbon reservoirs in response to deglacial warming were small (<19 teragrams of methane per year, 95% confidence interval) and argue against similar methane emissions in response to future warming. Our results also indicate that methane emissions from biomass burning in the pre-Industrial Holocene were 22 to 56 teragrams of methane per year (95% confidence interval), which is comparable to today.

Abstract. Deglaciations are characterized by relatively fast and near-synchronous changes in ice sheet volume, ocean temperature, and atmospheric greenhouse gas concentrations, but glacial inception occurs more gradually. Understanding the evolution of ice sheet, ocean, and atmosphere conditions from interglacial to glacial maximum provides insight into the interplay of these components of the climate system. Using noble gas measurements in ancient ice samples, we reconstruct mean ocean temperature (MOT) from 74 to 59.7 ka, covering the Marine Isotope Stage (MIS) 5a–4 boundary, MIS 4, and part of the MIS 4–3 transition. Comparing this MOT reconstruction to previously published MOT reconstructions from the last and penultimate deglaciation, we find that the majority of the last interglacial–glacial ocean cooling must have occurred within MIS 5. MOT reached equally cold conditions in MIS 4 as in MIS 2 (−2.7 ± 0.3 ∘C relative to the Holocene, −0.1 ± 0.3 ∘C relative to MIS 2). Using a carbon cycle model to quantify the CO2 solubility pump, we show that ocean cooling can explain most of the CO2 drawdown (32 ± 4 of 40 ppm) across MIS 5. Comparing MOT to contemporaneous records of benthic δ18O, we find that ocean cooling can also explain the majority of the δ18O increase across MIS 5 (0.7 ‰ of 1.3 ‰). The timing of ocean warming and cooling in the record and the comparison to coeval Antarctic isotope data suggest an intimate link between ocean heat content, Southern Hemisphere high-latitude climate, and ocean circulation on orbital and millennial timescales.

New ice cores retrieved from the Taylor Glacier (Antarctica) blue ice area contain ice and air spanning the Marine Isotope Stage (MIS) 5-4 transition, a period of global cooling and ice sheet expansion. We determine chronologies for the ice and air bubbles in the new ice cores by visually matching variations in gas-and ice-phase tracers to preexisting ice core records. The chronologies reveal an ice agegas age difference ( age) approaching 10 ka during MIS 4, implying very low snow accumulation in the Taylor Glacier accumulation zone. A revised chronology for the analogous section of the Taylor Dome ice core (84 to 55 ka), located to the south of the Taylor Glacier accumulation zone, shows that age did not exceed 3 ka. The difference in age between the two records during MIS 4 is similar in magnitude but opposite in direction to what is observed at the Last Glacial Maximum. This relationship implies that a spatial gradient in snow accumulation existed across the Taylor Dome region during MIS 4 that was oriented in the opposite direction of the accumulation gradient during the Last Glacial Maximum.