Global chemistry–climate models (CCMs) play an important role in assessing the climate and air pollution implications of aerosols and chemically reactive gases. Evaluating these models under past conditions and constraining historical sources and sinks necessitate reliable records of atmospheric mixing ratios spanning preindustrial times. Such precious records were recently obtained for carbon monoxide (CO), documenting for the first time the evolution of this reactive compound over the industrial era. In this study, we compare the simulated atmospheric surface CO mixing ratios ([CO]) from two different sets of chemistry–climate models and emissions within the frameworks of CMIP5 and of CMIP6 (Coupled Model Intercomparison Project Phases 5 and 6) to recent bipolar ice archive reconstructions for the period spanning 1850 to the present. We analyse how historical (1850–2014) [CO] outputs from 16 ACCMIP (Atmospheric Chemistry and Climate Model Intercomparison Project) models and 7 AerChemMIP (Aerosol Chemistry Model Intercomparison Project) models over Greenland and Antarctica are able to capture both absolute values and trends recorded in multi-site ice archives. While most models underestimate [CO] at northern high latitudes, a reduction in this bias is observed between the ACCMIP and the AerChemMIP exercise. Over the 1980–2010 CE period (common era; all subsequent years in the paper are reported in CE), trends in ice archive and firn air observations and AerChemMIP outputs align remarkably well at northern and southern high latitudes, indicating improved quantification of anthropogenic CO emissions and the main CO sink (OH oxidation) compared to ACCMIP. From 1850 to 1980, AerChemMIP models and observations consistently show increasing [CO] in both the Northern Hemisphere (NH) and Southern Hemisphere (SH), suggesting a robust understanding of the CO budget evolution. However, a divergence in the [CO] growth rate emerges in the NH between models and observations over the 1920–1980 period, attributed to uncertainties in CO emission factors (EFs), particularly EFs for the RCO (residential, commercial, and other) and transportation sectors, although we cannot totally rule out the possibility that the CO record based on the Greenland ice archives may be biased high by CO chemical production processes occurring in the ice prior to the measurements (i.e. in situ CO production). In the Southern Hemisphere, AerChemMIP models simulate an increase in atmospheric [CO] from 1850 to 1980 that closely reproduces the observations (22 ± 10 ppb and 13 ± 7 ppb, respectively). Such agreement supports CMIP6 biomass burning CO emission inventories, which do not reveal a peak in CO emissions in the late 19th century. Furthermore, both SH models and observations reveal an accelerated growth rate in [CO] during 1945–1980 relative to 1850–1945, likely linked to increased anthropogenic transportation emissions.

Galactic cosmic rays (GCRs) interact with matter in the atmosphere and at the surface of the Earth to produce a range of cosmogenic nuclides. Measurements of cosmogenic nuclides produced in surface rocks have been used to study past land ice extent as well as to estimate erosion rates. Because the GCR flux reaching the Earth is modulated by magnetic fields (solar and Earth's), records of cosmogenic nuclides produced in the atmosphere have also been used for studies of past solar activity. Studies utilizing cosmogenic nuclides assume that the GCR flux is constant in time, but this assumption may be uncertain by 30 % or more. Here we propose that measurements of 14C of carbon monoxide (14CO) in ice cores at low-accumulation sites can be used as a proxy for variations in GCR flux on timescales of several thousand years. At low-accumulation ice core sites, 14CO in ice below the firn zone originates almost entirely from in situ cosmogenic production by deep-penetrating secondary cosmic ray muons. The flux of such muons is almost insensitive to solar and geomagnetic variations and depends only on the primary GCR flux intensity. We use an empirically constrained model of in situ cosmogenic 14CO production in ice in combination with a statistical analysis to explore the sensitivity of ice core 14CO measurements at Dome C, Antarctica, to variations in the GCR flux over the past ≈ 7000 years. We find that Dome C 14CO measurements would be able to detect a linear change of 6 % over 7 ka, a step increase of 6 % at 3.5 ka or a transient 100-year spike of 190 % at 3.5 ka at the 3σ significance level. The ice core 14CO proxy therefore appears promising for the purpose of providing a high-precision test of the assumption of GCR flux constancy over the Holocene.

Measurements of carbon-14-containing carbon monoxide ((CO)-C-14) in glacial ice are useful for studies of the past oxidative capacity of the atmosphere as well as for reconstructing the past cosmic ray flux. The (CO)-C-14 abundance in glacial ice represents the combination of trapped atmospheric (CO)-C-14 and in situ cosmogenic (CO)-C-14. The systematics of in situ cosmogenic (CO)-C-14 production and retention in ice are not fully quantified, posing an obstacle to interpretation of ice core (CO)-C-14 measurements. Here we provide the first comprehensive characterization of (CO)-C-14 at an ice accumulation site (Summit, Greenland), including measurements in the ice grains of the firn matrix, firn air and bubbly ice below the firn zone. The results are interpreted with the aid of a firn gas transport model into which we implemented in situ cosmogenic C-14. We find that almost all (approximate to 99.5 %) of in situ (CO)-C-14 that is produced in the ice grains in firn is very rapidly (in <1 year) lost to the open porosity and from there mostly vented to the atmosphere. The timescale of this rapid loss is consistent with what is expected from gas diffusion through ice. The small fraction of in situ (CO)-C-14 that initially stays in the ice grains continues to slowly leak out to the open porosity at a rate of approximate to 0.6 % yr(-1). Below the firn zone we observe an increase in (CO)-C-14 content with depth that is due to in situ (CO)-C-14 production by deep-penetrating muons, confirming recent estimates of (CO)-C-14 production rates in ice via the muon mechanisms and allowing for narrowing constraints on these production rates.

Here we present a newly developed ice core gas-phase proxy that directly samples a component of the large-scale atmospheric circulation: synoptic-scale pressure variability. Surface pressure changes weakly disrupt gravitational isotopic settling in the firn layer, which is recorded in krypton-86 excess (Kr-86(xs)). The Kr-86(xs) may therefore reflect the time-averaged synoptic pressure variability over several years (site "storminess "), but it likely cannot record individual synoptic events as ice core gas samples typically average over several years. We validate Kr-86(xs) using late Holocene ice samples from 11 Antarctic ice cores and 1 Greenland ice core that collectively represent a wide range of surface pressure variability in the modern climate. We find a strong spatial correlation (r=-0.94, p < 0.01) between site average Kr-86(xs) and time-averaged synoptic variability from reanalysis data. The main uncertainties in the analysis are the corrections for gas loss and thermal fractionation and the relatively large scatter in the data. Limited scientific understanding of the firn physics and potential biases of Kr-86(xs) require caution in interpreting this proxy at present. We show that Antarctic 86Krxs appears to be linked to the position of the Southern Hemisphere eddy-driven subpolar jet (SPJ), with a southern position enhancing pressure variability.We present a Kr-86(xs) record covering the last 24 kyr from the West Antarctic Ice Sheet (WAIS) Divide ice core. Based on the empirical spatial correlation of synoptic activity and Kr-86(xs) at various Antarctic sites, we interpret this record to show that West Antarctic synoptic activity is slightly below modern levels during the Last Glacial Maximum (LGM), increases during the Heinrich Stadial 1 and Younger Dryas North Atlantic cold periods, weakens abruptly at the Holocene onset, remains low during the early and mid-Holocene, and gradually increases to its modern value. The WAIS Divide Kr-86(xs) record resembles records of monsoon intensity thought to reflect changes in the meridional position of the Intertropical Convergence Zone (ITCZ) on orbital and millennial timescales such that West Antarctic storminess is weaker when the ITCZ is displaced northward and stronger when it is displaced southward. We interpret variations in synoptic activity as reflecting movement of the South Pacific SPJ in parallel to the ITCZ migrations, which is the expected zonal mean response of the eddy-driven jet in models and proxy data. Past changes to Pacific climate and the El Nino-Southern Oscillation (ENSO) may amplify the signal of the SPJ migration. Our interpretation is broadly consistent with opal flux records from the Pacific Antarctic zone thought to reflect wind-driven upwelling.We emphasize that Kr-86(xs) is a new proxy, and more work is called for to confirm, replicate, and better understand these results; until such time, our conclusions regarding past atmospheric dynamics remain speculative. Current scientific understanding of firn air transport and trapping is insufficient to explain all the observed variations in Kr-86(xs). A list of suggested future studies is provided.

Abstract. Cosmic rays entering the Earth’s atmosphere produce showers of secondary particles such as neutrons and muons. The interaction of these neutrons and muons with oxygen-16 (16O) in minerals such as ice and quartz can produce carbon-14 (14C). Analyses of in situ produced cosmogenic 14C in quartz are commonly used to investigate the Earth’s landscape evolution. In glacial ice, 14C is also incorporated through trapping of 14C-containing atmospheric gases (14CO2, 14CO, and 14CH4). Understanding the production rates of in situ cosmogenic 14C is important to deconvolve the in situ cosmogenic and atmospheric 14C signals in ice, both of which contain valuable paleoenvironmental information. Unfortunately, the in situ 14C production rates by muons (which are the dominant production mechanism at depths of > 6 m solid ice equivalent) are uncertain. In this study, we use measurements of in situ 14C in ancient ice (> 50 kilo-annum before present, ka BP) from the Taylor Glacier ablation site, Antarctica in combination with a 2D ice flow model to better constrain the rates of 14C production by muons. We find that the commonly used values for muogenic 14C production rates (Heisinger et al., 2002a, 2002b) in ice are too high by factors of 5.7 (3.6–13.9, 95 % confidence interval) and 3.7 (2.0–11.9 95 % confidence interval) for negative muon capture and fast muon interactions, respectively. Our constraints on muogenic 14C production rates in ice allow for future measurements of 14C in ice cores to be used for other applications and imply that muogenic 14C production rates in quartz are overestimated as well.

Historical atmospheric H2 levels were reconstructed using firn air measurements from two sites in Greenland (NEEM and Summit) and two sites in Antarctica (South Pole and Megadunes). A joint reconstruction based on the two Antarctic sites yields H2 levels monotonically increasing from about 330 ppb in 1900 to 550 ppb in the late 1990s, leveling off thereafter. These results are similar to individual reconstructions published previously (Patterson et al., 2020, 2021). Interpretation of the Greenland firn air measurements is complicated by challenges in modeling enrichment induced by pore close-off at these sites. We used observations of neon enrichment at NEEM and Summit to tune the parameterization of enrichment induced by pore close-off in our firn air model. The joint reconstruction from the Greenland data shows H2 levels rising 30 % between 1950 and the late 1980s, reaching a maximum of 530 ppb. After 1990, reconstructed atmospheric H2 levels over Greenland are roughly constant, with a small decline of 3 % over the next 25 years. The reconstruction shows good agreement with the available flask measurements of H2 at high northern latitudes.

The results of theoretical and experimental studies of the fundamental possibility of joint use of deep diagnostics and flexible restoring procedures to ensure rational control of the operability of the model unit of electric flywheel motors are presented. Diagnostic models for 11 types of destabilizing effects are described. A dichotomous tree for searching for types of destabilizations is proposed. Algorithms for restoring the operability of a unit of electric flywheel motors are given. The results of experimental studies of one channel of the model unit of electric flywheel motors in the form of transient characteristics at step input actions are presented.

Carbon monoxide (CO) is a regulated pollutant and one of the key components determining the oxidizing capacity of the atmosphere. Obtaining a reliable record of atmospheric CO mixing ratios ([CO]) since preindustrial times is necessary to evaluate climate–chemistry models under conditions different from today and to constrain past CO sources. We present high-resolution measurements of CO mixing ratios from ice cores drilled at five different sites on the Greenland ice sheet that experience a range of snow accumulation rates, mean surface temperatures, and different chemical compositions. An optical-feedback cavity-enhanced absorption spectrometer (OF-CEAS) was coupled with continuous melter systems and operated during four analytical campaigns conducted between 2013 and 2019. Overall, continuous flow analysis (CFA) of CO was carried out on over 700 m of ice. The CFA-based CO measurements exhibit excellent external precision (ranging from 3.3 to 6.6 ppbv, 1σ) and achieve consistently low blanks (ranging from 4.1±1.2 to 12.6±4.4 ppbv), enabling paleoatmospheric interpretations. However, the five CO records all exhibit variability that is too large and rapid to reflect past atmospheric mixing ratio changes. Complementary tests conducted on discrete ice samples demonstrate that these variations are not artifacts of the analytical method (i.e., production of CO from organics in the ice during melting) but are very likely related to in situ CO production within the ice before analysis. Evaluation of the signal resolution and co-investigation of high-resolution records of CO and total organic carbon (TOC) suggest that past atmospheric CO variations can be extracted from the records' baselines with accumulation rates higher than 20 cm w.e.yr-1 (water equivalent per year). Consistent baseline CO records from four Greenland sites are combined to produce a multisite average ice core reconstruction of past atmospheric CO for the Northern Hemisphere high latitudes, covering the period from 1700 to 1957 CE. Such a reconstruction should be taken as an upper bound of past atmospheric CO abundance. From 1700 to 1875 CE, the record reveals stable or slightly increasing values in the 100–115 ppbv range. From 1875 to 1957 CE, the record indicates a monotonic increase from 114±4 to 147±6 ppbv. The ice core multisite CO record exhibits an excellent overlap with the atmospheric CO record from Greenland firn air which spans the 1950–2010 CE time period. The combined ice core and firn air CO history, spanning 1700–2010 CE, provides useful constraints for future model studies of atmospheric changes since the preindustrial period.

Here we use high-precision carbon isotope data (δ 13 C-CO 2 ) to show atmospheric CO 2 during Marine Isotope Stage 4 (MIS 4, ~70.5-59 ka) was controlled by a succession of millennial-scale processes. Enriched δ 13 C-CO 2 during peak glaciation suggests increased ocean carbon storage. Variations in δ 13 C-CO 2 in early MIS 4 suggest multiple processes were active during CO 2 drawdown, potentially including decreased land carbon and decreased Southern Ocean air-sea gas exchange superposed on increased ocean carbon storage. CO 2 remained low during MIS 4 while δ 13 C-CO 2 fluctuations suggest changes in Southern Ocean and North Atlantic air-sea gas exchange. A 7 ppm increase in CO 2 at the onset of Dansgaard-Oeschger event 19 (72.1 ka) and 27 ppm increase in CO 2 during late MIS 4 (Heinrich Stadial 6, ~63.5-60 ka) involved additions of isotopically light carbon to the atmosphere. The terrestrial biosphere and Southern Ocean air-sea gas exchange are possible sources, with the latter event also involving decreased ocean carbon storage.

Abstract. Deglaciations are characterized by relatively fast and near-synchronous changes in ice sheet volume, ocean temperature, and atmospheric greenhouse gas concentrations, but glacial inceptions occur more gradually. Understanding the evolution of ice sheet, ocean, and atmospheric conditions from interglacial to glacial maximum provides important insight into the interplay of these components of our climate system. Using noble gas measurements in ancient ice samples, we reconstruct mean ocean temperature (MOT) from 74 to 59.5 ka BP, covering the Marine Isotope Stage (MIS) 5-4 boundary, MIS 4, and part of the MIS 4-3 transition. Comparing this MOT reconstruction to previously published MOT reconstructions from the last glacial cycle, we find that the majority of interglacial-glacial ocean cooling occurred across MIS 5, and MOT reached full glacial levels by MIS 4 (−2.7 ± 0.3 °C relative to the Holocene). Comparing MOT to contemporaneous records of CO2 and benthic 𝛿18O, we find that ocean cooling and the solubility pump can explain most of the CO2 drawdown and increase in 𝛿18O across MIS 5. The timing of ocean warming and cooling in our record indicates that millennial scale climate variability plays a crucial role in setting mean ocean temperature during this interval, as seen during other periods, such as the last deglaciation.

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.

Carbon-14 (^14C) is produced in the atmosphere when neutrons from cosmic-ray air showers are captured by ^14N nuclei. Atmospheric ^14C becomes trapped in air bubbles in polar ice as compacted snow (firn) transforms into ice. ^14C is also produced in situ in ice grains by penetrating cosmic-ray neutrons and muons. Recent ice core measurements indicate that in the ^14CO phase, the ^14C is dominated by the in situ cosmogenic component at most ice coring sites. Thus, it should be possible to use ice-bound ^14CO to reconstruct the historical flux of cosmic rays at Earth, without the transport and deposition uncertainties associated with ^10Be or the carbon cycle uncertainties affecting atmospheric ^14CO_2. The measurements will be sensitive to the cosmic-ray flux above the energy range most affected by solar modulation. We present estimates of the expected sensitivity of ^14CO in ice cores to the historical flux of Galactic cosmic rays, based on recent studies of ^14CO in polar ice.

Methane is a powerful greenhouse gas and a key player in atmospheric chemistry. Important uncertainties remain in the global atmospheric methane budget, with natural geologic emissions being one of the particularly uncertain terms. In recent bottom-up studies, geologic emissions have been estimated to comprise up to 10% of the global budget (40–60 Teragrams of methane per year, Tg CH4 yr–1). In contrast, top-down constraints from 14C of methane in preindustrial air extracted from ice cores indicate that the geologic methane source is approximately an order of magnitude lower. Recent bottom-up inventories propose microseepage (diffuse low-level flux of methane through soils over large areas) as the largest single component of the geologic methane flux. In this study, we present new measurements of methane microseepage from the Appalachian Basin (Western New York State) and compare these with prior microseepage measurements from other regions and with predicted values from the most recent bottom-up inventory. Our results show lower microseepage values than most prior data sets and indicate that positive microseepage fluxes in this region are not as widespread as previously assumed. A statistical analysis of our results indicates that mean microseepage flux in this region has very likely been overestimated by the bottom-up inventory, even though our measurements more likely than not underestimate the true mean flux. However, this is a small data set from a single region and as such cannot be used to evaluate the validity of the microseepage emissions inventory as a whole. Instead, the results demonstrate the need for a more extensive network of direct geologic emission measurements in support of improved bottom-up inventories.

Important uncertainties remain in our understanding of the spatial and temporal variability of atmospheric hydroxyl radical concentration ([OH]). Carbon-14-containing carbon monoxide (14CO) is a useful tracer that can help in the characterization of [OH] variability. Prior measurements of atmospheric 14CO concentration ([14CO] are limited in both their spatial and temporal extent, partly due to the very large air sample volumes that have been required for measurements (500–1000 L at standard temperature and pressure, L STP) and the difficulty and expense associated with the collection, shipment, and processing of such samples. Here we present a new method that reduces the air sample volume requirement to ≈90 L STP while allowing for [14CO] measurement uncertainties that are on par with or better than prior work (≈3 % or better, 1σ). The method also for the first time includes accurate characterization of the overall procedural [14CO] blank associated with individual samples, which is a key improvement over prior atmospheric 14CO work. The method was used to make measurements of [14CO] at the NOAA Mauna Loa Observatory, Hawaii, USA, between November 2017 and November 2018. The measurements show the expected [14CO] seasonal cycle (lowest in summer) and are in good agreement with prior [14CO] results from another low-latitude site in the Northern Hemisphere. The lowest overall [14CO] uncertainties (2.1 %, 1σ) are achieved for samples that are directly accompanied by procedural blanks and whose mass is increased to ≈50 µgC (micrograms of carbon) prior to the 14C measurement via dilution with a high-CO 14C-depleted gas.

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 .

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.