Atmospheric methane and nitrous oxide of the Late Pleistocene from Antarctic ice cores

Carbonic Anhydrase Robustness for Use in Nanoscale CO2 Capture Technologies

Continued dependence on crude oil and natural gas resources for fossil fuels has caused global atmospheric carbon dioxide (CO2) emissions to increase to record-setting proportions. There is an urgent need for efficient and inexpensive carbon sequestration systems to mitigate large-scale emissions of CO2 from industrial flue gas. Carbonic anhydrase (CA) has shown high potential for enhanced CO2 capture applications compared to conventional absorption-based methods currently utilized in various industrial settings. This study aims to understand structural aspects that contribute to the stability of CA enzymes critical for their applications in industrial processes, which require the ability to withstand conditions different from those in their native environments. Here, we evaluated the thermostability and enzyme activity of mesophilic and thermophilic CA variants at different temperature conditions and in the presence of atmospheric gas pollutants like nitrogen oxides and sulfur oxides. Based on our enzyme activity assays and molecular dynamics simulations, we see increased conformational stability and CA activity levels in thermostable CA variants incubated week-long at different temperature conditions. The thermostable CA variants also retained high levels of CA activity despite changes in solution pH due to increasing NO and SO2 concentrations. A loss of CA activity was observed only at high concentrations of NO/SO2 that possibly can be minimized with the appropriate buffered solutions.

North African humid periods over the past 800,000 years

The Sahara region has experienced periodic wet periods over the Quaternary and beyond. These North African Humid Periods (NAHPs) are astronomically paced by precession which controls the intensity of the African monsoon system. However, most climate models cannot reconcile the magnitude of these events and so the driving mechanisms remain poorly constrained. Here, we utilise a recently developed version of the HadCM3B coupled climate model that simulates 20 NAHPs over the past 800 kyr which have good agreement with NAHPs identified in proxy data. Our results show that precession determines NAHP pacing, but we identify that their amplitude is strongly linked to eccentricity via its control over ice sheet extent. During glacial periods, enhanced ice-albedo driven cooling suppresses NAHP amplitude at precession minima, when humid conditions would otherwise be expected. This highlights the importance of both precession and eccentricity, and the role of high latitude processes in determining the timing and amplitude of the NAHPs. This may have implications for the out of Africa dispersal of plants and animals throughout the Quaternary.

Seasonal temperatures in West Antarctica during the Holocene

The recovery of long-term climate proxy records with seasonal resolution is rare because of natural smoothing processes, discontinuities and limitations in measurement resolution. Yet insolation forcing, a primary driver of multimillennial-scale climate change, acts through seasonal variations with direct impacts on seasonal climate¹. Whether the sensitivity of seasonal climate to insolation matches theoretical predictions has not been assessed over long timescales. Here, we analyse a continuous record of water-isotope ratios from the West Antarctic Ice Sheet Divide ice core to reveal summer and winter temperature changes through the last 11,000 years. Summer temperatures in West Antarctica increased through the early-to-mid-Holocene, reached a peak 4,100 years ago and then decreased to the present. Climate model simulations show that these variations primarily reflect changes in maximum summer insolation, confirming the general connection between seasonal insolation and warming and demonstrating the importance of insolation intensity rather than seasonally integrated insolation or season duration^2,3. Winter temperatures varied less overall, consistent with predictions from insolation forcing, but also fluctuated in the early Holocene, probably owing to changes in meridional heat transport. The magnitudes of summer and winter temperature changes constrain the lowering of the West Antarctic Ice Sheet surface since the early Holocene to less than 162 m and probably less than 58 m, consistent with geological constraints elsewhere in West Antarctica^4-7.

Rapid timescale for an oxic transition during the Great Oxidation Event and the instability of low atmospheric O2

Understanding the rise of atmospheric oxygen on Earth is important for assessing precursors to complex life and for evaluating potential future detections of oxygen on exoplanets as signs of extraterrestrial biospheres. However, it is unclear whether Earth’s initial rise of O₂ was monotonic or oscillatory, and geologic evidence poorly constrains O₂ afterward, during the mid-Proterozoic (1.8 billion to 0.8 billion years ago). Here, we used a time-dependent photochemical model to simulate oxygen’s rise and the stability of subsequent O₂ levels to perturbations in supply and loss. Results show that large oxygen fluctuations are possible during the initial rise of O₂ and that Mesoproterozoic O₂ had to exceed 0.01% volume concentration for atmospheric stability.

The Great Oxidation Event (GOE), arguably the most important event to occur on Earth since the origin of life, marks the time when an oxygen-rich atmosphere first appeared. However, it is not known whether the change was abrupt and permanent or fitful and drawn out over tens or hundreds of millions of years. Here, we developed a one-dimensional time-dependent photochemical model to resolve time-dependent behavior of the chemically unstable transitional atmosphere as it responded to changes in biogenic forcing. When forced with step-wise changes in biogenic fluxes, transitions between anoxic and oxic atmospheres take between only 10² and 10⁵ y. Results also suggest that O₂ between ~10−8 and ~10−4 mixing ratio is unstable to plausible atmospheric perturbations. For example, when atmospheres with these O₂ concentrations experience fractional variations in the surface CH₄ flux comparable to those caused by modern Milankovich cycling, oxygen fluctuates between anoxic (~10−8) and oxic (~10−4) mixing ratios. Overall, our simulations are consistent with possible geologic evidence of unstable atmospheric O₂, after initial oxygenation, which could occasionally collapse from changes in biospheric or volcanic fluxes. Additionally, modeling favors mid-Proterozoic O₂ exceeding 10−4 to 10−3 mixing ratio; otherwise, O₂ would periodically fall below 10−7 mixing ratio, which would be inconsistent with post-GOE absence of sulfur isotope mass-independent fractionation.

Rainfall and sea level drove the expansion of seasonally flooded habitats and associated bird populations across Amazonia

Spatial arrangement of distinct Amazonian environments through time and its effect on specialized biota remain poorly known, fueling long-lasting debates about drivers of biotic diversification. We address the late Quaternary sediment deposition that assembled the world's largest seasonally flooded ecosystems. Genome sequencing was used to reconstruct the demographic history of bird species specialized in either early successional vegetation or mature floodplain forests. Sediment deposition that built seasonally flooded habitats accelerated throughout the Holocene (last 11,700 years) under sea level highstand and intensification of the South American Monsoon, at the same time as global increases in atmospheric methane concentration. Bird populations adapted to seasonally flooded habitats expanded due to enlargement of Amazonian river floodplains and archipelagos. Our findings suggest that the diversification of the biota specialized in seasonally flooded habitats is coupled to sedimentary budget changes of large rivers, which rely on combined effects of sea level and rainfall variations.

This study found that millennial periods of higher rainfall combined with rising sea level enhanced sediment accumulation in Amazonian rivers valleys. This fuelled synchronous expansion of vegetation adapted to seasonally flooded substrates and its specialized bird populations, showing how global climate changes can affect specific Amazonian species.

Variations by degrees: Western European paleoenvironmental fluctuations across MIS 13-11

Marine Isotope Stages (MIS) 13-11 saw a major transformation in the hominin occupation of Europe, with an expansion in the scale and geographical distribution of sites and artifact assemblages. That expansion is explored here in the context of paleoenvironmental variability, focusing on geographical and chronological trends in climatic and habitat conditions at and between key Lower Paleolithic sites in Western Europe. Climatic conditions at British sites are compared across MIS 13-11, and used to test predicted values from the Oscillayers data set. Conditions at hominin and nonhominin sites are compared to explore possible limitations in hominin tolerances during MIS 13-11. Trends in conditions are explored with reference to long-term global patterns, short-term substage events, and seasonal variations. The apparent increase in the scale of hominin activity in north-western Europe during MIS 13 is surprising in light of the relatively harsh conditions of late MIS 13, and is likely to reflect significant physiological and/or behavioral adaptations, a mild south-north temperature gradient in western Europe during MIS 13, and the relatively mild, sustained conditions spanning MIS 15-13. The expanded occupation of north-western Europe during MIS 11 probably reflects the extended mild conditions of MIS 11c, since marked seasonal temperature differences and substantial behavioral changes between hominin sites in MIS 13 and 11 are not clearly evident. Site-specific conditions in south-western Europe during MIS 11 suggest milder winters, warmer summers, and reduced seasonal variability compared to north-western Europe. Some or all of these conditions may have supported larger, core populations, as may the relatively mild conditions associated with south-western European sites during MIS 12. Finally, comparisons between north-western and north-central European sites indicate relatively small differences in seasonal temperatures, suggesting that climate may only be a partial factor behind the smaller-scale occupations of north-central Europe during MIS 13-11.

What do we know about the global methane budget? Results from four decades of atmospheric CH4 observations and the way forward

Atmospheric CH₄ is arguably the most interesting of the anthropogenically influenced, long-lived greenhouse gases. It has a diverse suite of sources, each presenting its own challenges in quantifying emissions, and while its main sink, atmospheric oxidation initiated by reaction with hydroxyl radical (OH), is well-known, determining the magnitude and trend in this and other smaller sinks remains challenging. Here, we provide an overview of the state of knowledge of the dynamic atmospheric CH₄ budget of sources and sinks determined from measurements of CH₄ and δ¹³C_CH4 in air samples collected predominantly at background air sampling sites. While nearly four decades of direct measurements provide a strong foundation of understanding, large uncertainties in some aspects of the global CH₄ budget still remain. More complete understanding of the global CH₄ budget requires significantly more observations, not just of CH₄ itself, but other parameters to better constrain key, but still uncertain, processes like wetlands and sinks. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 1)'.

Antarctic surface temperature and elevation during the Last Glacial Maximum

Water-stable isotopes in polar ice cores are a widely used temperature proxy in paleoclimate reconstruction, yet calibration remains challenging in East Antarctica. Here, we reconstruct the magnitude and spatial pattern of Last Glacial Maximum surface cooling in Antarctica using borehole thermometry and firn properties in seven ice cores. West Antarctic sites cooled ~10°C relative to the preindustrial period. East Antarctic sites show a range from ~4° to ~7°C cooling, which is consistent with the results of global climate models when the effects of topographic changes indicated with ice core air-content data are included, but less than those indicated with the use of water-stable isotopes calibrated against modern spatial gradients. An altered Antarctic temperature inversion during the glacial reconciles our estimates with water-isotope observations.

Regional impacts of climate change and its relevance to human evolution

The traditional concept of long and gradual, glacial-interglacial climate changes during the Quaternary has been challenged since the 1980s. High temporal resolution analysis of marine, terrestrial and ice geological archives has identified rapid, millennial- to centennial-scale, and large-amplitude climatic cycles throughout the last few million years. These changes were global but have had contrasting regional impacts on the terrestrial and marine ecosystems, with in some cases strong changes in the high latitudes of both hemispheres but muted changes elsewhere. Such a regionalization has produced environmental barriers and corridors that have probably triggered niche contractions/expansions of hominin populations living in Eurasia and Africa. This article reviews the long- and short-timescale ecosystem changes that have punctuated the last few million years, paying particular attention to the environments of the last 650,000 years, which have witnessed key events in the evolution of our lineage in Africa and Eurasia. This review highlights, for the first time, a contemporaneity between the split between Denisovan and Neanderthals, at ~650-400 ka, and the strong Eurasian ice-sheet expansion down to the Black Sea. This ice expansion could form an ice barrier between Europe and Asia that may have triggered the genetic drift between these two populations.

Abrupt CO2 release to the atmosphere under glacial and early interglacial climate conditions

Pulse-like carbon dioxide release to the atmosphere on centennial time scales has only been identified for the most recent glacial and deglacial periods and is thought to be absent during warmer climate conditions. Here, we present a high-resolution carbon dioxide record from 330,000 to 450,000 years before present, revealing pronounced carbon dioxide jumps (CDJ) under cold and warm climate conditions. CDJ come in two varieties that we attribute to invigoration or weakening of the Atlantic meridional overturning circulation (AMOC) and associated northward and southward shifts of the intertropical convergence zone, respectively. We find that CDJ are pervasive features of the carbon cycle that can occur during interglacial climate conditions if land ice masses are sufficiently extended to be able to disturb the AMOC by freshwater input.

Glacial-Interglacial Precipitation Changes

Glacial-interglacial cycles have constituted a primary mode of climate variability over the last 2.6 million years of Earth's history. While glacial periods cannot be seen simply as a reverse analogue of future warming, they offer an opportunity to test our understanding of the response of precipitation patterns to a much wider range of conditions than we have been able to directly observe. This review explores key features of precipitation patterns associated with glacial climates, which include drying in large regions of the tropics and wetter conditions in substantial parts of the subtropics and midlatitudes. I describe the evidence for these changes and examine the potential causes of hydrological changes during glacial periods. Central themes that emerge include the importance of atmospheric circulation changes in determining glacial-interglacial precipitation changes at the regional scale, the need to take into account climatic factors beyond local precipitation amount when interpreting proxy data, and the role of glacial conditions in suppressing the strength of Northern Hemisphere monsoon systems.

A simulated Northern Hemisphere terrestrial climate dataset for the past 60,000 years

We present a continuous land-based climate reconstruction dataset extending back 60 kyr from 0 BP (1950) at 0.5° resolution on a monthly timestep for 0°N to 90°N. It has been generated from 42 discrete snapshot simulations using the HadCM3B-M2.1 coupled general circulation model. We incorporate Dansgaard-Oeschger (DO) and Heinrich events to represent millennial scale variability, based on a temperature reconstruction from Greenland ice-cores, with a spatial fingerprint based on a freshwater hosing simulation with HadCM3B-M2.1. Interannual variability is also added and derived from the initial snapshot simulations. Model output has been downscaled to 0.5° resolution (using simple bilinear interpolation) and bias corrected. Here we present surface air temperature, precipitation, incoming shortwave energy, minimum monthly temperature, snow depth, wind chill and number of rainy days per month. This is one of the first open access climate datasets of this kind and can be used to study the impact of millennial to orbital-scale climate change on terrestrial greenhouse gas cycling, northern extra-tropical vegetation, and megaflora and megafauna population dynamics.

PaleoClim, high spatial resolution paleoclimate surfaces for global land areas

High-resolution, easily accessible paleoclimate data are essential for environmental, evolutionary, and ecological studies. The availability of bioclimatic layers derived from climatic simulations representing conditions of the Late Pleistocene and Holocene has revolutionized the study of species responses to Late Quaternary climate change. Yet, integrative studies of the impacts of climate change in the Early Pleistocene and Pliocene - periods in which recent speciation events are known to concentrate - have been hindered by the limited availability of downloadable, user-friendly climatic descriptors. Here we present PaleoClim, a free database of downscaled paleoclimate outputs at 2.5-minute resolution (~5 km at equator) that includes surface temperature and precipitation estimates from snapshot-style climate model simulations using HadCM3, a version of the UK Met Office Hadley Centre General Circulation Model. As of now, the database contains climatic data for three key time periods spanning from 3.3 to 0.787 million years ago: the Marine Isotope Stage 19 (MIS19) in the Pleistocene (~787 ka), the mid-Pliocene Warm Period (~3.264-3.025 Ma), and MIS M2 in the Late Pliocene (~3.3 Ma).

Land-use and abandonment alters methane and nitrous oxide fluxes in mountain grasslands

Grasslands cover more than one fifth of total land area in Europe and contribute significantly to the total greenhouse gas budget. The impact of management and land use on the carbon cycle and carbon sequestration in grasslands has been well-studied, however effects on emissions of N₂O and CH₄ remain uncertain. Additionally, the majority of studies have focussed on management differences between intensively managed grasslands, with few results available for lightly managed grasslands and in particular grassland abandonment. We present N₂O and CH₄ flux measurements for an abandonment trajectory at low land-use intensity, comparing meadow (fertilized and cut), pasture (grazed) and abandoned (unmanaged since 1983) grassland sites located in the Austrian Alps. Mean growing season N₂O fluxes were 0.07, 0.07 and - 0.13 nmol m^-2 s^-1 and CH₄ fluxes were - 1.0, - 0.5 and - 1.6 nmol m^-2 s^-1 for the meadow, pasture and abandoned sites respectively. Variability for both gases at the abandoned site was dominated by 'hot moments', while 'hot spots' dominated at the managed meadow and pasture sites. Consideration of the diurnal cycle observed at the abandoned site, linear correlations within all data sets, and principal components analyses of the full data set revealed increased consumption of both N₂O and CH₄ with increasing temperature, but hardly any relationship between fluxes and soil moisture. Upscaled over a year, the observed fluxes correspond to enhanced non-CO₂ greenhouse gas uptake of 172 g CO₂-equiv. m^-2 y^-1 following abandonment. These results show that non-CO₂ greenhouse gases form an important part of the total climate impact of land use change and grassland abandonment, such that abandoned grassland is a net sink for both CH₄ and N₂O.

Pollen from the Deep-Sea: A Breakthrough in the Mystery of the Ice Ages

Pollen from deep-sea sedimentary sequences provides an integrated regional reconstruction of vegetation and climate (temperature, precipitation, and seasonality) on the adjacent continent. More importantly, the direct correlation of pollen, marine and ice indicators allows comparison of the atmospheric climatic changes that have affected the continent with the response of the Earth's other reservoirs, i.e., the oceans and cryosphere, without any chronological uncertainty. The study of long continuous pollen records from the European margin has revealed a changing and complex interplay between European climate, North Atlantic sea surface temperatures (SSTs), ice growth and decay, and high- and low-latitude forcing at orbital and millennial timescales. These records have shown that the amplitude of the last five terrestrial interglacials was similar above 40°N, while below 40°N their magnitude differed due to precession-modulated changes in seasonality and, particularly, winter precipitation. These records also showed that vegetation response was in dynamic equilibrium with rapid climate changes such as the Dangaard-Oeschger (D-O) cycles and Heinrich events, similar in magnitude and velocity to the ongoing global warming. However, the magnitude of the millennial-scale warming events of the last glacial period was regionally-specific. Precession seems to have imprinted regions below 40°N while obliquity, which controls average annual temperature, probably mediated the impact of D-O warming events above 40°N. A decoupling between high- and low-latitude climate was also observed within last glacial warm (Greenland interstadials) and cold phases (Greenland stadials). The synchronous response of western European vegetation/climate and eastern North Atlantic SSTs to D-O cycles was not a pervasive feature throughout the Quaternary. During periods of ice growth such as MIS 5a/4, MIS 11c/b and MIS 19c/b, repeated millennial-scale cold-air/warm-sea decoupling events occurred on the European margin superimposed to a long-term air-sea decoupling trend. Strong air-sea thermal contrasts promoted the production of water vapor that was then transported northward by the westerlies and fed ice sheets. This interaction between long-term and shorter time-scale climatic variability may have amplified insolation decreases and thus explain the Ice Ages. This hypothesis should be tested by the integration of stochastic processes in Earth models of intermediate complexity.

High-Precision Measurement of N2O Concentration in Ice Cores

Atmospheric nitrous oxide (N₂O) is a greenhouse gas and ozone-depleting substance whose emissions are substantially perturbed by current human activities. Although air trapped in polar ice cores can provide direct information about N₂O evolution, analytical precision was not previously sufficient for high temporal resolution studies. In this work, we present a highly improved analytical technique with which to study N₂O concentrations in ancient-air-trapped ice cores. We adopt a melt-refreezing method to extract air and use a gas chromatography-electron capture detector (GC-ECD) to determine N₂O concentrations. The GC conditions are optimized to improve the sensitivity for detecting N₂O. Retrapped N₂O in ice during the extraction procedure is precisely analyzed and corrected. We confirmed our results using data from the Styx Glacier ice core in Antarctica by comparing them with the results of a dry-extraction method. The precision estimated from the pooled standard deviation of replicated measurements of the Styx ice core was 1.5 ppb for ∼20 g of ice, a smaller sample of ice than was used in previous studies, showing a significant improvement in precision. Our preliminary results from the Styx Glacier ice core samples have the potential to define small N₂O variations (a few parts per billion) at centennial time scales.

Arctic Ocean sea ice cover during the penultimate glacial and the last interglacial

Coinciding with global warming, Arctic sea ice has rapidly decreased during the last four decades and climate scenarios suggest that sea ice may completely disappear during summer within the next about 50–100 years. Here we produce Arctic sea ice biomarker proxy records for the penultimate glacial (Marine Isotope Stage 6) and the subsequent last interglacial (Marine Isotope Stage 5e). The latter is a time interval when the high latitudes were significantly warmer than today. We document that even under such warmer climate conditions, sea ice existed in the central Arctic Ocean during summer, whereas sea ice was significantly reduced along the Barents Sea continental margin influenced by Atlantic Water inflow. Our proxy reconstruction of the last interglacial sea ice cover is supported by climate simulations, although some proxy data/model inconsistencies still exist. During late Marine Isotope Stage 6, polynya-type conditions occurred off the major ice sheets along the northern Barents and East Siberian continental margins, contradicting a giant Marine Isotope Stage 6 ice shelf that covered the entire Arctic Ocean.

Coinciding with global warming, Arctic sea ice has rapidly decreased during the last four decades. Here, using biomarker records, the authors show that permanent sea ice was still present in the central Arctic Ocean during the last interglacial, when high latitudes were warmer than present.

Computational Investigation on the Role of Disilene Substituents Toward N2O Activation

The effect of substituents in disilene mediated N₂O activation was studied at the M06-2X/QZVP//ωB97xD/TZVP level of theory. The relationship between structural diversity and the corresponding reactivity of six disilenes (I_A-F^t) in the presence of four different substituents (-NMe₂, -Cl, -Me, -SiMe₃) is addressed in this investigation. We primarily propose two plausible mechanistic routes: Pathway I featuring disilene → silylene decomposition followed by N₂O coordination and Pathway II constituting the N₂O attack without Si-Si bond cleavage. Depending on the fashion of N₂O approach the latter route was further differentiated into Pathway IIa and Pathway IIb detailing the "end-on" and "side-on" attack to the disilene scaffold. Interestingly, the lone pair containing substituents (-NMe₂, -Cl,) facilitates disilene → silylene dissociation; on the contrary it reduces the electrophilicity at Si center in silylene, a feature manifested with higher activation barrier during N₂O attack. In the absence of any lone-pair influence from substituents (-Me, -SiMe₃), the decomposition of disilenes is considerably endothermic. Therefore, Pathway I appears to be the less preferred route for both types of substituents. In Pathway IIa, the N₂O moiety uniformly approaches via O-end to both the silicon centers in disilenes. However, the calculations reveal that Pathway IIa, although not operational for all disilenes, is unlikely to be a viable route due to the predominantly higher transition barrier (ca. 36 kcal/mol). The most feasible route in this current study accompanying moderately low activation barriers (∼19-26 kcal/mol) is Pathway IIb, which involves successive addition of two N₂O units proceeding via terminal N, O toward the Si centers and is applicable for all disilenes. The reactivity of substituted disilenes can be estimated in terms of the first activation barrier of N₂O attack. Surprisingly, in Pathway IIb, the initial activation barrier and hence the reactivity shows negligible correlation with Si-Si bond strength, indicating toward the versatility of the reaction route.

Contribution of Antarctica to past and future sea-level rise

Polar temperatures over the last several million years have, at times, been slightly warmer than today, yet global mean sea level has been 6-9 metres higher as recently as the Last Interglacial (130,000 to 115,000 years ago) and possibly higher during the Pliocene epoch (about three million years ago). In both cases the Antarctic ice sheet has been implicated as the primary contributor, hinting at its future vulnerability. Here we use a model coupling ice sheet and climate dynamics-including previously underappreciated processes linking atmospheric warming with hydrofracturing of buttressing ice shelves and structural collapse of marine-terminating ice cliffs-that is calibrated against Pliocene and Last Interglacial sea-level estimates and applied to future greenhouse gas emission scenarios. Antarctica has the potential to contribute more than a metre of sea-level rise by 2100 and more than 15 metres by 2500, if emissions continue unabated. In this case atmospheric warming will soon become the dominant driver of ice loss, but prolonged ocean warming will delay its recovery for thousands of years.

Structure and Abundance of Nitrous Oxide Complexes in Earth's Atmosphere

We have investigated the lowest energy structures and binding energies of a series of atmospherically relevant nitrous oxide (N2O) complexes using explicitly correlated coupled cluster theory. Specifically, we have considered complexes with nitrogen (N2-N2O), oxygen (O2-N2O), argon (Ar-N2O), and water (H2O-N2O). We have calculated rotational constants and harmonic vibrational frequencies for the complexes and the constituent monomers. Statistical mechanics was used to determine the thermodynamic parameters for complex formation as a function of temperature and pressure. These results, in combination with relevant atmospheric data, were used to estimate the abundance of N2O complexes in Earth's atmosphere as a function of altitude. We find that the abundance of N2O complexes in Earth's atmosphere is small but non-negligible, and we suggest that N2O complexes may contribute to absorption of terrestrial radiation and be relevant for understanding the atmospheric fate of N2O.

Macroevolutionary consequences of profound climate change on niche evolution in marine molluscs over the past three million years

In order to predict the fate of biodiversity in a rapidly changing world, we must first understand how species adapt to new environmental conditions. The long-term evolutionary dynamics of species' physiological tolerances to differing climatic regimes remain obscure. Here, we unite palaeontological and neontological data to analyse whether species' environmental tolerances remain stable across 3 Myr of profound climatic changes using 10 phylogenetically, ecologically and developmentally diverse mollusc species from the Atlantic and Gulf Coastal Plains, USA. We additionally investigate whether these species' upper and lower thermal tolerances are constrained across this interval. We find that these species' environmental preferences are stable across the duration of their lifetimes, even when faced with significant environmental perturbations. The results suggest that species will respond to current and future warming either by altering distributions to track suitable habitat or, if the pace of change is too rapid, by going extinct. Our findings also support methods that project species' present-day environmental requirements to future climatic landscapes to assess conservation risks.

Isotopic constraints on marine and terrestrial N2O emissions during the last deglaciation

Nitrous oxide (N2O) is an important greenhouse gas and ozone-depleting substance that has anthropogenic as well as natural marine and terrestrial sources. The tropospheric N2O concentrations have varied substantially in the past in concert with changing climate on glacial-interglacial and millennial timescales. It is not well understood, however, how N2O emissions from marine and terrestrial sources change in response to varying environmental conditions. The distinct isotopic compositions of marine and terrestrial N2O sources can help disentangle the relative changes in marine and terrestrial N2O emissions during past climate variations. Here we present N2O concentration and isotopic data for the last deglaciation, from 16,000 to 10,000 years before present, retrieved from air bubbles trapped in polar ice at Taylor Glacier, Antarctica. With the help of our data and a box model of the N2O cycle, we find a 30 per cent increase in total N2O emissions from the late glacial to the interglacial, with terrestrial and marine emissions contributing equally to the overall increase and generally evolving in parallel over the last deglaciation, even though there is no a priori connection between the drivers of the two sources. However, we find that terrestrial emissions dominated on centennial timescales, consistent with a state-of-the-art dynamic global vegetation and land surface process model that suggests that during the last deglaciation emission changes were strongly influenced by temperature and precipitation patterns over land surfaces. The results improve our understanding of the drivers of natural N2O emissions and are consistent with the idea that natural N2O emissions will probably increase in response to anthropogenic warming.

Tropical Atlantic temperature seasonality at the end of the last interglacial

The end of the last interglacial period, ~118 kyr ago, was characterized by substantial ocean circulation and climate perturbations resulting from instabilities of polar ice sheets. These perturbations are crucial for a better understanding of future climate change. The seasonal temperature changes of the tropical ocean, however, which play an important role in seasonal climate extremes such as hurricanes, floods and droughts at the present day, are not well known for this period that led into the last glacial. Here we present a monthly resolved snapshot of reconstructed sea surface temperature in the tropical North Atlantic Ocean for 117.7±0.8 kyr ago, using coral Sr/Ca and δ¹⁸O records. We find that temperature seasonality was similar to today, which is consistent with the orbital insolation forcing. Our coral and climate model results suggest that temperature seasonality of the tropical surface ocean is controlled mainly by orbital insolation changes during interglacials.

The last interglacial has been suggested as a test bed for models developed for future climate prediction, yet many climatic parameters remain unknown. Here, the authors present a precisely dated fossil coral and show that temperature seasonality in the southern Caribbean Sea 118 ka was similar to today.

North Atlantic forcing of tropical Indian Ocean climate

The response of the tropical climate in the Indian Ocean realm to abrupt climate change events in the North Atlantic Ocean is contentious. Repositioning of the intertropical convergence zone is thought to have been responsible for changes in tropical hydroclimate during North Atlantic cold spells, but the dearth of high-resolution records outside the monsoon realm in the Indian Ocean precludes a full understanding of this remote relationship and its underlying mechanisms. Here we show that slowdowns of the Atlantic meridional overturning circulation during Heinrich stadials and the Younger Dryas stadial affected the tropical Indian Ocean hydroclimate through changes to the Hadley circulation including a southward shift in the rising branch (the intertropical convergence zone) and an overall weakening over the southern Indian Ocean. Our results are based on new, high-resolution sea surface temperature and seawater oxygen isotope records of well-dated sedimentary archives from the tropical eastern Indian Ocean for the past 45,000 years, combined with climate model simulations of Atlantic circulation slowdown under Marine Isotope Stages 2 and 3 boundary conditions. Similar conditions in the east and west of the basin rule out a zonal dipole structure as the dominant forcing of the tropical Indian Ocean hydroclimate of millennial-scale events. Results from our simulations and proxy data suggest dry conditions in the northern Indian Ocean realm and wet and warm conditions in the southern realm during North Atlantic cold spells.

Rapid reductions in North Atlantic Deep Water during the peak of the last interglacial period

Deep ocean circulation has been considered relatively stable during interglacial periods, yet little is known about its behavior on submillennial time scales. Using a subcentennially resolved epibenthic foraminiferal δ(13)C record, we show that the influence of North Atlantic Deep Water (NADW) was strong at the onset of the last interglacial period and was then interrupted by several prominent centennial-scale reductions. These NADW transients occurred during periods of increased ice rafting and southward expansions of polar water influence, suggesting that a buoyancy threshold for convective instability was triggered by freshwater and circum-Arctic cryosphere changes. The deep Atlantic chemical changes were similar in magnitude to those associated with glaciations, implying that the canonical view of a relatively stable interglacial circulation may not hold for conditions warmer and fresher than at present.

Climate sensitivity, sea level and atmospheric carbon dioxide

Cenozoic temperature, sea level and CO2 covariations provide insights into climate sensitivity to external forcings and sea-level sensitivity to climate change. Climate sensitivity depends on the initial climate state, but potentially can be accurately inferred from precise palaeoclimate data. Pleistocene climate oscillations yield a fast-feedback climate sensitivity of 3±1(°)C for a 4 W m(-2) CO2 forcing if Holocene warming relative to the Last Glacial Maximum (LGM) is used as calibration, but the error (uncertainty) is substantial and partly subjective because of poorly defined LGM global temperature and possible human influences in the Holocene. Glacial-to-interglacial climate change leading to the prior (Eemian) interglacial is less ambiguous and implies a sensitivity in the upper part of the above range, i.e. 3-4(°)C for a 4 W m(-2) CO2 forcing. Slow feedbacks, especially change of ice sheet size and atmospheric CO2, amplify the total Earth system sensitivity by an amount that depends on the time scale considered. Ice sheet response time is poorly defined, but we show that the slow response and hysteresis in prevailing ice sheet models are exaggerated. We use a global model, simplified to essential processes, to investigate state dependence of climate sensitivity, finding an increased sensitivity towards warmer climates, as low cloud cover is diminished and increased water vapour elevates the tropopause. Burning all fossil fuels, we conclude, would make most of the planet uninhabitable by humans, thus calling into question strategies that emphasize adaptation to climate change.

Carbonic Anhydrase: An Efficient Enzyme with Possible Global Implications

As the global atmospheric emissions of carbon dioxide (CO2) and other greenhouse gases continue to grow to record-setting levels, so do the demands for an efficient and inexpensive carbon sequestration system. Concurrently, the first-world dependence on crude oil and natural gas provokes concerns for long-term availability and emphasizes the need for alternative fuel sources. At the forefront of both of these research areas are a family of enzymes known as the carbonic anhydrases (CAs), which reversibly catalyze the hydration of CO2into bicarbonate. CAs are among the fastest enzymes known, which have a maximum catalytic efficiency approaching the diffusion limit of 108 M−1s−1. As such, CAs are being utilized in various industrial and research settings to help lower CO2atmospheric emissions and promote biofuel production. This review will highlight some of the recent accomplishments in these areas along with a discussion on their current limitations.

The flux of dissolved nitrogen from the UK--evaluating the role of soils and land use

Fluvial dissolved nitrogen (dissolved organic nitrogen [DON], nitrate and ammonium) fluxes from the terrestrial biosphere of the UK to surrounding oceans are explained on the basis of combined predictions of soil to water transfer and in-stream loss. The flux of different nitrogen species from land to surface waters is estimated using an export coefficient model employing catchment soil, land use and hydroclimatic characteristics, fitted to flux estimates derived from the Harmonised Monitoring Scheme between 2001 and 2007 for 169 UK catchments. In-stream losses of DON, nitrate and ammonium were estimated using a transit time filter in the fluvial network. Comparisons of modelled land to water N flux (2125 ktonnes N yr(-1)) with estimates of N fluxes to estuarine and ocean systems at the tidal limit (791 ktonnes N yr(-1)) suggest that significant in-channel N losses occur. These in transit losses are equivalent to up to 55 kg N ha(-1) yr(-1).

Deglacial rapid sea level rises caused by ice-sheet saddle collapses

The last deglaciation (21 to 7 thousand years ago) was punctuated by several abrupt meltwater pulses, which sometimes caused noticeable climate change. Around 14 thousand years ago, meltwater pulse 1A (MWP-1A), the largest of these events, produced a sea level rise of 14-18 metres over 350 years. Although this enormous surge of water certainly originated from retreating ice sheets, there is no consensus on the geographical source or underlying physical mechanisms governing the rapid sea level rise. Here we present an ice-sheet modelling simulation in which the separation of the Laurentide and Cordilleran ice sheets in North America produces a meltwater pulse corresponding to MWP-1A. Another meltwater pulse is produced when the Labrador and Baffin ice domes around Hudson Bay separate, which could be associated with the '8,200-year' event, the most pronounced abrupt climate event of the past nine thousand years. For both modelled pulses, the saddle between the two ice domes becomes subject to surface melting because of a general surface lowering caused by climate warming. The melting then rapidly accelerates as the saddle between the two domes gets lower, producing nine metres of sea level rise over 500 years. This mechanism of an ice 'saddle collapse' probably explains MWP-1A and the 8,200-year event and sheds light on the consequences of these events on climate.

Geobiological constraints on Earth system sensitivity to CO₂ during the Cretaceous and Cenozoic

Earth system climate sensitivity (ESS) is the long-term (>10³ year) response of global surface temperature to doubled CO₂ that integrates fast and slow climate feedbacks. ESS has energy policy implications because global temperatures are not expected to decline appreciably for at least 10³ year, even if anthropogenic greenhouse gas emissions drop to zero. We report provisional ESS estimates of 3 °C or higher for some of the Cretaceous and Cenozoic based on paleo-reconstructions of CO₂ and temperature. These estimates are generally higher than climate sensitivities simulated from global climate models for the same ancient periods (approximately 3 °C). Climate models probably do not capture the full suite of positive climate feedbacks that amplify global temperatures during some globally warm periods, as well as other characteristic features of warm climates such as low meridional temperature gradients. These absent feedbacks may be related to clouds, trace greenhouse gases (GHGs), seasonal snow cover, and/or vegetation, especially in polar regions. Better characterization and quantification of these feedbacks is a priority given the current accumulation of atmospheric GHGs.

Non-CO2 greenhouse gases and climate change

Earth's climate is warming as a result of anthropogenic emissions of greenhouse gases, particularly carbon dioxide (CO(2)) from fossil fuel combustion. Anthropogenic emissions of non-CO(2) greenhouse gases, such as methane, nitrous oxide and ozone-depleting substances (largely from sources other than fossil fuels), also contribute significantly to warming. Some non-CO(2) greenhouse gases have much shorter lifetimes than CO(2), so reducing their emissions offers an additional opportunity to lessen future climate change. Although it is clear that sustainably reducing the warming influence of greenhouse gases will be possible only with substantial cuts in emissions of CO(2), reducing non-CO(2) greenhouse gas emissions would be a relatively quick way of contributing to this goal.

Enhanced chemistry-climate feedbacks in past greenhouse worlds

Trace greenhouse gases are a fundamentally important component of Earth's global climate system sensitive to global change. However, their concentration in the pre-Pleistocene atmosphere during past warm greenhouse climates is highly uncertain because we lack suitable geochemical or biological proxies. This long-standing issue hinders assessment of their contribution to past global warmth and the equilibrium climate sensitivity of the Earth system (E(ss)) to CO(2). Here we report results from a series of three-dimensional Earth system modeling simulations indicating that the greenhouse worlds of the early Eocene (55 Ma) and late Cretaceous (90 Ma) maintained high concentrations of methane, tropospheric ozone, and nitrous oxide. Modeled methane concentrations were four- to fivefold higher than the preindustrial value typically adopted in modeling investigations of these intervals, even after accounting for the possible high CO(2)-suppression of biogenic isoprene emissions on hydroxyl radical abundance. Higher concentrations of trace greenhouse gases exerted marked planetary heating (> 2 K), amplified in the high latitudes (> 6 K) by lower surface albedo feedbacks, and increased E(ss) in the Eocene by 1 K. Our analyses indicate the requirement for including non-CO(2) greenhouse gases in model-based E(ss) estimates for comparison with empirical paleoclimate assessments, and point to chemistry-climate feedbacks as possible amplifiers of climate sensitivity in the Anthropocene.

Greenhouse gases in the Earth system: a palaeoclimate perspective

While the trends in greenhouse gas concentrations in recent decades are clear, their significance is only revealed when viewed in the context of a longer time period. Fortunately, the air bubbles in polar ice cores provide an unusually direct method of determining the concentrations of stable gases over a period of (so far) 800,000 years. Measurements on different cores with varying characteristics, as well as an overlap of ice-core and atmospheric measurements covering the same time period, show that the ice-core record provides a faithful record of changing atmospheric composition. The mixing ratio of CO(2) is now 30 per cent higher than any value observed in the ice-core record, while methane is more than double any observed value; the rate of change also appears extraordinary compared with natural changes. Before the period when anthropogenic changes have dominated, there are very interesting natural changes in concentration, particularly across glacial/interglacial cycles, and these can be used to understand feedbacks in the Earth system. The phasing of changes in temperature and CO(2) across glacial/interglacial transitions is consistent with the idea that CO(2) acts as an important amplifier of climate changes in the natural system. Even larger changes are inferred to have occurred in periods earlier than the ice cores cover, and these events might be used to constrain assessments of the way the Earth could respond to higher than present concentrations of CO(2), and to a large release of carbon: however, more certainty about CO(2) concentrations beyond the time period covered by ice cores is needed before such constraints can be fully realized.

Late Holocene methane rise caused by orbitally controlled increase in tropical sources

Considerable debate surrounds the source of the apparently 'anomalous' increase of atmospheric methane concentrations since the mid-Holocene (5,000 years ago) compared to previous interglacial periods as recorded in polar ice core records. Proposed mechanisms for the rise in methane concentrations relate either to methane emissions from anthropogenic early rice cultivation or an increase in natural wetland emissions from tropical or boreal sources. Here we show that our climate and wetland simulations of the global methane cycle over the last glacial cycle (the past 130,000 years) recreate the ice core record and capture the late Holocene increase in methane concentrations. Our analyses indicate that the late Holocene increase results from natural changes in the Earth's orbital configuration, with enhanced emissions in the Southern Hemisphere tropics linked to precession-induced modification of seasonal precipitation. Critically, our simulations capture the declining trend in methane concentrations at the end of the last interglacial period (115,000-130,000 years ago) that was used to diagnose the Holocene methane rise as unique. The difference between the two time periods results from differences in the size and rate of regional insolation changes and the lack of glacial inception in the Holocene. Our findings also suggest that no early agricultural sources are required to account for the increase in methane concentrations in the 5,000 years before the industrial era.

Flux measurements of benzene and toluene from landfill cover soils

Carbon dioxide and CH(4), C(6)H(6) and C(7)H(8) fluxes from the soil cover of Case Passerini landfill site (Florence, Italy) were measured using the accumulation and static closed chamber methods, respectively. Results show that the CH(4)/CO(2), CH(4)/C(6)H(6) and CH(4)/C(7)H(8) ratios of the flux values are relatively low when compared with those of the 'pristine' biogas produced by degradation processes acting on the solid waste material disposed in the landfill. This suggests that when biogas transits through the cover soil, CH(4) is affected by degradation processes activated by oxidizing bacteria at higher extent than both CO(2) and mono-aromatics. Among the investigated hydrocarbons, C(6)H(6) has shown the highest stability in a wide range of redox conditions. Toluene behaviour only partially resembles that of C(6)H(6), possibly because de-methylation processes require less energy than that necessary for the degradation of C(6)H(6), the latter likely occurring via benzoate at anaerobic conditions and/or through various aerobic metabolic pathways at relatively shallow depth in the cover soil where free oxygen is present. According to these considerations, aromatics are likely to play an important role in the environmental impact of biogas released into the atmosphere from such anthropogenic emission sites, usually only ascribed to CO(2) and CH(4). In this regard, flux measurements using accumulation and static closed chamber methods coupled with gas chromatography and gas chromatography-mass spectrometry analysis may properly be used to obtain a dataset for the estimation of the amount of volatile organic compounds dispersed from landfills.

Tropical ocean temperatures over the past 3.5 million years

Determining the timing and amplitude of tropical sea surface temperature (SST) change is an important part of solving the puzzle of the Plio-Pleistocene ice ages. Alkenone-based tropical SST records from the major ocean basins show coherent glacial-interglacial temperature changes of 1 degrees to 3 degrees C that align with (but slightly lead) global changes in ice volume and deep ocean temperature over the past 3.5 million years. Tropical temperatures became tightly coupled with benthic delta18O and orbital forcing after 2.7 million years. We interpret the similarity of tropical SST changes, in dynamically dissimilar regions, to reflect "top-down" forcing through the atmosphere. The inception of a strong carbon dioxide-greenhouse gas feedback and amplification of orbital forcing at approximately 2.7 million years ago connected the fate of Northern Hemisphere ice sheets with global ocean temperatures since that time.

Volatility of unevenly sampled fractional Brownian motion: an application to ice core records

The analysis of many natural time series and especially those related to ice core records often suffers from uneven sampling intervals. For fractional Brownian motion, we show that standard estimates of the volatility can be strongly biased due to uneven sampling. Taking these limitations into account, we study high-resolution records of temperature proxies obtained from Antarctic ice cores. We find that the volatility properties reveal a strong nonlinear component in the temperature time series for time scales of 5-200 kyr extending earlier results. These findings suggest in particular that temperature increments over these time scales appear in clusters of big and small increments-a big (positive or negative) change is most likely followed by a big (positive or negative) change and a small change is most likely followed by a small change.

A new method for high-resolution methane measurements on polar ice cores using continuous flow analysis

Methane (CH4) is the second most important anthropogenic greenhouse gas in the atmosphere. Rapid variations of the CH4 concentration, as frequently registered, for example, during the last ice age, have been used as reliable time markers for the definition of a common time scale of polar ice cores. In addition, these variations indicate changes in the sources of methane primarily associated with the presence of wetlands. In order to determine the exact time evolution of such fast concentration changes, CH4 measurements of the highest resolution in the ice core archive are required. Here, we present a new, semicontinuous and field-deployable CH4 detection method, which was incorporated in a continuous flow analysis (CFA) system. In CFA, samples cut along the axis of an ice core are melted at a melt speed of typically 3.5 cm/min. The air from bubbles in the ice core is extracted continuously from the meltwater and forwarded to a gas chromatograph (GC) for high-resolution CH4 measurements. The GC performs a measurement every 3.5 min, hence, a depth resolution of 15 cm is achieved atthe chosen melt rate. An even higher resolution is not necessary due to the low pass filtering of air in ice cores caused by the slow bubble enclosure process and the diffusion of air in firn. Reproducibility of the new method is 3%, thus, for a typical CH4 concentration of 500 ppb during an ice age, this corresponds to an absolute precision of 15 ppb, comparable to traditional analyses on discrete samples. Results of CFA-CH4 measurements on the ice core from Talos Dome (Antarctica) illustrate the much higher temporal resolution of our method compared with established melt-refreeze CH4 measurements and demonstrate the feasibility of the new method.

Crops and climate change: progress, trends, and challenges in simulating impacts and informing adaptation

Assessments of the relationships between crop productivity and climate change rely upon a combination of modelling and measurement. As part of this review, this relationship is discussed in the context of crop and climate simulation. Methods for linking these two types of models are reviewed, with a primary focus on large-area crop modelling techniques. Recent progress in simulating the impacts of climate change on crops is presented, and the application of these methods to the exploration of adaptation options is discussed. Specific advances include ensemble simulations and improved understanding of biophysical processes. Finally, the challenges associated with impacts and adaptation research are discussed. It is argued that the generation of knowledge for policy and adaptation should be based not only on syntheses of published studies, but also on a more synergistic and holistic research framework that includes: (i) reliable quantification of uncertainty; (ii) techniques for combining diverse modelling approaches and observations that focus on fundamental processes; and (iii) judicious choice and calibration of models, including simulation at appropriate levels of complexity that accounts for the principal drivers of crop productivity, which may well include both biophysical and socio-economic factors. It is argued that such a framework will lead to reliable methods for linking simulation to real-world adaptation options, thus making practical use of the huge global effort to understand and predict climate change.