Patricia Martinerie

Physical and climatic parameters which influence the air content in polar ice

Abstract Under present-day climatic conditions, the air content of polar ice ( V ) generally shows a high sensitivity to the atmospheric pressure and hence to the surface elevation of the ice sheet wherethe ice is formed. The results presented here are from sixteen different sites (fourteen in Antartica, one in Greenland and one in the Yukon Territories, Canada), and they allow a better understanding of the parameters which influence the air content in ice. It is demonstrated the V can be expressed very simply as a function of the atmospheric pressure ( P i ), the temperature ( T i ) and the porous volume of ice ( V i ) at which the air in the firn becomes isolated in terms of pressure from the atmosphere during the process of pore close-off. Our results confirm a V i increase with temperature and show no clear V i dependence on snow accumulation rate. The possible non-linearity of the V i − T i relation we obtain could be due to a wind influence on V i and/or to a second-order effect of the accumulation rate. Our V i results are compared to measurements of the volume of closed pores versus depth in the firn. To close, we discuss the parameterization that we can obtain to interpret the air content variations observed in deep ice cores (over periods covering glacial-interglacial transitions) in terms of paleoclimatic conditions prevailing at the surface of the ice sheet.

Air content paleo record in the Vostok ice core (Antarctica) : a mixed record of climatic and glaciological parameters

Under present-day climatic conditions the air content of ice shows a high sensitivity to the atmospheric pressure and hence to the elevation at the surface of the ice sheet. This observation has been used to infer past ice sheet thickness variations of Antarctica and Greenland. A high-resolution air content profile (more than 1000 measurements) covering approximately the last 200,000 years was obtained along the 2546-m long Vostok ice core. Three analytical techniques were used, leading to consistent results which show large amplitude and rapid air content variations. The Vostok results support thicker/thinner ice in the central part of East Antarctica during warm/cold periods

The chemical composition of ancient atmospheres: A model study constrained by ice core data

A coupled chemistry radiation transport two-dimensional model of the lower and middle atmosphere was adapted to study the chemical composition of the atmosphere at preindustrial time and last glacial maximum (LGM). The model was constrained by trace gas concentrations (CO2, CH4, and N2O) inferred from polar ice core records. The formulation of tropospheric dynamics and chemistry was improved in order to more accurately simulate the transport and the oxidation processes below the tropopause. Our objectives are to infer the changes in middle-atmosphere temperature, ozone layer, and oxidation capacity of the atmosphere (e.g., methane lifetime) over the last 18,000 years. A middle-atmosphere cooling was obtained between LGM and preindustrial Holocene (PIH) as well as between PIH and present time. This is mainly due to changes in the CO2 and chlorofluorocarbon (CFC) concentrations, respectively. CFCs are also the main contributors to the middle-atmosphere ozone decrease since PIH. Between LGM and PIH the compensating effects of CO2 and N2O lead to little variation in stratospheric ozone. A 17% decrease in tropospheric OH was obtained between LGM and PIH, whereas the model provides a 6% OH increase since PIH. The corresponding changes in the methane sink are too small to have played a dominant role in the past methane concentration changes. Our model derived methane emissions for LGM, PIH, and present time are in good agreement with methane sources evaluated during these three periods.

Natural and anthropogenic variations in methane sources during the past two millennia

Methane is an important greenhouse gas that is emitted from multiple natural and anthropogenic sources. Atmospheric methane concentrations have varied on a number of timescales in the past, but what has caused these variations is not always well understood. The different sources and sinks of methane have specific isotopic signatures, and the isotopic composition of methane can therefore help to identify the environmental drivers of variations in atmospheric methane concentrations. Here we present high-resolution carbon isotope data (δ13C content) for methane from two ice cores from Greenland for the past two millennia. We find that the δ13C content underwent pronounced centennial-scale variations between 100 bc and ad 1600. With the help of two-box model calculations, we show that the centennial-scale variations in isotope ratios can be attributed to changes in pyrogenic and biogenic sources. We find correlations between these source changes and both natural climate variability—such as the Medieval Climate Anomaly and the Little Ice Age—and changes in human population and land use, such as the decline of the Roman empire and the Han dynasty, and the population expansion during the medieval period.

Air content along the Greenland Ice Core Project core: A record of surface climatic parameters and elevation in central Greenland

We present here measurements of the air content of the ice, V, performed along the Greenland Ice Core Project (GRIP) ice core. The main features of the long-term trends are (1) a decrease of 13% between the last glacial maximum (LGM) and the earliest part of the Holocene, and (2) an increase of 8% during the Holocene. The results are discussed in terms of changes in atmospheric pressure, surface elevation and porosity at close-off. The V record contains a significant signal of past changes of surface elevation in qualitative agreement with ice sheet modeling simulations. It suggests a thickening of central Greenland during the transition from the LGM to the early Holocene, and a significant thinning through the Holocene period. It also stresses the large influence on past V variations of changes in ice porosity, which are not explained by the present-day spatial relationship with temperature and may reflect changes in other surface climatic parameters (like precipitation seasonality or wind stress). The potential role of temporal variations of atmospheric pressure patterns is also evaluated. Air content results in the GRIP ice older than 110 ka indicate values approximately in the same range as those observed during the last 40,000 years, with generally higher air content corresponding to isotopically warmer ice.

Glacial-interglacial mean sea level pressure change due to sea level, ice sheet and atmospheric mass changes

Abstract The change in the global mean atmospheric pressure between glacial and interglacial periods is evaluated at sea level. This change originates in a modification of topography and in a possible variation in the atmospheric mass. In this calculation the atmosphere is at hydrostatic equilibrium, and the parameters describing the glacial period are varied in a plausible range. The result, with constant atmospheric mass, is a mean sea level pressure decrease of 9–15 hPa linked with the deglaciation. The corresponding pressure change at the reference level corresponding to the present day sea level does not exceed one hPa. When considering only the change in the atmospheric mass, an increase which does not exceed 2 hPa is found, linked with the deglaciation.

Research paperGlacial-interglacial mean sea level pressure change due to sea level, ice sheet and atmospheric mass changes

The change in the global mean atmospheric pressure between glacial and interglacial periods is evaluated at sea level. This change originates in a modification of topography and in a possible variation in the atmospheric mass. In this calculation the atmosphere is at hydrostatic equilibrium, and the parameters describing the glacial period are varied in a plausible range. The result, with constant atmospheric mass, is a mean sea level pressure decrease of 9–15 hPa linked with the deglaciation. The corresponding pressure change at the reference level corresponding to the present day sea level does not exceed one hPa. When considering only the change in the atmospheric mass, an increase which does not exceed 2 hPa is found, linked with the deglaciation.

Changes in Trace Gas Concentrations during the last 2,000 years and more generally the Holocene

One possible forcing factors of the climate variability observed over the last 2,000 years is changing radiatively active trace gas concentrations in the atmosphere. Direct measurements of atmospheric trace gas concentrations over the last few decades indicate increasing global trends for CO2, CH4 N2O and various halocarbons (CFC’s), due to anthropogenic activities. Our knowledge of the evolution of atmospheric trace gases for older periods arises mainly from the analysis of the air trapped in polar ice. The documentation of the 2,000 yr — record is uneven. A large effort concerns the CO2 and CH4 records over the last millennium, which indicate fluctuations of about 10 ppmv for CO2 and 70 ppbv for CH4. Such fluctuations can possibly reflect oceanic and/or continental biospheric responses to climatic changes such as the Little Ice Age or the Medieval Optimum. Another explanation lies in the response of the oceanic or continental biospheric reservoirs to high frequency natural climate variability. The CO2 and CH4 records extended to the Holocene period show significant variations (up to about 40 ppmv for CO2 and 150 ppbv for CH4). Estimates of the climatic impact of such CO2 and CH4 fluctuations are given. Data concerning other less documented greenhouse trace gases, like N2O or tropospheric ozone (over about the last 100 years), are also reviewed.

A variational approach for optimal diffusivity identification in firns

Trace gas measurements in interstitial air from polar firn allow to reconstruct their atmospheric concentration time trends over the last 50 to 100 years. This provides a unique way to reconstruct the recent anthropogenic impact on atmospheric composition. Converting depth-concentration profiles in firn into atmospheric concentration histories requires models of trace gas transport in firn. A fundamental parameter of these models is firn diffusivity. Here we propose a new method to evaluate the diffusivity of polar firns using automatic control analysis techniques. More precisely, the diffusivity identification is formulated as an optimization problem in terms of partial differential equations (PDE). The proposed theorems generally apply to transport phenomena in non-homogeneous media (space-dependent coefficients), a variational approach is proposed and the optimization problem is solved with an adjoint-based gradient-descent algorithm.

Input Estimation from Sparse Measurements in LPV Systems and Isotopic Ratios in Polar Firns* *This work was partially supported by the CNRS/INSU LEFE program.

We adress the problem of inverse input reconstruction for linear parameter-varying (LPV) systems when only a limited amount of data (i.e. sparse measurements at final time) is available. We include the LPV property by deriving a time-varying Greens function that models the input/output behavior. The estimation is achieved by solving a least-squares optimization problem parameterized in terms of the input rugosity (regularization term) to take into account the under-constrained nature of the problem. Several automatic tuning methods for the rugosity are described, based on stochastic analysis of the data. A new LPV model is derived for the isotopic ratio of chemical species and our results are applied to the atmospheric history reconstruction of trace gases from polar firn measurements, a problem of prime interest in the environmental sciences community.