Methane: A bunch of information for climate research

Methane is a radiatively and chemically active trace gas of the earth's atmosphere. Its atmospheric concentration has been measured continuously and directly since 1978 (STEELE et al., J. Atmos. Chem., 5,125,1987; BLAKE and ROWLAND, Science, 239,1129,1988; DLUGOKENCKY et al., J. Geophys. Res., 99,17021,1994). Before that the CH_4 concentration history has to be reconstructed from paleo records. Ice sheets and glaciers are so far the only archives which have stored the atmospheric gas composition directly. Results from various ice cores are consistent and prove the reliability of this archive. The pre-industrial CH_4 concentration changes, with some exceptions, in concert with the main climate features of the northern hemisphere. That is, lower concentrations of about 350 ppbv during ice ages, and higher concentrations, around 700 ppbv, during warm periods. Natural variations are most probably linked to the extent and source intensity of wetlands, the main natural source. The anthropogenic increase from 700 to over 1700 ppbv over the last 200yr is caused by emissions linked to the human population growth like domestic ruminants, rice paddies, human-induced fires, landfills, and fossil fuel exploitation (FUNG et al., J. Geophys. Res., 96,13033,1991 and references therein). CH_4 can, due to its global signal (neglecting a pole to pole difference of a few percent), be used to synchronise ice core data from various sites. This is extremely useful in investigating the causes of climate change

CH\u3csub\u3e4\u3c/sub\u3e and δ\u3csup\u3e18\u3c/sup\u3eO of O\u3csub\u3e2\u3c/sub\u3e records from Antarctic and Greenland ice: A clue for stratigraphic disturbance in the bottom part of the Greenland Ice Core Project and the Greenland Ice Sheet Project 2 ice cores

The suggestion of climatic instability during the last interglacial period (Eem), based on the bottom 10% of the Greenland Ice core Project (GRIP) isotopic profile, has been questioned because the bottom record from the neighboring Greenland Ice Sheet Project 2 (GISP2) core (28 km away) is strikingly different over the same interval and because records of the δ18O of atmospheric O2 from both cores showed unexpected rapid fluctuations. Here we present detailed methane records from the Vostok (Antarctica), GRIP, and GISP2 cores over the relevant intervals. The GRIP and GISP2 data show rapid and large changes in methane concentration, which are correlative with variations of the δ18O of the ice, while the Vostok record shows no such variations. This discrepancy reinforces the suggestion that the bottom sections of the Greenland records are disturbed. By combining the methane data with measurements of δ18O of O2 in the same samples, we attempt to constrain the nature of the stratigraphic disturbance and the age of the analyzed ice samples. Our results suggest that ice layers from part of the last interglacial period exist in the lower section of both ice cores and that some of the apparent climate instabilities in the GRIP core would be the result of a mixture of ice from the last interglacial with ice from the beginning of the last glaciation or from the penultimate glaciation

A new set-up for simultaneous high-precision measurements of CO₂,δ¹³C-CO₂ and δ¹⁸O-CO2 on small ice core samples

Palaeoatmospheric records of carbon dioxide and its stable carbon isotope composition (<i>δ</i>¹³C) obtained from polar ice cores provide important constraints on the natural variability of the carbon cycle. However, the measurements are both analytically challenging and time-consuming; thus only data exist from a limited number of sampling sites and time periods. Additional analytical resources with high analytical precision and throughput are thus desirable to extend the existing datasets. Moreover, consistent measurements derived by independent laboratories and a variety of analytical systems help to further increase confidence in the global CO₂ palaeo-reconstructions. Here, we describe our new set-up for simultaneous measurements of atmospheric CO₂ mixing ratios and atmospheric <i>δ</i>¹³C and <i>δ</i>¹⁸O-CO₂ in air extracted from ice core samples. The centrepiece of the system is a newly designed needle cracker for the mechanical release of air entrapped in ice core samples of 8–13 g operated at −45 °C. The small sample size allows for high resolution and replicate sampling schemes. In our method, CO₂ is cryogenically and chromatographically separated from the bulk air and its isotopic composition subsequently determined by continuous flow isotope ratio mass spectrometry (IRMS). In combination with thermal conductivity measurement of the bulk air, the CO₂ mixing ratio is calculated. The analytical precision determined from standard air sample measurements over ice is ±1.9 ppm for CO₂ and ±0.09 ‰ for <i>δ</i>¹³C. In a laboratory intercomparison study with CSIRO (Aspendale, Australia), good agreement between CO₂ and <i>δ</i>¹³C results is found for Law Dome ice core samples. Replicate analysis of these samples resulted in a pooled standard deviation of 2.0 ppm for CO₂ and 0.11 ‰ for <i>δ</i>¹³C. These numbers are good, though they are rather conservative estimates of the overall analytical precision achieved for single ice sample measurements. Facilitated by the small sample requirement, replicate measurements are feasible, allowing the method precision to be improved potentially. Further, new analytical approaches are introduced for the accurate correction of the procedural blank and for a consistent detection of measurement outliers, which is based on <i>δ</i>¹⁸O-CO₂ and the exchange of oxygen between CO₂ and the surrounding ice (H₂O)

Rapid growth of HFC-227ea (1,1,1,2,3,3,3-Heptafluoropropane) in the atmosphere

We report the first measurements of 1,1,1,2,3,3,3-heptafluoropropane (HFC-227ea), a substitute for ozone depleting compounds, in remote regions of the atmosphere and present evidence for its rapid growth. Observed mixing ratios ranged from below 0.01 ppt in deep firn air to 0.59 ppt in the northern mid-latitudinal upper troposphere. Firn air samples collected in Greenland were used to reconstruct a history of atmospheric abundance. Year-on-year increases were deduced, with acceleration in the growth rate from 0.026 ppt per year in 2000 to 0.057 ppt per year in 2007. Upper tropospheric air samples provide evidence for a continuing growth until late 2009. Fur- thermore we calculated a stratospheric lifetime of 370 years from measurements of air samples collected on board high altitude aircraft and balloons. Emission estimates were determined from the reconstructed atmospheric trend and suggest that current "bottom-up" estimates of global emissions for 2005 are too high by more than a factor of three

Changes in the atmospheric CH4 gradient between Greenland and Antarctica during the Holocene

High-resolution records of atmospheric methane over the last 11,500 years have been obtained from two Antarctic ice cores (D47 and Byrd) and a Greenland core (Greenland Ice Core Project). These cores show similar trapping conditions for trace gases in the ice combined with a comparable sampling resolution; this together with a good relative chronology, provided by unequivocal CH4 features, allows a direct comparison of the synchronized Greenland and Antarctic records, and it reveals significant changes in the interpolar difference of CH4 mixing ratio with time. On the average, over the full Holocene records, we find an interpolar difference of 44±7 ppbv. A minimum difference of 33±7 ppbv is observed from 7 to 5 kyr B.P. whereas the maximum gradient (50±3 ppbv) took place from 5 to 2.5 kyr B.P. A gradient of 44±4 ppbv is observed during the early Holocene (11.5 to 9.5 kyr B.P). We use a three-box model to translate the measured differences into quantitative contributions of methane sources in the tropics and the middle to high latitudes of the northern hemisphere. The model results support the previous interpretation that past natural CH4 sources mainly lay in tropical regions, but it also suggests that boreal regions provided a significant contribution to the CH4 budget especially at the start of the Holocene. The growing extent of peat bogs in boreal regions would also have counterbalanced the drying of the tropics over the second half of the Holocene. Finally, our model results suggest a large source increase in tropical regions from the late Holocene to the last millennium, which may partly be caused by anthropogenic emissions

Consistent Greenland temperature variability over the past 2000 years from NGRIP and GISP2

第4回極域科学シンポジウム横断セッション：[IC] 極域アイスコア・地形地質・モデリングから迫る古環境変動とそのメカニズム11月12日（火） 統計数理研究所 ３階セミナー室１（D305

Reduction of radiation biases by incorporating the missing cloud variability by means of downscaling techniques: a study using the 3-D MoCaRT model

Handling complexity to the smallest detail in atmospheric radiative transfer models is unfeasible in practice. On the one hand, the properties of the interacting medium, i.e., the atmosphere and the surface, are only available at a limited spatial resolution. On the other hand, the computational cost of accurate radiation models accounting for three-dimensional heterogeneous media are prohibitive for some applications, especially for climate modelling and operational remote-sensing algorithms. Hence, it is still common practice to use simplified models for atmospheric radiation applications. &lt;br&gt;&lt;br&gt; Three-dimensional radiation models can deal with complex scenarios providing an accurate solution to the radiative transfer. In contrast, one-dimensional models are computationally more efficient, but introduce biases to the radiation results. &lt;br&gt;&lt;br&gt; With the help of stochastic models that consider the multi-fractal nature of clouds, it is possible to scale cloud properties given at a coarse spatial resolution down to a higher resolution. Performing the radiative transfer within the cloud fields at higher spatial resolution noticeably helps to improve the radiation results. &lt;br&gt;&lt;br&gt; We present a new Monte Carlo model, MoCaRT, that computes the radiative transfer in three-dimensional inhomogeneous atmospheres. The MoCaRT model is validated by comparison with the consensus results of the Intercomparison of Three-Dimensional Radiation Codes (I3RC) project. &lt;br&gt;&lt;br&gt; In the framework of this paper, we aim at characterising cloud heterogeneity effects on radiances and broadband fluxes, namely: the errors due to unresolved variability (the so-called plane parallel homogeneous, PPH, bias) and the errors due to the neglect of transversal photon displacements (independent pixel approximation, IPA, bias). First, we study the effect of the missing cloud variability on reflectivities. We will show that the generation of subscale variability by means of stochastic methods greatly reduce or nearly eliminate the reflectivity biases. Secondly, three-dimensional broadband fluxes in the presence of realistic inhomogeneous cloud fields sampled at high spatial resolutions are calculated and compared to their one-dimensional counterparts at coarser resolutions. We found that one-dimensional calculations at coarsely resolved cloudy atmospheres systematically overestimate broadband reflected and absorbed fluxes and underestimate transmitted ones