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International audienceThree Digital Elevation Models (DEM) of the Glacier Blanc have been calculated using photogrammetric restitutions of aerial photographs realised in 1952, 1981 and 2002 by IGN (the French administration for geography) and SINTEGRA (private company, Meylan, France). The aerial triangulation and the orientation of the stereographic images based on GPS control points lead to an absolute orientation whose residual standard deviation is between 0.25 to 1.25 m in planimetry and between 0.4 and 1.55 m in altitude (Lambert III, IGN69). Spot heights and 5 meter contour lines have been drawn (120 points/ha) which enables to build a Triangular Irregular Network (TIN) according to Delauney algorithm. This vector network has been linearly interpolated to obtain a DEM which is a raster grid of 10 meter spatial resolution. The subtraction of those DEM yields altitudinal variations on the surface of the glacier from which cumulative mass balance has been deduced. Results show a mass balance increasing of 2.1 m between 1952 and 1981 and decreasing of 10.8 m between 1981 and 2002. Results are compared with those of neighbour glaciers

Refined temporal signals are extracted from a glacier winter and summer mass balance series recorded at Glacier de Sarennes (French Alps) using variance decomposition. They are related to local and synoptic meteorological data in terms of interannual variability and structured trends. The winter balance has increased by +23% since 1976 due to more precipitation in early and late winter. The summer balance has decreased since 1982 due to a 43% increase in snow and ice melt. A 24-day lengthening of the ablation period – mainly due to longer ice ablation – is the main component in the overall increase in ablation. In addition, the last 25 yr have seen increases in ablation rates of 14 and 10% for snow and ice respectively. A simple degree-day analysis can account for both the snow/ice melt rate rise and the lengthening of the ablation period as a function of higher air temperatures. From the same analysis, the equilibrium line altitude of this 45° North latitude south-facing glacier has sensitivity to temperature of +93 m °C⁻¹ around its mean elevation of 3100 m a.s.l. over 6 decades. The sensitivity of summer balance to temperature is −0.62 m w.e. yr⁻¹ °C⁻¹ for a typical 125-day long ablation season. Finally, the time structure of winter and summer mass balance terms are connected to NAO anomalies. Best correlations are obtained with winter NAO anomalies. However, they strongly depend on how the NAO signal is smoothed, so that the link between mass-balance seasonal terms and NAO signal remains tenuous and hard to interpret

Abstract. Mass balance observations are very useful to assess climate change in different regions of the world. As opposed to glacier-wide mass balances which are influenced by the dynamic response of each glacier, point mass balances provide a direct climatic signal that depends on surface accumulation and ablation only. Unfortunately, major efforts are required to conduct in situ measurements on glaciers. Here, we propose a new approach that determines point surface mass balances from remote sensing observations. We call this balance the geodetic point surface mass balance. From observations and modelling performed on the Argentière and Mer de Glace glaciers over the last decade, we show that the vertical ice flow velocity changes are small in areas of low bedrock slope. Therefore, assuming constant vertical velocities in time for such areas and provided that the vertical velocities have been measured for at least 1 year in the past, our method can be used to reconstruct annual point surface mass balances from surface elevations and horizontal velocities alone. We demonstrate that the annual point surface mass balances can be reconstructed with an accuracy of about 0.3 m of water equivalent per year (m w.e. a−1) using the vertical velocities observed over the previous years and data from unmanned aerial vehicle images. Given the recent improvements of satellite sensors, it should be possible to apply this method to high-spatial-resolution satellite images as well.

Glacier-wide mass balance has been measured for more than sixty years and is widely used as an indicator of climate change and to assess the glacier contribution to runoff and sea level rise. Until present, comprehensive uncertainty assessments have rarely been carried out and mass balance data have often been applied using rough error estimation or without error considerations. In this study, we propose a framework for re-analyzing glacier mass balance series including conceptual and statistical toolsets for assessment of random and systematic errors as well as for validation and calibration (if necessary) of the glaciological with the geodetic balance results. We demonstrate the usefulness and limitations of the proposed scheme drawing on an analysis that comprises over 50 recording periods for a dozen glaciers and we make recommendations to investigators and users of glacier mass balance data. Reanalysis of glacier mass balance series needs to become a standard procedure for every monitoring programme to improve data quality and provide thorough uncertainty estimates

For reasons other than the climate, 2020 was an extraordinary year. The COVID-19 pandemic has affected almost all of us, changing the lives of many people around the globe. While the economic disruption associated with COVID-19 led to modest estimated reductions of 6-7% (e.g., le Quere et al. 2020;Friedlingstein et al. 2020; BP Statistical Review of the World Energy 2021) in global anthropogenic carbon dioxide (CO 2 ) emissions, atmospheric CO 2 levels continued to grow rapidly-a reminder of its very long residence time in the atmosphere and the challenge of reducing atmospheric CO 2 . As we show in this chapter, the climate has continued to respond to the resulting warming from these increases in CO 2 and other greenhouse gases such as methane and nitrous oxide, which also experienced record increases in 2020.The year 2020 was one of the three warmest since records began in the mid-to-late 1800s, with global surface temperatures around 0.6°C above the 1981-2010 average, despite the El Niño-Southern Oscillation progressing from neutral to La Niña conditions by August (see section 4b). Lower tropospheric temperatures matched those from 2016, the previous warmest year. Meanwhile, stratospheric temperatures continued to cool as a result of anthropogenic CO 2 increases. Along with the above-average surface temperatures, an unprecedented (since instrumental records began) geographic spread of heat waves and warm spells occurred. Antarctica observed its highest temperature on record (18.3°C) at Esperanza in February. In August, Death Valley, California, reported the highest temperature observed anywhere on Earth since 1931 (preliminary value of 54.4°C).Consequently, many permafrost measurement sites experienced their highest temperatures on record; Northern Hemisphere (NH) snow cover was below the 51-year average and the fourthleast extensive on record. Glaciers in alpine regions experienced their 33rd consecutive year of negative mass balance and 12th year of average losses of more than 500 mm depth. On average, NH lakes froze over 3 days later and thawed 5.5 days earlier than the 1981-2010 average during the 2019/20 winter, which was the third-shortest ice cover season since 1979/80.The atmosphere responded to higher temperatures accordingly by holding more water. Total column water vapor was high relative to the 1981-2010 average, ranging from 0.75 to 1.06 mm over ocean and 0.58 to 0.94 mm over land, but did not reach the record values of 2016. At the surface, specific humidity over oceans was at record high levels (0.23 to 0.41 g kg −1 ) and was well above average over land (0.14 to 0.36 g kg −1 ). Conversely, relative humidity was well below average over land (-1.28 to -0.68 %rh), continuing the long-term declining trend. Precipitation increased compared to 2019, driven largely by land values, but there were few exceptional extreme precipitation events, coupled with below-average cloudiness over most of the land. More lakes showed positive water level anomalies than 2019, and in East Africa, Lake Victoria's level ...

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Abstract. Mass balance observations are very useful to assess climate change in different regions of the world. As opposed to glacier-wide mass balances, which are influenced by the dynamic response of each glacier, point mass-balances provide a direct climatic signal that depends on surface accumulation and ablation only. Unfortunately, major efforts are required to conduct in situ measurements on glaciers. Here, we propose a new approach that determines point surface mass balances from remote sensing observations. We call this balance the geodetic point surface mass balance. From observations and modelling performed on Argentière and Mer de Glace glaciers over the last decade, we show that the vertical ice flow velocity changes are small in areas of low bedrock slope. Therefore, assuming constant vertical velocities in time for such areas and provided that the vertical velocities have been measured for at least one year in the past, our method can be used to reconstruct annual point surface mass balances from surface elevations and horizontal velocities alone. We demonstrate that the annual point surface mass balances can be reconstructed with an accuracy of about 0.3 m w.e. a−1 using the vertical velocities observed over the previous years and data from Unmanned Aerial Vehicle images. Given the recent improvements of satellite sensors, it should be possible to apply this method to high spatial resolution satellite images as well.