. The mountain cryosphere of mainland Europe is recognized to have important impacts on a range of environmental processes. In this paper, we provide an overview on the current knowledge on snow, glacier, and permafrost processes, as well as their past, current, and future evolution. We additionally provide an assessment of current cryosphere research in Europe and point to the different domains requiring further research. Emphasis is given to our understanding of climate–cryosphere interactions, cryosphere controls on physical and biological mountain systems, and related impacts. By the end of the century, Europe's mountain cryosphere will have changed to an extent that will impact the landscape, the hydrological regimes, the water resources, and the infrastructure. The impacts will not remain confined to the mountain area but also affect the downstream lowlands, entailing a wide range of socioeconomical consequences. European mountains will have a completely different visual appearance, in which low- and mid-range-altitude glaciers will have disappeared and even large valley glaciers will have experienced significant retreat and mass loss. Due to increased air temperatures and related shifts from solid to liquid precipitation, seasonal snow lines will be found at much higher altitudes, and the snow season will be much shorter than today. These changes in snow and ice melt will cause a shift in the timing of discharge maxima, as well as a transition of runoff regimes from glacial to nival and from nival to pluvial. This will entail significant impacts on the seasonality of high-altitude water availability, with consequences for water storage and management in reservoirs for drinking water, irrigation, and hydropower production. Whereas an upward shift of the tree line and expansion of vegetation can be expected into current periglacial areas, the disappearance of permafrost at lower altitudes and its warming at higher elevations will likely result in mass movements and process chains beyond historical experience. Future cryospheric research has the responsibility not only to foster awareness of these expected changes and to develop targeted strategies to precisely quantify their magnitude and rate of occurrence but also to help in the development of approaches to adapt to these changes and to mitigate their consequences. Major joint efforts are required in the domain of cryospheric monitoring, which will require coordination in terms of data availability and quality. In particular, we recognize the quantification of high-altitude precipitation as a key source of uncertainty in projections of future changes. Improvements in numerical modeling and a better understanding of process chains affecting high-altitude mass movements are the two further fields that – in our view – future cryospheric research should focus on.

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The European mountain cryosphere: a review of its current state, trends, and future challenges

Global mean sea level is an integral of changes occurring in the climate system in response to
unforced climate variability as well as natural and anthropogenic forcing factors. Its temporal
evolution allows detecting changes (e.g., acceleration) in one or more components. Study of
the sea level budget provides constraints on missing or poorly known contributions, such as
the unsurveyed deep ocean or the still uncertain land water component. In the context of the
World Climate Research Programme Grand Challenge entitled “Regional Sea Level and
Coastal Impacts”, an international effort involving the sea level community worldwide has
been recently initiated with the objective of assessing the various data sets used to estimate
components of the sea level budget during the altimetry era (1993 to present). These data sets
are based on the combination of a broad range of space-based and in situ observations, model
estimates and algorithms. Evaluating their quality, quantifying uncertainties and identifying
sources of discrepancies between component estimates is extremely useful for various
applications in climate research. This effort involves several tens of scientists from about fifty
research teams/institutions worldwide (www.wcrp-climate.org/grand-challenges/gc-sea-
level). The results presented in this paper are a synthesis of the first assessment performed
during 2017-2018. We present estimates of the altimetry-based global mean sea level (average
rate of 3.1 +/- 0.3 mm/yr and acceleration of 0.1 mm/yr2 over 1993-present), as well as of the
different components of the sea level budget (http://doi.org/10.17882/54854). We further
examine closure of the sea level budget, comparing the observed global mean sea level with
the sum of components. Ocean thermal expansion, glaciers, Greenland and Antarctica
contribute by 42%, 21%, 15% and 8% to the global mean sea level over the 1993-present. We
also study the sea level budget over 2005-present, using GRACE-based ocean mass estimates
instead of sum of individual mass components. Results show closure of the sea level budget
within 0.3 mm/yr. Substantial uncertainty remains for the land water storage component, as
shown in examining individual mass contributions to sea level.

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Global sea-level budget 1993 - present

Himalayan glaciers supply meltwater to densely populated catchments in South Asia, and regional observations of glacier change over multiple decades are needed to understand climate drivers and assess resulting impacts on glacier-fed rivers. Here, we quantify changes in ice thickness during the intervals 1975–2000 and 2000–2016 across the Himalayas, using a set of digital elevation models derived from cold war–era spy satellite film and modern stereo satellite imagery. We observe consistent ice loss along the entire 2000-km transect for both intervals and find a doubling of the average loss rate during 2000–2016 [−0.43 ± 0.14 m w.e. year−1 (meters of water equivalent per year)] compared to 1975–2000 (−0.22 ± 0.13 m w.e. year−1). The similar magnitude and acceleration of ice loss across the Himalayas suggests a regionally coherent climate forcing, consistent with atmospheric warming and associated energy fluxes as the dominant drivers of glacier change.

Acceleration of ice loss across the Himalayas over the past 40 years

The Tibetan Plateau and its surroundings are known as the Third Pole (TP). This region is noted for its high rates of glacier melt and the associated hydrological shifts that affect water supplies in Asia. Atmospheric pollutants contribute to climatic and cryospheric changes through their effects on solar radiation and the albedos of snow and ice surfaces; moreover, the behavior and fates within the cryosphere and environmental impacts of environmental pollutants are topics of increasing concern. In this review, we introduce a coordinated monitoring and research framework and network to link atmospheric pollution and cryospheric changes (APCC) within the TP region. We then provide an up-to-date summary of progress and achievements related to the APCC research framework, including aspects of atmospheric pollution's composition and concentration, spatial and temporal variations, trans-boundary transport pathways and mechanisms, and effects on the warming of atmosphere and changing in Indian monsoon, as well as melting of glacier and snow cover. We highlight that exogenous air pollutants can enter into the TP's environments and cause great impacts on regional climatic and environmental changes. At last, we propose future research priorities and map out an extended program at the global scale. The ongoing monitoring activities and research facilitate comprehensive studies of atmosphere-cryosphere interactions, represent one of China's key research expeditions to the TP and the polar regions and contribute to the global perspective of earth system science.

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Linking atmospheric pollution to cryospheric change in the Third Pole region: current progress and future prospects

Editor’s note: For easy download the posted pdf of the State of the Climate for 2016 is a very low-resolution file. A high-resolution copy of the report is available by clicking here. Please be patient as it may take a few minutes for the high-resolution file to download.

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State of the Climate in 2015

Observations show that glaciers around the world are in retreat and losing mass. Internationally coordinated for over a century, glacier monitoring activities provide an unprecedented dataset of glacier observations from ground, air and space. Glacier studies generally select specific parts of these datasets to obtain optimal assessments of the mass-balance data relating to the impact that glaciers exercise on global sea-level fluctuations or on regional runoff. In this study we provide an overview and analysis of the main observational datasets compiled by the World Glacier Monitoring Service (WGMS). The dataset on glacier front variations (�42000 since 1600) delivers clear evidence that centennial glacier retreat is a global phenomenon. Intermittent readvance periods at regional and decadal scale are normally restricted to a subsample of glaciers and have not come close to achieving the maximum positions of the Little Ice Age (or Holocene). Glaciological and geodetic observations (�5200since 1850) show that the rates of early 21st-century mass loss are without precedent on a global scale, at least for the time period observed and probably also for recorded history, as indicated also in reconstructions from written and illustrated documents. This strong imbalance implies that glaciers in many regions will very likely suffer further ice loss, even if climate remains stable.

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Historically unprecedented global glacier decline in the early 21st century

To answer questions about linkages between changes in glaciers and climate change—e.g., How much of the current global sea-level rise can be attributed to melting glaciers?—more precise and quantitative studies of glaciers are required. This includes systematically extending the available in situ and remote sensing data, putting together a more-detailed world glacier inventory (WGI), continuing and strategically enlarging the global mass balance monitoring network, and conducting a rigorous uncertainty assessment of the available data series.

O. Sigurdsson

Papers

Historically unprecedented global glacier decline in the early 21st century

Summary of International Glacier Monitoring Summit