Editing IPCC:AR6/SROCC/Chapter-2 (section)

== 2.5 Key Gaps in Knowledge and Prospects ==

<div id="article-2-5key-gaps-in-knowledge-and-prospects-block-1"></div>

Impacts associated with climate-related changes in the high mountain cryosphere are evident in the observations reported in this chapter (Section 2.3). However, uncertainties remain with detection and attribution of key atmospheric drivers that influence much of these climate-related changes (Section 2.2.1), due to limited spatial density and/or temporal extent of observation records at high elevations. For example, trends in total or solid precipitation at high elevation remain highly uncertain, due to intrinsic uncertainties with ''in situ'' observation methods, and large natural variability. There are clear knowledge gaps in the distribution and characteristics of cryospheric variables, in particular the extent and ice content of permafrost in mountains, but also current glacier ice volumes, trends in lake and river ice, and the spatial and temporal variation of snow cover. These knowledge gaps persist despite a wealth of new data since AR5 especially from Earth observation satellites, which overcome much of the remoteness and inaccessibility of high mountains yet still face challenges for observations in mountains such as dealing with cloud cover and rugged terrain. Along with improved capacities to generate and integrate diverse observation data, initiatives such as citizen science (e.g., Dickerson-Lange et al., 2016; Wikstrom Jones et al., 2018 <sup>[[#fn:r1060|1060]]</sup> ) or Indigenous knowledge and local knowledge (Section 1.8.2, Cross Chapter Box 4 in Chapter 1) can also complement some observations that are based on conventional instruments and models. Radiative forcing effects of light absorbing particles, and understanding their spatiotemporal dynamics, is a key knowledge gap for the attribution of changes in high mountain snow and glaciers and the understanding of regional feedbacks (Section 2.2.2, Box 2.2).

These observational knowledge gaps currently impede efforts to quantify trends, and to calibrate and evaluate models that simulate the past and future evolution of the cryosphere and its impacts. Specific uncertainties are associated with projections of future climate change trends at high elevations due mostly to current limits in regional climate models and downscaling methods to capture the subtle interplays between large-scale climate change and local phenomena influenced by complex topography and high relief (Section 2.2.1). Coarse-scale simulations of future permafrost conditions in high mountains are fraught with difficulties in capturing fine-scale variation of topography, surface cover and near-surface materials (Section 2.2.4). Improved cross-disciplinary studies bringing together current observation and modelling approaches in each specific field hold potential to contribute to addressing these gaps in the future.

Experiences with changes in water availability, and with changes in frequency and/or magnitude of natural hazards, demonstrate the relevance of integrated approaches to understand past impacts and prepare for future risks, where exposure and the underpinning existing vulnerabilities of mountain socioecological systems influence the extent of these impacts (Section 2.3.2.3). However, there is insufficient understanding of the effects of cryospheric change on some natural hazards such as glacier outburst floods and on infrastructure, for example for transportation. Increased wildfire risk with a shrinking cryosphere is an uncertainty both spatially and temporally and with consequent effects on mountain ecosystems, particularly with respect to soil carbon and potential biome shifts. Overall, few studies have taken a comprehensive risk approach to systematically characterise and compare magnitude and extent of past impacts and future risks across high mountain regions, including compound risks and cascading impacts where instances of deep uncertainty in responses and outcomes may arise (Cross Chapter Box 5 in Chapter 1). Furthermore, a key knowledge gap is the capacity to economically quantify cryosphere-specific impacts and potential risks.

With ecosystems, particularly the terrestrial component, uncertainty exists at which community changes can be directly linked to cryospheric change as distinct from those due to atmospheric warming. In some cases, the changes can be linked, for example, where a receding glacier creates new habitat, but rising air temperature allow some species to establish that would not otherwise be able to. A major research gap is in our understanding of the fate of legacy pollutants such as mercury downstream of their release from glaciers and permafrost in terms of quantity and regional differences, freshwater sinks, and potential effects to ecosystems and human health. Similarly, the effect of permafrost thaw on water quality and ecosystems due to the increasing release of natural heavy metals and nutrients represents a gap in knowledge.

While adaptation measures are reported for high mountain cryosphere changes (Figure 2.9 a), it stands as a relatively new and developing area of research since AR5 (Figure 2.9 b), with particular gaps in terms of systematically evaluating their cost-benefits and long-term effectiveness as ‘fit-for-purpose’ solutions in the mountain context. Improved inter-comparability of successful adaptation cases, including the transferability of evidence for how adaptation can address both climate change and sustainable development objectives in different mountain regions, are prospects to support an evidentiary basis for future assessments of adaptation to cryosphere changes in the high mountains (Adler et al., 2019 <sup>[[#fn:r1061|1061]]</sup> ; McDowell et al., 2019 <sup>[[#fn:r1062|1062]]</sup> ).

<span id="a-acknowledgements"></span>