The Big Thaw: gauging the past, present and future of our mountain water resources

The world's mountains store and release frozen water when it is most valuable, as summer meltwater in the growing season. This service is an extraordinary generator of wealth and well-being, sustaining a sixth of the global population and a quarter of global GDP, but is highly vulnerable to climate change. Over the next 30 years, the Alps, Western North America, Himalayas and Andes will lose 10-40% of their snow, hundreds of cubic kilometres of summer water supply, and by end of century, mountain glaciers will lose 20-60% of their ice.

To map our mountain water resources and predict their future, we must rely on models of snowfall, seasonal snowpacks, glacier gains and losses, and river runoff. The skill of these models is, however, fundamentally limited by the quality and availability of observations needed to test and develop them, and the mountain cryosphere is so large, varied and inhospitable that we lack many of these key observations. In most mountain ranges, snowfall is underestimated by 50-100%, and weather records are too short to have captured a history of their climate extremes. The thickness of only 6 of 41,000 glaciers has been surveyed in the Himalayan headwaters of the Brahmaputra, Indus and Ganges basins, so the lifespan of a water resource used by 800 million people remains unpredictable.

This project aims to fill four of the key observation gaps: 1) snowfall, 2) glacier thickness, 3) runoff, and 4) weather extremes, by taking a targeted approach to provide not blanket coverage of the mountain cryosphere but carefully-selected datasets designed to test and improve model skill. Importantly, through the calibration and refinement of relevant model processes at these target sites we can eliminate gross biases and reduce uncertainties in model outputs that can then apply not just locally but across all model scales, in the past, present and future.

We will make new snowfall observations with a pioneering method that, for the first time, makes unbiased measurements over areas thousands to billions of times larger than rain gauges, and use these to test and improve snowfall models that are run worldwide. To capture and understand the extremes of mountain precipitation, we will extend the decades-long instrumental record back by centuries to millennia by identifying the signals of wet and dry years preserved in high, undisturbed Himalayan-lake sediments that we will core and analyse at very high resolution. In parallel, we will use a recently acquired and uniquely extensive glacier survey from Nepal to improve glacier-thickness models on the mountain-range scale.

We will use our new snowfall maps and projections to drive detailed models of snowpack and glacier evolution over the 21st century for two targeted catchments in the Alps and Himalayas. We will apply our models to our glacier thickness maps to determine how long these glaciers will survive under a changing climate, how much meltwater will flow into their catchments and how this will change. We will test the performance of our models against cutting-edge new flux and hydrochemistry observations of the contribution of different water sources to downstream river flow. Finally, we will determine which climate factors affect the frequency and severity of extreme wet and dry years for the two catchments, and how these events are likely to change through the 21st century.

Together, our targeted, data-driven modelling advances will demonstrably improve our ability to quantify how much seasonal snow accumulates in the mountain cryosphere and predict how it will change in the future, what the timescales and potential trajectories for change are for glacier-ice resources, how frequently dry and wet years occur, what climate factors cause this, and how these extremes will change. By making the mountain cryosphere more predictable, we will support societies in managing change in this critical but vulnerable water resource.