The availability of water under ever-increasing climate stress has never been more important. Nowhere is this more apparent than in glacial mountain regions where runoff from glaciers provides water in times of drought or low river flows. As glaciers retreat due to climate change, the water supplied to these basins will diminish. To better understand these hydrological changes, a recent study published in Nature Climate Change examined the world’s largest glacierized drainage basins under future climate change scenarios.
Expansive in scale, the study differentiates itself from previous research that assessed the hydro-glacier issue at more localized scales like specific mountain ranges, for example. This study analyzes 56 glacierized drainage basins on four continents excluding Antarctica and Greenland. The basins examined were selected based on their size: they needed to be bigger than 5,000 km2, in addition to having at least 30 km2 of ice cover and greater than 0.01 percent of total glacier cover during the chosen base period of 1981 to 2010.
The motivation behind the study’s global scale, the first ever completed, according to Regine Hock, one of the study’s authors, is that “at a local scale you can only cover a fraction of the glaciers/catchments that may be relevant.” She told GlacierHub that while there are advantages to local studies because they can be more detailed and accurate, the advantage of a global study is that spatial patterns across regions can be identified and analyzed.
In order to calculate changes in glacial mass and accompanying runoff, defined as water that leaves a glacierized area, the authors utilized the Global Glacier Evolution Model to simulate relevant glacial processes including mass accumulation and loss, changes in glacial extent, and glacier elevation. The glacier model was driven by three of the IPCC’s Representative Concentration Pathways (RCP). These are future greenhouse gas (GHG) concentration scenarios based on different socio-economic pathways. The RCP’s chosen by the authors were the 2.6 scenario, which they note is the most similar to the 2015 Paris agreement, the 4.5 scenario where GHG concentrations stabilize by 2100, and the 8.5 or “business-as-usual” scenario where GHG concentrations continue to increase past 2100.
How do these three scenarios impact glacial volume in the study’s glacierized basins? After running the glacier model, total volume was projected to decrease in all three with a decrease of 43±14 percent for the 2.6, 58±13 percent for the 4.5, and 74±11 percent for the 8.5, respectively.
A decrease in glacial volume will in the short term mean an increase in water for a basin as runoff increases, that is until the point of “peak water,” where the amount of glacial runoff begins to decrease as glacier volume declines. Distressingly, peak water has already been reached in 45 percent of the basins examined in the study including most of the Andes, Alps, and Rocky Mountains.
Three factors— total glacial area, ice cover as a fraction of the basin, and the basin’s latitude— influence the timing of peak water occurrence in a basin. Basins with many large glaciers at higher latitudes like in coastal Alaska were projected to reach peak water near the end of the century whereas basins closer to the equator with small glaciers like the Peruvian Andes have already experienced or will soon experience peak water. Furthermore, the Himalayas are projected to experience peak water around mid-century as their high elevation tempers the effect of their relatively low latitude.
The study also examined changes to glacial runoff on a monthly timescale for the years 2050 and 2100, focusing specifically on the melt season from June to October in the Northern Hemisphere and December to April in the Southern Hemisphere. The monthly results showed spatial consistency, which surprised the authors, according to Hock, with runoff increasing in almost all basins at the beginning of the melt season (June/December) and decreasing toward the end (August and September/February and March). Another unexpected finding was the significant reduction in overall runoff, up to a 10 percent decrease by 2100 in at least one month, in basins with very low glacial cover, a phenomenon that was observed in a third of the basins, Hock added.
It is important to remember that these changes in basin runoff mean more than just changing numbers and statistics: there are people and communities that rely on water provided by glaciers. The authors note that 26 percent of the Earth’s land surface is covered by glacierized drainage basins, impacting a third of the world’s population.
The ramifications of glacier retreat will not be felt equally across the basins observed in this study. When asked what regions are most at risk, Hock identified both the Andes and Central Asia as places of concern. In the Andes, runoff is decreasing in almost all basins. This is of particular concern due to the limited water resources of the South American west coast. In Central Asia, glaciers contribute to basin runoff in all months, leading to potential problems if runoff is significantly reduced.
These regions, along with other glacier reliant places, face an uncertain and atypical water future, one that will likely see an increase in glacial runoff, followed by a sharp decline.To prepare for these forthcoming challenges, further study is needed, particularly with a focus on the human dimensions of glacial retreat.