4.5 Mega Aquifer Systems and Climate Change
The current state of individual mega aquifer systems may still reflect climatic conditions of past geological periods because complete adaptation to the current climate has not yet been achieved. This is because physical processes tend to propagate extremely slowly across the entire domain of mega aquifer systems, which is partly due to their huge size. Examples are the slow decay of natural groundwater discharge from the Nubian Aquifer System (Figure 13) and the continuous degradation of permafrost (thawing) in the West Siberian Basin (Box 1), both in progress for approximately 10,000 years.
Current and projected anthropogenic climate change (IPCC, 2021a) is super-imposed on the long-term natural climate variations. Significant impacts are expected to develop during the present century. This includes a reduction in groundwater recharge due to warming (which causes an increase in evaporation and evapotranspiration) that will be exacerbated in regions where the annual precipitation also decreases. The impacts will be amplified by increased groundwater abstraction to meet water demands given the higher temperatures and lower precipitation. Both the reduced groundwater recharge and increased abstraction will contribute to higher groundwater development stress in many of the mega aquifer systems. Fact sheets prepared by the Intergovernmental Panel on Climate Change (IPCC, 2021b) provide a summary of the climate change prognosis for each of the mega aquifer systems.
Finally, some comments related to the influence of climate change on three specific categories of mega aquifer systems. First, aquifer systems containing only non-renewable groundwater are not threatened by changes in recharge, which implies that accelerated groundwater resource declines there will result only from increases in groundwater abstraction. Second, mega aquifer systems bordering the sea will be affected by sea-level rise induced by climate change, which leads to increased risk of seawater intrusion and seawater flooding. Finally, degeneration of permafrost in the three northernmost mega aquifer systems in Asia will, in principle, cause groundwater recharge to increase and the discharge of groundwater and its dissolved solids will also increase (Box 1).