Water stress in +2C vs. +4C climate changed worlds

(BY HUGO)

The Tyndall Centre for Climate Change Research published the Four Degrees and Beyond Special Issue of the Philosophical Transactions of the Royal Society A Journal (link), with an interesting article by Fai Fung, Ana Lopez & Mark New on «Water availability in +2°C and +4°C worlds».

The paper contrasts water availability and water stress in a world where warming is limited to 2◦C as per the Copenhagen undertakings and one where policy fails and warming reaches 4◦C, a likely scenario according to scientific sources quoted.

The paper defines a water stress index (WSI) based on water resources per capita calculated as the ratio of mean annual surface run-off (MAR) to population. This is recognized as simplistic and reference is made to more complex models in the literature on water stress or scarcity, but a simpler index is justified to reduce uncertainty and limit data requirements. Some interesting parts of the paper discussion are as follows:

«changes in mean annual run-off in a +2◦C world are generally amplified in a +4◦C world: drier areas dry further and wetter areas become wetter. Moreover, as these changes in MAR become amplified, both the consensus and spatial coherence of these changes strengthen. By investigating the changes in water stress in 112 of the world’s major river basins, we have also found that the majority of these river basins are projected to suffer greater water stress in a +4◦C world than in a +2◦C world. However, as we move from a +2◦C to a +4◦C world, there are also a small but increasing number of basins that may experience less water stress, as they are located in regions where rainfall is projected to increase. By using population growth scenarios for the 2030s and 2060s, we find that in a +2◦C world, water stress is dominated by the change in population. However, as we move to +4◦C world and the climate change signal becomes stronger, climate change can play a more dominant role in determining water stress in a river basin. (...)

By examining a subset of the world’s major river basins, we have shown that the picture for water stress in each river basin is dependent on the magnitude of the climate change and the nature of the population growth. For some river basins, the effects of climate change become large enough to offset the large increases in demand in a +4◦C world, e.g. in the Ganges; in most basins, however, climate and population growth combine to increase stress or climate change is insufficient to offset increased demand.

We have also found that seasonality in run-off may be more pronounced in a +4◦C world compared with a +2◦C world; thus, even where annual average runoff increases, dry seasons can become more stressed. This could mean that more sophisticated infrastructure projects may be required in a +4◦C world compared with a +2◦C world in order to prevent flooding and droughts.»

Given the business as usual approach to adaptation in Québec, this could mean that trends in dam building and supply oriented solutions will continue to amplify. Possible impacts of dams on the environment at the provincial level might depend on changes to reservoir release regimes under which the dams must operate (inversion, homogenisation, natural) in order to adapt to climate change. As most large dams operate for power generation, modifications in energy demand to face climate change could be an important factor influencing flow regimes and alterations patterns to freshwater ecosystems. Reductions in heating during winter, increase in climatisation duing summer and increased seasonal run-off variability might all point to a move away from natural reservoir release patterns and towards homogenisation or inversion regimes. The Ouranos Consortium has published studies regarding some aspects of climate change on Québec water resources.