Water availability in +2°C and +4°C worlds

Abstract

While the parties to the UNFCCC agreed in the December 2009 Copenhagen Accord that a 2 • C global warming over pre-industrial levels should be avoided, current commitments on greenhouse gas emissions reductions from these same parties will lead to a 50 : 50 chance of warming greater than 3.5 • C. Here, we evaluate the differences in impacts and adaptation issues for water resources in worlds corresponding to the policy objective (+2 • C) and possible reality (+4 • C). We simulate the differences in impacts on surface run-off and water resource availability using a global hydrological model driven by ensembles of climate models with global temperature increases of 2 • C and 4 • C. We combine these with UN-based population growth scenarios to explore the relative importance of population change and climate change for water availability. We find that the projected changes in global surface run-off from the ensemble show an increase in spatial coherence and magnitude for a +4 • C world compared with a +2 • C one. In a +2 • C world, population growth in most large river basins tends to override climate change as a driver of water stress, while in a +4 • C world, climate change becomes more dominant, even compensating for population effects where climate change increases runoff. However, in some basins where climate change has positive effects, the seasonality of surface run-off becomes increasingly amplified in a +4 • C climate.

Potential Threats from Variations of Hydrological Parameters to the Yellow River and Pearl River Basins in China over the Next 30 Years

“…This is because the ensemble mean of WSI fluctuates over the next 30 years with the decrease or increase of population, whereas the ensemble mean of annual runoff does not show such fluctuation. The suggestion is similar to some previous studies that found the changes in water stress will be dominated primarily by population changes rather than climate changes over the next few decades in a 2 • C or <2 • C warming world [2,8].…”

“…Here, annual river runoff per capita was used to measure the water stress of the two river basins. Similarly, per capita annual runoff in a given basin was applied as a measure of water resources per capita with which to investigate water stress [8]. Considering the bias of the projection, a simple method was used to correct the values.…”

“…In addition, alterations of seasonal discharge have also been projected [2,6,7]. In some basins, wet seasons are expected to become wetter and dry seasons drier in a world changing from 2 • C warming to 4 • C warming [8]. River basins dominated by a monsoonal in annual runoff, seasonal runoff, and hydrological extremes during different time slices during the 21st century based on the IPCC AR5 [6,7,26].…”

Rethinking adaptation for a 4 ° C world

“…Potential impacts associated with an increase of more than 4 • C in global average temperatures are severe, particularly as even higher levels of change may be experienced locally (e.g. [3][4][5]). There is no doubt, therefore, that the subject the different types of adaptation decisions and responses which may be needed in a 4 • C world, and presents some examples.…”

Special issue “Adaptation to climate change: analysing capacities in Africa”

“…With climate and population change, water availability per person is likely to decline in these regions. While population growth is a major driver of water stress, in a ?2°C and ?4°C world, climate change would be the dominant cause of water stress in African river basins (Fung et al 2011). Sea level rise and saltwater intrusion also threaten parts of Africa (Nicholls and Cazenave 2010).…”