Global climate change and water resources: to 2025 and beyond

2 February 2000
Public event

Chair:

Tony Allan, SOAS

Speakers:

Nigel Arnell, Department of Geography, University of Southampton
Declan Conway, Climatic Research unit, University of East Anglia

Description

This event provided an insight into the 'underlying science' over global water resources.

1. Tony Allan said the session would provide insight into the 'underlying science' over global water resources and would address some of the major assumptions the world would have to come to terms with respect to climate change. The purpose of the presentations was to see where climate might be causing significant change with respect to water resources.

2. The first speaker, Professor Arnell, divided his presentation into an analysis of one modelling approach to global climate change and the implications for future water resources availability. At present, he stated, around 1.75 billion people were living in countries suffering from water scarcity, defined as withdrawing more than 20% of their available water resources each year. Population growth and economic developments would mean that by 2025 this could increase to 5 billion people (about 60% of the world's population), mainly because China and other very populous countries would move into the 'stress' category.

3. Turning to the data sets and indicators to see what the additional consequences of climate change might be he stated that the Second Assessment Report of the Intergovernmental Panel on Climate Change (1996) (WMO/UNEP) suggested, on balance, that the evidence indicated human activities are having a 'discernible effect' on climate, leading to the Kyoto Protocol signed in 1997. The main challenge was knowing how that pattern of change would continue.

4. The approaches used and the major uncertainties involved in meeting this challenge were his next focus. The first assumption related to the future rate of greenhouse gas emissions; then how to work out their affect on the radiation balance; then how to translate these energy inputs into implications for climate. Whilst climate models were used, one limitation was that human reaction had yet to be 'factored in', providing a feedback loop. Professor Arnell presented a set of models (Hadley Centre Climate Models - HADCM 2 and 3) and stated that a general, but significant current trend, was the convergence of different models towards some 'broad' patterns. These showed a general pattern of more rain at low latitudes and equatorial zones and high latitudes and less rain in between.

5. Looking at the impact on the hydrological cycle he said that climate change due to global warming was likely to result in a general increase in rainfall, and therefore streamflow, although large parts of the world in mid-latitudes and semi-arid regions might see substantial reductions in rainfall. Scenarios for climate change, based on Hadley Centre climate models, indicated that climate change may exaggerate water resources stresses in many parts of the world, particularly in southern Asia, northern and western Africa, and the Middle East. Whilst water resources stresses may lessen in other parts of the world - particularly parts of east Asia - but this may be at the expense of increased flooding. In fact the most 'robust signal', he said, was that associated with snow cover and higher temperatures. Hence, total streamflow in some parts of the world might not vary considerably, but seasonal distribution could. The HADCM3 scenario he presented indicated suggests that by the 2020s there would be another 50m people living in water-stressed countries, and would fall towards the 2050s before rising again in the 2080s. By middle of this century about 2.5 billion of the 6 billion projected population would be in a worse position due to climate change, (although less under the HADCM3), nevertheless, indicating the potentially serious impact of climate change on water stress.

6. Moving on to responses to climate change Professor Arnell distinguished mitigation (emphasised in the Kyoto Protocol) and adaptation (implying reducing exposure to adverse impacts). Implementing the Kyoto Protocol would have no significant effect on the numbers of people adversely affected in the middle of the century, he said, and that mitigation would have to go 'an awful long way' before counteracting the effects of climate change. If, by contrast, adaptation meant withdrawing 10% less water in 2020s than originally thought necessary, the number living in water-stressed countries would fall by substantially more than the climate change effect. He concluded, therefore, that even in the absence of climate change global water stresses are increasing in many vulnerable countries, but that climate change is likely to exacerbate these problems (though in some countries it may improve things). Nevertheless, reducing significantly climate change impacts required severe stabilisation of carbon dioxide emissions. Reducing exposure to impacts lay in adapting to change, though not through developing new tools, rather through using more effectively the existing 'toolkit' to meet the challenge of uncertainty.

7. The next speaker, Declan Conway, focused on the African scenario, and on the uncertainties surrounding the implications of climate change. He acknowledged the input of Mike Hulme and other colleagues to the ideas he would be presenting. His presentation covered a review of the historical variability of climate with respect to rainfall over Africa and an analysis of the rainfall impact on river flows.

8. Considerable spatial and temporal variability in rainfall existed across Africa. Rainfall records for this century, he said, showed varying levels of short (annual) to longer term (decadal) variability throughout Africa and recent lake level reconstructions for East Africa over the last 1000 years indicated even greater levels of variability than those observed during the last century. Significant fluctuations in river flows and lake levels had occurred as a result of rainfall variability and many examples suggested that water resources in Africa were highly sensitive to climate variability. For instance, fluctuations in White Nile (Kenya, Uganda and Tanzania) and Blue Nile (Ethiopian Highlands) flows have caused fluctuations in Main Nile discharge of up to ±20 per cent during the last century which have had major implications for water resources management in the downstream riparians, particularly Egypt. Since the 1960s, for instance, gradual and prolonged decline in rainfall over the Ethiopian highlands had caused a significant impact on Blue Nile flows which reached their lowest point in 1984, before recovering in the 1990s. Drawing on other examples he concluded that there was clearly sensitivity of river flows to rainfall variability.

9. What did this mean in terms of climate change over Africa? A recent study of future climate change scenarios for Africa using Global Climate Models suggested warming would occur by between 2° and 6°C by 2100. Predicting the rainfall changes that would occur was far harder, partly due to models not incorporating the role of the El Ninio Southern Oscillation (ENSO) and other atmospheric processes affected by land cover change and dust loadings. Understanding ENSO - which has a significant correlation with rainfall over the Ethiopian highlands and hence with implications for extremely vulnerable rural livelihoods - is a pre-requisite to assessing the full implications of future climate change for Africa, he argued.

10. At present we do not know how much the world would warm up for a given amount of greenhouse gas emissions, and, by extension, how global change would affect regional change. It is only possible to say with confidence that frequency will change, although there is uncertainty over which direction. Nevertheless, the main indication for Africa was an increase in rainfall in equatorial areas and decreases in southern African areas for the December to February rainy season by the 2080s. Dr Conway emphasised, in conclusion, that adaptive responses would require the strengthening of present-day coping mechanisms and the ability to predict variability and manage uncertainty as a first step in addressing the impacts of future climate change.

11. The ensuing discussion introduced by Tony Allan following a comment that the presentations had happily not presented a 'doomsday scenario', ranged from the meaning of this change and uncertainty for decision makers 'tomorrow', at the regional level what could this tell us about adaptation, to the ability to predict fluctuations in lake levels and the implications of using different measures of scarcity. On the latter point, Professor Arnell stated that 20% of available water being used to indicate 'water stress' was one such indicator amongst many. Using different indicators led to different results and that this particular one was recognised to be a 'crude' indicator. Nevertheless, the same analysis carried out using resources per capita had tended to miss apparent irrigation stresses in certain countries. Get a different picture from different indicators. Then this would indicate greater stress.

12. One of the key points on uncertainty, raised by the audience was how far and to what extent were human actions incorporated into the models? Recognised by Professor Arnell to be 'virtually impossible' given the complexity of predicting human responses, at present only emissions scenarios were incorporated. But what should we be doing on 'Monday morning' in the light of the evidence presented? The speakers suggested that as a starting point planning based on resource base complexity and uncertainty should be recognised implying the need for flexible planning processes. Professor Arnell stated that the 'what should we de' was the theme of the Third Assessment Report. Dr Conway suggested that the major implication for policy makers was in looking forwards to future resources implications (namely, in the case of a dam project, not basing future yields on maximal projections).

13. Described as 'worrying' that impacts were not being dis-aggregated further to a regional level, particularly given the potential impact on crop yields, and because inevitably adaptations were going to be driven by national policy decisions and responses from people at a local level, the question was asked how reliable were such general statements? Dr Arnell said that no information was guaranteed and that the key was to guide policy makers in asking the right questions. Whilst a lot of work had been undertaken on the regional impact of climate change in some parts of the world, there was little funding available for developing world studies of this nature.

14. Tony Allan concluded by saying that politicians are professionals at dealing with uncertainty and that the importance of social relations in managing 'mistakes', as much as getting it 'right' was very important. The session had, he said, been extremely good and provided an important update on how things had developed in last 10-15 years, revealing a growing confidence in predicting climate change.