Climate Change – 2: Effects on Freshwater Resources

Human Activities Affect Freshwater Resources• Both quantity and quality

Emissions of greenhouse gases Climate

Land Use

Food Demand

Population, life style, economy,

technology

Terrestrial part of hydrological cycle

Water Use

Water Resources Management

Observed climate-related trendsPrecipitation Increasing over land north of 30°N over the period 1901–2005.

Decreasing over land between 10°S and 30°N after the 1970s. Increasing intensity of precipitation

Cryosphere Snow

cover Decreasing in most regions, especially in spring

Glaciers Decreasing almost everywhere Permafrost Thawing between 0.02 m/yr (Alaska) and 0.4 m/yr (Tibetan Plateau)

Surface waters

Streamflow Increasing in Eurasian Arctic, significant increases or decreases in some river basins. Earlier spring peak flows and increased winter base flows in Northern America and Eurasia.

ET Increased actual evapotranspiration in some areas Lakes Warming, significant increases or decreases of some lake levels, and

reduction in ice cover Groundwater No evidence for ubiquitous climate-related trend Floods and droughts

Floods No evidence for climate-related trend, but flood damages are increasing Droughts Intensified droughts in some drier regions since the 1970s

Water quality No evidence for climate-related trend Erosion and sediment

No evidence for climate-related trend

Irrigation water demand

No evidence for climate-related trend

Current Vulnerabilities of Freshwater Resources and Their Management

Projected Vulnerabilities of Freshwater Resources

Impacts due to increases in temperature, sea level and precipitation variability

• Seasonal shift in streamflow• Increase in the ratio of winter to annual flows• Reduction in low flows caused by decreased

glacier extent or snow water storage• Sea-level rise will extend areas of salinisation of

groundwater and estuaries, decrease in freshwater availability in coastal areas

• Increased precipitation variability will increase risks of flooding and drought

Semi-arid and arid areas particularly exposed to impacts of climate change on freshwater

• Mediterranean basin, western USA, southern Africa, and north-eastern Brazil) will suffer a decrease in water resources due to climate change

• Groundwater recharge will decrease considerably

• Effects will be exacerbated by the rapid increase in population and water demand (very high confidence)

Water Quality

• Higher water temperatures• Increased precipitation intensity• Longer periods of low flows • Exacerbate water pollution• Impacts on – Ecosystems, – Human health, – Water system reliability – Operating costs

Climate change affects infrastructure as well as water management

• Aggravates impacts of other stresses– population growth, changing economic activity, land-use

change, and urbanisation• Water demand will grow due to population growth and

increased affluence• Large changes in irrigation water demand are likely• Current water management practices are very likely to be

inadequate to reduce the negative impacts of climate change on – water supply reliability, flood risk, health, energy, and aquatic

ecosystems

Driver - Temperature• Will increase by – 2020s ~ 1°C– 2100 ~ 2 - 4°C

• Greatest increases at high northern latitudes and over land

• Increases will be stronger in summer than in winter• Sea-level rise will be between 14 and 44 cm within

this century (does not take ice sheet melting into account)

Driver - Precipitation• Will increase at high latitudes and in the

tropics – e.g., the south-east monsoon region and over the

tropical Pacific• Will decrease in sub-tropics – e.g., over much of North Africa and the northern

Sahara• Variability will increase

Water Stress

• Population at risk of increased water resources stress (A2 scenario):– 2020s 0.5-1.7 billion; – 2050s 1.5-2.0 billion; – 2080s 2.4-3.2 billion

• By the 2050s water stress on global land area is projected to – Decrease on 20-29% and – Increase on 62-76%

Zbigniew W. Kundzewicz, Sept. 2007

Water ScarcityRegion Problem

Africa Increased water shortages due to reduced rainfall leading to large increases in the number of people suffering water scarcity

Asia In some large basins: water availability and agricultural yields are projected to decline

Australia and NZ Ongoing water security problems are very likely to increaseEurope In Southern Europe, negative impacts on water resources,

hydropower potential, agriculture, and wildfire outbreaksLatin America In Andean countries, glacier melt leads to changes in the

seasonal pattern and amount of runoff, hence affecting water resource availability and hydropower

North America

Warming in the western mountains is very likely to cause decreases in snowpack and summer flows, expanding water management challenges for competing uses

Small Islands Current water shortages are due to pressures from population growth, agriculture and tourism. Sea level rise leads to groundwater salinization and any reduction in rainfall will intensify pressures

Zbigniew W. Kundzewicz, Sept. 2007

FloodsRegion Problem

Asia In the Himalaya, glacier melt will lead to increasing numbers and severity of melt-related floods, such as GLOFs (Glacial Lake Outburst Floods), ice and rock avalanches from destabilized slopes and disruption of water resources

Australia and NZ

Higher risks to major infrastructure such as floodplain protection and urban drainage/sewerage

Europe Flash floods are likely to increase in all of Europe, while snowmelt-related floods in Central and Eastern Europe are expected to decline

Latin America In Andean countries, melting glaciers causeflooding and changes in the seasonal pattern and amount of runoff

North America

Projected warming in the western mountains by the mid 21stcentury is very likely to cause increased peak winter flows and flooding.

Zbigniew W. Kundzewicz, Sept. 2007

Water-related Hotspots on Map of Relative Changes in Runoff

Uncertainties• Increase with the length of the time horizon

– Near term (e.g., the 2020s), model uncertainties– Longer time horizons, emissions scenario

• GCMs subject to uncertainties in the modeling process• Climate projections not easy to incorporate into hydrological impact

studies• For the same emissions scenario, different GCMs produce different

geographical patterns of change, particularly with respect to precipitation• Agreement with respect to projected changes of temperature is much

higher than with respect to changes in precipitation• GCM structure is the largest source of uncertainty, next are the emissions

scenarios, and finally hydrological modeling

Some GCM Issues• Most climate change studies consider only changes in precipitation

and temperature• Time series of observed climate values are adjusted with the

computed change in climate variables to obtain scenarios that are consistent with present-day conditions to minimise the error in GCMs

• Mismatch of spatial grid scales between GCMs (typically a few hundred kilometers) and hydrological processes

• Techniques to downscale GCM outputs to finer spatial (and temporal) resolution have been developed– Dynamic downscaling - based on links between the climate at large and at

smaller scales– Statistical downscaling - using empirical relationships between large-scale

atmospheric variables and observed daily local weather variables

Model resolutions as function of spectral truncation2.8o x 2.8o

(200 x 300 km)1.4o x 1.4o

(100 x 150 km)1990’s

2005

2011?

3 x 3 o o

Change in Annual Runoff by 2041 (relative to 1900-70)

Change in Annual Runoff by 2090 (relative to 1980-99)

Adaptation