DOI:10.2478/v10104-009-0015-y Vol. 8 No 2-4, 195-203 2008
Climate change impacts on the hydrological cycle
Ecohydrological Processes and Sustainable Floodplain Management
Zbigniew W. Kundzewicz Research Centre for Agricultural and Forest Environment, Polish Academy of Sciences, Bukowska 19, 60-809 PoznaĔ, Poland e-mail: [email protected]
and Potsdam Institute for Climate Impact Research, Telegrafenberg, 14 412 Potsdam, Germany e-mail: [email protected]
Abstract Climate and water on the planet Earth are intimately linked. Water influences the climate, and is influenced by the climate. Every change in the climatic system induces a change in the water system, and the other way round. Climate change has been observed and even a stronger change is projected for the future by climate models. Discussion of changes is presented, with reference to such variables as temperature, precipitation, sea level, river flow, soil moisture, evapotranspiration, groundwater, and cryospheric characteristics. The weight of observational evidence indicates an ongoing intensification of the water cycle, with increasing rates of evaporation and precipitation. Climate change will alter the future world’s freshwater resources in several aspects, such as freshwater availability, quality, and destructive potential. The likelihood of deleterious impacts, as well as the cost and difficulty of adaptation, would increase with the extent and the speed of global climate change. One of the effects of climate change is that hydrological extremes become more extreme. This leads to emergence of hot-spots and vulnerable areas, and the need for difficult adaptation. Globally, the negative impacts of climate change on freshwater systems are very likely to outweigh their benefits. Key words: global warming; greenhouse effect; hydrological processes; change detection; model-based projections
1. Introduction Water resources of the planet Earth take part in the infinitely recurrent hydrological cycle, the largest movement of matter in the Earth’s system. Since water is the basic element of the life support system of the planet, it is of utmost importance to understand the impacts of the ongoing and projected climate change on water resources and water availability. Under balance of evi-
dence, global warming is unequivocal and most of it is very likely due to the increase in atmospheric greenhouse gas concentrations. Observed climate change has extended beyond temperature. However, there are multiple pressures of global change, such as: population growth, land use and land cover changes (urbanization, especially in coastal areas, deforestation), and environmental pollution, which in many areas are exacerbated by climate change. Hence, there are
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reasons for concern related to impacts of changes in the climate system on freshwater resources.
2 Hydrological cycle: water stores and water fluxes Hydrological cycle can be interpreted as a set of water fluxes (hydrological processes), which transfer water between stores (reservoirs) in the geosphere (hydrosphere proper – oceans, seas, lakes, rivers, wetlands, and marshes; cryosphere - ice and snow; lithosphere – groundwater, water in rocks, and Earth crust; and atmosphere – clouds) and biosphere (water contained in living organisms, plants and animals). The total global water resources constitute approximately 1.385 billion km3, with 96.5% of their volume (1.338 billion km3) contained in oceans, the largest water store (Shiklomanov, Rodda 2003). Salty oceans and seas, covering nearly 71% of the Earth’s surface area, play an essential role in the water cycle as the main source of water in the atmosphere. Glaciers and permanent snow cover contain 24.4 million km3 of water, while groundwater constitutes 23.4 million km3, but more than half of it is not fresh. Other water stores contain much smaller volumes (lakes – by two orders of magnitude smaller; soil, atmosphere, and wetlands – by three orders of magnitude each; rivers, and biological water – by four orders of magnitude each). The total volume of water in the hydrosphere has been nearly constant over a longer time-scale. Hydrosphere is a closed system and water takes part in recycling processes. The water is on a perpetual move. Major hydrological processes partaking in the water cycle are: evapotranspiration, precipitation, runoff, sublimation, infiltration, snowmelt, interception, subsurface flow, and capillary rise. Precipitation water falling down on land is the main source of the formation of land waters: rivers, lakes, groundwater, and glaciers. Precipitated water may be intercepted by vegetation, infiltrate into the ground, be stored in ponds, lakes and depressions at the Earth’s surface, or run off. A portion of atmospheric precipitation evaporates, a part infiltrates and contributes to groundwater, and most of the rest gets as river flow to the ocean, and then evaporates, so that the process repeats again and again. Water moves also in the biosphere, in oceans, seas and lakes, in snow pack and even in seemingly immobile glaciers. The mean sojourn time of a water particle in different stores varies from hours to millennia. Slow turnover is typical in ocean bodies, large lakes, and deep groundwater, where mean residence time of a water particle depends on the depth, and in ice sheet and glaciers in cold (low-energy) climates. A water particle spends, on average, about
10 thousand years in underground ice in the permafrost area or the aeternal snows and polar ice, 2500 years in the ocean, 1600 years in mountain glaciers, and 1400 years in groundwater. In lakes, wetlands, and the soil, the mean residence times of a water particle read: 15–17, five and one year, respectively. Much faster is the turnover of water in rivers (16 days), atmospheric water (8–10 days, hence the atmosphere recycles its contents approximately 40 times per year) and biological water (a few hours), cf. Shiklomanov, Rodda (2003). Vertical processes, evaporation and precipitation, are the two main, in volumetric terms, fluxes in the water cycle, responsible for the water transfer between the hydrosphere and the atmosphere. The water cycle is powered by solar energy mainly through direct vaporization. Water evaporates from the Earth’s surface, vapour is lifted to form clouds and transported in the atmosphere. Water vapour, which has risen from tropical seas to the atmosphere, is carried by winds away from the tropics, where it condenses, releasing latent heat and precipitates over oceans and land. Every year, solar energy lifts about 500 000 km3 of water, evaporating from the Earth’s surface, 86% of which (i. e. 430 000 km3) evaporates from the oceanic surface and 14% (i. e. 70 000 km3) from land. About 90% of the volume of water evaporating from oceans precipitates back onto oceans, while 10% is transported to areas over land, where it precipitates. About two-thirds of the latter evaporate again and one-third runs off to the ocean. The global volume of precipitation is equal to that of evaporation, i. e. 500 000 km3 (thereof 390 000 km3 on the ocean and 110 000 km3 on land). The resulting imbalance, i.e. difference between precipitation on and evaporation from land surface, 40 000 km3 per year, represents the water vapour movement from oceans to terrestrial atmosphere over continents and islands, being equal to the total runoff of Earth’s rivers and direct groundwater runoff to the ocean. The rate of global water cycle can be conveniently expressed via thickness of water layer. On average, a layer of 140 cm of water evaporates from the oceans annually, and 127 cm of water precipitates onto the oceans. The difference of 13 cm is very important, as it explains the continental phase of the water cycle, the surplus of moisture moves over land and precipitates there. Precipitation on the land (80 cm) is much higher than evaporation from the land (48 cm), cf. Shiklomanov, Rodda (2003). One can conceptually divide precipitated water into „green” and “blue” water. The former is a part of precipitation that evaporates and sustains plant growth, while the latter is liquid water in surface and subsurface water bodies, which can be withdrawn for human use (e.g. for irrigation). “Blue” water turns to “green” water in the natural and managed ecosystems.
Climate change impacts on the hydrological cycle
At times, volumes of water in stores and fluxes of water between stores take extremely high or extremely low values. For over a century, highest precipitation values have been recorded in meteorological stations worldwide, for various time intervals. The world records measured in a point range from 38 mm of precipitation in one minute interval, through 1825 mm in one day (24 hours), to 9.3 m in one month, 22.45 m in six months, 26.46 m in one year and 40.76 m in two years. Too much rainfall can cause excess runoff, or (possibly destructive) flooding, while too little rainfall leads to drought, decrease in surface water level (or even drying out of surface water bodies), drop of groundwater level and soil moisture. Drought is often accompanied by failure of crops, problems with water supply, navigation, hydropower, and wild fires. There is a quality dimension in the water cycle. Water is an excellent solvent, able to dissolve many chemical compounds, e.g. mineral salts. It plays a substantial role in biogeochemical cycles of carbon, phosphorus, nitrogen, as a solvent and a carrier. Water interacts with both the atmosphere and the lithosphere, acquiring solutes from each. Dissolved and particulate substances are transported in the hydrological cycle (e.g. via overland flow, river runoff). Water cycle contains natural purification mechanisms. The process of evaporation purifies (distills) salty oceanic water. Evaporate is freshwater, but salt remains in the oceans as water evaporates. Moreover, natural water self-purification takes place in rivers and wetlands.
3 Climate and hydrological processes Climate and water on the planet Earth are closely linked. Water takes part in a large-scale exchange of mass and heat between the atmosphere, the ocean, and the land surface, thus influencing the climate, and also being influenced by the climate. Every change in the climatic system induces a change in the water system, and the other way round. Beside oceans and seas, also surface water bodies, such as lakes, wetlands, and large rivers, affect the local, or regional, climate. Enhanced evaporation in large water storage reservoirs is an important component of the water balance, especially in arid and semiarid areas, being an essential part of the regional water budget. Hydrological processes depend either explicitly or implicitly on climatical variables. Precipitation is, in fact, a variable at the interface between the two domains, being the output signal from climatical (meteorological) systems and the input signal to hydrological systems. The hydrological cycle affects the energy budget of the Earth. Clouds alter Earth’s radiation balance. Atmospheric water vapor (along with
carbon dioxide and methane) is a powerful greenhouse gas, playing a significant role in the greenhouse effect. The atmospheric transport of water from equatorial to subtropical regions (where latent heat is released from water vapour) serves as an important mechanism for the transport of thermal energy. Physical processes contributing to the hydrological cycle can be described in terms of mathematical equations. The most essential, and universal, law guiding the water cycle is the rule of balance, also called the law of conservation of mass (expressed by the continuity equation), cf. Dooge (1973). It reads, for any fixed control volume: Inflow – Outflow = Change of storage
Considering only the most essential hydrological processes, that is the total precipitation on the basin, evaporation from the basin, runoff (river flow in a cross-section terminating the basin) and change of storage in the basin (manifesting itself via surface waters – rivers, lakes, ponds, wetlands; soil moisture, groundwater, and intercepted water), one can formulate the continuity equation for a river basin as: Precipitation – Evaporation - Runoff = Change of storage (2) The primary characteristics of climate change are changes in temperature and precipitation, which influence all components of equation (2). In general, warming is expected to accelerate the hydrological cycle, speeding hydrological processes up. Temperature rise enhances evapotranspiration, precipitation, and intense precipitation. Evaporation is a complex process, dependent on several factors. A plethora of equations have been proposed to estimate potential evaporation and actual evapotranspiration, e.g. based on energy balance, or approximate empirical relationships. Evapotranspiration is sensitive to several climate characteristics, such as temperature, relative humidity, and wind speed. Use of simplified equations based on a small number of variables depends on the availability of input data (even in extreme cases when temperature alone is available) and can only be treated as very rough, orientation-type, relation which can be conditionally used for longer time intervals, where the neglected secondary factors are expected to level out. An important law illustrating the precipitation changes caused by changes in temperature is the Clausius-Clapeyron equation (Iribarne, Godson, 1973), describing the water holding capacity of the atmosphere. It reads: des(T) / es(T) = L dT / R T2
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where es(T) is the saturation vapor pressure at temperature T, L is the latent heat of vaporization, and R is the gas constant. According to equation (3), saturated vapour pressure depends only on temperature and it increases with temperature (eq. 4, Fig. 1). Onedegree warming increases the saturated vapour pressure by 6.6%. Es = 611 exp [17.27 T / (237.3 + T) ] [Pa]
Snowmelt clearly depends on temperature and, in a rough formulation is proportional to degree-days, with rated of the order of a few mm per oC per day. Many hydrological variables depend on precipitation. According to the approximate SCS runoff curve number method (cf. Dooge, 1973), there is: Q = (P – 0.2 S)2 / (P + 0.8 S)
where Q is the runoff, P is the precipitation and S is related to soil and cover conditions via the relationship: S = 1000/CN – 10
The “rational formula” (cf. Dooge, 1973) used in rough assessments for well over a century expressed the maximum river discharge as a function of precipitation intensity: Qmax = C I A
Carbon dioxide enrichment improves efficiency of plant water use, reducing stomatal conductance and leaf-scale evaporation, but this is partly offset by increased plant growth. Climate-change induced sea-level rise influences the location of the freshwater-saltwater interface in function of the location of the groundwater table above sea level. The Ghyben-Herzberg relation states that if the groundwater table is one unit (e.g. meter) above sea level, then the freshwater-saltwater interface is approximately 40 units below the sea level. Thus, there is an amplification effect: a small sea-level rise results in a 40-fold decrease of the depth of the freshwatersaltwater interface. Not only fluxes (input–output processes) depend on climate, but also system properties, e.g. parameters of the instantaneous unit hydrograph depend explicitly on effective rainfall (even if determination of effective precipitation is not straightforward).
4 Observed and projected changes Earth’s climate has always been changing, reflecting regular, periodic, shifts in Earth’s orbit and such factors as solar activity and radiation, and volcanic eruptions. However, most of the climate change observed recently is very likely due to human activity (IPCC 2007). People have been carrying out a planetary-scale experiment, disturbing the natural composition of the atmosphere by increasing emissions of greenhouse gases. This takes place because of the unprecedented, and increasing, burning of fossil carbon (coal) and hydrocarbons (oil and natural gas), and large-scale deforestation (reduction of carbon sink). In consequence, carbon dioxide concentration in the Earth’s atmosphere increases and the greenhouse effect
where C is the runoff coefficient (higher for impermeable surfaces, lower for non-repellent sands) and I is the precipitation intensity. Other simple empirical approaches of this type have been introduced. The Universal Soil Loss Equation (USLE) shows dependence of soil loss on the rainfall erosivity, which in turn depending on rainfall intensity. Regionalization approaches (of use in ungauged drainage basins) yield regional models for flood discharge dependent on catchment characteristics, including precipitation intensity. Water quality can be degraded by higher water temperature (firstorder rate coefficient for reaction kinetics is clearly temperature-dependent), but this may be offset regionally by dilution where flows increase. Saturation concentration Fig. 1 Dependence of saturated water pressure on temperature, according of dissolved oxygen decreases with to Clausius-Clapeyron law (illustration of equations 3-4), cf. Iribarne, temperature growth. Godson (1973).
Climate change impacts on the hydrological cycle
becomes more intense, leading to global warming. Apart from the warming, there are several further manifestations of climate change and its impacts, of direct importance to the hydrosphere. In the history of Earth’s climate there were time periods when much of the hydrosphere on the surface of the planet was in the solid form of glacial ice. There have been several ice ages in the history of the Earth, and the most recent retreat of glaciation is dated at some 10 000 years ago. Range and extent of ice sheets, glacier and permanent snow areas remain a sensitive indicator of changes in the Earth’s climate. After expansion during the Little Ice Age, mountain glaciers have been shrinking increasingly fast in response to the ongoing global warming.
Temperature Warming of the global climate system is unequivocal (IPCC 2007). This is now evident from observations of increases in air and water temperatures in all regions. Most of the observed increase in global mean air temperature since the mid-20th century is very likely due to the rise in atmospheric greenhouse gas concentrations,
caused by increasing anthropogenic emissions of such gases as carbon dioxide, methane, nitrous oxide, and land-use changes (such as deforestation). The updated 100-year linear trend (1906 to 2005) shows a 0.74 °C global mean temperature increase, while the linear warming trend over the last 50 years (0.65 °C) is nearly twice as strong as that for the last 100 years (IPCC 2007). Among 14 globally warmest calendar years in the global instrumental observation record, available since 1850, there are 13 years from the period 19952008. Each of the years 2001–2008 belongs to the set of the ten warmest years on record. Kundzewicz et al. (2007a, 2009) proposed to analyze the mean temperature of consecutive 12 months as the input to the global climate debate. They showed that in 2006-2007, the highest mean average temperature over consecutive 12 months has been reached at a number of scales (Fig. 2), from local through continental (Europe) to hemispheric (Northern Hemisphere). For the next two decades a warming of about 0.2 °C per decade is projected for a range of SRES emission scenarios (for review of scenarios, see IPCC 2007). We are commited to further warming of about 0.1 oC per decade, even if the concentra-
Fig. 2 Recent records of 12-month mean air temperature; for the large Central European region (60–44°N, 5°W–22°O), January 1901 – September 2007. Black line represents the mean for the whole period considered and dashed lines represent three fold standard deviation. Lower graph - temperature anomalies (deviations from 1961-1990 mean) for Northern Hemisphere, land only, January 1850 – June 2007. Data from CRUTEM3 data base available at http://www. cru.uea.ac.uk/cru/data/temperature/. Black line – 6th order polynomial smoothing (based on Kundzewicz et al. 2009).
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tions of all greenhouse gases and aerosols had been kept constant at the year 2000 levels (IPCC 2007). According to IPCC (2007), the likely range of global mean temperature for 2100 without climate policy is from 1.1 to 6.4°C, while “best values” of warming for 2090-2099 (relative to 1980-1999) for all non-mitigation scenarios span the range from 1.8 to 4.0 °C. All climate models project warming everywhere, but the magnitude of temperature change varies among models. All climate models predict that hot extremes and heat waves will become more frequent in the future, while cold extremes (e.g. number of frost days, or icy days) will become less frequent (IPCC 2007).
Precipitation Global temperature changes are accompanied by changes in other climatic variables. Patterns of precipitation change are more spatially and temporally variable than temperature change. There is no statistically significant linear long-term trend in the time series of global precipitation in the period from 1900 to 2005 (Trenberth et al. 2007). There was an overall increase of global mean land precipitation, until the 1950s, with peaks in 1950s and then in 1970s, a decline from 1970s until the early 1990s and a recovery afterwards. As summarized in Trenberth et al. (2007), long-term precipitation trends have been found in many large regions where sufficient data exist. Precipitation has generally increased over land in most areas of higher latitudes of Northern Hemisphere (north of 30°N), and in the eastern part of North and South America. It decreased from 30°N to 10°S, especially after 1977, as well as in South Africa. With respect to precipitation variability, more intense and longer droughts have been observed over wider areas since the 1970s, particularly in the tropics and subtropics. The frequency of heavy precipitation events has increased over most land areas (IPCC 2007). As regards future projections of precipitation, projections made by different climate models do not agree well for several areas of the globe. In high latitudes and parts of the tropics, climate models are consistent in projecting precipitation increases, while in some subtropical and lower mid-latitude regions, they are consistent in projecting precipitation decreases. In areas between these regions of robust increase and decrease, there is much uncertainty in projections, and predicted magnitudes of change differ very strongly. For precipitation changes until the end of the 21 st century, the multi-model ensemble mean exceeds the inter-model standard deviation only at high latitudes (IPCC 2007). However, climate models project increasing precipitation variability. The frequency of heavy
precipitation and the maximum number of consecutive days without precipitation are projected to increase in the future (Kundzewicz et al. 2006). This holds even for some regions where the mean precipitation is projected to decrease. Precipitation is the essential driver controlling the effect of climate change on streamflow, lake levels and groundwater recharge.
Sea level rise Global sea level has risen, on average, by 17 cm over the 20th century, with the rate of 1.8 mm per year from 1961 to 2003 and about 3.1 mm per year from 1993 to 2003. Most of the sea level rise in 1993-2003 has been caused by thermal expansion (1.6 mm per year), less than half of this (0.77 mm per year) stems from glaciers and ice caps, while a quarter (0.42 mm per year) from Greenland and Antarctic ice sheets. However, while uncertainty as to the contribution of Greenland Ice Sheet is moderate (0.21 ± 0.07 mm per year), the estimate for the contribution of Antarctic Ice Sheet (0.21 ± 0.35 mm per year) is highly uncertain (IPCC 2007). Over the 21st century, sea level is projected to rise faster, so that the range of growth for the time horizon 20902099, in comparison to 1980-1999 is 18-59 cm. However, the upper value of the range of sea level rise, stated in IPCC (2007), 59 cm, has been recently challenged by several scientists who admit the possibility of much higher sea level rise in this century.
River flow According to Milly et al. (2008), in view of the magnitude and ubiquity of the hydroclimatic change apparently now under way, stationarity should no longer serve as a central, default assumption in water resources planning. The changes in temperature, evapotranspiration, and precipitation, affect river runoff significantly. An additional impact stems from the widespread mass losses from glaciers and reductions in snow cover over recent decades that are projected to accelerate throughout the 21 st century (IPCC 2007). Similar to precipitation, significant trends, both increases and decreases, in some regional indicators of river flow have been found, but no globally homogeneous trend has been reported. Often these trends cannot be definitively attributed to changes in climate, due to existence of several other factors. In some regions, interannual variability of river flows is strongly influenced by largescale atmospheric circulation patterns associated with ENSO, NAO and other oceanic-atmospheric variability systems that operate at within-decadal and multi-decadal time scales. At a large scale,
Climate change impacts on the hydrological cycle
there is evidence of a broadly coherent pattern of change in mean annual river runoff (Milly et al. 2005). Many regions at higher latitudes of the Northern Hemisphere, such as southeastern through central North America, parts of western North America and northern Eurasia, the La Plata Basin of South America, the southeast quadrant of Africa and northern Australia experience runoff increase. Decrease of river runoff in parts of West Africa, sub-Saharan Africa, southern Europe and southernmost South America was observed. There is abundant evidence that warming leads to changes in the timing of river flows where much of the winter precipitation currently falls as snow. The effect is greatest at lower elevations (where snowfall is more marginal). There has been an earlier occurrence (by 1-2 weeks during the last 65 years in North America and northern Eurasia) of spring peak river flows and an increase in winter base flow in basins with important seasonal snow cover in North America and northern Eurasia. The early spring shift in runoff leads to a shift in peak river runoff away from summer and autumn, which are normally the seasons with the highest water demand (Rosenzweig et al. 2007). Consistent with the precipitation projections, runoff is projected (Milly et al. 2005; Nohara et al. 2006) to increase by 10-40% by midcentury at higher latitudes and in some wet tropical areas, including populous areas in East and South-East Asia, and decrease by 10-30% over some dry regions at mid-latitudes and dry tropics, due to decreases in rainfall and higher rates of evapotranspiration. Water resources in many semiarid areas (e.g. the Mediterranean Basin, western United States, southern Africa and northeastern Brazil) will suffer a decrease due to climate change. Semi-arid and arid areas are particularly exposed to adverse impacts of climate change. However, climate models are not consistent as to the magnitude of change. The reliability of surface water supply is very likely to decrease due to higher temporal flow variations that stem from increased precipitation variability and reduced summer low flows in snowdominated basins (Kundzewicz et al. 2007b). The frequency of floods and droughts are projected to increase in the future, due to the increased precipitation variability (Kundzewicz et al. 2007b; Bates et al. 2008). The frequency of heavy precipitation events (or proportion of total rainfall from heavy falls) will very likely increase over most areas, and the intensity of precipitation events is projected to increase, particularly in tropical and high latitude areas. This will augment flood risks. Globally, drought-affected areas, the proportion of the land surface in extreme drought at any one time, and the temporal frequency of extreme drought events will likely increase.
Cryosphere Warming generates increased glacier melt, hence, widespread glacier retreat has been already observed worldwide, and many small glaciers have been disappearing. Water supplies stored in glaciers and seasonal snow cover have already declined as a result of warming, and are projected to decline further as warming proceeds. High reductions in the mass of Northern Hemisphere glaciers are expected in the warming climate. As glaciers retreat, the contribution of glacier melt to river runoff will gradually fall over the next few decades. Access of population living in glacier-fed basins to usable water supplies will be affected by a seasonal shift in streamflow, entailing – in many regions – an increase of the ratio of winter to annual flows, and reduction of low flows in summer and autumn.
Groundwater Groundwater reacts to climate change mainly due to changes in groundwater recharge. Where trends in river runoff have been identified, a similar trend in groundwater recharge is expected. However, due to lack of data, no ubiquitous climate-related trends for groundwater recharge or groundwater levels century could be determined for the 20th century (Kundzewicz et al., 2007b). Observed declines in groundwater tables are mostly due to unsustainable high groundwater abstraction rates. The climate-related effect on groundwater quality is the increase of groundwater salinity in coastal areas, due to sea level rise.
Water quality Water quality is projected to be degraded by higher water temperature, but this may be offset regionally by the dilution effect of increased flows. The increase of heavy precipitation events, together with higher water temperatures, is likely to exacerbate water quality problems, in particular by accelerating transport of pathogens and other pollutants (Kundzewicz et al. 2007b). Warming-enhanced sea level rise can lead to saltwater intrusion into fresh groundwater bodies. Thus, freshwater availability in coastal areas is likely to decrease in the warmer climate. Water pollution (sediments, nutrients, dissolved organic carbon, pathogens, pesticides, salt and thermal pollution) is likely to be exacerbated by higher water temperatures, increased precipitation intensity and floods, and more frequent low flows, with impacts on ecosystems, human health, and water system reliability and operating costs. However, at the local scale – relevant for water
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management – climate change impacts on water quality are not adequately understood, particularly in the context of extreme events and in developing countries.
5. Global and regional perspectives Many climate-change impacts on freshwater resources have already been observed, and further (and more pronounced) impacts have been projected, depending on region and season. The spatial and temporal distribution of river flows and groundwater recharge is affected by changes in temperature, evaporation, sea level, and, crucially, precipitation. The weight of observational evidence indicates an ongoing intensification of the water cycle, with increasing rates of evaporation and precipitation. Very dry or very wet areas have increased, globally. There is more water vapour in the atmosphere, hence there is potential for enhanced intense precipitation. There is a poleward shift of the belt of higher precipitation. Increase in midsummer dryness in continental interiors has been observed and further increase is projected. Climate change will alter the future world’s freshwater resources in several aspects, such as freshwater availability, quality, and destructive potential. There is growing evidence that, in many regions, climate change may cause significant harm to water resources. The adverse changes may affect the adequacy of water supplies, the health of aquatic ecosystems, and risks of floods and droughts. The likelihood of deleterious impacts, as well as the cost and difficulty of adaptation, would increase with the extent and the speed of global climate change. Globally, the negative impacts of climate change on freshwater systems are very likely to outweigh their benefits. Water stress is modelled to increase on most of the global land area (Kundzewicz et al. 2007b, 2008b; Bates et al. 2008). The climate-water links strongly influence many systems and sectors, such as the water supply and sanitation, agriculture, energy, human health, ecosystems, settlements, infrastructure, industry, transportation, tourism, insurance, and financial services. Among water-related vulnerability hot spots, where climate change impacts on freshwater resources in the decades to come are a threat to the pursuit of sustainable development, are the following areas in developing countries (Kundzewicz et al. 2007b, 2008): - Many of the presently water stressed semi-arid and arid areas are likely to suffer from decreasing water resource availability due to climate change, as both river flows and groundwater recharge decline. Efforts to offset declining sur-
face water availability due to increasing precipitation variability in such areas as the Northeast of Brazil will likely be hampered by the fact that groundwater recharge is projected to decrease there by over 70% by mid-century. - As glaciers retreat due to warming, river flows will decline once the glaciers disappear. More than one billion people (one sixth of the world population) live in river basins supplied by meltwater (glacier- or snowmelt) from major mountain ranges, such as the Himalaya, Hindukush and Andes, and changes in the timing of streamflow in these areas (e.g. reduction of low flows in summer and autumn) may have large impacts on resource availability. - The beneficial impacts of projected increases in annual runoff in such areas as eastern and southeastern Asia will be tempered by adverse impacts of increased variability and seasonal runoff shifts on water supply, water quality and flood risk, in particular in heavily populated low-lying river deltas (e.g. in Bangladesh). Furthermore, additional precipitation during the wet season in such regions may not alleviate dry season problems if the extra water cannot be stored. - Saline intrusion due to excessive water withdrawals from aquifers is expected to be exacerbated by the effect of sea-level rise, leading to reduction of freshwater availability in the coasts. Even a small sea-level rise may induce very large decreases in the thickness of the freshwater lens below small islands. - Key consequences of declining water quality due to climate change include increasing water withdrawals from low-quality sources; greater pollutant loads from diffuse sources due to heavy precipitation (via higher runoff and infiltration); water infrastructure malfunctioning during floods; and overloading the capacity of water and wastewater treatment plants during extreme rainfall. Model-based projections for the future, particularly related to expected changes in precipitation, are highly uncertain, hence directly unusable for credible assessment of future freshwater availability. There is a great deal of uncertainty in projections and some of this uncertainty (e.g. refering to future socio-economic development) cannot be substantially reduced. Although quantitative projections of climate change impacts on water resources are available, they should be understood as plausible scenarios, whose probability of occurrence is difficult to estimate.
Acknowledgements The author has greatly benefited from the work within the IPCC process. Collaboration with co-authors of the IPCC WG2 AR4 chapter on freshwater resources and their management
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and of the IPCC Technical Paper on Climate Change and Water is gratefully acknowledged.
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