Appl Envir & Organismal Bio
Appl Envir & Organismal Bio ISB 202
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Date Created: 09/19/15
Vol 438l17 November 2005idoi101038nature04141 nature REVIEWS Potential impacts of a warming climate on water availability in snowdominated regions T P Barnettl J C Adam2 amp D P Lettenmaier3 All currently available climate models predict a nearsurface warming trend under the influence of rising levels of greenhouse gases in the atmosphere In addition to the direct effects on climate for example on the frequency of heatwaves this increase in surface temperatures has important for the 39 39 39 g39 39 cycle 39 39 39 In regions where water supply is currently dominated by melting snow or ice In a warmer world less winter precipitation falls as snow and the melting of winter snow occurs earlier in spring Even without any changes in precipitation intensity both of these effects lead to a shift in peak river runoff to winter and early spring away from summer and autumn when demand is highest Where storage capacities are not sufficient much of the winter runoff will immediately be lost to the oceans With more than onesixth of the Earth39s population relying on glaciers and seasonal snow packs for their water supply the consequences of these hydrological changes for future water quot 39 quot39 p d39 t d with high quot4 and already diagnosed in some regions are likely to be severe ater is essential to human sustenance Well over half of the world s potable water supply is extracted from rivers either directly or from reservoirs The discharge of these rivers is sensitive to long term changes in both precipitation and temperature particularly in the snowmelt dominated parts of the world Changes in the amount of precipi tation tend to affect the volume of runoff and particularly the maximum snow accumulation which usually occurs near the end of the winter at the onset of the melt season On the other hand temperature changes mostly affect the timing of runoff Increasing temperatures lead to earlier runoff in the spring or winter and reduced ows in summer and autumn at least in the absence of changes in precipitation In general the direction and to a lesser extent the magnitude of surface temperature chan es are much mo temperature predictions from existing global climate models that are forced with anthropogenic increases in atmospheric greenhouse gas concentrations imply a high degree of con dence that future changes to the seasonality in water supply will occur in snowmelt dominated regions Even for models with temperature sensitivities near the lower end of the predicted range impacts on snowmelt dominated regional water resources are substantialz Indeed such changes are already obvious in the observational records of key components of the hydrological cycle such as snow pack in the western USA3 5 Taken together the predictions and observations portend important issues for the water resources of a substantial fraction of the world s population It is generally thought that increasing greenhouse gases will cause the global hydrological cycle to intensify with bene ts for water availability although a possible exacerbation of hydrological extremes may counteract the bene ts to some degree However in regions where the land surface hydrology is dominated by winter snow accumulation and spring melt the performance of water management systems such as reservoirs designed on the basis of the timing of runoff is much more strongly related to temperature than to precipitation changes Even though there is relatively little agreement among the global models as to the magnitude and even direction of precipitation changes regionally7 m there is no indica tion for a seasonal shift of precipitation to the summer and autumn The projected changes in L l imply iutuie changes of seasonal runoff patterns in snowmelt dominated regions The hydrological cycle at the land surface includes the processes of snowice accumulation and melting as well as the impact these processes will have on regional changes in evaporative demand In a warmer climate snow will melt earlier in the year than it did before and in some places this has already happened quot Taken together these impacts mean less snow accumulation in the winter and an earlier peak runoff in the spring n a global scale the largest changes in the hydrological cycle due to warming are predicted for the snow dominated basins of mid to higher latitudes because adding or removing snow cover fundamen tally changes the snow pack s ability to act as a reservoir for water storage Studies in various regions of the globe indicate that the stream ow regime in snowmelt dominated river basins is most sensitive to wintertime increases in temperature Because of this and also because there is little certainty in precipitation predic tions7 m we focus here on the sensitivity of water resources in snowmelt dominated regimes to temperature All models show warming with increasing greenhouse gases so we can begin to say with some certainty ow some critical components of the hydrological cycle will respond in the future Global distribution of snowmeltdominated runoff We used a spatially distributed macroscale hydrology model to identify the regions of the globe where snowmelt plays a dominant lClimate Research Division Scripps institution of Oceanography ta lolla California 92093 USA 2Department of Civii and Environmental Engineering 3Department of Civii and Environmentai Engineering University of Washington Seattle Washington 98i95 2700 USA 303 2005 Nature Publishing Group REVIEWS role in the seasonal patterns of stream ow The model was run over all global land areas excluding Antarctica and Greenland at a spatial resolution of 05 latitudelongitude for a twenty year 1980 1999 period We approximated the importance of snow to annual runoff by using the ratio R of the accumulated annual snowfall to annual runoff Fig 1 colour scale This allowed us to determine whether or not runoff for each grid cell is snowmelt dominated by using the criterion that R gt 05 for these cells compared for each of the world s major river basins the simulated annual runoff to the estimated reservoir storage capacity s in order to determine cases where reservoir storage capacity is adequate to buffer large seasonal stream ow shifts and hence exclude basins that in spite of being snowmelt dominated would be insensitive to shifts in runoff timing Water sheds within the snowmelt dominated domain that meet these criteria include the Colorado River the Churchill River and the Grand River all in North America and the Angara River a tributary of the Yenisei River in Asia The red outline in Fig 1 shows the domain where runoff is snowmelt dominated minus the four basins identi ed as having large storage capacities relative to runoff Within this domain water resources are arguably susceptible to warming induced shifts in stream ow seasonality In general the snowmelt dominated regions occupy parts of the globe that are at latitudes greater than 45 North and South with some exceptions 1 Mountainous regions except those nearest the Equator are generally snowmelt dominated the inset of Fig 1 regions of the world that are topographically complex according to a criterion based on average slope 2 Some regions poleward of 45 North that are warmed by oceans do not experience enough snowfall to be snowmelt dominated for example parts of Euro e and the coastal regions of the USA Paci c Northwest and British Columbia 3 Cold dry regions that experience little wintertime precipitation also do not receive enough snowfall to be snowmelt dominated for example northeastern China The domain of in uence within t e red line of Fig l is almost certainly underestimated because the criterion we used is applied on a grid cell by grid cell basis and does not account for areas where water availability is predominantly in uenced by snowmelt that is generated upstream Therefore we extended the domain of in uence 180 W 90quotN NATURElVoI 438ll7 November 2005 into sub basins where the annual runoff originating in the snowmelt dominated cells accounts for at least 50 of the runoff for the entire sub basin black lines in Fig 1 These regions include parts of central USA and some coastal areas of western North and Europe According to a year 2000 population map approxi mately one sixth of the world s population lives within this combined snowmelt dominated low reservoir storage domain The population affected by warming induced shifts in water availability is most probably greater than this estimate because we do not account for populations that derive their water resources from outside the basins in which they dwell Note that the combined region in Fig l encompasses much of the industrial ized world accounting for roughly one quarter of the global gross domestic product Evapotranspiration in a warming climate Our discussion so far has focused on the direct effects of warming on stream ow seasonality in snowmelt dominated regions Warming induced changes to evapotranspiration may also affect regionalwater availability Unfortunately there is little agreement on the direction and magnitude of historical let alone one predicted evapotranspira tion trends Observations from various countries in the Northern Hemisphere show that pan evaporation has been steadily decreasing for the past fty years contrary to the expectation that warming o cause increased evaporation 22 Two proposals exist to explain this paradox First decreasing pan evaporation trends may be indicative of increasing actual as opposed to potential evapotranspiration in moisture limited regions because increased land surface evaporation alters the humidity regime surrounding the pan causing the air over the pan to be cooler and more humid23 25 Second consistent declines of pan evaporation diurnal temperature range and global solar irradiance suggest that actual evapotranspiration is also declining because of increased cloudiness and concentrations of atmospheric aerosols that systematically reduce surface energy availability for evaporation 27 29 Changes in wind speed or in the attenuation of wind at the surface due to changes in vegetation at observing sites 180 E 45 N Figure 1 l Accumulated annual snowfall divided by annual runoff over the global land regions L quot quot L J 1 and is given by the colour scale R The red lines indicate the regions where stream ow is snowmelt dominated and where there is not adequate reservoir storage capacity to buffer shifts in the seasonal hydrograph The 304 black lines indicate additional areas where water availability is n iueneed by g upstream but runoff generated within these areas is not snowmelt dominated The inset shows regions of the globe that have complex topography using the criterion of ref 17 2005 Nature Publishing Group NATURElVol 438l17 November 2005 may also play some role in apparent downward trends in pan evaporation data Ohmura and Wild28 discuss some complications that impede our understanding of global trends in evapotranspiration In snowmelt dominated regions though these uncertainties are arguably of reduced importance because changes in the timing of snowmelt runoff induce a negative feedback on changes in evapotranspiration Earlier melt results in increased soil moisture and so also the water available for evapotranspiration earlier in the season a time when potential evaporation dominated by net radiation is low Later in the year when potential evaporation is higher the shift in snowmelt timing reduces soil moisture and hence evaporative resistance is increased again reducing the effect of evaporation changes There fore although changes in evapotranspiration are critical to runoff production in most hydrological regimes their effect and hence the effects of the above noted uncertainties are attenuated in the snowmelt dominated regions of the globe Impacts on regional water supplies We examine three case studies from different parts of the world that are in the snowmelt dominated domain These case studies were selected to help provide an appreciation for the magnitude of the potential regional water problems that may be associated with shifts in the seasonality of runoff associated with climate change Western USA The Accelerated Climate Prediction Initiative ACPI2 demonstration project was launched in 00 o investigate the impacts of greenhouse warming on water supplies in the western United States The methods and detailed results are included in 16 papers in a special volume ofthe journal Climatic Chungez The most obvious signature of climate change in the simulations generated by this project was a general warming over the western USA a warming that by the middle of the 21st century was projected to be 08 17 C greater than present values This warming is projected to be accompanied by little or no change in precipitation according to the climate change scenarios generated for the project by the NCAR DOE Parallel Climate Modelz In the western USA much of the annual precipitation falls as snow in the mountains during the winter and then melts during the spring and summer that is it is within the red lines shown in Fig 1 The most signi cant impact of a general warming was found to be a large reduction in mountain snow pack and a substantial shift in stream ow seasonality so that by 2050 the spring stream ow maximumwill come about one month earlier in the year There is not enough reservoir storage capacity over most of the West to handle this shift in maximum runoff and so most of the early water will be passed on to the oceans These hydrological changes have consider able impacts on water availability and are discussed in the literaturez For example in the Columbia River system less winter snowfall and earlier melting will force residents and industries to face by 2050 or before a choice of water releases for summer and autumn hydro electric power or spring and summer releases for salmon runs The ACPI research shows that with the predicted climate change the river cannot be managed to accommodate both unless we are ready to accept substantial 10 20 reductions of hydropower generation or serious harm to the federally protected salmon population of the region Fig 2 T e Rhine River in Europe Climate change simulations project a warming in the Rhine River basin of 10 24 over present values by the middle of the century Hydrological simulations suggest that this warming will shift the Rhine River basin from a combined rainfall and snowmelt regime to a more rainfall dominated regime resulting in an increase in winter discharge a decrease in summer discharge increases in the frequency and height of peak ows and longer and more frequent periods of low ow during the summer Socio economic implications include a reduction in water availability for industry agriculture and domestic use during the season of peak demand which is further stressed by an increase in summertime REVIEWS demand due to higher temperatures an increase in the number of low ow days during which ships cannot be fully loaded on major transport routes causing an increase in transportation costs a decrease in the level of ood protection given no additional implementation of ood defence measures a decrease in annu hydropower generation in some parts of the basin and a loss in revenue due to a shortened ski season Canadian prairiesi Climate studies for the Canadian prairies gener ally agree that a doubling of atmospheric C02 will result in an increase in surface air temperature possibly as much as 8 C during winter a decrease in snow ck an earlier snowmelt and a decrease in summer soil moisture These effects and a longer period of low ows during summer and autumn could lead to an increase in the frequency and severity of droughts Historically nearly 50 of the water use over the Canadian prairies has been for agriculture through irrigation and this demand has been met primarily with surface water unlike the prairies of the USA which rely also on ground water For this reason and because stream ows are limited and extremely variable from year to year agriculture in the Canadian prairies is very sensitive to drought Although global climate models do not predict great changes in precipitation for Canada an earlier spring runoff peak will probably cause agriculture in the Canadian prairies to become more at risk in a warming climate Furthermore increased water demand for irrigation will also lead to heightened competition with other water needs including stream ow requirements to maintain aquatic habitat and the needs of water users downstream of the Alberta Saskatchewan border under a 1969 agreement Alberta must allow 50 of stream ow to pass downstream of the border ummary of regional impacts The studies summarized above show that current demands for water in many parts of the world will not be met under plausible future climate conditions much less the demands of a larger population and a larger economy The physics behind this statement is temperature driven not precipitation driven and this makes the conclusions robust because all current models redict a warmer future world The other key factor affecting water availability is the lack of enough reservoir storage to manage a s ift in the seasonal cycle of runoff Current information about the climate related water challenges facing much of the world although by no means perfect is suf ciently robustthat major future problem areas can now be de ned The matter takes on a greater urgency because the model predicted signals are already being observed 20702098 E 140 i 2 o POM control climate and E 120 current operations E D POM projected cllmate g 100 7 and current operations 1 I PCM projected climate 5 with adaptive E management a 60 o Firm Annual ow hydropower de cit at McNery dam Fi ure 2 l Tradeoff between firm hydropower and streamflow requirements The effect of Parallel Climate Model PCM climate change projections for the period of2070 to 2098 on Columbia River Basin reservoir system reliabilities as compared to the PCM control climate and operations scenario Implementing adaptive management reduces the annu environmental ow de cit at McNary Dam in southeastern Washin on USA bene ting salmon but decreases rm reliable hydropowen Figure created by Al Hamlet using results from ref 32 305 2005 Nature Publishing Group REVIEWS Will changes in precipitation patterns offset the problems asso ciated with warming The most likely answer is no If less rain falls over a region water availability will decrease If more rain falls and the reservoir storage capacity is much less than the annual runoff then the water willbe lost downstream to the ocean in many cases particularly in regions like the western USA where precipitation is mainly in winter and the effective storage capacity of winter snow pack will be lost The changes in precipitation required to ameliorate the problem would have to come through a shift in the seasonal cycle of rainfall towards the dry season a feature that is not usually exhibited by anthropogenically forced climate models Two examples of impacts on glaciersl The results for the regional water resources case studies discussed above and the simple physics behind them seem likely to be qualitatively reproduced in virtually all regions where snowmelt is important to local water availability5 and where annual runoff exceeds storage capabilities Our results in the western USA suggest that even more serious problems may occur in regions that depend heavily on glacial meltwater for their main dry season water supply This is because once the glaciers have melted in a warmer world there will be no replacement for the water they now provide in contrast to the present snow pack dependent water supply that is renewed seasonally In this case the natural storage of fossil water in the glaciers has even more importance than seasonal storage in just the snow pack It is well documented that glaciers are in retreat over most but not all ofthe worldquot so the threat here seems both real and immediate a situation also well documented in the world s press over the past several years Himalaya Hindu Kush region Perhaps the most critical region in which vanishing glaciers will negatively affect water supply in the next few decades will be China and parts of Asia including India together forming the Himalaya Hindu Kush HKH region because of the region s huge population about 50 60 of the world s population The ice mass over this mountainous region is the third largest on earth after the Arctid Greenland and Antarctic regions The hydrological cycle of the region is complicated by the Asian monsoon but there is little doubt that melting glaciers provide a key source of water for the region in the summer months as much as 70 ofthe summer ow in the Ganges and 50 60 ofthe ow in other major rivers In China 23 of the population lives in the western regions where glacial melt provides the principal dry season water source There is little doubt that the glaciers of the HKH region are melting and that the melting is accompanied by a long term increase of near surface air temperature ref 44 and Figs 29 and 210 in ref 1 the same level of warming we saw impacting the western USA After 25 years of study the China Glacier Inventory was recently released It showed substantial melting of virtually all glaciers with one of the most marked retreats in the last 13 years 750 m of the glacier that acts as one of the major sources of the Yangtze River the largest river in China In total it is estimated that the entire HKH ice mass has decreased in the last two decades Furthermore the rate of melting seems to be acceleratin 45 The few analytical studies that exist for the region suggest both a regression of the maximum spring stream ow period in the annual cycle by about 30 days ref 47 and an increase in glacier melt runoff by 33 38 ref 48 These numbers seem consistent with what is being observed and bear striking similarities to the stream ow results from the western USA T e huge inconsistency however occurs in the impacts on local water supplies In the western USA model predicted impacts are already being seen in the hydrological cycle The models suggest that the impacts will appear as a long term trend in snow amount and runoff But in the HKH region there may for the next several decades appear to be normal even increased amounts of available melt water to satisfy dry season needs The shortage when it comes will likely arrive much more abruptly in time with water systems going from plenty to want in perhaps a few decades or less 306 NATURElVol 438117 November 2005 It appears that some areas of the most populated region on Earth are likely to run out of water during the dry season if the current warming and glacial melting trends continue for several more decades This may be enough time for long term planning to see just how the region can cope with this problem Unfortunately the situation here is that when the glaciers melt and their fossil water is used or lost their contribution to the water supply of the region will C6356 South American Andes A large fraction of the population living west of the South American Andes relies on the glacial melt from those mountains to feed the area s rivers to supply water and hydro power Without the glacier supplied river water the people and economies of the region would have to undergo tremendous adjust physics governing the Andean glaciers are more complicated than simple temperature forcing Depending on the latitude and on which side of the Andes we consider the glaciers mass balance can be controlled by different factorss Although air temperature changes are still important in most areas other pro cesses such as moisture ux and precipitation dominate in some regions This makes the prediction of what might happen in the Andes much more dif cult Although all greenhouse models predict warming air temperatures they can disagree on predicted changes in rainfall moisture ux and so on In spite of this complexity melting of the glaciers is well docu mented for the Andessis In Peru alone the glacier covered area has been reduced by 25 in the last three decades as reported at the Conference on Mass Balance of Andes Glaciers Huaraz Peru 6 9 July 2004 httpwwwinrenagobpeserususerusu ppointhtm At current rates some of the glaciers may disappear in a few decades if not sooner The high frequency surges and retreats and the uneven spatial distribution of the general glacier retreat makes understand ing and predicting the behaviour of glaciers in this area uncertain The melting tarted 1 1 The 39 i Panel for Climate Change IPCC shows a long term trend in increasing air temperature in the region ref 38 and Figs 29 and 210 in ref 1 Higher resolution more detailed analysis of many stations in the region show a similar temperature increase one that seems to be increasing Consider the case of Quelccaya in the Andes Fig 3 When the summit core was originally drilled in 1976 it contained clear annual cycles in its layering that extended back in time for approximately 1500 years ref 38 When it was re drilled in 1991 the annual layers in the upper 20 m of the core had been obliterated by percolation of meltwater Together these two results show that melting at the summit had occurred a condition that had not previously occurred in the last 1500 years The probability seems high that the current glacier melting in the Andes will continue just as it will in Asia and other regions ofthe world It is fossil water that has been lost and will not be replaced anytime soon especially not in the context of anthropogenically induced greenhouse warming The results and projections suggest that current dry season water resources will be heavily depleted once the glaciers have disappeared ome uncertainties in estimating impacts All of the future climate predictions have uncertainties We touch on only a few of the more important ones below with the goal of seeing whether they might overcome the warming signal and make the conclusions above moot We do not however attempt here a complete discussion of all the uncertainties that attend climate models In some cases the uncertainties have to do with the mod NATUREiVol 438H7 November 2005 be required for good quantitative estimates of potential future water problems Such high resolution regional hydrological studies have not yet been undertaken for either the HKH region or South America One of the greatest uncertainties in future prediction has to do with how the models are forced Stated more directly what are the implications of omitting forcings that we strongly suspect or know are important but cannot yet reliably be included in the model physics Of these the most important is thought to be the incomplete inclusion of aerosols and their impacts especially on clouds Excellent discussions of the current state o the aerosol problem may be found in refs 57 and 58 and ref 59 shows the sensitivity of climate model predictions to uncertainties in indirect aerosol forcing The key question for this paper is Can the aerosolcloud problem overwhelm the direct greenhouse gas induced temperature forcing that affects the regional hydrological cycle giving net cooling as opposed to warming We consider below some of these uncertainties qualitatively to see how they might impact the results discussed a 0V6 Aerosols and clouds Aerosols are thought to cool the planet s surface through increased scattering and cloud cover and re radiation of solar energy to space The representation of clouds in CGCMs carries alarge uncertainty all by itself but the joint interaction of clouds and I 1978 Figure 3 i Changes in the Qori Kalis Glacier Quelccaya Ice Cap Peru between 1978 a and 2002 b Glacier retreat during this time was 1100 m L Thompson personal communication Photographs courtesy of L Thompson REVIEWS aerosols represents one of the major challenges to climate modellers today Virtually all climate models have some representation of direct aerosol effects that is re ectivity of the particles in them but none have yet fully included the indirect effects for example the effect of aerosols on cloud distributions via their role as cloud condensation nuclei or other effects discussed below A preliminary study6D suggests that indirect aerosol impacts on clouds are important but even given the uncertainty in estimating these impacts this mechanism is not strong enough to counter greenhouse warming ffects Recent observational studies show that locally over India the total aerosol effect direct plus indirect has been associated with a surface cooling of 03 C over the last three decades This is close to the warming expected from greenhouse gases However the aerosols are observed to be associated with warming in the lower to middle troposphere the regions inhabited by the glacier elds In this case the aerosols may be enhancing the direct temperature forcing by contributing to the melting of the higher glaciers of the HKH region Sm all amounts Aerosols are found to alter cloud physics in a manner that reduces precipitation downstream from the pollution source This also reduces the snow particle rime growth resulting in lower snow water equivalent a result obtained from direct eld measurements 54 Properly represented aerosols in climate models will apparently also work together with increasing temperature to reduce snow ice in regions where heavy air pollution exists for example China the western USA and Europe Snowice melt rates A common aerosol found in the atmosphere over many regions of the earth is black carbon This substance L L L Tti rruhlierl from L r L l i and because it is ubiquitous is likely to end up in the snow and ice elds of the planet There it could decrease the surface albedo causing the snowice to absorb solar energy more readily and thereby melt sooner Measurements of black carbon amounts and its budgets are only now being made By whatever means darkening the surface of a snowice eld will enhance melt rates Again it seems that proper inclusion of aerosols in global climate models will increase early melting of snow packs and especially glaciers and sea ice6 bottom line here is that other important but poorly rep resented atmospheric physical and chemical processes seem unlikely to neutralize or reverse greenhouse warming This is true even if we take the lower end of the estimated warming by the IPCC 14 C to be the net thermal forcing on the snowglacier packs Our ACPI study2 showed that such an increase coupled with inadequate containment is all it takes to invoke the water storage problems noted above a 585u Overview of expected regional water impacts In this review we suggestthat the simplest of changes associated with global warming a modest increase in near surface air temperature will be responsible for alterations of the hydrological cycle in snowmelt dominated regions via seasonal shifts in stream ow Without adequate water storage capacity these changes will lead to regional water shortages The model predicted changes are already being seen in the observed data If maintained at current levels these changes will lead to a serious reduction in dry season water availability in many regions of the Earth within the next few decades The physical principles found to apply in snowmelt dominated regions for example the western USA are one of the probable causes of the observed early snowmelt and more importantly deglaciation that is now occurring in most mountainous regions of the world The serious situations developing in the HKH region and South America have been brie y presented It is clear that both regions as well as others not mentioned are headed for a water supply crisis Better water management techniques can help but cannot solve the problem without signi cant changes to agriculture industry and lifestyle Detailed studies of the future impact of global warming on water resources in these regions are long overdue 307 2005 Nature Publishing Group VIEWS We have discussed brie y here some of the major uncertainties in the models in particular the impacts of aerosols and clouds as well as their suspected impacts on the aspects of the hydrological cycle hav ing to do with snow and ice In all the cases considered current scienti c evidence suggests that these processes which are currently either not included or are marginally included in IPCC scenario run the s will act to increase the impact of mere temperature increase on snow and ice elds ofthe planet ime is running out for nations in the sensitive areas we have T evaluated particularly those whose water supplies are dependent on mid latitude glaciers to understand just what the future might hold for dec gee r init them How much they can do is uncertain given the several ades of warming that will occur as a result of past actions even if nhouse emissions were halted at today s levels but perhaps the iation of strategic planning will be motivated by the prospect and what is rapidly becoming the reality of diminished water supplies Iquot E 5 P 9 N 9 gt9 5 Iquot 6 E G 5 3 0 5 M o it M M The lnternational Panel for Climate Change lPCC Climate Change 2001 The Scientific Basis eds Houghton J T et al Cambridge Univ Press Cambridge 001 Barnett T P amp Pennell W eds impact of global warming on Western US water supplies Clim Change 62 Spec Vol 2004 M te P W Hamlet A F Clark M P amp Lettenmaier D P Declining mountain snow pack in western North America Bull Am Jllet Soc 86 39 49 2005 Dettinger M D Cayan D R Meyer M K amp Jeton A E Simulated hydrologic responses to climate variations and change in the Merce Carson and American River Basins Sierra Nevada California 1900 2099 Clim Change 62 283 317 2004 Hamlet A F Mote P W Clark M P amp Lettenmaier D P Effects of temperature and precipitation variability on snow pack trends in the western USJ Clim inthe Douville H et al Sensitivity of the hydrological cycle in increasing amounts of greenhouse gases and aerosols Clim Dyn 20 45 68 2002 Giorgi F Whetton P H ampJones R G Emerging patterns of simulated regional climatic changes for the 21st century due to anthropogenic forcings eophys Res Lett 28 3317 3321 2001 Giorgi F amp Bi X Regional changes in surface climate interannual variability for the 21st century from ensembles of global model simulations Geophys Res Lett 32 L13701 doi1010292005GL023002 2005 Ruiz Barradas A amp Nigam S lPCC s 20th century climate simulations Varied representations of North American hydroclimate variability J Clim submitted Dai A Precipitation characteristics of eighteen coupled models J Clim submitte Cayan D R Kammerdiener S A Dettinger M D Caprio J M amp Peterson D H Changes in the onset of Spring in the Western United States Bull Am Met Soc 82 399 415 2001 Stewart l Cayan D C amp Dettinger M D Changes in snowmelt runoff timing in Western orth America under a business as usual climate change scenario Clim Change 62217 232 2004 Niissen B O Donnell G M Hamlet A F amp Lettenmaier D P Hydrologic vulnerability of global rivers to climate change Clim Change 50143 175 2001 Liang X Lettenmaier D P Wood E F amp Burges S J A simple hydrologically based model of land surface water and energy fluxes for general circulation models J Geophys Res 99D1714415 14428 1994 Vorosmarty C J K et al The storage and aging of continental runoff in large reservoir systems of the world Ambio 26 210 219 1997 Vorosmarty C J et al Anthropogenic sediment retention Maior global impact from registered river impoundments Glob Planet Change 39169 190 03 A m J C Clark E A Lettenmaier D P amp Wood E F Correction ofglobal precipitation products for orographic effects J Clim in the press Center for lnternational Earth Science lnformation Network ClESlN Socioeconomic Data and Applications Center SEDAC Gridded Population of the World ersion 3 Columbia University and Centro lnternacional de Agricultura Tropical Palisades New York 2004 available at http betasedacciesincolumbiaedugpw Peterson T C Golubev V S amp Groisman P V Evaporation losing its strength Nature 377 687 688 1995 Chattopadhyay N amp Hulme M Evaporation and potential evapotranspiration in lndia under conditions of recent and future climate change Agricult Forest Meteorol 87 55 73 1997 Thomas A Spatial and temporal characteristics of potential evapotranspiration trends over China Int J Clim 20 381 396 Golubev V S et al Evaporation changes over the contiguous United States and the former USSR a reassessment Geophys Res Lett 2813 2665 2668 2001 M U M is M P M 9 M N M on M a U o B U M U U U P U 57 U N U 00 U gt9 is m is ox is i E Lawrimore J H amp Ohmura A amp Wild M Ohm Payne J T Wood A W Hamlet A F Palmer R N amp Lett Gao Mark B G amp Mark B NATURElVoI 438117 November 2005 Brutsaert W amp Parlange M Hydrologic cycle explains the evaporation 8 paradox Nature 39630 199 Peterson T C Pan evaporation trends in dry and humid regions of the United States J Hydrometeorol 1 543 546 2000 Hobbins M T amp Ramirez J A Trends in pan evaporation and actual evapotranspiration across the conterminous US paradoxical or complementary Geophys Res Lett 31 doi 1010292004GL019846 2004 Walter M T Wilks D S Parlange J Y amp Schneider R L lncreasing evapotranspiration from the conterminous United States J Hydrometeorol 5 405 408 2004 Roderick M L amp Farquhar G D The cause of decreased pan evaporation over the past 50 years Science 2981410 1411 2002 Wild M ls the hydrological cycle accelerating Science 298 1345 1346 2002 ura A amp Gilgen H On the consistency of trends in radiation and temperature records and implications for the global hydrological cycle Geophys Res Lett 31doi1010292003GL0191882004 lnternational Panel for Climate Change Climate Change 2001 Impacts Adaptation and Vulnerability eds McCarthy J J et al Cambridge Univ Press C mbridge UK 2001 Barnett T P et al The effects of climate change on water resources in the West lntroduction and overview Clim Change 621 11 2004 enmaier D P Mitigating effects of climate change on the water resources of the Columbia River Basin Clim Change 62 233 256 2004 Middelkoop H et al lmpact ofclimate change on hydrological regimes and water resources management in the Rhine basin Clim Change 49105 128 2001 Gan T Y Reducing vulnerability of water resources of Canadian prairies to potential droughts and possible climatic warming Wat Res Manag 14111 135 2000 Burn D Hydrologic effects ofclimatic change in west central CanadaJ Hydrol 160 53 70 1994 de Loe R Kreutzwiser R amp Moraru L Adaptation options for the near term climate change and the Canadian water sector Glob Environ Change 11 231 245 2001 Schwindler D W The cumulative effects of climate warming and other human stresses on Canadian freshwaters in the new millennium Can J Fish Aauat Sci 5818 29 2001 Thompson L G et al Tropical glacier and ice core evidence of climate change on annual to millennial time scales Clim Change 59137 155 2003 Combes S Prentice M L Hansen L amp
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