GEF

9 Introduction UNIT4 | TheGlobalWaTerCrIsIs 85 WaTer CoNCePTs: DESALINATION Asa response towater scarcity, somecoastal regionsandcitieshave invested in seawaterdesalination,a technique forproducing freshwater from saltwater.The numberofdesalinationplants isgrowing rapidly,especially in thewater-starved MiddleEast. Severalmethodsofdesalinationareused.Themostcommon isvacuumdistillation, inwhich seawater isboiledandcondensedat lowatmosphericpressure, requiring lessenergy toheat thewater.Amoreenergy-efficientapproach is reverseosmosis, in which seawater is forced throughafinemembrane, trapping saltsandallowing fresh water topass through. Desalination isexpensiveand requiresa lotofenergy, so it isusuallynotanoption in less-developedcountrieswherewater issuesaremostpressing.Producing freshwater from saltwateralsohasnegativeeffectson theenvironment.When seawater is taken from theocean fordesalination,manymarineorganismscanbekilled.Desalination alsoproducesextremely saltywastewater thatmaycontaincleaningchemicalsand other toxins. Inaddition, theenergyused to rundesalinationplantsnormallycomes fromburning fossil fuels,contributing toclimatechangeandcreatingairpollution. Some innovativedesalinationplants, suchasone inPerth,Australia,usewindenergy toprovideaportionof theirpower. Whiledesalination isanecessity in somecountries thatcannotmeet theirdomestic waterneeds,most sustainabilityadvocatesmaintain that it isa last resort. Instead, the comprehensiveuseofwaterconservation to reducewaterwastecanoften reduce the need toobtainnew suppliesof freshwater. UNIT4 | TheGlobalWaTerCrIsIs 89 heat fromescaping to space, raising theaverage temperature of theplanet.Withhigherglobal temperaturescomea range of impacts, includingextremeweather,erratic rainfall, stronger hurricanes, sea level rise,andmelting sea iceandcontinental glaciers.Theeffectsofclimatechangearealreadybeing felt around theworld,and scientistspredict that theywillcontinue to intensify in thecomingdecades. Higher temperaturesmeanhigher ratesofevaporation from landand seaareas, ineffect speedingup theglobal water cycle .Morewater in theatmospherecausesmoreprecipita- tion,and scientistshave found thatglobalprecipitationhas increased significantlyover the last twentyyears. In some regions thishascaused increasedflooding.Forexample, historicfloods in theMississippiRiverbasin in2011were WATER CYCLE Abiogeochemicalcycle that tracks themovementof water throughearth systems. WaTCh & learN https://youtu.be/cu5RW9mDijw Watch thevideoand respond to thequestionsbelow. 1. Whatare threewaysamajorityof scientist sayhumanactivitiesarechanging the compositionof theatmosphere? 2. Whatare the twoprimarygreenhousegasesgeneratedbyhumanactivity? 3. Whatare three signs thatclimatechange isadverselyaffectingweather systems? video: GreenhouseGases andClimateChange Water Concepts features expand on the current topic, raising your awareness of challenges and successes surrounding water sustainability issues. Interactive Watch & Learn videos take you out of the text to expand on the current topic. Prompts encourage you to think critically about the message from each video and apply the information to water sustainability challenges.