【正文】 y nicely rounded at the top, so the water just ran right down their sides to thedesert therewere thousands of yearsof wind between thetwo rainyperiods, reshaping the dunes. So, during the second rainyperiod, the dunes werekind of chopped up at the top,full of hollows and ridges, and these hollows would39。re up to a kilometer long, but only a few meterswide, and they39。ve been impossible for all the waterto soak into theground. Second, as you know, sand dunes contain other typesof particles, besides sand, including clay and silt. Now,when the rain fell, waterran down the sides of thedunes, carrying clay and silt particles with whereverthese particles settled, theyformed a pan, a layer that water couldn39。s past. Keepin mind though that this studyonly looked at 19 formations. And about a thousand have been there39。m confused. Why would lakes form in the desert?It39。vebeen times in the past when monsoon rains soaked the EmptyQuarter and turned it from a desert into grassland that was dotted with lakesand home to various animals. Therewere 全國(guó)免費(fèi)咨詢電話： 4000123267 actually two periods ofrain and lake formation: thefirst one began about 35000 yearsago。s known as theEmptyQuarter.The EmptyQuarter is a huge area of sand that coversabout a quarter ofthe Arabian Peninsula. Today it39。t want to waste anytime getting it back. S: Thanks a lot. Now I don39。re not allowed tocheck anything out because of unreturned books. S: That39。t returned books. They39。sour policy. S: Oh, I didn39。re working on right now? And by the timeyou need the rest ofthe book, maybe it will have been returned. We can even do the photocopy for you because of thecircumstances. S: Oh, well, that would be great. L: Isee you39。ve been using lately for a section of mythesis. Why? L: Well,you can photocopyup to one chapterof the book. Why don39。s returned in a couple ofweeks. S: But I reallyneed this book right now. L: Do you needall of it or is therea certain section or chapteryou39。s been recalled. You can keepit all semester as long as no one else requestsit. But,someone else has. It looks like one of the professors in the sociology departmentrequested you have to bring it back, even though you39。m writing my senior thesis, I39。m supposed to return a book that I checked out back in January,it39。lldo do you need? S: Well, I39。sa circle, a loop. And because of this loop, which is promoted by warmertemperaturesin winterand spring, well, it looks like the tundra may be turning into shrub land. 全國(guó)免費(fèi)咨詢電話： 4000123267 Female student: But willit be long term?I mean maybe the shrubs will be abundant for a few years, and then it39。s caught by shrubs. And deep snow drifts often form around shrubs. And we39。t it? Lecturer:Well, much ofthe soil is usually still frozen during peak runoff. And the nutrientsare deep down in the active layer anyway, not high up near the surface, which is the part of theactive layer most affected byrunoff. Butas I was about to say, there39。s responsible for thegrowth ofshrubs in the summer and their spread tonew areasof the tundra. Areaswith more new nutrients arethe areas with the largest increase in shrubs. Female student: But,what about runoff in the spring, when thesnow finally melts? Won39。sbeen increase in nutrient production in the that39。retwo reasons for this: first, theylive in the active layer,which, remember,containswater that doesn39。s thepuzzle?Warmer temperaturesshould lead to increased vegetation growth, right? Well,the connections are not so simple. The temperature increase has occurred during the winter and spring, not during the summer. Butthe increase in shrubs has occurred in the how can increase temperaturesin thewinter and spring result in increased shrub growth in thesummer? Well, it may be biological processesthat occur in the soil in thewinter,that cause increased shrub growth in thesummer,and here39。t interfere with their ? Now since the temperatureshave been increasing in Arctic Alaska, thegrowth ofshrubs has increased. And thisis presented toclimate scientists with a puzzle... Male Student: I39。renot talland being low in the ground protect them from the cold and wind. And their rootsdon39。snot much precipitation. And because of the cold temperatures, the tundra has two layers: top layer,which is called the active layer,is frozen in thewinter and spring, but thawsin the summer. Beneaththis active layer is the second layer called permafrost,which is frozenall year around, and is impermeable towater. Female Student: So because of thepermafrost, none ofthe plants that grow therecan have deep roots, can they? 全國(guó)免費(fèi)咨詢電話： 4000123267 Lecturer:No, and that39。rearound the topic ofglobal climate change and its effects,in Alaska, in the northern Arcticpart ofAlaska, over the last thirtyyears or so, temperature has increased about half a deg