COOLING DRYING 19 4 CT WT CT AR. mean global temperature levels of aridity latitudinal stratification have all changed appreciably

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1 as we have seen. BIOMES are the biota's adaptive response to earth's climate zones but climate too has a history - - it has evolved through time mean global temperature levels of aridity latitudinal stratification have all changed appreciably 1 C M P C Global climate has changed dramatically during earth's history. Here we see earth's mean temperature through the Phanerozoic Era Note generally warm conditions in Mesozoic & most of the Paleozoic, and the massive drop in later part of Cenozoic 2 here we look at climate development since the Mesozoic, focusing on North America, as a preparation to our look at specific biomes FIRST : global picture SECOND : local picture note the take-home message: 3 ~90, +/- all N. Am. warm & moist NO ICE-CAPS COOLING DRYING 4 at K/T, sudden global cooling & drying begins ~60, Mexico becomes arid ~30, much N. Am. becomes arid; cool temperate expands to N. Am. then in Eocene, Antarctic icecap grows 5 6 ~10 C drop in N. America 1

2 ~15, e. N. Am. becomes moist; cool temp. expands south; new COLD so, by mid-miocene (= ~15 ), global climate is much as today, (though Ice Ages yet to come) cooler; dryer; marked latitudinal zonation during Pliocene (~6 ~6 ) & Pleistocene (~2 ~2 ) strong, rapid climate changes as both ice-caps grow 7 MORPHOLOGY OF U.S.A. this cooling and drying caused a great shrinking of forests, and the expansion of savannas - a largely new biome Cascades Basin further drying caused these savannas to become grasslands and shrublands phanerophytes decreased; decreased chamaephytes, hemicryptophytes & therophytes increased 8 Sierra Nevada 9 local N. American factors Appalachians Rockies Plains Colorado Plateau Ozarks 10 Cascades America west of Plains has long been geologically active: Rockies vulcanism + uplift + faulting but up to Miocene, uplift almost matched by erosion - domed uplands Sierra Nevada in early Miocene, great crustal stretching as California moves northwest (25-10 ) Basin forming Basin 11 Colorado Plateau 12 2

3 Grand Canyon - cut in 2-3 m.y.! 10-8 m.y.a, uplift accelerates, strongly lifting Rockies & Colorado Plateau; Grand Canyon begins to form 4-6 m.y.a, Sierra Nevada also rises, to 4.3km. 2-4 m.y.a, Rockies & Colorado Plateau rise rapidly again; Rockies to similar heights; Grand Canyon completes 13 what impact does this new topography have? these massive N-S ranges force precipitation on their w. slopes, but to their east cause MASSIVE RAIN-SHADOWS this explains why N. Am. has longitudinal belts of precipitation 14 NORTH AMERICA precipitation wet slopes arid shadow vegetation types show same pattern 15 moist east dry interior Forest cover across the U.S. 16 finally, we add to this the PLEISTOCENE GLACIATIONS arid lands great plains 80-90% of last 1.5 m.y. has been occupied by glacial times forested east life-zones pushed south and down sea-level drops 17 last interglacial began ~10K y.a. - almost at an end? 18 3

4 Last glacial maximum ~18,000 y.a K y.a. -last glacial maximum 13K y.a. - note Beringia and ice-free passage K y.a. western icefields largely gone 8K y.a. - though warmer than now, much ice remains

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