most of history, general pattern of earth was hot with no permanent ice anywhere. current "ice epoch" began 40mya, ranged from murderously bad to not bad at all. further back you go more sketchy pict is b/c ice ages tend to wipe out older evidence of. at least 17 severe glacial episodes in last 2.5my [pd that coincides w/ rise of homo erectus in Africa then modern humans]. Two common culprits, rise of himalayas, formation of isthmus of panama - first disrupted air flows, second ocean currents [higher landscape cooler, but also diverted winds in a way that made them flow north and toward NrthAmer, making it more susceptible to long-term chills; panama rose, closing gap between NA and SA, disrupting the flows of warming currents btwn PAcific and Atlantic, changing patterns of oprecipitation across at least half the world, one conseq was drying out of Africa, causing apes to climb down from trees], so with oceans and continents arranged as they are, looks like ice is long-term part of future, according to John McPhee, about 50 more glacial episodes can be expected, each lasting 100,000 yrs or so, before we can hope for long thaw. We're still in an ice age - somewhat shrunken one... at height of last pd of glaciation, 20000yrs ago, 30% of land's surface area was under ice, 10% still is, with further 14% in state of permafrost, 3/4 of all fresh water is locked up in ice now, and two polar ice caps [possibility of it being first time in earth's history]; snowy winters through most of world and permanent glaciers even in New Zealand might seem usual, but's actually rather unusual for planet. Unnerving to note that all of human history taken place within atypical patch of fair weather for our ice age; previous interglacials lasted as little as 8000yrs, ours already 10000 anniversary.
Long time thought ice ages come/went gradually over thousands yrs; ice cores from greenland give us detailed record of climate for over hundred thousand years, disproved this theory. Says for most recent history, earth nothing like stable/tranquil place civilization knows, rather lurched violently btwn pds of warmth, brutal chill. Toward end of last big glaciation, 12000yrs agoish, earth warmed rapidly, then plunged back into cold for thousandish years [event known to science as Younger Dryas; name from arctic plant dryas, one of first to recolonize land after ice sheet withdraws. Also older dryas period, but not so sharp]. At end of this, temps leapt again, 7 degrees in 20 years [think exchanging climates of scandinavia and mediterranean in two decades]. Some localities even faster, greenland 15 degrees in 10 yrs. We have no idea what could so quickly change temps. Elizabeth Kolbert: "No known external force, or even any that has been hypothesized, seems capable of yanking teh temperature back and forth as violently, and as often, as these cores have shown to be the case." Apparently, she adds, there's "some vast and terrible feedback loop," prob involves oceans, disruptions of normal patterns of ocean circulation, but long way from understood. No reason to believe this stretch of climate stability will last much more. Global warming counterweight? Not likely. Kolbert pointed out, when confronted w/ fluctuating climate, "the last thing you'd want to do is conduct a vast unsupervised experiment on it." Suggested ice age could be induced by rise in temp's - slight warming'd enhance evaporation rates, increase cloud cover, leading in higher latitudes to more persistent accumulations of snow. Global warming might actually lead to considerable cooling in North America, northern Europe.
Climate's product of so many variables - rising/falling CO2 levels, shifts continents, solar activity, Milankovitch cycles - as difficult to comprehend events of past as to predict those of future. Ex, antartica for at least 20milyrs after settling over south pole, remained covered in plants, free of ice. If ice sheets start to form again, there's lot more water for them to draw on - great lakes, hudson bay, lakes of canada weren't there last time, 'cause last ice age created 'em. Could also be lots of melting of ice, though: if all ice sheets melted, sea levels'd rise by 200ft. Just collapse of West Antartic ice sheet - already in last 50 yrs water around up by 2.5degC, collapses increased dramatically, b/c of underlying geology, large-scale collapse much more possible. Sea levels globally would rise [quickly!] by 15-20ft.
Cause of ice ages - cool summers, b/c if they can't melt all the snow, more incoming sunlight reflected, so cooling effect's exaggerated, encourages more snow [self-perpetuating]. as snow accumulates -> ice sheet, region grows cooler, encourage even more. Gwen Schultz: "It is not necessarily the amount of snow that causes ice sheets but the fact that snow, however little, lasts." Thought that ice age could start from single unseasonal summer. "The process is self-enlarging, unstoppable, and once the ice is really growing it moves," John McPhee. Milankovitch cycles [tilt, pitch and wobble, which affect length/intensity of sunlight on any patch o' land... formally obliquity, precession, and eccentricity... cycles 20thou, 40thou, 100thou years, but varying in each case up to few thousand yrs; ess Milankovitch worked out angle&duration of incoming solar radiation at evry latitude on Erth, every season, for million yrs, adj for three ever-changing variables, took 'im 20 years to do calculations, in 1930 book Mathematical Climatology and the Astronomical Theory of Climatic Changes], cool summers noted by Kšppen, disposition of continents, esp landmasses over poles [suggested that if NA, eurasia and greenland go 300mi up, we'd have permanent and inescapable ice ages], specifics imperfectly understood of this and other factors.
Before 50mya earth had no regular ice ages, when we did they were huge. Massive freezing 2.2bya, followed by billionyrs of warmth. Then another known as Cryogenian or super ice age by some scientists, more popularly called "Snowball Earth." Fall in solar radiation of abt 6% [theory] and dropoff in production [or retention] of greenhouse gases, earth lost ability to hold onto heat; 80 degree F drop in temp all over world. Entire surface earth mighta froze over, ocean ice up to half-mile thick at higher latitudes and tens yards thick in tropics. Cynabacteria survived, tho, they photosynthesize, so needed sunlight; either there was some open ocean water [localized warming at hotspot?], or ice formed in way to remain translucent [this does happen sometimes in nature]. So how'd it get warm again? shouldve reflected so much heat stayed frozen forever, appears rescue from molten interior, idea that volcanoes pushed through surface, spewing lots of heat, gases that melted snow, reformed atmosphere. End of cryogenian era was Cambrian outburst. As earth warmed, prob wildest weather ever, hurricanes powerful enough to raise waves to heights of skyscrapers, rainfalls of indescribable intensity. Prob stuff at deep sea vents continued on ho hum, but rest of life was as close as ever to complete extinction.
Bill Bryson's A Short History of Nearly Everything, pages 425-432.