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  • Earth's climate shifts between short periods

  • of warm and long, long periods of frigid cold.

  • Based on past pans, there's reason

  • to think that the current warm period might be nearly done.

  • Is the Ice Age coming back, or will human activity

  • swing us wildly in the opposite direction?

  • We live in an ice age.

  • Our geological period is the Quaternary,

  • and is characterized by a massive glaciation-- vast ice

  • sheets stretching from the Arctic

  • all the way down to the Missouri River through Siberia, much

  • of Europe, and spreading out from all major mountain ranges.

  • OK, sure.

  • Right now, we're in a brief interglacial phase--

  • a relatively summery stretch in which

  • the glaciers have retreated.

  • These interglacial periods are short lived.

  • The Quaternary Ice Age has lasted 2.5 million years

  • so far.

  • It's 10,000 to 15,000-year warm patches are separated

  • by glacial periods that last several times as long as.

  • The current respite is called the Holocene era,

  • and it began around 11,000 years ago.

  • Temperatures rose, glaciers, and woolly mammoths migrated north,

  • and humans thrived.

  • This new era of warmth and plenty

  • saw the rise of agriculture, writing, cities,

  • and technology.

  • All of our recorded, even our remembered history,

  • is of the Holocene.

  • You might forgive us for imagining

  • that these relatively summery millennia are

  • normal for this planet.

  • That is not the case.

  • The current interglacial is already long.

  • Does this mean that the glaciers are overdue?

  • Is winter coming?

  • To answer these questions, we need

  • to understand what triggers the march of the glaciers

  • and why they eventually retreat.

  • In fact, we know the broad answer to this,

  • even if the details are under debate.

  • Earth's motion around the sun changes, and with it,

  • the intensity and distribution of sunlight.

  • It was Serbian scientist Milutin Milankovitch

  • who realized that the gravitational tug of Jupiter

  • and Saturn would lead to three periodic shifts that

  • might explain the enormous climatic swings

  • of the Quaternary period.

  • These are the Milankovitch cycles.

  • Let me summarize.

  • One-- the elongation or the eccentricity

  • of Earth's elliptical orbit shifts from almost completely

  • circular to somewhat more elliptical in 100,000-years

  • cycle.

  • At the absolute maximum eccentricity,

  • Earth's most distant point from the sun--

  • the Aphelion-- is about 30% further

  • than the closest point, the Perihelion.

  • One hemisphere will experience summer at Aphelion and winter

  • at Perihelion and milder seasons all around.

  • That's the north at the moment.

  • The Southern Hemisphere is closer to the sun in summer

  • and further in the winter, so more extreme seasons.

  • However, the difference in sunlight intensity

  • due to this difference in distance from the sun

  • is much less than the simple difference

  • due to the seasons themselves.

  • So this shouldn't be a huge effect.

  • Two, the pointing of Earth's axis precesses.

  • It rotates 360 degrees over approximately 26,000 years.

  • In addition, the long axis of Earth's elliptical orbit

  • also precesses.

  • Together, these two effects define where in the orbit

  • the seasons occur.

  • They combine to produce a 21,000-year cycle called

  • the precession of the equinoxes.

  • So eventually, the north's mild Perihelion winter

  • will turn into a cold Aphelion winter.

  • And 3- Earth's tilt changes.

  • Our spin axis is now tilted at 23 1/2 degrees relative

  • to the axis of our orbit.

  • This obliquity oscillates between 22.1 and 24.5 degrees

  • over 41,000 years.

  • High obliquity means more extreme seasons.

  • But it's low obliquity that ultimately leads to a colder

  • global climate climate.

  • Because then the highest latitudes, where glaciation

  • begins, never get much sun.

  • Now, Milankovitch predicted that obliquity

  • would drive climate variations, because it governs

  • the strength of the seasons.

  • But how can we test this?

  • Paleoclimatology.

  • We can reconstruct our planet's climate history

  • by digging holes.

  • First, glacial ice cores.

  • The most famous is the nearly four-kilometer-deep hole

  • drilled in the Vostok Glacier in Antarctica.

  • This glacier was built up by millennia of snowfall.

  • Each year's layer carries bubbles of the Earth's

  • atmosphere from that time.

  • Isotope ratios and greenhouse gas content in those

  • bubbles traces global climate over the past 420,000 years.

  • Second-- oceanic sediment cores reveal the changes

  • in ocean floor sea life, whose composition also depends

  • sensitively on ocean temperatures

  • and salinity, and so also on global climate and ice volume.

  • Ocean cores get us a climate record back tens of millions

  • of years.

  • If you look back to the early Quaternary-- earlier than, say,

  • a million years ago-- it seems Milankovitch was right.

  • Temperature goes up and down on the roughly 40,000-year time

  • scale of changing obliquity.

  • But then, around 800,000 to 900,000 years ago,

  • something changed.

  • As Earth reached the depth of the current ice age,

  • the cycle shifted.

  • Now the warm periods come only once every 100,000 years.

  • They seem to follow the change in eccentricity, not obliquity.

  • Every time Earth's orbit becomes more circular, the planet warms

  • and the glaciers go away.

  • As eccentricity increases again, the glaciers return.

  • This is totally weird, because eccentricity

  • should produce a much smaller effect than obliquity.

  • So what changed?

  • It's not entirely clear.

  • But it may be that we're now so deep in the ice age

  • that it takes all of the Milankovitch cycles

  • together to cause the glaciers to retreat.

  • Eccentricity and obliquity and precession

  • must line up perfectly.

  • The eccentricity cycle is the longest,

  • and so the shifts correspond to its period.

  • OK.

  • So we're now in a warm interlude in the depth of an ice age.

  • You might be wondering, when are the glaciers going to rush down

  • from the north, bringing polar bears, white walkers, Tontons?

  • One thing is for sure-- the glaciers

  • will come from the north.

  • The vast oceans of the Southern Hemisphere

  • provide a powerful buffer against changes in temperature.

  • Ice struggles to build up on water.

  • But even now, northern winters see ice and snow cover the land

  • all the way down to the continental US, Europe,

  • and China.

  • In summer, it retreats completely.

  • But if the climate were a little bit cooler,

  • then summer may not be warm enough

  • to melt all of the winter snow.

  • Then it would build up year after year,

  • slowly creeping south.

  • Now, by themselves, shifts in Earth's orbit

  • aren't enough to radically change climate.

  • But they are enough to trigger positive feedback cycles.

  • As ice cover increases, Earth starts

  • to reflect more incoming sunlight.

  • Its albedo increases.

  • More ice means less absorbed sunlight,

  • lowering global temperature and allowing even more ice to grow.

  • The glaciation initiated by the Milankovitch cycles

  • accelerates.

  • A second feedback cycle is equally important.

  • Cooler oceans are better at absorbing carbon dioxide

  • from the atmosphere, and so the Earth's natural greenhouse

  • effect is diminished.

  • There is an unfortunate combination

  • of orbital properties that kickstarts this process.

  • First, low obliquity means less overall sun at high latitudes

  • where the glaciers start.

  • Second, high eccentricity means one hemisphere experiences

  • a bad winter at Aphelion, further from the sun.

  • Earth also moves slower at Aphelion, and so those long,

  • cold winters are not counteracted

  • by the short, warmer summers.

  • And third, the procession of the equinoxes