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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

  • sends the glacier-prone Northern Hemisphere

  • into a bitter Aphelion winter while the eccentricity is high.

  • So when does this happen next?

  • Well, right now, obliquity is decreasing,

  • and it will bottom out in around 12,000 years.

  • It's currently winter at Perihelion in the Northern

  • Hemisphere, but it'll persist completely

  • to the bad situation in 10,000 years.

  • So over 10,000 to 12,000 years, all of that points to cooling.

  • What about the 100,00-year eccentricity cycle that seems

  • to define the overall cycle?

  • Well, actually, we're just coming out

  • of a peak in eccentricity.

  • That should've been bad.

  • And perhaps it would have meant that the upcoming cooling

  • trend would bring the glaciers with it.

  • However, we may have dodged a bullet.

  • See, the recent eccentricity maximum was a sad little pig,

  • and our orbit remains pretty circular.

  • See, as well as the 100,000-year cycle,

  • there's a longer 400,000-year cycle on top of that.

  • Roughly, every fourth eccentricity peak is very low.

  • That just happened.

  • And the next peak will be weak, also.

  • We got lucky.

  • We're in a long, stable, low-eccentricity phase.

  • Because of this, climate models predict

  • that we have another 25,000 to 50,000 years

  • of interglacial period left And that's

  • only if you ignore anthropogenic climate change.

  • Human influence on the climate messes with the whole equation.

  • With CO2 now at 400 parts per million,

  • it's higher than at any point in the Quaternary period.

  • It's been predicted that this may

  • extent the current interglacial for 100,000 years.

  • So we've probably at least offset the next glaciation,

  • although it wasn't coming any time soon, anyway.

  • The real question is have we ended the entire Quaternary ice

  • age?

  • Also possible.

  • However, the recent increase in greenhouse gases

  • is so large and so sudden that there's no precedent anywhere

  • in the climate record.

  • This makes modeling our influence a huge challenge.

  • But don't mistake that for a lack of certainty.

  • Our influence is certainly enormous.

  • There is another climate extreme that's

  • much less fun than a long, mild interglacial.

  • That's a sweltering greenhouse climate,

  • like the one that dominated the Mesozoic when

  • the dinosaurs roamed, or Venus.

  • See you next week for more cold, hard facts on Space Time.

  • Last week, we wrapped up our conversation on dark energy,

  • talking about anti-gravity, negative pressure,

  • and conservation of energy.

  • You guys had some pretty deep comments.

  • 4798Alexander4798 asks, is the universe

  • behaving its way because math, or is math behaving its way

  • because universe?

  • Whoa.

  • Mind blown.

  • This is a pretty fundamental question.

  • My guess-- the universe doesn't know any math.

  • It failed pre-calc.

  • It wouldn't know a hypotenuse if you slapped it with one.

  • Mathematics is a model that we use to describe

  • the behavior of the universe.

  • The astonishing thing is that it has

  • such incredible predictive power.

  • Ryan Lidster and a few others have

  • wondered whether the energy lost in the cosmological redshift

  • of photons could account for the energy gained by dark energy.

  • OK.

  • So to summarize, as the universe expands,

  • the energy in matter in any one co-moving volume

  • or expanding volume is conserved.

  • It gets more spread out, but the method doesn't disappear.

  • But photons also get spread out and they get red shifted,

  • so they do lose energy inversely proportional

  • to the increasing scale factor.

  • Now, Physics Girl has an excellent video

  • describing this effect.

  • Link in the description.

  • So could this lost energy become dark energy?

  • No.

  • The scales are way off.

  • Photons make up only a tiny energetic contribution

  • to the modern universe-- far less, even,

  • than baryonic matter, which itself

  • is far less than dark energy.

  • The radiation-dominated era ended around 50,000 years

  • after the Big Bang.

  • These days, photons just don't have enough energy

  • left to contribute.

  • Yet dark energy continues to be created.

  • Eugene Khutoransky points out that the idea that energy

  • is not conserved in an expanding universe

  • is still pretty speculative.

  • And yeah, there is some speculation here,

  • but I don't think it's a speculative statement

  • to say that the law of conservation of energy,

  • as we learned when we studied Newtonian mechanics,

  • is a feature of flat spacetime.

  • Curved spacetime changes things.

  • Even gravity from a Newtonian perspective

  • requires the invention of a new quantity--

  • gravitational potential energy-- in order

  • to preserve energy conservation.

  • Described in general relativity, you

  • can still come up with conserved quantities--

  • energy analogies that are invariant

  • in, say, an expanding universe.

  • But, for example, a stress energy momentum pseudo tensor

  • isn't mathematically the same thing as classical energy.

  • This gets us back to the idea of whether the universe knows

  • math.

  • The universe is mechanistic and its behavior results

  • in emergent mathematical laws that

  • allow us to model and predict its behavior.

  • Conservation of energy is one such law

  • that work in flat space time.

  • But energy itself is not a thing.

  • We draw energy life bars in our animation sometimes,

  • but the universe doesn't have any hidden energy counter.

  • It just acts according to a deep, and presumably very

  • simple, set of fundamental rules that give rise

  • to mathematical relationships.

  • And we shouldn't mistake those relationships

  • as themselves being fundamental.

Earth's climate shifts between short periods

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