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.
Today's human-caused climate change will be tomorrow's human-directed climate control.