- 4093 - ICE
AGE - what caused it? What caused the last ice age to end
around 10,000 years ago? Nearly 10,000
years ago, Earth came out of its most recent ice age. Vast, icy swaths of land
around the poles thawed, melting the glaciers that had covered them for nearly
100,000 years.
-
--------------------------- 4093 - ICE AGE - what caused it?
- Why, after such a long period of cold, did
the ice age finally come to an end? The
traditional explanation for why ice ages begin and end is a series of
eccentricities and wobbles in the planet's orbit known as the “Milankovitch
cycles”.
-
- Named after Serbian scientist Milutin
Milankovitch, these cycles describe patterns in Earth's orbit and axial tilt.
Over time, our planet's orbit around the sun alternates from being more
circular to more egg-shaped. At the same time, our planet's axis tends to both
tilt and wobble.
-
- Milankovitch found that these factors
combine at regular intervals to cause land at 65 degrees north latitude (a
parallel that runs through Canada, Alaska and parts of Eurasia) to become
warmer than normal and theorized that this warming and then subsequent cooling
of the Northern Hemisphere explained the planet's cycle of ice ages, or
glacials, and warmer periods, or interglacials.
-
- However, while there is evidence that
Milankovitch cycles drive the ebb and flow of ice ages, many modern
glaciologists don't think the cycles' reported ties to ice ages completely
checks out. One issue is that when the glaciers in the Northern Hemisphere
melted, glaciers in the Southern Hemisphere melted too.
-
- But with the traditional explanation, it's
unclear how warming in one hemisphere would melt glaciers in the other. The Milankovich-driven changes in sunlight
intensity that would produce warmer temperatures in the north would cause
temperature drops in the south, counterbalancing any net warming.
-
- One possible explanation is that when the
Northern Hemisphere began to warm around 13,000 years ago, meltwater and
icebergs flooded the North Atlantic Ocean, causing a temporary cooling of the
Northern Hemisphere known as the “Younger Dryas period” (12,900 to 11,700 years
ago).
-
- There is some evidence that the “Younger
Dryas” affected ocean currents in a way that caused the Southern Atlantic to
warm up, stirring up the ocean in the process and releasing tons of stored
carbon dioxide into the atmosphere, which in turn caused glaciers in the
Southern Hemisphere to melt over the next 1,500 years. The end result was
likely a more carbon-rich atmosphere that continued to warm both hemispheres,
lifting the planet out of the glacial period.
-
- Another hypothesis suggests that the length
and intensity of the Southern Hemisphere's winters could dictate when ice ages
end. On the surface, it sounds like the polar opposite of the Milankovitch
theory, which suggests that Northern Hemispheric summers drive the climatic
changes.
-
- However, long winters in the Southern Hemisphere
alter wind patterns near the tropics, which can create frequent storms over an
area of the Pacific Ocean known as the “Tropical Warm Pool”, a region of ocean
that stores and releases great amounts of heat. Altered winds can create storms
in this area, which in turn releases massive amounts of water vapor that can
act as a greenhouse gas.
-
- Another idea is that salty water pouring
from the Indian Ocean into the Atlantic Ocean helped end the last ice age. The
Indian Ocean had become super salty because a drop in sea level had cut off a
critical current that flows from the Pacific to the Indian Ocean; normally,
this current diluted the Indian's very salty tropical waters. A change in wind
patterns and currents in the Indian Ocean could have caused the Indian Ocean to
dump tons of dense, salty water into the Atlantic Ocean, altering its currents
and temperatures in both the Northern and Southern hemispheres.
-
- Could it be caused by the “solstice”? The length of the day changes over the year
due to the slight tilt in the Earth’s axis.
The mid-year solstice in 2023 falls at 2:58 pm UTC on 21 June.
-
- The soltice is the way Earth whirls and
wobbles as it wends its way around the sun.
Earth is a moving platform, orbiting the sun in a little more than 365
days. Despite our incredible orbital speed (around 30 kilometers per second),
we don't feel this motion. Instead, it appears to us as though the sun is
moving through the year.
-
- The stars, incredibly distant, rise and set
every 23 hours, 56 minutes and 4 seconds, the true rotation period of
Earth. The sun, though, rises and sets
roughly every 24 hours, making the "solar day" 3 minutes and 56
seconds longer than Earth's true rotation period.
-
- That difference is the result of the sun's
apparent motion against the background stars. From our imaginary airless Earth,
we would see the sun gradually sliding through the constellations of the
zodiac, making one full lap of the sky in one year.
-
- However, our moving platform is tipped over,
tilted on its side by about 23.5 degrees.
As we move around the sun, our planet alternately tilts one hemisphere
towards our star, then away again. This is the cause of the seasons.
-
- When your hemisphere is tilted towards the
sun, you have summer, long days, with the noonday sun high in the sky. Six
months later, when you are tilted away, you have winter, the noonday sun is
low, days are shorter, and there is a chill in the air.
-
- Between those extremes, the sun gradually
drifts north and south. At the extremes of its motion, it would be overhead
from 23.5° north of the Equator (northern hemisphere midsummer) or 23.5° south
(southern midsummer). In total, the
sun's motion moves it between two extremes some 47° apart. Low in the sky in
winter, and high in summer.
-
- The two solstices are the points at which
the sun is either the farthest north in the sky (which is what we have today),
or at its most southerly location. When
the sun is farthest north in the sky, it will appear lowest in the sky at noon
from locations in the southern hemisphere. This also means the shortest period
of daylight of the calendar year.
-
- For the northern hemisphere, the situation
is reversed, the summer solstice places the noonday sun high in the sky, with
the longest period of daylight of the year.
In six months' time, on December 22, 2023, we will have the other
solstice, marking the point at which the sun is at its most southerly point in
the sky. That will bring with it the longest day for those in the southern
hemisphere, and the shortest for those in the north.
-
- By the astronomical definition for the
seasons, summer runs from midsummer to the autumnal equinox (when the sun
crosses the Equator). Autumn runs from that equinox to midwinter's day. Winter
goes from midwinter to the spring equinox, and spring goes from the spring
equinox through to midsummer.
-
- The reason is down to how our climate
behaves. In a simple universe, one would expect the longest day to be the
hottest (with most time for the sun to heat the Earth) and the shortest day to
be the coldest (the most hours of darkness for things to cool down).
-
- However, things are somewhat more complex.
The atmosphere, the ground, and particularly the oceans, take a long time to heat
up and to cool down. The result? The warmest time of the year for many places
comes a few weeks after midsummer. While
the days are getting shorter, the ocean, ground and air continue to warm up.
Similarly, the coldest time in winter is usually a few weeks after midwinter.
-
- Not all months are alike. Some are shorter
than others. The southern hemisphere
summer (northern winter), from December 22 to March 21, lasts just 89 days. The
southern winter (northern summer), by contrast, is almost 94 days long!
-
- The southern autumn (March to June) is
almost 93 days long, while the northern autumn (September to December) is only
90 days.
-
- The reason behind these variations is, once
again, all down to Earth's orbit. As we move around the sun, the distance to
our star varies slightly. Sometimes,
we are closer to our star, and Earth moves faster in its orbit. At other times,
we are more distant, and move slower.
-
- On July 7, 2023, Earth will reach its
farthest point from the sun, which astronomers call "aphelion." On
that date, we will be more than 152 million kilometers from our star.
- Six months later, on January 3, 2024, we
will be at our closest to the sun, perihelion", just over 147 million
kilometers distant.
-
-
July 16, 2023 ICE AGE -
what caused it? 4093
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Sunday, July 16, 2023
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