Summary of
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Climate effects on human evolution
The Milankovich cycle
The 100 kyr model (albedo-temperatureprecipitation feedback scheme)
By Agnes Barszcz
Week 4
Climate on human evolution:
Behrensmeyer, Trauth and Potts presented the effects of climate on human
evolution. They looked at the data during the Cenzoic epoch, more precisely
during the Pliocene. It happens that 2.8 Mys ago the climate went into a colder
period. This was accompanied with larger variability as the dominant feature
driving the climate changed from oscillations due to precession to oscillations
due to obliquity. There are three general hypotheses on how the environment
affects adaptations. First a change in evolution may have occurred during a
period when the climate was stable for a long time. Second, adaptations
happened when the climate was slowly changing in one direction. Third,
adaptations occurred when the climate had high variability, which was the case
during the Pliocene. This forced the hominids (name of the taxonomy humans
belong to) to endure more extreme changes. It gave them a stronger gear for
the future since characteristics developed were able to endure the most
environmental variability. Some examples of adaptation that may have been
forced by climate during the Pliocene are: bipedal motion, stone tools, and
variability in tools. This would be explained by a change and an increase in the
variability of vegetation and animals which links back again to the climate's
variability. To survive humans had to go further and faster and thus developed
new skills.
Milankovich cycles:
It is a theory that explains the 100 kyr cycle. This cycle represents the
oscillations between glacial and interglacial periods. The Milankovich theory
explains it by a variation in the amount of solar radiation received by the earth
from the sun. Various factors contribute to this variation, namely the eccentricity
in the earth’s orbit, the precession of earth’s axis, and the change in the earth’s
tilt. Each of these factors have a different period of oscillation (precession: 23
kyr; tilt: 21 kyr, eccentricity 100 kyr). The earth's climate sinks with the weakest
and longest of the signals: the eccentricity. This fact could be explained by
some non-linear interactions of the sun’s isolation with the earth’s system (Like
the yoga balls example). On another note, here are some insights about our
solar system that were pointed out in class. The sun is located in one of the
focal points of the earth’s orbit. Aphelion is the largest sun-earth distance
during the year while Perihelion is the smallest. The earth always takes the
same amount of time to complete one orbit. The second Kepler’s law tells us
that equal orbital areas are swept in the same amount of time. Thus when the
earth is far from the sun it travels slower, and when it is closer it travels faster.
The summer solstice is June 21st, and it occurs when the North Pole is the
closest to the sun. It is to note that this is not when the sun earth distance is the
shortest, but only when the North Pole is oriented towards the sun. The winter
solstice occurs on December 21 and is when the South Pole is the closest to
the sun. The equinoxes are when the day has the same length as the night.
The polar circle delimits the area of the earth that gets at least one full day of
sunshine during the year. The tropic delimits the region that sees the sun
overhead at least once during the year.
The 100 KYR cycle:
Hezi Gildor and Eli Triperman elaborated a model which explains a 100 kyr
cycle in a way that is internal to the earth’s system. They programmed a simple
feedback model that exploits an albedo-temperature-precipitation feedback.
The key to their model is the assumption that the ablation rate of glaciers is
always constant with time and that the poles are always colder than zero
degree. Thus precipitation over land in the poleward region is always solid. The
model run starts in an interglacial period. Temperatures close to the equator
are mild. The ocean SST are larger then zero everywhere. There is a lot of
evaporation. Over the poles this precipitation falls as snow, and compacts to
ice in a few decades. Therefore the land glaciers grows. This leads to an
increase of the albedo of the earth. The earth absorbs less solar radiation and
the temperatures are globally falling. When the SST falls to the freezing point
ice forms very rapidly. The climate goes into a glacial period. When the earth is
in the glacial period the sea ice is preventing evaporation. There is less
moisture in the air and therefore less precipitation. Since the ablation rate is
constant, it becomes dominant, and the land ice retreats. As the land ice
retreats, the albedo also goes down, and the earth absorbs more solar
radiation. The global temperatures rise slowly. As the equator always remains
ice free, the sun heats up the equatorial waters. These warm waters get mixed
pole ward, and will also contribute to the raise of the ocean’s temperature.
Once the SST goes above zero all the sea ice melts very rapidly, and the earth
goes back into an interglacial mode. This oscillation lasts approximately 100
kyrs.