Summary of
Climate effects on human evolution
The Milankovich cycle
The 100 kyr model (albedo-temperature-precipitation 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 anincrease 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 climatesinks 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 icein 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 iceis 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 thesea ice melts very rapidly, and the earth goes back into an interglacial mode. This oscillation lasts approximately 100 kyrs.