There are different hypothesis that have been tested in global cooling model simulation to check their potential importance in the glacial inception of the Northern Hemisphere. One of the major hypotheses is the closure of both the Indonesian (~3-5 Ma, Cane and Molnar, 2001) and Panama seaways (~3 Ma, Bartoli et al, 2005). The closure of the Panama isthmus, which began 13 Ma, was very slow. When the connection between the Pacific and the Atlantic Oceans closed, it intensified the thermohaline circulation in the Atlantic intensifying the heat transport from the equator toward high latitudes. Such hypotheses tested in GCMs circulation show that, even if a larger heat export could bring more precipitation and lead to the built of an ice sheet, the difference in ice sheet volume accumulated between an “open” or “closed” isthmus is small (Klocker et al., 2005; Lunt et al., 2008). On the contrary, the closure of the Indonesian seaway stopped the warm waters from the South Pacific from flowing into the Indian Ocean. This increased the amount of the North Pacific cold waters involved in circulation into the Indian Ocean and thus reduced the heat transport from the tropics toward the higher latitudes, finally triggering a global cooling (Cane and Molnar, 2001). http://www.climatescienceandpolicy.eu/2011/01/the-three-million-years-ago-dilemma-the-beginning-of-the-ice-ages/
a) For several million years the Earth has experienced regular galciations followed by shorter warmer periods roughly every 100,000 years. What causes this seemingly regular climate oscillation ? The textbook answer is that they are initiated by changes in the Earth’s orbit and axis tilt called Milankowitch cycles and are then enhanced by a CO2 feedback effect. However the details are complex. For the last 1 million years Ice ages have occurred more or less every 100,000 years which corresponds to the change in eccentricity of the Earth’s orbit around the sun. Looking at the detailed effects on changes to incident solar radiation we find:
1. A 41,000 year variation in the tilt of the Earth’s axis to the sun. This effects the severity of winters and summers during the year.
2. A 23,000 year precession of the same axis of rotation which changes the season within the year. 13,000 years ago Winter in the Northern hemispheer was in June.
3. A 100,000 year oscillaton in the elipticity of the Earth’s orbit around the sun. Most important is the change in elipticity of the Earth’s orbit which changes the distance from the Sun during the year. So when winter in the northern hemisheper corresponds to a large distince from the sun we can expect more severe cold winters. http://clivebest.com/blog/?p=2732
|How Antarctica got its ice sheets–In the continual movement of Earth’s tectonic plates, Antarctica was severed from the southern tip of South America about 34 million years ago, creating the Drake Passage. Antarctica became completely surrounded by ocean. The powerful Antarctic Circumpolar Current began to sweep around the continent, isolating Antarctica from the warmth of the global oceans and provoking large-scale cooling. Illustration by Jack Cook. http://www.whoi.edu/services/communications/oceanusmag.050826/v42n2/haug-en1.html|
c) 3-22-04 How could the Gulf Stream–which transports not only moisture but also heat to the North Atlantic–lead to major Northern Hemisphere cooling and the formation of ice?
Neal Driscoll and Gerald Haug proposed one solution. They postulated that moisture carried northward by the Gulf Stream was transported by prevailing westerly winds to Eurasia. It fell as rain or snow, eventually depositing more fresh water into the Arctic Ocean–either directly, or via the great Siberian rivers that empty into the Arctic Ocean.
The added fresh water would have facilitated the formation of sea ice, which would reflect sunlight and heat back into space. It would also act as a barrier blocking heat stored in the ocean from escaping to the atmosphere above the Arctic. Both these phenomena would further cool the high latitudes. In addition, Arctic waters flowing back into the North Atlantic would have become less cold and salty–short-circuiting the efficiency of the Ocean Conveyor belt as a global heat pump to North Atlantic regions.
These preconditions–moisture plus an Arctic nucleus for cooling–would have made the climate system highly susceptible to ice sheet growth. Even modest changes in the global environment would have been sufficient to tip the scales and lead to the onset of major Northern Hemisphere glaciation.
Just such a change occurred between 3.1 and 2.5 million years ago, as Earth’s axis fluctuated so that the planet’s tilt toward the sun was less than today’s angle of 23.45 degrees. Less tilt to the Earth would have reduced the amount and intensity of solar radiation hitting the Northern Hemisphere, leading to colder summers and less melting of winter snows.
The onset of Northern Hemisphere glaciation also affected the Subarctic Pacific. It led to the formation about 2.7 million years ago of a freshwater lid at the surface of the ocean, called a halocline. This Arctic halocline would have created a barrier to upwelling, which blocked deep carbon-dioxide-rich deep waters from rising to the surface. The “leak” of heat-trapping carbon dioxide into the atmosphere was stemmed, further cooling the planet.
Many other ocean-atmosphere feedback mechanisms, resulting from the opening and closing of oceanic gateways, remain imperfectly understood. http://www.whoi.edu/services/communications/oceanusmag.050826/v42n2/haug.html
d) Their case rests on temporal correlations between tectonic and climatic phenomena. Particularly impressive is the correlation at 50 Ma of the Indian–Asian collision and the consequent shutdown of what they call the “carbon factory” during the climate optimum, which is immediately followed by the temperature decline of the Middle and Late Eocene and the transition to a glacial state. Correlations do not necessarily imply causation, but they are strongly suggestive when linked in the manner that Kent and Muttoni (1) do. http://www.pnas.org/content/105/42/16061.full
III. The mechanisms behind sudden climate transitions.
It is still unclear how the climate on a regional or even global scale can change as rapidly as present evidence suggests. It appears that the climate system is more delicately balanced than had previously been thought, linked by a cascade of powerful mechanisms that can amplify a small initial change into a much larger shift in temperature and aridity (e.g., Rind and Overpeck, 1993). At present, the thinking of climatologists tends to emphasize several key components:
III.1. North Atlantic circulation as a trigger or an amplifier in rapid climate changes.
The circulation of the north Atlantic Ocean probably plays a major role in either triggering or amplifying rapid climate changes in the historical and recent geological record (Broecker 1995, Keigwin et al., 1994, Jones et al., 1996; Rahmstorf et al., 1996).
II.2 Carbon dioxide and methane concentration as a feedback in sudden changes.
Analysis of bubbles in ice cores shows that at the peak of glacial phases, CO2 was about 30% lower than during interglacial conditions (e.g., Jouzel et al., 1993). We do not at present know whether the lower glacial CO2 levels were a cause or merely an effect of the ice ages.
III.3 Surface reflectivity (albedo) of ice, snow and vegetation.
The intensely white surface of sea ice and snow reflects back much of the sun’s heat, hence keeping the surface cool. Presently, about a third of the heat received from the sun is reflected back into space, and changes in this proportion thus have the potential to strongly influence global climate (e.g., Crowley and North, 1991). In general the ice cover on the sea, and the snow cover on the land, have the potential to set off rapid climate changes because they can either appear or disappear rapidly given the right circumstances.
III.4 Water vapour as a feedback in sudden changes.
Water vapour is a more important greenhouse gas than carbon dioxide, and as its atmospheric concentration can vary rapidly, it could have been a major trigger or amplifier in many sudden climate changes.
III.5. Dust and particulates as a feedback in sudden changes.
Particles of mineral dust, plus the aerosols formed from fires and from chemicals evaporating out of vegetation and the oceans, may also be a major feedback in co-ordinating and amplifying sudden large climate fluctuations.
III.6. Seasonal sunlight intensity as a background to sudden changes.
A major background factor in pacing climate switches on timescales of tens of thousands of years seems to have been the set of ‘Milankovitch’ rhythms in seasonal sunlight distribution or insolation (Imbrie and Imbrie, 1992; Imbrie et al., 1992, 1993). Although the insolation values change gradually over thousands of years, they may take the earth’s climate to a ‘break point’ at which other factors will begin to amplify change into a sudden transition. http://www.esd.ornl.gov/projects/qen/transit.html
f) Last but certainly not least in factors causing the last ice age is that the continent of Antarctica moved to its locale near the South Polar Axis, via continental plate drift.
8-27-09 According to calculations by geologist Professor Christopher Scotese of the University of Texas, Antarctica could
move significantly away from its current location and become at least partially ice-free again within the next 50 million years. http://www.sciencefocus.com/qa/antarctica-moving-away-south-pole