This is an excellent, intelligent question!!!
The Toba eruption, although enormous, was a dwarf compared to the more well known extinction events, such as the one which wiped out the dinosaurs, or the one which preceded the dinosaurs and wiped out well over 90% of all life on earth.
There are several explanations as to why the Toba eruption may not have killed off as many species as one would suppose.
1) As I already stated, this event was not on such a grand scale as the others.
2) Events can affect certain ecological conditions, which will only affect certain life forms that live in those certain ecological niches. The Toba eruption certainly affected the climate, which had a devastating impact on agriculture. Since humans depend heavily on agriculture, that it dramatically reduced our population is no surprise. It is possible that the only bottleneck that occurred was among humans, but I doubt this.
3) There is a likely possibility that the effects of the Toba eruption did indeed affect a wide range of species, but it wasn't and hasn't been studied at the same depth as in humans. Or perhaps it was, and we are simply unaware of the data, either because no attempt was made to study the bottleneck among other animals, or because the wrong animals were studied. According to
http://en.wikipedia.org/wiki/Toba_eruption, "Greenland ice cores record a pulse of starkly reduced levels of organic carbon sequestration. Very few plants or animals in Southeast Asia would have survived, and it is possible that the eruption caused a planet-wide die-off." I think this explanation is the most viable.
4) Fossilization is a very rare event. Happening upon these fossils is another very rare likelihood. Coupled together, that we have any fossil record at all is pure luck. Most fossils were catastrophically buried and preserved in an instantly cataclysmic event. The Toba eruption might have likely preserved dead animals nearby as they were buried by the pyroclastic flow, or wherever the tsunami might have reached, but other areas were covered with a thick ash over a longer period of time, and it is not likely that a large number of fossils would be discovered under those conditions in one place. Areas that are rich in fossils are usually former beds of ancient lakes and seas. Sediments are deposited quickly in these places. Chances are, the fossils we would expect to see do indeed exist, but have yet to be discovered.
5) That Toba contributed to an ice age is no surprise, since the resulting atmospheric changes would certainly cause a cooling of the earth from the blockage of the Sun?s rays. However, there are also other factors.
CAUSES OF ICE AGES?
A)
http://en.wikipedia.org/wiki/Ice_age#Changes_in_Earth.27s_atmosphere?The most relevant change is in the quantity of greenhouse gases in the atmosphere. There is evidence that greenhouse gas levels fell at the start of ice ages and rose during the retreat of the ice sheets, but it is difficult to establish cause and effect (see the notes above on the role of weathering). Greenhouse gas levels may also have been affected by other factors which have been proposed as causes of ice ages, such as the movement of continents and vulcanism.?
B)
http://en.wikipedia.org/wiki/Ice_age#Position_of_the_continents?The geological record appears to show that ice ages start when the continents are in positions which block or reduce the flow of warm water from the equator to the poles and thus allow ice sheets to form. The ice sheets increase the Earth's reflectivity and thus reduce the absorption of solar radiation. With less radiation absorbed the atmosphere cools; the cooling allows the ice sheets to grow, which further reduces reflectivity in a positive feedback loop. The ice age continues until the reduction in weathering causes an increase in the greenhouse effect.
There are three known configurations of the continents which block or reduce the flow of warm water from the equator to the poles:
?A continent sits on top of a pole, as Antarctica does today.
?A polar sea is almost land-locked, as the Arctic Ocean is today.
?A supercontinent covers most of the equator, as Rodinia did during the Cryogenian period.
Since today's Earth has a continent over the South Pole and an almost land-locked ocean over the North Pole, geologists believe that Earth is likely to experience further glacial periods in the geologically near future. Estimates of the timing vary widely, from 2,000 to 50,000 years depending on other factors.
Some scientists believe that the Himalayas are a major factor in the current ice age, because these mountains have increased Earth's total rainfall and therefore the rate at which CO2 is washed out of the atmosphere, decreasing the greenhouse effect. The Himalayas' formation started about 70 million years ago when the Indo-Australian Plate collided with the Eurasian Plate, and the Himalayas are still rising by about 5mm per year because the Indo-Australian plate is still moving at 67 mm/year. The history of the Himalayas broadly fits the long-term decrease in Earth's average temperature since the Paleocene-Eocene Thermal Maximum.?
C)
http://en.wikipedia.org/wiki/Ice_age#Variations_in_Earth.27s_orbit_.28Milankovitch_cycles.29?The Milankovitch cycles are a set of cyclic variations in characteristics of the Earth's orbit around the sun. Each cycle has a different length, so at some times their effects reinforce each other and at other times they (partially) cancel each other.
It is very unlikely that the Milankovitch cycles can start or end an ice age (series of glacial periods):
?Even when their effects reinforce each other they are not strong enough.
?The "peaks" (effects reinforce each other) and "troughs" (effects cancel each other) are much more regular and much more frequent than the observed ice ages.
In contrast, there is strong evidence that the Milankovitch cycles affect the occurrence of glacial and inter-glacial periods within an ice age. The present ice ages are the most studied and best understood, particularly the last 400,000 years, since this is the period covered by ice cores that record atmospheric composition and proxies for temperature and ice volume. Within this period, the match of glacial/interglacial frequencies to the Milankovi? orbital forcing periods is so close that orbital forcing is generally accepted. The combined effects of the changing distance to the Sun, the precession of the Earth's axis, and the changing tilt of the Earth's axis redistribute the sunlight received by the Earth. Of particular importance are changes in the tilt of the Earth's axis, which affect the intensity of seasons. For example, the amount of solar influx in July at 65 degrees north latitude varies by as much as 25% (from 400 W/m2 to 500 W/m2, see graph at [1]). It is widely believed that ice sheets advance when summers become too cool to melt all of the accumulated snowfall from the previous winter. Some workers believe that the strength of the orbital forcing is too small to trigger glaciations, but feedback mechanisms like CO2 may explain this mismatch.
While Milankovitch forcing predicts that cyclic changes in the Earth's orbital parameters can be expressed in the glaciation record, additional explanations are necessary to explain which cycles are observed to be most important in the timing of glacial/interglacial periods. In particular, during the last 800,000 years, the dominant period of glacial-interglacial oscillation has been 100,000 years, which corresponds to changes in Earth's eccentricity and orbital inclination. Yet this is by far the weakest of the three frequencies predicted by Milankovitch. During the period 3.0 - 0.8 million years ago, the dominant pattern of glaciation corresponded to the 41,000 year period of changes in Earth's obliquity (tilt of the axis). The reasons for dominance of one frequency versus another are poorly understood and an active area of current research, but the answer probably relates to some form of resonance in the Earth's climate system.?
D)
http://en.wikipedia.org/wiki/Ice_age#Variations_in_the_sun.27s_energy_output?There are at least two types of variation in the sun's energy output:
?In the very long term, astrophysicists believe that the sun's output increases by about 10% per billion (109) years. In about one billion years the additional 10% will be enough to cause a runaway greenhouse effect on Earth - rising temperatures produce more water vapour, water vapour is a greenhouse gas (much stronger than CO2), the temperature rises, more water vapour is produced, etc.
?Shorter-term variations, some possibly caused by "hunting". Since the sun is huge, the effects of imbalances and negative feedback processes take a long time to propagate through it, so these processes overshoot and cause further imbalances, etc. - "long time" in this context means thousands to millions of years.
The long-term increase in the Sun's output cannot be a cause of ice ages.
The best known shorter-term variations are sunspot cycles, especially the Maunder minimum, which is associated with the coldest part of the Little Ice Age. Like the Milankovitch cycles, sunspot cycles effects' are too weak and too frequent to explain the start and end of ice ages but very probably help to explain temperature variations within them.?
E)
http://en.wikipedia.org/wiki/Ice_age#Vulcanism?The largest known volcanic events, the flood basalt events which produced the Siberian Traps and Deccan traps and are both associated with mass extinctions, are not associated with ice ages. At first sight this implies that vulcanism cannot have produced ice ages.
But 70% of Earth's surface is covered by sea, and the theory of plate tectonics predicts that all of the Earth's oceanic crust is completely replaced about every 200 million years. Hence it is impossible to find evidence of submarine flood basalts or other extremely large undersea volcanic events more than 200 million years old, and evidence of more recent extremely large undersea volcanic events may already have been erased. In other words, our failure to find evidence of other extremely large volcanic events does not prove that they did not happen.
It is theoretically possible that undersea volcanos could end an ice age by causing global warming. One suggested explanation of the Paleocene-Eocene Thermal Maximum is that undersea volcanoes released methane from clathrates and thus caused a large and rapid increase in the greenhouse effect. There appears to be no geological evidence for such eruptions at the right time, but this does not prove they did not happen.
It is harder to see how vulcanism could cause an ice age, since its cooling effects would have to be stronger than and to outlast its warming effects. This would require dust and aerosol clouds which would stay in the upper atmosphere blocking the sun for thousands of years, which seems very unlikely. And undersea volcanos could not produce this effect because the dust and aerosols would be absorbed by the sea before they reached the atmosphere.
Regarding your question about the last few ice ages?
From
http://www.pbs.org/wgbh/nova/ice/chill.html?Ever since the Pre-Cambrian (600 million years ago), ice ages have occurred at widely spaced intervals of geologic time - approximately 200 million years - lasting for millions, or even tens of millions of years. For the Cenozoic period, which began about 70 million years ago and continues today, evidence derived from marine sediments provide a detailed, and fairly continuous, record for climate change. This record indicates decreasing deep-water temperature, along with the build-up of continental ice sheets. Much of this deep-water cooling occurred in three major steps about 36, 15 and 3 million years ago - the most recent of which continues today. During the present ice age, glaciers have advanced and retreated over 20 times, often blanketing North America with ice. Our climate today is actually a warm interval between these many periods of glaciation. The most recent period of glaciation, which many people think of as the "Ice Age", was at its height approximately 20,000 years ago.?
Hope this helps, and again, this was a great question.
You get a star from me!!
Quick Reply
