Did the Earth enter the current ice age gradually or suddenly?

[bluemoonKY]

During the history of Earth, there have been several ice ages. What caused the climate to cool so much to create these ice ages?

 

[jz92wjaz]

Depends on the Ice age.

Extremely massive and explosive volcanoes can put large amounts of stuff high into the atmosphere, reducing the amount of sunlight reaching the earth. It can take centuries for everything to clear.

Anything that significantly reduces greenhouse gases in the atmosphere can trigger an Ice Age.

Ice Ages seem to follow a pattern that coincides with cyclical changes in the orbit of the earth.

 

[Ophiolite]

The Milankovicth cycles referred to by jz92wjaz influence the amount of solar insolation (incoming EM radiation from the sun) that falls on different parts of the Earth's surface. These are cycles in the eccentricity of the Earth's orbit, precession of the poles and changes in axial tilt.

These changes are sufficient, at present, to periodically tip the balance of heating in such a way that slightly more snow falls in winter in some parts of the world than melts in summer and an ice age ensues.

The at present requires at least two further conditions. jz92wjaz has already mentioned the need for greenhouse gases to be comparatively low: typically because a significant proportion of carbon is sequestered in biomass and carbonate rocks.

Secondly, the distribution of continents and the oceanic circulation patterns should combine to encourage ice build up. Thus, Antarctica being parked over the south pole for several million years, or the closure of the Panama Straits altering global currents.

 

[D H]

It will help to be a bit clearer on terminology.

First I want to distinguish between greenhouse earth, icehouse earth, and snowball earth. The Earth was almost entirely ice-free during greenhouse conditions. Even areas near the poles were relatively warm during these times. The climate is considerably cooler globally and much more uneven from equator to pole during icehouse Earth conditions. This is when ice ages occur. Finally, there's snowball earth, where ice reaches well into the tropics, possibly all the way to the equator.

Ice ages are long periods of time (several millions of years) that occur during icehouse Earth conditions. During an ice age, ice coverage semi-periodically alternates between glacial periods (or glaciations) and interglacial periods. During glaciations, landmasses can be covered with ice to within 40 degrees of the equator. During interglacial periods (e.g., now), ice coverage is limited to high altitudes and high latitudes. Right now we have ice covering Antarctica, Greenland, and very high mountains, but nothing else.

Most scientists mark the current icehouse Earth as beginning about 49 million years ago when CO₂ levels plummeted from about 3400 ppm to 650 ppm. The most likely cause: A bloom of freshwater ferns in the then very warm Arctic Ocean that sequestered massive amounts of CO₂. This was the Azolla event. Wiki article: http://en.wikipedia.org/wiki/Azolla_event.

The current ice age started a bit later, 31 to 34 million years ago, and scientists disagree on both the timing and the cause. Some attribute it to CO₂ reaching some critical low threshold, perhaps caused by the formation of the Himalaya, that enabled the formation of the Antarctic ice sheet. Others attribute it to the formation of the Tasmanian Gateway, others attribute it to the formation of the Drake Passage, yet others to the closure of the Panama Straits, even others to a combination of the above. Most agree that a number of conditions are needed to have an ice age:

• Low atmospheric CO₂ levels,
• A continent parked over one of the poles, and
• Continents arranged such that ocean circulation engenders cooling.

The climate oscillates between glacial and interglacial periods once ice age conditions are reached. That's where the Milankovitch that Ophiolite mentioned come into play. The continents are currently emplaced such that there is a marked difference between the northern and southern hemispheres. Glaciations occur when the Earth's orbit and obliquity create cool summers and warm winters in the northern hemisphere.

There are a number of challenges to Milankovitch theory. One key issue is a switch from a 40,000 glacial/interglacial cycle prior to a million years ago the current 100,000 year cycle. Milankovitch theory nicely explains the 40,000 cycle, but the 100,000 year cycle doesn't quite fit. There is a roughly 100,000 year cycle in Milankovitch theory, but it is rather weak compared to the dominant 40,000 year cycle.

 

[Andre]

...
Most scientists mark the current icehouse Earth as beginning about 49 million years ago when CO₂ levels plummeted from about 3400 ppm to 650 ppm. ...http://en.wikipedia.org/wiki/Azolla_event.
.

Wikipedia sometimes misrepresent the orginal sources like Pearson & Palmer 2000. See fig 4 where such a reduction is dated just before 51Ma, hence some 2 Ma before the Azolla event:

"We note that the termination of North Atlantic volcanism at about 54±53 Myr ago corresponds approximately to the initial drop that we record in pCO2"

See also how pCO2 increases from about 600ppm to about 2600ppm 'shortly' after 49Ma, in the grey band. The authors note:

Our data do not support a precise covariation of pCO2 and temperature; indeed we record a pCO2 peak during the cooling phase at approximately 45.5 Myr ago.

Note that the more recent Zachos et al 2008 (fig 2) is consistent with those spikes.

 

[Andre]

Wikipedia...

This post requires further discussion and rectification when it turns out that the references are doubtful.

When I read about Pearson and Palmers methods (d11B in foraminifera shells) to determine ancient oceanic pH, I realized that the method was rather complicated and could be subject to a lot of noise. Therefore I continued searching until I found Zachos et al (2008) with the same spikes but presumably from other sources. However, that turned out not to be the case. Instead, I found out that my reservations about boron were shared by others. Therefore a rectification is in order.

An excellent study of the limits of boron proxies for oceanic pH can be found in the chapter 17 of this sylabus by Heming and Hönisch and explains why we can consider the Eocene boron proxies for CO2 unreliable. However this also removes the reference for the Wiki Eocene Azolla bloom artice about the alleged reduction of Atmospheric pCO2 in the Early Eocene. Therefore we should check other publications like Doria et al 2011. Check out fig 1 on page 64 (page 2 of the pdf).

Note that there is a cluster of proxy data around the Paleocene Eocene boundary around 55 million years that seem to center around some 500ppm. Then, moving to middle Eocene, there are only a few data -mainly stomata proxies- around 800ppm CO2. Obviously, it appears that there is nothing here that substantiates the statement of a pCO2 drop from 3400 ppm to 650 ppm in the Early Eocene.

Also Doria et al introduce the current "ice house" state with:

Paleotemperature records (multiple refs) demonstrate that the inception of the current icehouse climate mode was not strictly coincident with the Eocene-Oligocene boundary (33.9Ma); instead the Earth "staggered"into the boundary, punctuated by short lived (<1 m.y.) warm and cool phases (fig 1)