"Global" this and that: Paleoclimatologists use the word "global" in a very literal-minded way, to refer to trends in average climate conditions throughout the world. This contrasts with regional trends. For example, the Little Ice-Age, the cool climate conditions that prevailed in Western Europe from the 16th to the 19th centuries, was balanced by warmer conditions elsewhere on Earth. The Little Ice-Age was a regional event, not a global one. Note: at its coolest, Europe was only 1 deg. C cooler on average than it is now.

Global cooling and ice-ages:

Definitions:

Ice ages seem to be driven by three basic considerations: Milankovitch cycles, continental configurations, and positive feedback.

Cool animation.

Solar forcing: The sum of the effects of these cycles gives the general tendency for glaciers to form. Note: Solar forcings are different at different latitudes and in different hemispheres.
Requirements for the growth of continental glaciers:

The process: If the above conditions are met, then the interaction of the Milankovitch cycles can initiate a glaciation. That is only the beginning. The factors that cause ice ages form a positive feedback loop. In the following schematics, keep track of these variables:

Snowball Earth in a Nutshell: We start with the example of the most extreme, but also the simplest example, the Snowball Earth episode of the later Proterozoic in which the supercontinent Rodinia was gripped by the greatest known glaciations. Note: The Proterozic world was simpler, because there was no significant biosphere to move carbon around.

  • To begin: Earth is at equilibrium and global climate is warm. Solar energy is constant. Some is reflected to space, some is captured by greenhouse gasses. CO2 is produced by volcanoes, but mostly dissolves in the ocean or is removed by weathering reactions.
  • Milankovitch cycles cause mild summers: Incoming solar energy is reduced in summer. In some places, continental glaciers start to form, initiating a positive feedback loop. As albedo increases, more solar energy is reflected into space, and climate cools farther than simple orbital parameters would lead one to predict. Ath the same time, as glaciers grow, less bedrock is exposed to atmosphere for CO2 weathering reactions. CO2 begins to accumulate in atmosphere. Over the short term, the cooling effect of albedo overwhelms the greenhouse effect of the CO2.
  • Glacial conditions prevail: As orbital cycles progress, incoming solar energy is restored to summer hemisphere, but it is not enough to counteract the cooling effect of the great albedo of vast continental ice sheets. CO2 weathering reactions are greatly curtailed as bedrock is covered by ice, accelerating CO2's accumulation in the atmosphere. Icebergs raft dropstones into the deep oceans, laying down a distinct layer of oceanic glacial sediment.
  • Greenhouse warming takes over: Eventually, as CO2 accumulates in atmosphere, a point is reached in which greenhouse warming overwhelms albedo-driven cooling as the dominant climatic effect. Now a new positive feedback loop accelerates greenhouse warming, as CO2 is released from previously frozen soil and from the warming oceans. Melting glaciers expose bare bedrock to atmospheric CO2. Opposing this feedback loop, CO2 weathering reactions return with a vengeance, causing copious amounts of CaCO3 to be transported to oceans as CaCO3, where they form thick layers of chemical precipitates, and atmospheric CO2 drops.
  • Equilibrium restored: Atmospheric CO2 levels return to normal as excess CO2 is used up by weathering reactions. The previous glaciation is recorded in the rock record as a layer of marine glacial sediments (dropstones, etc.). In extreme cases, the recovery is recorded as cap carbonates, copious layers of limestone capping the glacial deposits.

    The end? Only until solar forcing upsets the equilibrium again.


    How long does it take? From the time that the ice starts to melt and CO2 weathering reactions can resume, it takes up to 150,000 years for the atmosphere to return to equilibrium. Why doesn't the CO2 just dissolve in the oceans? Much of it does, but to get it into the deep oceans requires passing it through the small oceanic "windows" where surface waters sink into the depths. (Today = North Atlantic and around Antarctica.) That also takes millenia.

    Cooling without solar forcing? Possibly. There may be another way to force a cooling event. Simply expose a huge amount of unweathered bedrock to the atmosphere, and CO2 will be drawn out by weathering reactions. The resulting anomalous reduction in greenhouse gasses can cause global cooling. In fact, the Neogene collision of India and Eurasia, with the elevation of the bare bedrock of the Himalayas and Tibetan Plateau has been suggested as a contributor to the Neogene ice-house world. This correlates well with previous great ice-ages that were characterized by supercontinents with enormous interior orogenies. Recovery would occur when erosion rates eventually diminished and atmospheric CO2 returned to equilibrium levels.
    Global warming:

    We have already seen examples of global warming - the recoveries at the end of glacial intervals. These tend to happen rapidly. More recent ice-ages (Carboniferous and Quaternary) have been less extreme than the Snowball Earth episode, so that the restoration of strong incoming solar energy in summer has been enough to trigger deglaciations. Just as cooling triggers a positive feedback loop over a short time scale, so does warming:

    • Incoming sunlight warms summer hemisphere causing ice to begin to melt.
    • Warming oceans can hold less CO2, so this comes out of solution, increasing atmospheric greenhouse effect.
    • On warming continents, decomposition of (previously frozen) plant material releases more CO2.

    BIG CONCEPT: We see that just as the albedo of growing ice sheets amplified cooling through solar forcing, the release of CO2 amplifies warming through solar forcing.

    Caveat: Yes, we just said that warming exposes bedrock that drives CO2 weathering reactions that remove CO2 from the atmosphere. Now we're saying that warming adds CO2. WTF? The difference is the time scales involved. The positive feedback processes described above work on an order to decades. The weathering reactions work on the order of tens of millenia. Weathering reactions win in the end, but not before the positive feedback elements have had their day.

    Indeed, CO2 concentrations and isotope data from the Greenland Ice Core suggests that during the last deglaciation, global temperatures rose rapidly, on a time scale of decades. Note: at the glacial maximum, average global temperature was 5-6 degrees C lower than today.

    Can we have a warming event without first cooling the Earth off? In fact, it's happened.
    The Paleocene-Eocene Thermal Maximum (PETM): 55 m.a., the boundary of the Paleocene and Eocene Epochs of the Paleogene was marked by proxy data suggesting a rapid warming of 5-6 deg. C within 20,000 years. This was caused by the sudden appearance in the atmosphere of a high concentration of CO2. Over the following 150,000 years, temperatures returned to their previous (greenhouse) equilibrium. This event is of great interest because it strongly resembles the anticipated effects of contemporary anthropogenic global warming. The big question: Where did the "extra" CO2 that drove the PETM come from? Two possibilities:

    Extinction event: The PETM coincided with a minor extinction event, and with the origin of several modern mammal groups (even and odd-toed ungulates, carnivorans.) For foraminiferans, it was a true disaster. The PETM is quite visible in the core sample at right. The change in color coincides with a global extinction of foraminiferans.

    Why were they extinguished? Arguably, the acidification of the oceans, as large amounts of CO2 reacted with water to form carbonic acid.
    The future: Had the industrial revolution not occurred, this would still be a complex topic. What is known is that:


    Key concepts and vocabulary: