Climate change as a reality of nature: By the 18th century, literate people recognized that climate conditions described by Classical and Medieval authors were often different from those that they witnessed. Today we note that the canals of the low countries are no longer the reliable winter highways for skaters depicted by Peter Brueghel or described in Hans Brinker, or the Silver Skates. It is apparent that climate changes over time. As the science of geology arose, the attention of geologists was drawn to ancient climates for which no eye-witness accounts existed.
Louis Agassiz: (1807 - 1873) Swiss geologist, paleontologist, paleoclimatologist. Investigated reports of glacial erratics (glacier-transported boulders) in places where contemporary glaciers couldn't possibly transport them, such as the Jura Mts. of France and Switzerland. In 1840, published Etudes sur les glaciers (Study of Glaciers), proposing that the prehistoric Earth had experienced an ice age in which a continental glacier similar to that of Greenland had covered the Alps and had lapped against the Juras. As more information rolled in, it became clear that the the ice age glaciations had occurred at high elevations throughout the world and throughout high latitudes.

To know what kind of landforms led to this conclusion, one needs to understand the deposition of continental glaciers today:

Land forms resulting from continental glaciers: Continental scale glaciation creates interesting opportunities for ice to interact with large volumes of sediment. Note: Glacial sediment takes three forms:

Results include:

Periglacial features: Beyond features created by glacial ice, itself, the regions adjacent to continental glaciers display characteristic features owing to:

Resulting land-forms reflect interaction of regolith and ice:

  • Patterned ground - Polygons formed by ice wedges. extending into the soil.
  • Pingoes - Bodies of ice that rise up through the regolith in response to burial pressure.
    Pleistocene temperature and "proxy data": The first reliable thermometers went into use in Italy in the late 17th century. The first continuous records of daily temperature didn't begin until the early 19th century in England, so how do we know about ancient temperatures? We infer them indirectly through proxy data:

    Of course, the evaluation of glacial and periglacial landforms represented our first use of proxy data to assess climate change. This has subsequently been augmented by the other proxy records we ahve discussed, including:

    Ice ages:

    Putting this together reveals that climate over the last two million years has alternated between glacial and interglacial states, with atmospheric CO2 (plotted over the last 0.8 my, right) serving as a good general proxy for temperature.

    We note the following:

    Note: The end of each glacial interval is marked by an abrupt warming. As the precision of our measurements improves, we find this transition to be more and more abrupt - measurable on the order of decades. Transitions to glacial conditions are abrupt also, but somewhat more gradual.

    Condition 18,000 years ago during the last glacial maximum:

  • Average global temperature was 6 deg. C colder than currently.

  • Today's ice caps grew to 3x their current area and were up to 3 km thick

  • Cold weather zones expanded and warm weather zones contracted into a thinner strip of warm tropical weather.

  • Regions of highest rainfall shifted to higher latitudes, forming large, rain-fed pluvial lakes. (These can be mapped using ancient wave-cut platforms).

  • Bedrock was deformed by being loaded with heavy ice sheets. In some locations, the land is still slowly rebounding from the removal of the ice. E.g. Scandinavia (1m / century), Ohio valley.

  • As a consequence, the land buckled downward near the edge of the ice. Glacial meltwater pooled up at margins of glaciers forming large meltwater lakes. The Great Lakes are stranded remnants of such.

  • At the climax of the last glaciation, 18,000 years ago, accumulation of all that ice and snow - sea level dropped 120 m. Some of the major geographic differences from the modern Earth (besides the presence of so much continental ice):

    How does climate shift between glacials and interglacials?

    To address this we need to consider three factors:

    Solar forcings:

    Milankovitch cycles: In the 1920s, the Yugoslavian meteorologist Milutin Milankovitch realized The Earth's movement through space is subject to three kinds of cycles:

    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. The graphic at right shows cumulative solar forcings for 65 deg. north, where most of Earth's land is concentrated.

    Solar forcings are irrelevant unless they favor mild summers in high latitudes: Each cycle influences the severity of winters to some degree, but what really matters is the mildness of summers. It snows hard in many places, and in polar regions, it always snows during winter. That's irrelevant. Glaciers only form in regions where at least some of the previous winter's snow survives the summer. Thus, when Milankovitch cycles favor mild summers, as when:

    ...then glaciers may begin to grow.

    The effects of continental positions:

    The process with feedback effects:

    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. Albedo increases, more solar energy is reflected into space. Climate cools farther than simple orbital parameters would lead one to predict. As glaciers grow, less bedrock is exposed to atmosphere for CO2 weathering reactions. CO2 begins to accumulate in atmosphere.
  • 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. CO2 continues to accumulate in atmosphere. Icebergs raft dropstones into the deep oceans, laying down a distinct layer of oceanic glacial sediment.
  • Greenhouse warming takes over: As CO2 accumulates in atmosphere, a point is reached in which greenhouse warming overwhelms albedo-driven cooling as the dominant climatic effect. Melting glaciers expose bare bedrock to atmospheric CO2. CO2 weathering reactions return with a vengeance. At first, CO2 continue to increase as CO2 escapes from the warming oceans, but soon, copious amounts of CaCO3 are 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. (Link to image from Proterozoic of Namibia.)

    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 = NADW and ABW.) That also takes millenia.

    the warming phase:

    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:

    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.

    Fine-tuning cycles within cycles:

    Heinrich events: Within the ~100,000 year glacial-interglacial cycle hide smaller, more subtle cycles. Sedimentologists note that during the Quaternary, we see occasional deep ocean sediments full of glacial dropstones, a sign that during an interval of less than 1000 years, vast flotillas of icebergs detached from glaciers and spread out to melt in the northern oceans. Heinrich events are occur on average every 7000 years. Cause is debated, but may be connected with complex chains of events where:

    Indeed, some speculate that the Younger Dryas introduced last week is the most recent Heinrich event.

    Climate change on larger Time-Scales: The Plate-Tectonics has its say

    When we consider climate change in the deeper past, we see larger-scale patterns in which global warming and cooling is effected directly by the moving continents. This works in two major ways:
    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 PETM is a good general model for the carbon emissions going on now. It is, of course, now part of a cycle. Such one off events can be especially scary. E.G. the volcano-global-warming driven Permian-Triassic extinction event.

    The future: Had the industrial revolution not occurred, this would still be a complex topic. What is known is that: