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. 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. 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 sediment in response to burial pressure.
    Ice ages:

    So what has the global distribution and age of the glacial and periglacial features taught us?

    The Cretaceous and early Paleogene was a greenhouse world. From the Late Plaeogene onward, we have been in an ice house conditions occurred during that interval. During last 2 million years (the Quaternary Period), the situation has become extreme, an ice age with major continental glaciations alternating with interglacials. The interval from 2 mya to 10,000 years ago is called the Pleistocene Epoch. The last interglacial of the Quaternary Period (the one we are in) is the Holocene Epoch.

    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:


    The Oxygen isotope record: Holtz has discussed how we can reconstruct the ocean's isotopic history by looking at the ratio of oxygen isotopes present in foraminiferan shells deposited at different times. That ratio, in turn, tells us how much water was locked up as continental ice.
  • The result:

    Apparently there were closer to thirty distinct glacials and interglacials. On a longer time scale, we see that these cycles have gradually increased in severity.
    "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.

    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.
    Factors Influencing 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. 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.

    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:

    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.

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


  • 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).

  • The land deformed isostatically upon loading by the ice sheets. In some locations, the land is still responding isostatically to the removal of the ice. E.g. Scandinavia (1m / century), Ohio valley.

  • As a consequence of isostatic deformation, 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):
  • Last remnant of North American Ice sheets melted roughly 10,000 years ago.

    Quaternary biota of North America

    During the last glacial, plant communities were both shifted southward from their current ranges, and altered by different combinations of rainfall and temperature. Compare recent biomes (right) with those of the glacial max. The big losers in the glacial were temperate forests, which were replaced by boreal (northern) forests and evergreen taiga, and the deserts of the southwest, which were occupied by grasslands and scrub. Naturally, animal communities were also different, with significant grasslands and boreal/taiga forest communities.
    At the end of the last ice age lots of North American animals went extinct - especially large plant-eating mammals and the predators that fed on them, including:

    these large mammals were scavenged by large vultures including:


    Was this extinction due to:

    An irrationally emotional debate rages. (Psst! Merck bets on the third option in most cases.)


    No one has a freaking clue what the equilibrium ecology of North American forests was like back when they were crawling with elephants. In Africa and Asia these creatures radically transform forests into open savannahs. This probably happened in North America as well. Some people speak of "old-growth" forests as if they were the pristine primeval state of this land, but they, too, are an invention of the Holocene.


    Note: Absent from North America were familiar animals such as elk, brown bears, and timber wolves, which invaded the continent at the same time as humans.

    Ice-Age details

    The Younger Dryas: By fifteen thousand years ago, climate was warming rapidly and glacial ice melting quickly. Then, thirteen thousand years ago, glacial conditions returned to the lands around the North Atlantic, as evidenced by the presence of pollen of Dryas, an arctic weed. WTF? Apparently, huge North American meltwater lakes had penetrated an ice barrier separating them from the ocean, reducing ocean salinity to where the formation of Atlantic deep water was curtailed.

    The Holocene optimum: In contrast, despite local climate fluctuations, the last ten thousand years have been a time of unprecedented climatic stability. Coincidentally, this is the interval in which humans developed a sophisticated material civilization.


    Dansgaard-Oeschger events: In contrast, cliamte during glacial intervals was far from stable. Glacials are punctuated by Dansgaard-Oeschger events, abrupt warming to almost interglacial levels followed by slow cooling. These occur roughly every 1500 years. Some climatologists regard them as indications of a recurring cyclicity - the Bond cycle, although their cause is not absolutely clear. Some D-O events are correlated with Heinrich events in which warming is correlated with the advance of icebergs across the northern oceans. Why would warming lead to the proliferation of icebergs? Possibly regional warming in the southern ocean prompted melting of antarctic ice that raised sea level, floating northern glacial ice off of its bedrock foundation. Icebergs raft large pieces of sediment into the deep oceans.


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