The Reality of Climate Change Dawns


A 1565 winter landscape by Pieter Brueghel from Mirades
Climate change as a reality of nature: 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? In fact, 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 Pieter Brueghel or described in Hans Brinker, or the Silver Skates. It is, therefore, 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.

Physical Evidence for Glaciers

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 soil and ice:

  • Patterned ground - Polygons formed by ice wedges. extending into the soil. (Note: patterned ground occurs in polar regions of Mars, also.)
  • Pingoes - Bodies of ice that rise up through the soil in response to burial pressure.

    Proxy Data for ancient temperature

    The problem with the glacial landforms discussed above is that they are very blunt instruments:

    As a result, ancient glacial landforms tell us only that glaciers were present. We would like to know more about the timing of ancient climate change, and with greater precision. To this end, we use proxy data - information in the rock record that is connected to temperature and climate, including: We can't discuss all of these now, except for one primary example:

    The Oxygen isotope record: During the late 20th century, a new technique allowed us to refine this sequence. The method:


    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:

    Ice-ages: Longer interval characterized by the prolonged alternation of glacials and interglacials. Earth has been in an ice-age for the last 2.5 million years, roughly.

    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 less abrupt.

    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:

    Note: These parameters effect different latitudes differently at any given time.

    Solar forcing: The sum of the effects of these cycles at a given latitude gives the general tendency for glaciers to form. 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.

    Formation of continental glaciers:

    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.

    World Geography, Greenhouses, and Ice-houses:

    Ice-house world conditions prevail: Over deep time, Earth's pattern of ocean circulation has alternated between:

    The slight differences in summer weather caused by Milankovitch cycles doesn't make a lick of difference in greenhouse world conditions in which no winter snow has a prayer of surviving the summer. Thus, ocean circulation must bring water to the poles before we can think about having glaciations. Moreover....

    Large land masses (snow-catchers) must exist in high latitudes Consider past ice ages:

    The process and its 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 glacials form a positive feedback loop. In the following schematics, keep track of these variables:

    A Glacial in a Nutshell: We keep it simple, without considering the effects of the biosphere.

  • 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 overlap to 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, therefore, 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 the 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, 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.), and the onset of the interglacial as a "cap carbonate" deposit.

    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:

    The recoveries at the end of glacial intervals tend to happen rapidly. 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 from oceans and biosphere amplifies warming through solar forcing.


    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 occur on average every 7000 years. Cause is debated, but may be connected with complex chains of events where:

    A well documented example is the Younger Dryas, an interval of north Atlantic cooling provoked by the sudden infusion of North American meltwater into the Atlantic.


    Rancho La Brea, Los Angeles, CA, during the last glacial from The Page Museum
    A final note: The climate changes associated with glacials and interglacials occur on an ecological time-scale, allowing critters to respond by changing their geographic ranges. Something about the beginning of the Holocene, however, was different and resulted in an (almost) world-wide mass extinction of large mammals. We will discuss this next week.