GEOL 204 Dinosaurs, Early Humans, Ancestors & Evolution:
The Fossil Record of Vanished Worlds of the Prehistoric Past
Spring Semester 2014
A Song of Ice and Fire: Ice Ages and Greenhouse Warming
From the whole of these phenomena, and the circumstance of analogous striae having been observed in Sweden by M.
Sefstroem, I conclude, that at a certain epoch the whole of Europe was covered with ice: that this epoch marks the date
of the disappearance of the large mammifera which have been discovered among the ice of northern regions ; that it must
have preceded the elevation of the Alps; but that the recedence of the ice, the polished surfaces, the moraines, and the
dispersion of the boulders to the very summits of high mountains, must all have been posterior to the elevation of the
Alps to their present level.. -- "Remarks on Glaciers" (1839) Louis Agassiz
BIG QUESTION:What do ancient events show about the effect of climate change on the living world?
These are, in fact, real fossils.Coal is the body fossil of vast amounts of plants; petroleum
and natural gas are "chemical" fossils derived from the decay and distillation of organic material
buried and trapped in sedimentary rocks.
In a very real sense, these are "buried sunlight", as the chemical energy in coal, petroleum, and natural gas was originally produced
by photosynthesis in ancient photosynthesizers (plants, algae, phytoplankton) driven by solar energy. This energy is stored in either
relatively pure carbon (in coal) or some form of hydrocarbon (compounds of hydrogen and carbon).
With the rise of the steam engine (at the end of the 1700s) and the internal combustion engine (in the late 1800s), fossil
fuels have become the primary source of energy for modern industrial society:
To release the energy in fossil fuels, the coal, gasoline, or gas must be burned. Burning of these fossil fuels releases carbon dioxide as
a waste product. Unfortunately, carbon dioxide is a potent greenhouse gas, and so the addition of so much CO2 into the atmosphere has
produced a marked change in Earth's climate. But this is not the only reason that climate change happens.
What is Climate? What is Climate Change?
Climate is the average weather for a particular region over some time. That region may be as small as a county or as big as the whole
planet (global climate). Climate therefore includes weather but also variations in weather over time. Some researchers specify the amount
of time represented for the averaging (30 years is a commonly used numbers), but there is no hard and fast rule.
A folksy way of saying this is "Climate is what you expect, weather is what you get."
What CAUSES climate? There are three major factors:
Insolation (incoming sunlight), the energy source for weather, and thus for climate
Atmospheric composition (including particulate matter, clouds, and especially greenhouse gases), which
may trap or reflect heat within the atmosphere depending on the composition
Albedo (Latin for "whiteness"), the reflectivity of a surface (and thus whether the sunlight is just
reflected back or absorbed to be reradiated as infrared radiation (aka "heat")
Every single one of these factors can change over various time scales.
Climate change is, therefore, changes in the statistics of weather for a particular region over some time. Nothing in this definition
requires climate change to be anthropogenic (human made), nor tied into changes in greenhouse gasses, nor be especially about
warming per se. (As we will see, however, recent climate change is all three!)
Of concern to human affairs are three main consequences of climate change, which can perturb human affairs directly or via their effects on
other parts of global systems. These three consequences of climate change are:
Changes in local temperatures (minima, maxima, average)
Changes in precipitation distribution (increases and decreases in frequency and intensity)
Changes in sea level
Here is a graph of the distribution of summer temperatures relative to a 1951-1980 mean of Northern Hemisphere summer months (June-July-August):
Here is another look: the Berkely Earth Surface Temperature (BEST) Project's reconstruction of changing temperatures over the course of the
instrumental record (that is, the period of time when there are thermometers and such to actualy take the temperature of the environment):
But (you might say), I've heard that the temperature has actually been cooling! That is a common myth spread by
climate change denialists (we'll investigate this community later this semester, and in detail in the third semester.)
Below is a figure that demonstrates one of the problem with their arguments:
And here is a video that explains this in even more detail:
How are climate and and climate change documented? After all, we are talking about averaged phenomena (rainfall, temperature, pressure,
etc., etc., etc.) over the surface of the planet and over long periods of time. There are two main forms of observations we make
concerning climates and climate systems:
Instrumental: Thermometers, hygrometers, barometers, satellite remote sensing equipment, and anything else that takes
direct readings of some factor of the climate system at some location at a particular time.
Proxies: Migration patterns, plant flowering times, tree ring widths, isotope ratios in fossils or ice bubbles, borehole temperatures,
the presence or absence of certain species, density of stomata on leaves, and anything else that scales proportionately in some manner to
a climate parameter. Since the instrumental record has only been in operation for a few hundred years in the Europe--and much shorter time
periods for the rest of the planet--our knowledge of past climates (paleoclimates) is derived mostly from proxies. However, depending
on the proxy system used, we might be able to estimate past climate factors hundreds, thousands, millions, or even billions of years ago.
There is more to understanding climates than our current observations and past estimations. If we want to forecast future events
(that is, make predictions), we can rely on two major steps:
Theory: mathematical models representing the interactions and forcings of different components of the climate system over
time, based on experimental observations and theoretical interrelationships of these factors
Computer Models: taking the theories of climate systems, modeling them as gigantic multifactorial spreadsheets, and running
these models for some period of computational time to generate maps or other outputs representing changes in climate system over time.
Paleoclimates Climate (the average weather and its variable for a region over time) is the product of a number of factors, most especially:
The amount of incoming sunlight (mostly a product of the latitude, although also subject to change as the Sun's output varies over time, and the Earth's orbit varies over time)
The distribution of the heat from this incoming sunlight, affected by:
The reflectivity or absorbance of the land, ice, or water surface
The ability of the fluid surface (atmosphere and ocean) to transfer heat
The degree to which the energy reflected or reemitted from the Earth's surface is trapped by the atmosphere (the greenhouse effect)
None of these features are constant over time, and thus the climate of the world has changed: sometimes quite dramatically. Changes
can be produced by:
Plate tectonics: distribution of the landmasses change; sea levels rise and fall; mountain ranges are built up to redirect prevailing winds; etc.
Related to this, the large scale ocean currents: redirecting heat to alternately warm up or cool down the planet
Changes in the abundance and diversity of life, sequestering or releasing greenhouse gases
Changes in the position of Earth's orbit: a primary driver in wet-and-dry cycles (and during Ice Ages, glacial increase vs. glacial decrease)
Short term events, like the sudden emission of lots of methane from the sea floor due to volcanism (or of carbons dioxide from one primate species' discovery that burning fossil fuels provides a lot of energy…)
As a result, climates vary over Earth history. On the biggest scale, we see a change between generally hot (greenhouse) conditions with no substantial ice anywhere and generally cold (icehouse) conditions where ice builds up in the mountains and at the poles. The latter can sometimes be shifted further into ice ages, when large glaciers cover much of the continents.
The discovery that climates of the past were different than today was one of the first major discoveries of paleontology. Scientists found some
strata in temperate regions with tropical animals and plants, and others in the same region with cold-adapted organisms. So they recognized that the environments of the
past were often very different than today.
One specific important discovery was the existence of ice ages: intervals of time when great continental glaciers covered vast
regions. In the 19th Century Louis Agassiz and colleagues began to amass data
to show that many otherwise puzzling features of European and American geology made sense if large sections of those regions were
(geologically-speaking) once under vast ice sheets the way Greenland and Antarctica today. This evidence included:
Glacial erratics: boulders moved vast distances from their sources
Striations: gouges in the bedrock
Drift: collections of sediment ranging from boulders to ultra-fine grained "rock flour"
And various other features
Agassiz and colleagues had noted that these same sediments and structures are common in the mountain glaciers of the Alps and the like, and so their
presence in the plains of Eurasia and North America must have been produced by vast continental glaciers. Agassiz speculated that there was a singular period
of cold--an "Ice Age"--that wiped out the last of the fossil mammals (mammoths, cave bears, Irish elk, etc.)
But subsequent discoveries showed both these ideas were wrong. Firstly, these fossil mammals were inhabitants OF the Ice Age world, not
wiped out by it. Furthermore, it was discovered that whole "Ice Age" represented a series of cold snaps (glacials) and
warm phases (interglacials) that cycled over time:
This ice age cyclicity characterizes the Quaternary Period (2.588 Ma to the present). However, the Quaternary ice ages are
not the only one: earlier periods of Earth History also experienced their own pulses of ice age cycles. But ice ages are only one
subset of different conditions under which the world operates. Additionally, there are:
Icehouse Worlds: times when oceanic currents are directed towards the poles
Some continental glaciers; alpine glaciers present; temperatures may still be warmer than present, but cooler
than a greenhouse world
Greenhouse gas levels are low
Deep water generated at the poles
Greenhouse (or Hothouse) Worlds: times when oceanic currents do not get deflected towards the poles:
No continental glaciation; very few alpine glaciers; temperatures much hotter than present.
Greenhouse gas levels are many times higher than present
If there is equatorial circulation, can get deep water generated at the equator: water gets very
hot --> evaporation increases --> saltier, denser water sinks. Unlike the modern world, this deep water
is warm, and thus contains very little oxygen or carbon dioxide
Study of the Ice Ages and paleoclimate change were critical in inspiring many lines of scientific research:
In 1859 John Tyndall published a study of how radiant heat (what we now call infrared
radiation) travels through different gasses. He discovered that water vapor and carbon dioxide were particularly good at trapping radiant
heat, but are transparent to visible light. This he recognized is the reason that the surface of the Earth is on average much warmer than it should
be based on black-body radiation theory given its distance from the Sun.
Half a century later (1907), Svante August Arrhenius predicted that the release of
carbon dioxide from the burning of fossil fuels (and other industrial processes) was reaching sufficient level to raise global temperatures
against what would have been otherwise. However, he miscalculated the rate: he predicted it would take 3000 years to double pre-industrial
conditions, whereas in reality we will reach that point by the early 22nd Century at the latest.)
Alfered Wegener's discovery of continental drift was in large part inspired by tracing out ancient (Paleozoic) glacially-produced
geological and paleontological phenomena, and inspired by his work on modern glaciers.
Milankovitch cycles: In the 1920s, the Yugoslavian meteorologist Milutin Milankovitch
realized the Earth's movement through space is subject to three kinds of cycles:
Orbital eccentricity: The orbit around the Sun is an ellipse that changes shape (becoming more and less
circular) in a cycle of 100,000 years
Axial inclination: The axis of rotation is tilted. The angle of tilt varies from 21.5 deg. to 24.5 deg. in a cycle of 41,000.
Axial precession: The axis of rotation wobbles around an axis like that of a toy top. So, today the axis points toward Polaris, the north star, but in earlier times, it didn't. One full precessional wobble takes 23,000 years.
The sum of the effects of these cycles gives the general
tendency for glaciers to form. In particular, the amount of sunlight in summertime at the high northern latitudes drives the Quaternary advance
and retreat of glaciers
(And other institutions are doing the same elsewhere.)
Modern Climate Change
Consumption rates of fossil fuels, production rates of concrete, and other industrial processes are used to independently predict the total
amount of anthropogenic (human-generated) greenhouse gasses:
Both theory and data are consistent with the observation of increasing global temperatures since the Industrial Revolution:
Modeling of temperature changes using only natural climate forcings (blue) fail to match observed temperature patterns (black line); in contrast, models
that incorporate anthropogenic forcings (pink) show a very good match to observed:
But modern human experience is vastly too short a time to understand the natural variability in climate systems. So there is a vital need for using
geological and fossil proxies to see how Earth's climate systems operate under conditions different than the last 6000 years (the age of written records).
For example, we find a very different pattern of carbon dioxide when we expand from instrumental readings of the atmosphere and begin to include longer-term
records in glacial ice:
Fossils as Proxies
Fossil data is used to approximate some of the climate conditions of the ancient world. There are different aspects of different fossils that make them useful in this context.
For example, many organisms have very narrow habitat preferences: that is, requirements of temperature, salinity, and the like outside of which they cannot live. So
for species of known habitat requirement are found at a fossil site, we can infer that the environment at the time of deposition was within those parameters. For example, compare the modern distribution of these three species of foraminifera (armored amoebas). Each species is associated with a different temperature range. If we were to find a fossil site in the Chesapeake with Neogloboquadrina pachyderma, we could infer that the waters of the Chesapeake at that time was frigid; if we found Globigerinoides ruber, then it was tropical at that time. This method works
best if these fossils are of species which are still present or their very close relative (so that we can see the life habits of these creatures in the modern world), or are related to aspects of the physiology that are unlikely to have changed over long periods of geologic time.
Additionally, isotope data can be used in various types of fossils. Because we are all ultimately comprised of what we eat (and breathe), changes in the isotopic composition of the atmosphere and oceans due to changes in temperatures and the like will be reflected in the shells, leaves, bones, and teeth of fossils.
Putting It Together
Using various paleoclimate markers, we can estimate the paleoclimatic conditions of any given fossil site. If we have several sites stacked on top of each other, we can reconstruct the changing climate condition of that location. If we reconstruct many different sites at the same slice of time, we can create paleoclimate maps of the ancient world. And by stacking the paleoclimate maps one after another, we can see shifts in global climate pattern.
While the terrestrial record can occasionally be good for this sort of study, the ocean record is much more continuous. There have been a series of national, and international, programs to systematically drill the ocean floor to (among other things) assess changing paleoclimates. These include:
Furthermore, there are many different projects to specifically reconstruct particular moments in Earth's history. Some of these are of interest, since, they are possible models for future climate change, such as the Paleocene-Eocene Thermal Maximum.
A Case Study in Rapid Global Warming: PETM
The Paleocene-Eocene thermal maximum (PETM) was a very short term global warming event
55.8 Ma in which for a geologically-short period of time (<150,000 years) the level of CO2 jumped to
3-4x the previous background level. (This jump is comparable to the upper end of modern anthropogenic greenhouse
gas models, but from a higher starting temperature (a greenhouse rather than ice age world.)
PETM was first discovered in deep sea cores, where the carbonate-rich sediments were suddenly replaced by carbonate-poor clays. It turned out
that the oceans had been stressed out (probably by ocean acidification) so rapidly that many organisms died out. When investigated in good stratigraphic
sections, a similar event was discovered on land.
PETM saw a temperature increase of +4-5K at the tropics, +6-8K
at the poles, and even +4-5 K in the deep sea.
Some consequences of this event in the biosphere:
Extinction of many species of plankton, and of additional sea and land organisms
A jump of ~3x background level of the amount of insect damage seen on leaves, lasting for about 10 kyr
Short term (~a few ky) dwarfing of terrestrial animals, probably as a result of environmental stress
Invasive species: major changes in the habitat ranges of species: in particular, warming polar regions allowed the spread of
subtropical and tropical animals from continent to continent over corridors which were once too
cold for them to enter
The likely cause of PETM was due to a massive release of methane clathrates from the sea floor.
Methane clathrates (sometimes called methane hydrates) are ices on the sea floor, containing tremendous amounts of the greenhouse gas methane (CH4). In fact, while some of the sudden rise in greenhouse gases may be from burning of terrestrial material or oxidizing of some exposed marine sediments, it was almost certainly primarily driven by tremendous amounts of methane. In turn, this is likely due to a sudden
burst of submarine volcanism in the northern Atlantic (opening up the limb of the northern Atlantic between Greenland and Europe). Melting of the sea ices led to methane degassing, which increased atmospheric temperature, which warmed the ocean floor, which released more methane, and so forth. Methane itself is a stronger greenhouse gas than carbon dioxide, and quickly (on the matter of a few years) breaks down into carbon dioxide itself (which lasts for 10s of kyrs).
The PETM is used as a model for modern anthropogenic greenhouse warming because the amount of gases released and the amount of warming called is just about the same. However, there are some differences:
Although rapid as a geologic event, the degassing of PETM was much slower than the release of gases by fossil fuel burning in the modern world
The PETM was an event during a greenhouse world, whereas the anthropogenic event started in an interglacial of the Quaternary Ice Age
The anthropogenic warming is mostly in the form of carbon dioxide, while PETM was mostly methane
Nevertheless, this is one of the closest matches we have in the fossil record, so it is studied in great detail. And some of the biological consequences (such as insect damage on crops, and dwarfing of terrestrial mammals) may be features of the world of the near future.
So, How Does Life Respond to Climate Change
In general, Life's response to climate change depends on how quick it is:
If it is slow enough, then life may actually change in response to the environmental change. That is, it will evolve.
If it is moderate speed (the scale of ecology), then organisms might simply migrate with the shifting climates.
And if it is very rapid, then the "Game of Thrones" rule is in play: you either win (survive) or you die (go extinct). And how that happens is the next lecture.