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


Fossil Fuels
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:

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:

Here is a graph of the distribution of summer temperatures relative to a 1951-1980 mean of Northern Hemisphere summer months (June-July-August):

(From the Goddard Institute of Space Studies, NASA)

Or, if you prefer, the same data in video format:

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:

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:


Paleoclimates
Climate (the average weather and its variable for a region over time) is the product of a number of factors, most especially:

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:

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.

For the geologically recent past, the record of change is much better understood, and we see that climates can shift from one condition to another on a very short period of time. The degree of change of even the geologically recent past can greatly exceed the conditions under which human civilization arose and grew.

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:

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:

Study of the Ice Ages and paleoclimate change were critical in inspiring many lines of scientific research:


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.

In particular, some species make excellent paleothermometers. In some cases they can mark only a particular temperature. The foraminferan Neogloboquadrina pachyderma builds shells with a coil to the right at temperatures above 8°C, but to the left below 8°C. Other paleothermometers can give a range of temperatures. It has been found that the percent of leaves in an environment with smooth (also called "entire") margins is very tightly correlated with the mean annual temperature. So if you find enough leaves at a site to compare this ratio statistically, one can estimate the ancient temperature.

Leaves provide an estimate of an additional paleoclimate parameter: the amount of carbon dioxide. Leaves take in carbon dioxide and "exhale" oxygen through tiny openings called stomata. It has been discovered in laboratory studies that the density of stomata decreases with increasing levels of CO2 and increases with decreasing CO2. Furthermore, this can be used to infer temperature as well.

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:

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:

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:
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Last modified: 12 February 2014