Deep Time


This is a “minitext” that was originally written by Dr. Richard Alley for Geosciences 320, Geology of Climate Change, at Penn State. It provides a short history of the world’s climate over the last 4.6 billion years. It assumes a little more background knowledge than is expected in the rest of the class but may be useful.

“Deep time” is sometimes difficult to understand. The planet is 4.6 billion years old. If you substitute distance for time, and let the 100-yard length of a US football field (just under 100 m, and roughly the length of a full-sized soccer pitch) be all of the Earth’s history, start at one goal line and drive toward the other, then:

  • Trilobites and other shelly critters show up after you have gone 88 yards, just 12 yards from scoring;
  • The end-Permian extinction is reached after 95 yards, just 5 yards from the goal;
  • The dinosaurs died out less than 1.5 yards from the goal;
  • The warming from the coldest part of the last ice age started about 0.02 inches from the goal;
  • Humans have been writing down our history for something like 0.005 inches, the thickness of one or two sheets of paper;
  • The average college student, approximately 20 years old, has been alive for roughly 1/300th of the thickness of a sheet of paper.

Studying Earth’s history, and the physics, chemistry, biology, geology, and other “ics” and “istries” and “ologies”, provides many insights to the planet. A few of these include:

Earth shows long-term stability. The physics of radiation provide a powerful protector for the planet. Geologists generally can tell with high confidence whether sediments were deposited in liquid water. Such water-laid sediments dominate the geologic record. Furthermore, there are indications of life through most of geologic time. Together, liquid water and life show that the climate of the planet must have one or more stabilizing feedbacks (as noted below, without such feedbacks, bad things would have happened). One of these stabilizing feedbacks is easy-simple radiative balance. Because the radiation emitted by a black body is proportional to the fourth power of the absolute temperature, a 1% rise in temperature of the planet causes a 4% rise in the energy emitted to space by the planet (or a 1% drop in temperature causes a 4% drop in emitted energy — the Earth is not really a black-body, but close enough that you can work with that for now). This means that the hotter something is, the more energy you must supply to increase its temperature another degree. That is a powerful stabilizer.

But black-body physics does not provide enough stabilization alone — the “faint young sun” paradox shows the importance of the greenhouse effect. Solar physicists are confident that the aging of the sun, as it burns hydrogen to helium, has caused the sun’s energy output to increase smoothly over time, starting from about 70% of the modern solar output at the time when the Earth formed. (Hydrogen fuses to helium, packing almost as much mass into a much smaller space in the center of the sun. This increases the sun’s gravitational pull on its outer layers, pulling the surrounding hydrogen more tightly towards the sun’s center. The fusion that powers the sun and converts hydrogen to helium requires that the hydrogen be packed tightly together, so the rising gravity makes fusion run faster, producing more energy.)

This result from solar physics yields the “faint young sun” paradox — assuming modern albedo and greenhouse effect, most of the Earth’s surface water should have been frozen for most of its history, but the available evidence shows that this did not happen. With an active hydrological cycle (as shown by the sedimentary record), hence clouds, there is no known way to lower the albedo enough to solve this problem, so the early Earth must have had a stronger greenhouse effect. (To offset solar output only 70% as large as today with the same greenhouse effect would require a perfectly black planet, not physically possible.) (The distance of the Earth from the Sun has changed a tiny bit over time, but not enough to really matter; collision with a Mars-sized body, such as the one believed to have blasted out material to form the moon, might have moved the planet a couple of percent of its distance from the sun; the meteorite that killed the dinosaurs would have moved the planet less than an inch.)

Rock-weathering stabilizes, too. Many things may have contributed to the stronger early greenhouse. A wide range of evidence indicates that the early atmosphere lacked abundant oxygen. (For example, pieces of minerals that break down rapidly in the presence of oxygen are found, not broken down, in old sedimentary deposits. The huge banded iron formations that we mine in places such as Minnesota have precipitated from ocean water long ago, but getting a whole lot of iron to the ocean in a dissolved form rather than as chunks of rust requires that the water carrying the iron lacks oxygen or rust would have formed. Also, “red beds” — rusty soils and other rusty sedimentary layers deposited above sea level — have formed commonly in “recent” geologic history but are very rare or entirely absent from the early Earth. And, there are still other indications that the early atmosphere lacked abundant free oxygen.) Carbon dioxide is a greenhouse gas, but per molecule and at concentrations vaguely similar to modern, methane is a more potent greenhouse gas than is carbon dioxide. (Raise the methane concentration a lot, and adding still more methane causes the new molecules to partly duplicate the job of existing molecules, just as for CO2 , so the importance per molecule of methane drops as the abundance rises, just as for CO2 and other greenhouse gases.) In the modern atmosphere, oxygen combines with methane over a decade or so to form carbon dioxide; for the early Earth, there may have been more methane and other reduced greenhouse gases because the oxygen wasn’t there to break them down.

The best-understood stabilizer, and the one most likely to have been important, was discovered by Penn State’s Jim Kasting and coworkers. This is the silicate weathering feedback. Volcanoes release carbon dioxide and volcanic rock, which is mostly silicate with a lot of calcium. Chemical processes (many involving biology, and generally lumped together as “rock weathering”) then recombine the carbon dioxide and rock to make dissolved materials that are washed to the ocean, turned into shell by living things (or deposited inorganically if there are no living things around to do the job, with inorganic deposition requiring somewhat higher concentrations in the water than organic deposition), deposited, then (eventually, over time scales of order 100 million years) taken down subduction zones or squeezed in obduction zones, where heating produces carbon dioxide and volcanic rock. (Metamorphic rock also may be formed, releasing carbon dioxide. For this broad-brush approach, metamorphic and volcanic rock are interchangeable.)

The formula is often oversimplified to:

CaSiO3 + CO2 → CaCO3 + SiO2

which shows the volcanic rock and carbon dioxide being changed to shells (calcium carbonate is found in coral reefs, many foraminifera, clams and snails and others; silicon dioxide or silica is found especially in diatom and radiolarian shells and sponge spicules).

The transformation of these shells back to rock and carbon dioxide (draw the arrow the other way) doesn’t much care about the temperature at the surface of the Earth, but the recombination of volcanic rock and carbon dioxide goes faster in a warmer climate (almost all chemistry goes faster when it is warmer, and in this case the chemical kinetics are accelerated further by there being more rainfall on a warmer world, because the reactions typically happen in water). Thus, if the temperature at the Earth’s surface increases, chemistry happens faster, removing carbon dioxide from the atmosphere and lowering the temperature back toward the original value. If the temperature falls, the removal of carbon dioxide from the air slows, the release of carbon dioxide from volcanoes continues unaffected, so the concentration of carbon dioxide in the air rises, increasing the greenhouse effect, and the planet warms back toward the original value. The time scale for this to work is something like 0.5 million years (more or less the residence time of carbon in the combined atmosphere-ocean system). This time scale may have changed over geologic history, but probably by no more than a factor of a few, not orders of magnitude.

Notice that the stabilizer of black-body radiation is almost instantaneous. The stabilizer of rock weathering takes hundreds of thousands of years to matter much. In between, we will see that amplifiers are more important.