Our Slow-Changing Planet

Earth’s climate is greatly affected by its relative position to the Sun, which is the result of complex gravitational interactions with other celestial bodies. There are many variables, but three cycles dominate:

1. Eccentricity. The Earth’s orbit around the Sun is not always a circle. It cycles between nearly circular and mildly elliptical with a period of around 100,000 years.
2. Obliquity. The axial tilt of the Earth changes slightly with a periods of around 41,000 years.
3. Precession. The direction Earth’s axis points to in the sky. It changes with a period of around 25,700 years. Over time, North Star in the night sky will not be due north.

In words even a congressman can understand, the Earth wobbles.

These cycles are called the Milankovitch cycles. Throughout these cycles, the distance between Earth and the Sun, as well as the angle between Earth’s axis and the incident sunlight, vary. This means that not only does the total energy Earth receives from the Sun change periodically, but the allocation of that energy to different parts of Earth also fluctuates. The geophysicist Milankovitch hypothesized that these cycles combine to cause climate changes on Earth. More sunlight leads to higher temperatures. Uneven sunlight distribution leads to different temperatures in different parts of Earth. Temperature differences lead to stronger winds and ocean currents, which redistribute heat. The effects would be particularly strong at the 65th parallel north due to the large amount of land there. Land masses change temperature more quickly than the ocean for two reasons: One, natural convection occurs in seawater, so warm water and cold water mix quickly, dampening the speed of temperature change. Two, land mass has a lower specific heat capacity, so the same amount of added heat leads to a larger temperature change. We have studied the concepts of convection and heat capacity here.

In addition to interactions with other celestial bodies, Earth itself is constantly changing. The outer crust of Earth is composed of several large tectonic plates, which have been slowly moving for billions of years. Mount Everest was famously formed by the collision of two plates, whereas the East African Rift is the result of plates splitting. The Great Rift Valley is of particular interest to us because many fossils of early humans were discovered there, including the famous Lucy. The Rift consists of a series of continuous geographic trenches that stretch thousands of miles, running north to south, from the Red Sea to Mozambique. The parting of the plates has left many basins up and down the Great Rift Valley. The changing landscape exacerbates the climate cycle at the local level. Fossil records from deposits at different sites in the Great Rift Valley show a consistent pattern of lakes coming and going. The cycles range from tens of thousands of years to hundreds of thousands of years, depending on the size of the lake. Fossil evidence shows that as the lakes dried up, the types and numbers of plants and animals in the area changed.

The problem with relying on fossil evidence alone is that many things are not preserved as fossils. If we draw conclusions from only fossils, it would be like the man who looks for his lost keys only under the lamp post because that’s where the light is. We need corroborating evidence from more prevalent material.

There is nothing more prevalent than soil. The different layers of soil were deposited at different times. The carbon in those layers comes from the organic compounds in the plants that were widespread at the time. Different plants have different metabolic pathways for assimilating carbon during photosynthesis. Trees, shrubs, and cool-season grasses use the C3 carbon fixation process, while the C4 carbon fixation process is more efficient in places with higher temperatures, more light, and less water. All carbon atoms have 6 protons, but some have 6 neutrons (C-12), while others have 7 neutrons (C-13). The C3 process selects against C-13 more than the C4 process, so plants with different approaches to carbon fixation have different carbon isotope ratios. The changes in available plants and animals had implications for early human diets, a topic we will turn to later.

The soil samples from deep underground in Africa show different carbon isotope ratios at different layers, which is strong evidence that the climate, and the plants living in it, have fluctuated over time. For instance, we can tell the Sahara Desert was covered in lush vegetation as recently as 5,000 years ago.

Another isotope gave us more direct evidence of temperature change. Most oxygen atoms have eight protons and eight neutrons (O-16), but some of them have two extra neutrons (O-18). The water molecules with heavier oxygen atoms can’t stay aloft in clouds if it is cold. When global temperatures drop, cold fronts march towards the equator, and more and more heavy vapor falls into the ocean at lower latitudes. This leaves the poles with a higher concentration of O-16. By drilling the ice sheet of Greenland and comparing the ratios of oxygen isotopes, scientists can infer the temperatures of the past.

According to the Greenland ice cores, the end of the Paleolithic coincides with the end of the last ice age, which ended rather abruptly around 11,600 BP. Averaged annual temperatures increased by around 8 °C over 40 years. Anyone alive at the time would certainly have realized the Earth was warming. It wasn’t a one-off event either. The rapid climate changes happened so often that they even have a name: Dansgaard-Oeschger events. If Homo sapiens had tried to evolve to the environment they knew, they had just had the rug pulled out from under them.

Meanwhile, at lower latitudes, the heavier oxygen atoms that fell into the ocean were absorbed by marine plants and animals. Like the plants with different carbon isotopes, these organisms were buried in the sediments at the bottom of the ocean. Higher temperatures led to more heavy oxygen in the ocean sediments, and more light oxygen in the polar ice. The reverse was true for lower temperatures.

The sediments on the ocean floor not only tell us the temperature but also the amount of rainfall. When there is less rain, more dust is blown into the ocean. Because continental dust is easily magnetized, the intensity and length of dry seasons can be estimated from the magnetic properties and depth of each sediment layer. It’s not just a conjecture. It has been proved by evidence collected by the research vessel JOIDES Resolution (Joint Oceanographic Institutions for Deep Earth Sampling). The ship actually sailed all over the world to drill core samples from deep under the ocean floor, with advanced equipment that is an engineering marvel. The National Science Foundation and a motley crew of international governments have teamed up to bankroll these expeditions. That, depending on your perspective, can either be construed as affirming or challenging the efficacy of government resource allocation.

All evidence—oxygen isotope in the ice from Greenland, carbon isotopes in the soil from Africa, and magnetized dust from sediments on the ocean floor—points to repeated and dramatic climate changes in Earth’s history.