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Phanerozoic means “visible life.” When it was derived in 1930 by Halcott Chadwick, scientists thought that animal life originated in the Cambrian Period. The climate has been stable during the 541 million years of the Phanerozoic Eon. Average temperature remained within a +/- 100 C hospitable range for life on Earth. Several factors influence temperature within this range: greenhouse gases, solar and orbital dynamics, and volcanic emissions. Although the climate has been hospitable for life, periodic mass extinctions have been caused by asteroid impacts and large releases of volcanic magma. These events can change global atmospheric and water chemistry, or cover the sky with soot, eliminating photosynthesis.
Earth has four climatic zones: tropical (0-23), subtropical (23-40), temperate (40 - 60), and cold (60 - 90). Subtropical includes wet and humid regions. The following video by Christopher Scotese shows climate variations during the Phanerozoic. Climatic regions include blue - ice, brown - cool, light green - warm, yellow -arid, and dark green - hot and wet (tropical). This next video shows changes in vegetation and topography during the Phanerozoic. Notice that polar ice was minimal until the last few million years (Ice Ages).
During the Cambrian Period at the beginning of the Phanerozoic Eon (Figure 7-20), the temperature was 10 to 15 C hotter than the present temperature. After many temperature fluctuations during the Phanerozoic Eon, the temperature of the last few million years is the coldest temperature of the entire eon except for a brief span of the Permian Period that was equally cold. Although the Ice Age temperate of the last few million years has been colder than normal, this has been the habitable temperature during which humans expanded around the globe and established agriculture and human civilization. The temperature of the last 10,000 years, which is slightly warmer than the coldest Ice Age periods, has been particularly habitable. It is worrisome that the projected temperature increase due to global warming will take us out of that range and into unknown territory for humans (lower graph, right side, Figure 7-20) .
Figure 7-20. Phanerozoic average surface temperature during greater 541 million years of Phanerozoic Eon (top) and with a focus on the last 66 million years (bottom) of Cenozoic Era. Credit: DeWikiMan. Used here per CC BY-SA 4.0.
The Cenozoic Eon, which is the period of mammals, began 66 Ma (lower graph, Figure 7-20). The major global warming event due to greenhouse gases in the Cenozoic Eon was the Paleocene-Eocene Thermal Maximum (PETM), which took place 56 million years ago. it is the spike at the Paleocene (Pa)-Eocene (Eo) boundary in Figure 7-20. This event was particularly hard on the ocean food web because atmospheric carbon dioxide acidified the ocean and did not allow invertebrates to form calcium carbonate skeletons. Thus, it caused mass extinction in the ocean. On the other hand, many modern orders of mammals evolved during the high temperatures in the early Eocene so the high temperature was beneficial for land mammal evolution. In particular, a world covered in tropical and subtropical trees was beneficial for the primate evolution. https://youtu.be/ldLBoErAhz4
Ocean sediments indicate that the PETM is correlated with an extremely high CO2 spike in the atmosphere. There are several proposed causes of the PETM: wildfires, volcanic activity, or release of methane hydrates in melting permafrost. Sediments indicate that 1.1 billion metric tons of carbon per year were released into the atmosphere for 4,000 years. Humans in the modern world are currently releasing 9 billion metric tons of carbon per year into the atmosphere. The climate is warming faster today than during the PETM.
Temperatures dropped for tens of millions of years since the PETM (Figure 7-20) until the world entered the Ice Ages of the last 2 million years. The temperature began to rise 20,000 years ago. Since the temperature finished rising at the end of the last ice age, approximately 13,000 years ago, the temperature has remained relatively stable. Thus, the entire period of civilization, which began 6,000 years ago, has experienced relatively stable temperatures. However, in the last 100 years, there has been an increase of almost 1 0C (Figure 7‑21), mostly due to anthropogenic global warming. Temperatures will increase several more degrees in the next century unless countries reduce output of greenhouse gases. The recent increase in temperature correlates with the rise in greenhouse gas concentration in the atmosphere during the industrial age (Figure 7‑23). Scientists are able to split the natural and manmade causes of climate change in climae models. If not for human released carbon dioxide and methane, the temperature would have remained constant for the last 50 years. Instead, it rose by almost 1 0C (Figure 7-22).
Figure 7-21. Global temperatures in last 2,000 years. Credit: Fbrazil. Used here per CC BY-SA 4.0
Figure 7-22. Natural and man-made global warming effects on climate. Credit: Fbrazil. Used here per CC BY-SA 4.0
Figure 7‑23. Changes in greenhouse gases during last millenium. Credit: NOAA.
While climate modeling is complex, it does not require a climate model to see that there is a clear connection between temperature rise (Figure 7-21) and greenhouse gas concentrations in the atmosphere (Figure 7-23). Climate models predict major changes in climates due to global warming.
The major cause of climate change in the last 2 million years has been solar cycles and orbital cycles (see video below); however, global warming has caused Earth's temperature to break out of the normal climate cycles.
If humans keep emitting greenhouse gases at the current rate, then temperature might increase by 5 0C by the end of the century. There also might be feedback effects such as release of methane from tundra or the deep ocean due to rise in temperature of the soil and ocean, respectively. A typical 106 0F summer day in Tucson will become an average 116 0F day (1 0C ~ 2 0F), and temperature will range up to 125 0F.
During the ice ages, the wooly rhino was sensitive to habitat change due to global warming. https://youtu.be/gUdtcx-6OBE Modern humans are not as susceptible as animals to climate change. We are more adaptable to change; however, there will be catastrophic loss of life due to climate change, especially in cultures that have lower capability to adapt. Even in America, there will be a degradation of life due to climate change. Examples of detrimental effects of global warming are higher temperatures in summer, wildfires and associated smoke, drought and water stress, sea level rise, and stronger hurricanes and storms.
Figure 7‑24. Ice coverage during last ice age and at present. Credit: NOAA.
Shifts in the angle of obliquity, orbital distance from the sun, cause climate change. Earth’s angle of obliquity (Figure 7‑25) ranges from 22.10 to 24.50 and causes changes in temperatures at the poles and the average temperature of Earth. Changes in eccentricity occur over hundreds of thousands of years. Eccentricity changes cause changes in the intensity of the seasons. Because of these factors, solar radiation intensity on different parts of the globe has varied by approximately 20% over the last 400,000 years. Thus, ice ages have come and gone, as seen in the temperature changes in the last million years in Figure 7-21.
When global temperature drops to the low end of the 100 C range, the polar ice sheet descends over the northern half of North America. (Figure 7-24). There is almost no ice in the arctic in the modern day. Between the end of the last glacial maximum 20,000 years ago and approximately 8,000 years ago, sea level rose by 120 m as the ice melted in the glaciers.
Figure 7‑25. Earth’s axial tilt and temperature at the poles. Credit: NOAA
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