Global Warming — The Historical Perspective
Global warming is one of the biggest challenges facing mankind. The burning of fossil fuels has significantly increased the CO2 in the atmosphere over the past two hundred years. This has resulted in an increase in global temperature, changes in weather patterns, an increase in wildfires, and the melting of glaciers.
Life plays a major role in controlling the amount of CO2 in the atmosphere. Plants, which includes algae and seaweeds, take CO2 from the atmosphere (or ocean) and convert the carbon into biomass and release oxygen. Animals use oxygen from the atmosphere to convert biomass (food) into energy and release CO2 back into the atmosphere. This is called the carbon cycle. The amount of CO2 in the atmosphere will vary depending on whether plants or animals are dominant.
The reason for the current crisis is that nature discovered two ways to permanently remove CO2 from the atmosphere. Man has recently discovered how to use these as a source of energy, unfortunately releasing huge quantities of CO2 back into the atmosphere in the process. These are of course the fossil fuels: coal, oil, and their associated gases.
The Historical Record
The graph below shows the temperature of the Earth over the past 500 million years.
It is actually five graphs with different scales on the X axis joined together. The first section on the left (orange) covers 500–100 million years. The next section (green) covers 100–10 million. Then we have 5–1 million (black) and 1 million to 20 thousand (blue). Finally, we have the last 20 thousand years which covers the era of human civilization since the last ice age.
As you can see there has been a fair bit of variation in temperature over these times. The Y-axis shows the average global temperature. The zero point is the average temperature between 1960–1990, which is about 14C (67F). Over the time period shown the average temperature has varied between 28C (tropical at the poles) and 10C (ice age). Scientists haven’t measured the actual temperature millions of years ago, but use geological evidence to estimate the temperature. For more information how the temperature is estimated see the article at… https://en.wikipedia.org/wiki/Geologic_temperature_record.
The amount of CO2 in the atmosphere is one factor determining the temperature, but there are others. The continents drift and their positions have a huge impact on the climate. The size of the ice caps is also a big factor. Ocean currents also play a large role. Vulcanism can have a big effect as can external events such as meteor impacts. The distance from the Earth to the Sun is also a factor.
Over the past 500 million years the activity of life on Earth removed considerable CO2 from the atmosphere and stored it in limestone, coal, and oil. CO2 levels in the atmosphere have gone from 5000 parts per million (ppm) to 250ppm. Over this period, the average temperature steadily declined and it is currently substantially colder than it was 500 million years ago.
The graph starts on the left (orange) with an era called the Cambrian which saw a huge expansion of animal and plant life. Prior to that life consisted mostly of bacteria, protozoa, algae and some primitive multi-celled creatures like sponges and jellyfish and everything lived in the sea. The CO2 level in the Cambrian was about 5000 parts per million, about 15 times what it is today. And you can see that it was pretty hot with average global temperatures around 28C which is well above the 14C we have today.
The period from 350–300 is called the Carboniferous period. During this period huge primitive forests dominated the earth and they converted a lot of CO2 to biomass. The bacteria and fungi of the day were unable to digest the lignin that made up the bark of these trees and convert it back to CO2, so the bark simply accumulated on the forest floor, eventually got buried, compressed, fossilized and turned into coal (carbon). The carbon was not returned to the atmosphere as CO2, the CO2 level dropped to about 500 ppm, the greenhouse effect was greatly reduced, and there was a huge ice age lasting 50 million years with global average temperature dropping from 28C to 10C. That’s the big dip in the middle of the orange section.
After that ice age the continents joined to form Pangea and things warmed considerably with one of the largest extinction events in Earth’s history, the Permian/Triassic extinction. This was caused by the Siberian Traps volcanic eruptions that sent the CO2 levels soaring back to 2500 ppm and temperatures rose by 8C. “It is the Earth’s most severe extinction event, with the extinction of 57% of biological families, 83% of genera, 81% of marine species and 70% of terrestrial vertebrate species. (Wikipedia)”
Like many extinctions, this opened up an opportunity for new species and after it, the dinosaurs flourished during the Triassic and Jurassic periods. The latter part of the orange section (100–66) is known as the Cretaceous period. The average global temperatures once more hit new highs. Then disaster struck. A meteor or comet struck the Earth and killed all the dinosaurs. This moves us to the second part of the graph (green) and the Paleocene and Eocene periods.
During this time the poles were free of ice caps, and palm trees and crocodiles lived above the Arctic Circle and much of the continental North America, Europe and Asia had a sub-tropical environment. The Paleocene-Eocene Thermal Maximum (PETM) is the peak at about 50 million years ago with the average global temperature near 28C again. Some paleo-geologists have suggested that this was caused by abrupt releases of methane from frozen methane ices from the bottom of the ocean. Methane is an even stronger greenhouse gas that CO2.
Then a weird thing happened. CO2 levels suddenly decreased from 2500 ppm to 500 ppm, setting the stage for a long period of cooling that has lasted until recent times. Scientists speculate…
“The Azolla event is a scenario hypothesized to have occurred in the middle Eocene epoch, around 49 million years ago, when blooms of the freshwater fern Azolla are thought to have happened in the Arctic Ocean. As they sank to the stagnant sea floor, they were incorporated into the sediment; the resulting draw-down of carbon dioxide has been speculated to have helped transform the planet from a “greenhouse Earth” state, hot enough for turtles and palm trees to prosper at the poles, to the current icehouse Earth known as the Late Cenozoic Ice Age.” — Wikipedia.
So, what happened to the Azolla? It turned into oil. Oil is the third way that CO2 gets permanently extracted from the atmosphere. Any time large quantities of biomass are deposited in an oxygen free environment where it cannot be decomposed, like the bottom of the ocean, it is turned into oil.
In the last sections of the graph, we see that after a huge drop at the end of the Eocene, global temperature declined steadily through the Miocene (30M-5M), Pliocene (5M-2.5M) and Pleistocene (2.5M-20K). A characteristic of the Earth’s climate throughout these periods can be seen in detail in the Pleistocene section of the graph.
The last 2 million years have been characterized by cycles of ice ages gradually deepening with time. The cycles are known as Milankovitch cycles and vary on 21, 41, and 100 thousand-year periods that are related to the orbit of the Earth and the energy output of the Sun. The graph above shows the temperatures and ice volumes during the last half of the Pleistocene that coincides with the development of hominids and Homo Sapiens. During the cold spells, parts of Europe, Asia and America were covered with up to a kilometer of ice. Then things would warm up and the ice caps would retreat. The last ice age ended about 20,000 years ago.
The last part of the graph covers the last 20,000 years, the period of human civilization and the temperature has been pretty stable for the past 10,000 years.
The graph below shows a close up of the recent temperatures.
There was a “little” ice age in 1600. Perhaps this would have been the start of the new ice age if it wasn’t for global warming due to carbon emissions at the onset of the industrial revolution round about 1800. Since then, there has been an increase in temperature of 1C. This is pretty consistent with other data and is what is known as global warming.
CO2 Levels
So what were CO2 levels doing during all this.
The chart shows four different estimates: Royer (2004), GEOCARB (2001), COPSE (2004), and Rothman (2001). The error bars are pretty big, but the general consensus is that CO2 levels were at about 5000–6000ppm in the Cambrian (Cm). With the advent of the Carboniferous (C) it plummeted to about 500ppm as coal deposits were laid down and we entered a 50-million-year ice age in the Permian (P). This ended with the volcanic eruptions that caused the Permian/Triassic extinction by increasing the CO2 levels to 2500ppm and raising the average temperature back to the 28C levels of the Cambrian era. Life recovered and dinosaurs ruled the Earth. At the end of the Cretaceous (K), CO2 levels once more fell, this time to between 180–280ppm and we enter Late Cenozoic age with its cyclic ice ages. However, since 1900 it has gone from 280ppm to 410ppm with a matching average temperature increase of 1C.
This is undoubtedly due to the burning of fossil fuels. It seems pretty clear that human activity has increased the CO2 level by half and the average temperature by 1C. However, CO2 levels are still about ten times lower that during most of the Earth’s history.
Conclusion
We can clearly see that in the past both CO2 level and the average temperature were significantly higher. When life conspired to remove carbon from the atmosphere, as in the Carboniferous, temperatures plummeted. When volcanos added carbon into the atmosphere, temperatures soared and the sea levels rose.
Over the past 500 million years the activity of living creatures reduced the CO2 in the atmosphere from 5000ppm to about 200ppm with the creation of limestone, coal, and oil. By exploiting fossil fuels for energy, human activity is pumping CO2 into the atmosphere which has caused and will continue to cause an increase in the mean global temperature, which will in turn change the climate. If we assume a worst-case scenario and put it all back, the average global temperature will increase to the historic levels of the Cretaceous period (28C) and sea-levels will rise by up to 70 meters, flooding many of our major cities. However, this will hopefully take thousands of years to happen which will give us some time to adapt, particularly if we take some action to slow down the process. There will certainly be mass migration, wars, famines, and disease, but I think humanity will survive. In the next essay we will investigate this in more detail.
