|The Grand View: 4 Billion Years Of Climate Change|
Doug L. Hoffman 08/27/2009
Two of the terms bandied about by global warming alarmists are “unprecedented” and “irreversible.” It is troubling that scientists, who should know better, persist in using these terms even though the history of our planet clearly shows that neither term is accurate. Proof of this inaccuracy is obvious if we look back over the history of Earth—the Phanerozoic Eon in particular—taking the “Grand View” of historical climate change.
According to Meg Urry, the head of the physics department at Yale University: “Scientists observe nature, then develop theories that describe their observations. Science is driven by nature itself, and nature gives us no choice. It is what it is.” While some of the dates presented here may change and scientists continue to argue some of the fine points, here is what science thinks it knows about life, the Universe and everything.
Around 13.7 billion years ago the Universe came into existence. Not long afterward the Milky Way galaxy was formed. Stars formed, transmuted elements in nuclear fire and ended their lives in supernovae explosions. This cycle was repeated many times for many different stars.
Then, 4.6 billion years ago our Sun was born out of the ashes of older dead stars. Along with the Sun a large brood of planets was also formed, including the one we call Earth. A million years after the birth of our sun, the violent explosion of a nearby supernova nearly ended life on Earth before it began. Over the next four and a half billion years, forces of nature shaped our planet and the life it harbored.
Buffeted by supernovae, barely surviving the traumatic birth of the Moon, bombarded by asteroids, the resilient Earth endured. And despite planet-freezing ice ages, devastating mass extinctions and ever changing climate life not only survived, it thrived. Even though meteors continued to rain down on the young planet there is evidence that as long as 4.2 billion years ago liquid water, the prerequisite for life as we know it, was present. The evidence also indicates that life has been present on our planet for close to 4 billion years, though for most of that time it was relatively simple single celled life. At the start, Earth's atmosphere was a toxic mix of methane, carbon dioxide and ammonia—oxygen was nearly absent in the atmosphere of early Earth. To humans and most of the world's familiar flora and fauna, this atmosphere would have been toxic.
Asteroid impacts, tremendous volcanic eruptions, and shifting tectonic plates resulted in drastic changes in climate and the emergence of new life forms. Somewhere along the way the simple microorganisms, which were ancient Earth's only inhabitants, developed photosynthesis that created a net gain of oxygen first in the ocean and later in the atmosphere. Then, 2.3 billion years ago, the world's first ecological disaster occurred when free oxygen established a permanent presence in the atmosphere. Known as the Great Oxidation or the Oxygen Catastrophe, almost every living thing on Earth died as a result of this massive bacteria-induced climate change.
Scientists know this from the minerals present in the rock record. Between 2.5-2.3 billion years ago, during the early Proterozoic Eon, extensive deposits of pyrite (iron sulfide) and uranite (Uranium oxide) can be found in river sediments. These minerals require low oxygen levels to form. From 2.3 billion years onward iron rust can be found, an indication of the presence of free oxygen. Even so, the oxygen levels were but a fraction of today's and intense radiation from the Sun sleeted down on the plant. Eventually, oxygen would solve the radiation problem as well as molecules of ozone (O3) were created in the stratosphere forming the protective ozone layer.
This first example of life radically changing Earth's environment, some times to the detriment of older life forms, was a good thing for our species for without the change in atmospheric composition we would never have existed at all. According to Thorne Lay, Professor of Earth Sciences at UC Santa Cruz : “Life itself modified the Earth system. As the system changed, more complex life forms became viable. Eventually diverse multi-cellular organisms flourished. But not without first being smacked in the face by a few snowballs.” And what snowballs they were!
Eight hundred million years ago, during the Neoproterozoic Era, Earth underwent a monstrous ice age. There is evidence of glacial ice in tropical latitudes, only 15° to 30° north of the equator. In our world, this would mean glaciers as far south as Miami, Florida. Earth would have looked like a different planet, with almost no open ocean and few areas of exposed rock. Only ice and snow, a world of almost pure white.
At that time, most of the land belonged to the super-continent of Rodinia, which formed around 1,100 mya. Rodinia contained the lands that makes up the modern continents today, but not in a configuration we would recognize. North America was in the middle. South America, Australia and Antarctica were packed around North America. Rodinia straddled the tropics, leaving a single vast ocean sweeping across the other side of the globe. There was no land at either pole.
In 1992, Joseph L. Kirschvink, of the California Institute of Technology in Pasadena, put forward a theory that our planet had almost completely frozen from pole to pole, with the only open ocean choked with pack ice. He named this condition “Snowball Earth.” Other researchers have calculated that some of the glacial periods during this time had lasted as long as 10 million years. During these periods the ocean may have frozen over completely, blocking all sunlight and killing most ocean life.
In fact, scientists now think that there have been ice ages dating back all the way to the middle of the Archean Eon, around 2.8 billion years ago. We have evidence of this from layers of sediment found in rock formations known to belong to that period. On occasion, these episodes lasted several hundred million years, and may have rivaled the ice age during the Neoproterozoic in intensity. There may have been several Snowball Earth periods in our planet's past.
The next major milestone for life on Earth occurred at the beginning of the Phanerozoic Eon, 542 million years ago, with the Cambrian Explosion. This event, with new multicellular organisms popping up in great profusion, resulted in an explosion of life. It marked the end of the Proterozoic Eon and the beginning of the Phanerozoic, Greek for “visible life.” This eon signals the rise of truly complex life, where individual organisms are large enough to be recognized without a microscope.
Different geologic time periods are marked by significant changes in the types of creatures living on Earth. The rock deposited during the Phanerozoic Eon contains evidence of fossilized hard body parts from living things and it is this fossil record that is used to date rock layers from the three eras. By reading the fossil record, scientists have constructed an outline of the development of life during the time following the Cambrian Explosion. Note that it is the changing cast of fossils that allows science to map the past—the history of our planet was written in rock by the fossil remains of uncounted extinct species.
So we see that there were mass extinctions, changes to atmospheric gas proportions and even multiple ice ages prior to the beginning of the Phanerozoic. However, the argument can be made that conditions during the Precambrian (the time prior to 542 million years ago) were not really representative of Earth's climate since complex life spread across the planet. So let's take a look at the “recent” past of the Phanerozoic.
Welcome to the Phanerozoic
To closely examine each era and period of the Phanerozoic would take a lot more space than I wish to commit to a single blog post so we will concentrate on the variation in several key environmental factors over that entire time span. These factors are temperature, carbon dioxide levels, ice age conditions, and species extinction and its impact on diversity. But before reviewing these data I do want to mention one period from the late Paleozoic Era that will give a flavor of the types of variation seen in the past.
In the late Paleozoic Era, during the Carboniferous Period, great forests of primitive plants thrived on land, forming extensive peat-swamps. These huge masses of plant matter were buried with sediment, eventually forming the great coal deposits found in North America, Europe and around the world. A global drop in sea level at the end of the Devonian reversed early in the Carboniferous, creating large shallow seas and huge deposits of carbonate minerals. These deposits trapped large quantities of atmospheric carbon that would later form vast beds of limestone.
During the later part of the Carboniferous, the amount of oxygen in Earth's atmosphere was about 35%, much higher than it is today. According to Robert Berner, levels atmospheric oxygen levels have varied between 15% and 30% over the past 550 million years (see “Atmospheric oxygen over Phanerozoic time” in PNAS September 28, 1999). At the same time, global CO2 went below 300 parts per million—a level which is now associated with glacial periods. The abundance of O2 led to the existence of the largest insects ever seen on Earth. Hawk-sized dragonflies, with 29 inch (75 cm) wing spans, spiders the size of house plants, 5 foot (1.5 m) long centipedes and soup bowl-sized crawling bugs. It was truly a time when insects ruled the planet. Perhaps it's a good thing the atmospheric oxygen level is only 21% today.
Carboniferous plants resembled the plants that live in tropical and mildly temperate areas today. From fossils, we know that many of them lacked growth rings, suggesting a uniform climate. But the climate was changing. By the middle of the Carboniferous, Earth was sliding into an Ice Age, the Permo-Carboniferous or Karoo Ice Age. The growth of large ice sheets at the southern pole locked up large amounts of water as ice. Because so much water was taken out of the environment, sea levels dropped, leading to a mass extinction of shallow marine invertebrates, the gradual decline of the swamps, and an increase in dry land.
Many times, these conditions were reversed when the glaciers receded. Glacial melt water was released back into the oceans, and again flooded the swamps and low-lying plains. Carboniferous rock formations often occur as a pattern of stripes, with alternating shale and coal seams indicating the cyclic flooding and drying of the land. Even under these stressful conditions, or perhaps because of them, life continued to develop. By the end of the era, the first large reptiles and the first modern plants, ancestors of today's conifers, had appeared.
In many ways the Carboniferous is unique in terms of its combination of atmosphere, climate and life forms, but each period of geologic time is unique—that's why they are distinguished with individual names by the ICS. Fact is, the thing that makes these remote periods in time similar is that they are all different from one another—the only constant factor running through the sweep of Earth history is change. For greater detail on the characteristics of these geologic periods see The Resilient Earth chapter 4, “Unprecedented Climate Change?,” or get a copy of our book from Amazon.
Now that we have a flavor of the types of change Earth has experienced in the past let's examine the temperature variation over the Phanerozoic Era. Below is a figure that shows science's best guess at how temperature has varied over the past 542 million years. Notice the wide variation in temperature over time, sometimes colder than the average 14°C of today but much of the time considerably warmer. Note also the blue rectangles along the bottom of the plot representing periods of ice house conditions. Even though there have been several extensive ice ages during the Phanerozoic, for the majority of the past half billion years there have been no permanent ice caps in either hemisphere. In that sense, the total melting of the Greenland and Antarctic glacial ice sheets would mark a return to historically normal conditions for our planet.
Next, take a look at variation in atmospheric CO2 levels shown in the graph below. Though the uncertainty in the measurments grows as we look farther into the past the general trend can be seen—there used to be much more CO2 in the air in most earlier times. There is an overall trend toward reduced levels but what is most interesting is to compare the CO2 graph with the temperature graph above.
At this scale, there is really no apparent correlation between carbon dioxide levels and global temperatures. What's more, there have been ice ages when CO2 has been as much as 10 to 15 times higher than modern levels (for example the end-Ordovician Ice Age). There have also been times when temperature was increasing but CO2 was decreasing and times when CO2 was increasing but temperatures decreasing (during the Silurian and Devonian and during the Triassic and Jurassic, respectively).
The dip in CO2 levels at the end of the Carboniferous and into the Permian can be attributed to the over active coal swamps that were busy accumulating the thick coal seams that provide energy for much of the world's power generation today. That dip persisted throughout the great Karoo Ice Age (360-260 mya) but started to rise following the Permian-Triassic Extinction (251 mya). Many have speculated that ice ages are a cause of ancient mass extinction events and there may be a connection. The timing of know extinction events is shown in the biodiversity graph below.
The Ordovician-Silurian extinction event, also called the end-Ordovician extinction, was the third-largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct and second largest overall in the overall loss of life. Somewhere between 450 and 440 mya, two bursts of extinction occurred, separated by about a million years. Notice how, after each major extinction (denoted by the red triangles) life bounces back with increased diversity. Clearly life rises to a challenge (for more about extinction see “Nature, Cruel and Uncaring”).
The graph below shows CO2, temperature and ice age timing information on a single plot. Did the cold have something to do with the extinction? Arguments rage on in the paleological community. Conversely, some have suggested that a sudden rise in CO2 levels at the end of the Permian was responsible for the Permian-Triassic Extinction. Science may never know.
What we do know is human CO2 emissions at their worst cannot approach the levels of natural GHG release, even events that did not trigger mass extinctions (see “Could Human CO2 Emissions Cause Another PETM?”). But what about the often mentioned link between CO2 and temperature? “In a nutshell, theoretical models cannot explain what we observe in the geological record,” says Rice University Oceanographer Gerald Dickens, “There appears to be something fundamentally wrong with the way temperature and carbon are linked in climate models.”
There have been many other factors at work during the past that affect climate change. This examination—being limited to carbon dioxide levels, temperature and the occurrence of ice ages—ignores the impact of shifting continents, variation in solar activity, orbital cycles and the possible impact of cosmic rays on Earth's climate. Greater detail on all of these factors are presented in our book. One interesting thing to notice is that having a continental land mass spanning either pole seems to help promote ice house climate conditions. During the Devonian the supercontinent Gondwana passed over the south pole, during the Carboniferous the polar ice cap covered the southern end of Pangaea, and today we have Antarctica astride the south polar region.
That concludes our whirlwind tour of Earth's climate history. There are a number of observations that can be made from our overview of the Phanerozoic:
•Earth’s temperature is always changing.
•Over time there have been periods when it has been colder than it is today.
•For most of the Phanerozoic it has been much warmer than it is today.
•Life has persisted during periods both hot and cold.
•There is no one “right” temperature.
•Carbon dioxide has always been present in Earth’s atmosphere.
•Over time there have been periods when CO2 has increased and decreased naturally.
•For most of the Phanerozoic it has been much higher than it is today.
•Life has persisted during periods with high CO2 and low CO2.
•CO2 levels will change with or without human contributions.
•Over time there have been a number of ice ages—Life has endured multiple ice ages.
•For most of the Phanerozoic there have been no persistent polar ice caps.
What the future holds climate scientists are unable to portend with all their computer models and IPCC consensus reports. The Earth and its climate are constantly changing—there is no one correct climate or temperature for our planet. Those who say CO2 is the most important factor in climate change, that human GHG emissions will cause runaway global warming, have no historical basis for such claims.
As Earth's climate history has shown, nothing predicted by the global warming alarmists would be unprecedented—Earth's climate has been colder than today's and much, much warmer. CO2 levels have also been many times higher than they currently are, even during ice ages. Ice ages come and go, caused by mechanisms mankind is powerless to control. And after every ice age the world warms and the glaciers disappear only to return millions of years later. No change in climate is irreversible. Given 4 billion years of Earth history and 542 million years of complex life, blaming mankind for 9,000 years of global warming seems rather silly. As it says in Ecclesiastes: “What has been is what will be, and what has been done is what will be done; there is nothing new under the sun.”
Be safe, enjoy the interglacial and stay skeptical.