HISTORY7 - Earth's Climate: The First 4.54 Billion Years

This blog is planned as the first of a two-part series on Earth’s climate.  This article will cover the history of the Earth’s climate, from the formation of the Earth up to today - using accepted scientific theory and facts as far as possible.  The second article, in a follow-up blog posting, will discuss Earth’s current and future climate and Man’s effect on it, including global warming and climate change - with a heavy dose of my opinions included.

My objective with this first article is to “get straight in my head” the historical background and key drivers of Earth’s climate to better understand what is happening today and lay the stage for talking about the future.

This is obviously a very complex subject and I have simplified wherever possible.  I discuss my interpretation of the history of the Earth’s climate.  I do not discuss the details of the study of Earth’s rock layers and sediment and glacial cores which support the climate history.  I also tried to keep the scientific vocabulary to a minimum. 

Formation of Earth

The generally accepted theory of how and when the Earth was formed is as follows, greatly simplified: 

The universe was created from a single point with infinite density by the “Big Bang” explosion roughly 13.8 billion years ago.  After an initial expansion, the Universe cooled sufficiently to allow the formation of subatomic particles, and later simple atoms. Giant clouds of these earliest elements later coalesced through gravity to form galaxies, stars, and planets.  Earth formed about 4.54 billion years ago by accumulation from a cloud of dust and gas left over from the formation of the Sun.

The geological time clock below shows the long span of time from the beginning of the Earth to the present, identifying some of the definitive events of Earth history.  (It takes some study, but this is an extremely useful graphic.)  Think of the geological clock as the more familiar time clock.  At the top, the Earth is formed.  Time runs clockwise around the graphic, identifying the major geological periods through Earth’s history.  (In the graphic, “Ga” means billion years ago; “Ma” means million years ago.)  

Earth was initially molten due to extreme volcanism and frequent collisions with other bodies.  Eventually the outer layer of the planet cooled to form a solid crust when water began accumulating in the atmosphere of hydrogen, helium, ammonia, and methane captured from the Sun.  That atmosphere was probably “blown away” by solar winds, streams of charged particles emanating from the Sun. 

The Moon formed about 4.527 billion years ago, probably from an impact of a planetoid with the Earth.  Over many years, the Moon stabilized the Earth from wobbling, tilted Earth on its axis to produce seasons, and caused ocean tides.

Intense volcanic activity and continued bombardment from asteroids and comets probably changed the Earth’s atmosphere to gases such as nitrogen, methane, sulfur dioxide, carbon monoxide, and carbon dioxide.  Increasing amounts of condensing water vapor, augmented by ice delivered by comets, produced the oceans.  There would have been only a tiny amount of oxygen. 

Life first developed on Earth in its oceans around four billion years ago.  These single-celled organisms did not require oxygen.  They took in amino acids and gave off methane and carbon dioxide.

About 3.2 billion years ago, blue-green algae began obtaining energy from sunlight by means of photosynthesis, where oxygen is a by-product.  This began a slow change to the composition of the Earth’s atmosphere, along with evolution of more complex organisms utilizing photosynthesis, towards the oxygen-rich atmosphere we know today.  Ultraviolet radiation from the Sun broke down some of the oxygen molecules to form an ozone layer in the atmosphere that prevents too much ultraviolet radiation from reaching the Earth’s surface, making the Earth a safe place for life to evolve - a delicate balance.

Plants and invertebrate animals first developed on land around 500 million years ago.  More complex oxygen-breathing animal life developed slowly with the first vertebrate land animals, dinosaurs, and mammals.  Early humans didn’t develop until about two million years ago, a tiny fraction of the Earth’s age.

This geological clock shows important periods in the Earth's development and notes major events.  (Courtesy of Wikimedia)

Magnetic Field

The Earth’s magnetic field was formed about 3.5 billion years ago, generated by the motion of liquid iron in the planet’s core.  The magnetic field extends from Earth’s interior several tens of thousands of miles into space, protecting the Earth from the charged particles of the solar wind and cosmic rays that would otherwise strip away the upper atmosphere, including the ozone layer that protects the Earth from harmful ultraviolet radiation.

For billions of years, the Earth’s magnetic field has exhibited remarkable consistency, with the strength varying within reasonable values and the polarity (north-south) flipping periodically over a few thousand years, with the time between reversals varying from tens of thousands of years to tens of millions of years.  Due to the Earth’s interior steadily cooling, its molten inner core began solidifying 0.5-2 billion years ago, and today is roughly 1,500 miles in diameter, compared the Earth’s diameter of about 8,000 miles.  The continued growth of the solid inner core, and its interactions with the remaining surrounding molten outer core, complicates our understanding of the Earth’s magnetic field.

Formation of Continents

About four billion years ago, as the Earth continued to cool, the Earth’s surface began to undergo a process of formation and migration of land masses that we call continents today.  These continental motions are explained by mechanisms of plate tectonics that describe the very slow movements of continents as interactions between the cold surface and the hot interior of the planet.  Supercontinents are formed when most of the Earth’s land mass at any given time is aggregated into a single continent.  This has happened at least four times in Earth’s geologic history at 2,500 Ma, 2,100 Ma, 1,100 Ma, and 300 Ma.  The last supercontinent, called Pangaea, started breaking up 200 million years ago, resulting in the continents and positions on the Earth’s surface that we have today.  The continents are continuing to move:  North America is headed toward Asia, Africa toward Europe, and Australia is headed toward Asia.  A new supercontinent is predicted within 50-200 million years.

The supercontinent Pangaea about 200 million years ago.  (Courtesy of the Institute of Geophysics, University of Texas)

The size and positions of the continents are important to the Earth’s climate because the geological record appears to show that ice ages start when continents are in positions which block or reduce the flow of warm ocean water from the equator to the poles and thus allow ice sheets to form.  Ice sheets increase Earth’s reflectivity and thus reduce the absorption of solar radiation.  With less radiation absorbed the atmosphere cools; the cooling allows the ice sheets to grow, which further increases reflectivity and so on. 

Ice Ages

An ice age is a long interval of time (millions to tens of millions of years) when global temperatures are relatively cold and large areas of the Earth are covered by continental ice sheets and alpine glaciers. Within an ice age, there are multiple shorter-term periods of warmer temperatures when glaciers retreat (called interglacials) and colder temperatures when glaciers advance (called glacials).  At least five major ice ages have occurred throughout Earth’s history: the earliest was over two billion years ago, and the most recent one began approximately three million years ago and continues today.  Three times during Earth’s history, the planet was completely covered by ice.  These “snowball Earths” occurred circa 2,220 Ma, 710 Ma, and 640 Ma, lasting a few million years each - with average global temperatures much colder than shown in the chart below.

Simplified chart showing when five major ice ages occurred in the past 2.4 billion years of Earth's history and how the average global temperature changed.  (Courtesy of Barry Saltzman)

We are now in the middle of the latest ice age.  Over the last 450,000 years we have cycled in and out of several glacial periods.  Glacials are characterized by cooler and dryer climates over most of the Earth and large land and sea ice masses extending outward from the poles.  Sea levels drop due the removal of large volumes of water above sea level in the icecaps. 

Five fairly regular glacial-interglacial cycles occurred during the last 450,000 years in the current ice age.  The colder temperatures shown - compared to the previous chart - are latitude specific for Antarctica, Alaska, and Greenland.  (Courtesy of nrdc.org)

The last period of glaciation peaked about 20,000 years ago. Vast ice sheets covered much of North America, northern Europe, and Asia.  At that time, the world was on average probably about 10°F colder than today.  Currently, we are in a warm interglacial that began about 11,000 years ago.  Both poles and most of Canada, and northern Russia are still covered in ice.

The maximum extent of ice cover in North America during the last glacial period, about 20,000 years ago.  (Courtesy of Tom Evslin)

The causes of long term ice ages and the ebb and flow of shorter intermediate glacial-interglacial periods are not fully understood.  The consensus is that several factors are important.  There is evidence that atmospheric greenhouse gas levels (primarily carbon dioxide and methane) fell at the start of ice ages and rose during the retreat of the ice sheets, but it has been difficult to establish cause and effect.  The geological record appears to show that ice ages start when the continents are in positions which block or reduce the flow of warm water from the equator to the poles, allowing ice sheets to form.  Additionally, the timing of glacials and interglacials coincides with cyclic variation of the Earth’s orbit around the Sun, changing the amount of sunlight reaching parts of the earth, directly affecting Earth temperatures.  Predicted changes in orbital forces suggest that the next glacial period would begin at least 50,000 years from now.  Volcanic eruptions may also have contributed to the end of ice ages by pouring high amounts of greenhouse gases such as water vapor and carbon dioxide into the atmosphere, thereby raising the Earth’s temperature.  Other potential factors affecting ice age intensity and timing include ocean current fluctuations, the uplift of the Tibetan plateau, above the glacial ice, decreasing Earth’s reflectivity during ice ages, and variations in the Sun’s energy output.

Greenhouse Gases

So-called greenhouse gases in the Earth’s atmosphere trap part of the warmth that would otherwise escape from the Earth’s surface into space.  Without them the Earth could not hold on to any of the energy that it receives from the Sun and our planet would soon enter a deep freeze.  The most significant greenhouse gases are water vapor, carbon dioxide, methane, and ozone - with carbon dioxide being the most important because of it long life in the atmosphere.

Carbon dioxide - associated with volcanic activity and plant growth - has been present in the Earth’s atmosphere for millions of years as shown in the chart below.

Carbon dioxide concentration in the Earth's atmosphere and the average global temperature over the last 600 million years.  (Courtesy of Paul Macrae)

Note the historically high carbon dioxide concentrations (over 5,000 ppm), but steadily decreasing over the last 150 million years or so, perhaps due to decreasing volcanic activity.  Throughout this period, the Earth’s average global temperature has varied between 50-75 degrees Fahrenheit, and on a geological scale (over millions of years), has been dropping steadily for the past 40 million years.  

Let’s take a closer look at a shorter time frame.  The figure below covers the last 420,000 years and shows how carbon dioxide levels and average global temperature have varied over five cycles of glacial and interglacial periods during the current ice age that began about 3 million years ago.  Carbon dioxide levels varied between 200-300 ppm during most of the period until a sudden rise in recent years.  Average global temperature varied between 50-60 degrees Fahrenheit, compared to the 20^th century average of 56.7 degrees Fahrenheit.  Note the higher temperatures in the current Holocene interglacial period and the previous Eemian interglacial period.

Variation of atmospheric carbon dioxide, average global temperature, and sea levels over the last 420,000 years.  (Courtesy of John Englander)

The above figure also shows cyclic sea level changes over the last 420,000 years.  As ice melted during the warmer interglacials, the water flowed into the sea, raising the level of the ocean.  The variation in sea level has been about 400 feet, greatly affecting shorelines around the world, as shown in the figure below.

America's eastern shoreline has changed dramatically over the years because of ice age activity.  (Courtesy of the U.S. Geological Survey)

Extinction Events

Extinction of individual species has occurred on Earth over hundreds of millions of years - at a steady, if uneven rate.  But scientists have identified at least five different major mass extinctions over history - where Earth’s climate changed dramatically, where there was a widespread and rapid decrease in the diversity and abundance of life on Earth, with 75-96% of existing plant and animal species lost.  These extinction events occurred 450-440 Ma, 375-360 Ma, 252 Ma, 201 Ma, and 66 Ma - caused by catastrophic events like asteroid impacts, enormous volcanic eruptions, and severe ice ages.  The Earth survived these events, and in each case was left ripe for evolutionary changes as new species developed to take the place of those lost.

Effect of Man

Humans have been present on Earth for about two million years, just an instant in geological time.  Scientists appear to agree that humans didn’t have much impact on the climate until the Industrial Revolution in 1750, when we began contributing to the carbon dioxide in Earth’s atmosphere.  The main source of carbon dioxide in the air from human activity is the burning of fossil fuels, formed from the fossilized remains of dead plants, and including petroleum, coal, natural gas, and commonly-used derivatives like kerosene and propane.  Fossil fuels contain high percentages of carbon and when burned, produce carbon dioxide as a waste product.  The production of electricity, transportation, and industry are the three areas that produce the most carbon dioxide.  

The figure below shows that atmospheric carbon dioxide levels have risen sharply since 1850, presumably as fossil fuel burning increased around the world.   Not shown on the chart, carbon dioxide levels have continued to rise, reaching 405.0 ppm in 2017, at the highest level since about three million years ago. 

Average global temperature variation is also shown in the figure, generally trending up with carbon dioxide levels.  In 2017 the average global temperature reached 58.3 degrees Fahrenheit, up about three degrees from the temperature in the 1850s, compared to the 20^th century average of 56.7 degrees Fahrenheit.

Atmospheric carbon dioxide levels and average global temperatures have risen dramatically since humans started contributing to atmospheric greenhouse gases.  (Courtesy of NASA)

Man has also affected Earth’s atmospheric ozone layer that protects humans against ultraviolet radiation from the Sun that could cause skin cancer, sunburn, and cataracts.  In the 1970s, scientists began noticing a steady depletion in the total amount of ozone in Earth’s atmosphere and a much larger seasonal depletion of ozone above the polar regions, the so-called “ozone hole.”  The main reason for ozone depletion was found to be manufactured chemicals, especially halocarbon refrigerants, solvents, propellants, and chlorofluorocarbons - transmitted into the stratosphere by the winds after being emitted from the surface, and then causing ozone to break down into oxygen.  This situation generated worldwide concern that resulted in the adoption of the Montreal Protocol in 1987 which bans the production of ozone-depleting chemicals.  Ozone levels stabilized by the mid-1990s, and began to recover to normal levels in the 2000s, with full recovery projected over the next century.

Conclusions

I’ve tried to make this Earth climate history discussion reflect generally accepted current scientific thinking.

The Earth’s climate has changed dramatically and often turbulently over its 4.54-billion year history.  The many influences on environmental change include the planet’s fiery birth, destructive asteroid impacts, violent volcanic action, reversals of Earth’s magnet field polarity, relentless movement of continents, surface and sea-level deforming ice ages, earth-warming greenhouse gases, depletion of the protective ozone layer, and extinction events. Earth has managed to not only survive these climate influences and the resulting climate changes, but has seen the birth and evolution of extensive plant and animal life, including Man. 

There are many unknowns or uncertainties in the historic climate change process.  The multiple critical climate influences are complicated individually and are often interrelated in even more complex ways that we do not yet fully understand, as we study them on a slow-paced multi-hundreds-of-million-year geologic time scale.

The impact of the effect of Man on Earth’s recent past and future climate must begin with the realization that we are currently in a natural warming cycle (interglacial) that normally would last perhaps another 50,000 years before the next cooling trend (glacial period).  Also, the relatively high carbon dioxide greenhouse gas levels (highest in last three million years) and average global temperatures that we are seeing today have been greatly exceeded in the past, during periods of thriving plant and animal life.  Moreover, for millions of years in our past, average global temperatures have not tracked (moved in the same direction:  up or down) atmospheric carbon dioxide levels.

We have to adjust our viewpoint from the geological timescale discussed above to assessing climate change issues and effects on a human-life-span timescale, while at the same time, trying to further our understanding of related geological-timescale issues.

In a future blog, I’ll present my assessment (opinion) of current climate change drivers, effects, and predictions.