The second history to review is climate.  As we can see from the recent news, the weather ids difficult to forecast, but easy to backcast as long as we are able to measure it.  We’ve only been measuring it for a little over a century, or about 1/32,000 the time life has been on Earth.

The temperature of places, especially how much ice and glaciers are over the landscape, affects the form of and composition of rocks we find in sediments form that era, Certain forms of rocks are ground onto shape by glaciers, and their presence at an aged layer of the earth indicates that glaciers covered that land or were nearby.

The concentration of isotopic Oxygen in core samples of the ocean floor is another indicator of a cold atmosphere.  O18 is heavier than O16, and is more likely to settle to the ocean floor the colder the oceans are.  Higher amounts of O18 in an aged stratum at the bottom of a present or past ocean is a relative indicator of colder seas, but not exact.  The difference in temperature between “Snowball Earth” (Before life came onto land) and “Hothouse Earth” (the rise of the dinosaurs) is thought to be only 10 degrees Celsius (18 degrees Fahrenheit)

That’s a basic primer on two major techniques of paleoclimatology.  I don’t want to get mired down in the methods of finding all this neat old information, so I will often just link to the source.  The sources I use, because I am a rank amateur, are often by academics, but distilled for a general audience like you and me.  I am neither a climatologist nor a paleontologist, but they should not have all the fun to themselves.

Here’s a review of the climate over the last 4 billion years.

As single-celled life was still subsisting on minerals, before even the formation of modern bacteria, the Pongolla (2.9 bya) ice age enveloped Earth for 30 million years.  Tens of millions of years of ice age makes our recent ice age look like a cold snap in winter, and it may be that other ice ages, of a few thousand years, have winked on an off between these major brutes, but have evaded detection.  The next glaciation we know about, the Huronian (2.5 bya), lasted for 100 million years, and was followed by almost 2 billion years of “normal” climate. The next cold period, the Cryogean (0.9 bya) lasted 200 million years, or the time between the dawn of the dinosaurs to when you are reading this.

It was finally time for a “Hothouse” Earth, (0.4 bya) that lasted 50 million years, followed by the Karoo Ice Age by 0.35 bya (only 50 million years later, or the amount of time it took us to evolve from mice to tailless apes). Finally, by 0.26 bya, the Earth was close to today’s climate, with expanded equatorial deserts and no ice cap at the south pole.  By 0.25 bya, a hot house earth had returned, while the dinosaurs developed and expanded from the smaller reptiles we still know today.  A hot, dry climate dominated from 0.22 bya, with monsoons developing by 0.18 bya.  The dry climate was enforced by the size of Pangaea at the time, with a vast arid interior thousand of miles from any ocean.  As Pangaea broke up, the landscape became wetter by 0.16 bya.

By 0.14 bya, the poles were covered in cold temperate forests akin to those in northern US, warmer than today, but colder than before. Instead of slouching back to the great glaciations of previous eras, a hothouse earth returned by 0.1 bya, during the pinnacle of dinosaur diversity and abundance.  Even after the extinction of the dinosaurs (0.066 bya), the climate remained hot and humid, with alligators near the poles as recently as 0.05 bya.  The configuration of the continents was also similar to today, with only minor changes, such as the collision of India with Asia, and South America with North America.

By 0.04 bya however, ice sheets had begun forming at the the south pole, and have remained there ever since.  There was still no ice at the north pole as recently as 0.03 bya, however.  By 0.02 bya, when cats and dogs were finally fighting, the tropics were still extensive, extending up to Maine.  The climate began to dry by 0.01 bya, but the climate was still mostly hot, not cold, until the last “hot maximum” (0.005 bya).  The most recent ice age, the Wisconsinian (starting only 0.00007 bya and lasting until 0.000015 bya after peaking 0.000025 bya), which history books consider to be so important, lasted just 55 thousand years, setting the stage for everything we think of as human history and prehistory.  Although the stone age (0.0025 bya) had been going on since before modern humans.

Some of these climactic changes could be accounted for by changes in the earths atmosphere, such as the relationship between rising oxygen (2.4 bya – 1 bya) and glaciation.  Other physical factors may also play a role.  The Earth is not static and fixed in its orbit or spin. Our orbit changes from circular to elliptic and back every 100,000 years.  This changes the difference between summer and winter proximity to the sun by as much as three times.  Axial tilt*, the angle between the earth’s orbit and its spin, varies between 22 and 24.5 degrees and back every 41,000 years.  The greater the tilt, the more pronounced our seasons.  Here in the US, the Earth is actually closer to the sun during the winter, but the Earth tilting away from the sun makes it cold in the northern Hemisphere between December and March. The focus of the orbit changes too, so that the US will be closer to the sun in the summer in 23,000 years, and again in 46,000 years, and so on.   These cycles may or may not force the ice ages and hot maximums that I describe above, but they do affect the yearly intensity of the seasons that life has had to endure over billions of years.

* The real reason for the season