The Evolution of the Modern Climate: New Evidence from Plant Remains

Things are just not like what they used to be. You know this. You know that the Age of Dinosaurs, for instance, was full of dinosaurs and stuff, and before transitional fossil forms crawled out of the sea to colonize the land, all animals were aquatic, etc. But did you know that from a purely modern perspective, the Miocene was the most important geological period?

First, lets get one thing straight. We are not in the so-called "Holocene." The so-called "Holocene" is a totally bogus geological period. Saying "Hey, we're in the Holocene, not the Pleistocene ... the Pleistocene is over, we're in the Holocene...." is like saying "Hey, it's not January, it's Tuesday! We're in Tuesday!!! January is over, man it's Tuesday."

If someone heard you saying that, they'd rightly think you were a moron. Don't be a moron. Deny the Holocene, embrace the Pleistocene. It's where we are at. It's when we are at.

Having said that, how did the Pleistocene get there? Well, it got there because of the Miocene. Back in the Eocene, things were generally warm and wet (at least near the end of the Eocene), the horses were still short. The mammalian fauna was utterly different than it is today, but it was there ... lots of mammals, but very very different. There were giant, mammal eating amphibians, like the size of small dinosaurs. There were dinosaur-like birds too.

The Miocene was really more of the same but without the mammal eating amphibians, and with a general enlargement of the mammal fauna. People think of the Pleistocene as being the age of giant mammals, but that is not true. The Miocene was the age of giant mammals. During the Pleistocene, perhaps you have the evolution of somewhat furrier giant mammals as it was colder, but the great diversification of enormous mammals had already happened.

Then the Pleistocene came along and ruined everything. The Pleistocene is also known as the "Ice Age" and, to rub salt in the wound, the climate system that dominates the Pleistocene is sometimes known as the Ice Box Model. The Pleistocene is not the age of the giant mammals because it was (still is!) the period during which the giant mammals went extinct. Saying that the Pleistocene is the age of the giant mammals is like saying ... oh never mind with the analogies, you get the point.

What is different between the Miocene and the Pleistocene? Mainly, the positioning of the continents. In the Eocene and most of the Miocene, the continents were more spaced apart from each other. Like when you are in line to go to the bathroom in Catholic school, or in formation in the Army, and you are supposed to be a certain distance from the next person so that the chief Nun or the drill sergeant can weave in between the ranks harassing you. The continents were like people in the American Midwest, where Ed Hall proved that we have a "large social bubble." ... Africa was not nuzzling up against Asia and Europe, India had not slammed high speed into Asia, North and South America were not connected to each other at all.

This meant that the process of heat moving from equator to pole was smooth and regular (compared to now) in both air and water currents. Once the continents bunched up into two big groups (Afroasiaeruope and Northcentralsouthamerica) the transfer of heat from equatorial regions to the poles became disrupted. Like fog forming out of nowhere on your glasses when you walk into a warm lobby from the bitter cold outside, like the kitchen cabinet being oddly chilly because it is built against an outside wall, so your coffee cup is cold in the morning, like the way there is sweat in your armpits but not on your knees .... there is an increasingly uneven distribution of heat and moisture across the planet with the continents all bunched up, the North Atlantic squeezed into a small space, and various mountain ranges blocking air currents. So you get polar ice caps.

The Miocene is the period during which we went from one kind of climate system to an entirely different kind of climate system, the time when grasslands emerged for the first time and spread across the planet, and so on. So, understanding the Miocene is very very important.

The research at hand is a paper by Kurschner et al, called The impact of Miocene atmospheric carbon dioxide fluctuations on climate and the evolution of terrestrial ecosystems. Here's part of the abstract:

The Miocene is characterized by a series of key climatic events that led to the founding of the late Cenozoic icehouse mode and the dawn of modern biota. The processes that caused these developments, and particularly the role of atmospheric CO2 as a forcing factor, are poorly understood. Here we present a CO2 record based on stomatal frequency data from multiple tree species. Our data show striking CO2 fluctuations of {approx}600-300 parts per million by volume (ppmv). Periods of low CO2 are contemporaneous with major glaciations, whereas elevated CO2 of 500 ppmv coincides with the climatic optimum in the Miocene. Our data point to a long-term coupling between atmospheric CO2 and climate. Major changes in Miocene terrestrial ecosystems, such as the expansion of grasslands and radiations among terrestrial herbivores such as horses, can be linked to these marked fluctuations in CO2.

Here is the main picture from the paper:

Essentially, the researchers were gazing at tea leaves to see the past rather than the future. Not really tea leaves, but fossil leaves. The frequency of stomata on leaves varies in a non linear inverse relationship with CO2 in the atmosphere. This can be demonstrated in modern contexts. Therefore, counting stomata can indicate CO2 levels. (Thus the data in the above graph.)

In the end....

As a result of climatic changes such as increased seasonality and cooling, [and] the decline in primary productivity due to the marked post-middle Miocene CO2 crisis, woodland biomes retreated, the species richness of browsing mammals declined, and grazers increased their diversity ... A large number of terrestrial mammalian herbivores, such as equids, camelids, antilocaprids, rhinos, and proboscideans show dramatic morphological changes of their dentitions during the Miocene (3-5). In particular, early Cenozoic horses are characterized by brachydont (short-crowned) dentitions but show, from the late middle Miocene on ... a rapid diversification of hypsodont (high-crowned) taxa, which has been attributed as an adaptation to include more fibrous and abrasive material, such as grasses, in the diet. This development implies the coevolution of large herbivores and plants in response to Miocene climate and atmospheric CO2 fluctuations.

The impact of Miocene atmospheric carbon dioxide fluctuations on climate and the evolution of terrestrial ecosystems

Kurschner, Wolfram M.Zlatko Kvacek, and David L. Dilcher (2008) The impact of Miocene atmospheric carbon dioxide fluctuations on climate and the evolution of terrestrial ecosystems. PNAS Published online before print January 3, 2008, 10.1073/pnas.0708588105. vol 105(2) Pp 449-453.