Catastrophes, Extinctions, and the Azolla Event: How Life Changes Climate as Often as Nature

    In light of rising global temperatures, I thought it would be fun to look at global climate data, going as far back as possible, to see if any patterns emerged in the many theories about mass extinction and climate change through Earth’s history.  The graph above shows detailed information on climate change over the last 500 million years.  It’s hard to find data before that, because the climate was much more extreme, alternating between periods of inhospitable heat and ones that were so cold that Earth was literally an ice planet.  That’s why it took a long time for life to evolve; prior to the Cambrian explosion- when most major animal phyla start appearing in the fossil record- the climate was too variable for species to dominate for long periods of time. 
            Looking at the graph, right away we can see that for most of the time our planet was relatively warm and had no polar ice caps.  Interestingly, there are periods spanning tens of millions of years where the Earth was too warm to have them.  We can also see that the story of the human race began during one of Earth's coldest periods.  Considering all the hysteria about climate change, it might surprise people to know that the planet is far cooler right now than it’s been on average. 
            Changes in the climate as seen on the graph can be explained by many things.  The most general of them is that animals increase global temperature while plants decrease it.  The oxygen-CO2 exchange has been out of balance many times, resulting in some of the most extreme temperatures seen in the last 500 million years.  The world's first animals are thought to have caused global warming after the Cryogenian period some 700 million years ago.  During the Cryogenian there was an overabundance of oxygen, which animals, then restricted to the ocean, used to their advantage.  The exchange of oxygen for CO2 caused a spike in the latter, which lead to global warming and the thawing of what's thought to have been a “Snowball Earth”.  
            In the late Ordivician, some 450 million years ago, there was a glaciation event, probably the first since the Cryogenian period.  It’s thought to be the leading cause of the first mass extinction in Earth’s history.  Scientists believe this event was caused by L chondrite meteorites from a nearby asteroid, that blocked the Earth from its usual sunlight for thousands of years.  Naturally, the climate slowly warmed once the meteorites finished dispersing through the atmosphere.  
            Then there was a long period of warming that lasted for 90 million years, until the Late Paleozoic Icehouse, which lasted even longer.  Temperatures were cooler than usual due to the evolution of land plants, occuring from 360-260 million years ago.  It’s the first known case of plants contributing to a global cooling trend.  
                The largest extinction of all time occurred at the same time as the largest swing in temperature variation.  About 250 million years ago, the End-Permian mass extinction resulted from a series of volcanic eruptions in Siberia, bringing the Earth out of the Icebox and into a severe greenhouse that killed over 90% of animal species.   Judging by the graph, it apparently took 50 million years to recover from this event, for temperatures did not return to normal until the Triassic ended. 
            When dinosaurs ruled the Earth, temperatures were higher than normal.  After the Jurassic began, there was a slow warming trend that eventually resulted in the Cretaceous Hot Greenhouse.  This is attributed to the Mid-Atlantic ridge opening up, causing a separation of the continents and much volcanic activity that kept the planet warm for the longest period in history.  More volcanism has been linked to the Paleocene-Eocene maximum, about 55 million years ago, yet the cause for this spike remains uncertain. 
            Next comes one of the more interesting events in Earth’s history.  During the Eocene, when temperatures were really high, the Arctic Ocean was born.  There was a species of plant called Azolla that dominated the ecosystem.  When these plants died and sank to the surface, they brought massive amounts of CO2 with them.  It was enough to cause global temperatures to fall dramatically, over a long period aptly called the Azolla Event, leading to the Late Cenozoic Ice Age that started 34 million years ago and is still happening.  The Azolla Event is only a theoretical explanation for the current Icebox Earth, but it may be the first case in history where a single species had an influence on long-term climate change. 
            The second of these cases should be self-explanatory, unless you are in denial about it.  Humans are directly responsible for the rapid increase in CO2 levels that have made global temperatures rise.  Obviously, no geologic or astronomic event is causing it, nor is there a widely accepted Earth cycle that would make it fluctuate at such an extreme rate.  Like the first animals, we are contributing to climate change on a level not seen in Earth’s history.  And the rate we are doing it could upset the balance so much that there would be an extinction event comparable to the disastrous End-Permian.  The reason it’s comparable is because we are at a low point in the graph, just like when the volcanoes in Siberia overturned it, sending it skyrocketing to the highest point.  The catastrophic rate of change spells disaster for the natural balance of things, which may result in an unprecedented number of extinctions, at least in the history of mammals. 
            Aside from geologic and astronomic events, it seems that individual species are certainly capable of changing the climate, if not collections of them.  Plants and animals both did it, hundreds of millions of years ago.  In the last 50 million years, there may have been two individual species responsible- possibly more to be discovered in the fossil record.  The more we discover, the more we're finding out that life changes Earth’s climate as often as natural events.