Carbon Dioxide - An Essential Ingredient of Life

Life on Earth formed a staggering 3.6 billion years ago. Conditions back then would have been pretty inhospitable to anything composed of more than just a few cells. In fact, that's so long ago that the crust had barely formed, asteroids were still pelting the Earth at an alarming rate and the atmosphere probably had upwards of 250 times more CO₂ than it does today.

If we dumped that much carbon into the atmosphere today we wouldn't be around long to see what happened. Yet instead of frying the planet, such a carbon-rich atmosphere probably allowed early life to develop. 3.6 billion years ago our sun was quite a bit younger and quite a bit cooler - 30% in fact - than it is today. Convenient then, that enough CO₂ was about to compensate and allow water to remain a liquid, the all essential prerequisite for life.

But this poses a problem. How does the atmosphere keep a 'convenient' amount of CO₂ in its atmosphere, not so much as to boil away our oceans but not too little as to freeze them solid? How was there so much more when it was needed and so much less now that it isn't?

An Unlikely Hero

Almost all of the carbon on Earth is safely locked away in the rocks and sediments that make up the crust. Occasionally though, some of that trapped carbon is ejected into the atmosphere by volcanoes. Recently, of course, humans have also accelerated this transfer by burning fossil fuels. Once in the atmosphere, we all know that it effectively traps some of the suns energy and warms the planet up a bit. Taking carbon back out of the atmosphere for the long term is down to chemical weathering, all in all a much more subtle process than volcanism. Indeed, the complexity of life owes a lot to a humble and unassuming chemical reaction that has occurred since the first raindrops fell to Earth.

Volcanic eruptions are a major natural source of CO₂ Source

Before water vapour falls as rain, it mixes with CO₂ in the atmosphere to form carbonic acid. This process basically produces acid rain, which then falls to Earth and starts eating away at silicate-based rocks (which make up most of Earth's crust). What comes out the other end of this reaction is calcium carbonate, the same compound that ends up as limescale on kettle's and washing machines.

But all this 'limescale' is actually a pretty fundamental source of life. Rivers eventually transport the calcium carbonate into the oceans, where it is used in the formation of marine shells and other little creatures such as diatoms, tiny pieces of algae which form the basis of the food chain. When these organisms die, their carbon-based bodies sink to the bottom of the ocean. The relentless process of sedimentation and tectonics then slowly put the carbon back from whence it came; stored safely in the Earth's crust until such time that it comes across a volcano and the whole process starts over.

Chemical weathering of a statue. Source

The Moral of the Story

The end-product of this incredibly long-winded process is a remarkably stable temperature on Earth. Chemical weathering acts as a thermostat, keeping the temperature more-or-less right. As temperature increases, there is generally more rainfall and more chemical weathering. In fact, a 10°C rise in temperature will double the amount of chemical weathering that occurs. Of course, chemical weathering also decreases as the temperature falls, transferring less carbon and keeping the atmosphere that little bit more carbon rich.

All things considered, the surface temperature of Earth has stayed more-or-less within the perfect margins required by life for billions of years. This is despite a multitude of factors, including a warming sun, that would otherwise have prevented life from forming or killed it off once it did. And all this is because CO₂ - that problem gas that is behind so many of our climate issues - has been there in just the right amount just when we need it.

Despite what we may think today, CO₂ is not the antichrist of life. Instead, it may well have helped form it.