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The carbon cycle

The cycle is the process by which carbon circulates around the and eventually becomes available to living . Carbon is essential for life because it can form stable, complex organic like nucleic acids, , and carbohydrates. Most of the carbon on earth is stored in the , which is the outer shell of the earth comprised of the crust and the upper layer of the mantle. The carbon cycle is what allows carbon from that vast source to work its way into the biosphere so it can be used to build the complex biomolecules required for life. The carbon cycle has 5 main components, which are the , the , the oceans, sediment, and the geology of the earth, such as the mantle and crust. Let's take a look at how carbon circulates between each of these stages, starting with the atmosphere. In the atmosphere, carbon exists primarily in the form of carbon dioxide and methane. Some carbon dioxide leaves the atmosphere when it is used for , which transfers the carbon into the biosphere. The biosphere is the vast web of organisms and ecosystems on earth. Carbon leaves the atmosphere and enters the biosphere when photosynthetic organisms use carbon dioxide for photosynthesis, at which point the carbon gets integrated into biomolecules in the plant. As the plants are eaten by other organisms, the carbon works its way up the food chain and throughout the other organisms in the biosphere. That carbon can then leave the biosphere through respiration, such as when we exhale carbon dioxide, sending the carbon back to the atmosphere. Carbon can also leave the biosphere through the process of combustion, including naturally occurring fires like forest fires or through human activity, such as burning fossil fuels. In addition to entering the biosphere, some carbon dioxide in the atmosphere is also dissolved into the oceans. A smaller amount of carbon also enters the oceans in the form of dissolved organic carbon in rivers. Once the carbon is in the ocean, it circulates through the ocean in a process called thermohaline circulation. In simple terms, that just means that as ocean currents move the water toward the north or south pole, the water begins to cool, changing its density. As that happens, it begins to sink, circulating the carbon throughout the ocean. The carbon then eventually leaves the oceans through photosynthesis by marine life, or in the form of calcium carbonate. Calcium carbonate is the substance that makes up the shells of marine organisms. When those organisms die, their shells sink to the bottom of the ocean, where they erode and become limestone. This is the primary way carbon reenters the lithosphere. It should be noted that the majority of the carbon in the lithosphere is still there from when the earth formed. The calcium carbonate deposits are the primary way carbon reenters the lithosphere, accounting for about 80% of the carbon reentry. The remaining 20% is from organic tissue that gets buried under high heat and pressure. The heat and pressure turns the organic tissue into a substance called kerogen, which is basically the mixture of organic compounds that eventually become fossil fuels. Once the carbon is in the lithosphere by either of those two methods, geologic activity causes parts of the crust to be pressed down into the mantle, where it melts into magma. Volcanic activity then ejects that carbon back into the atmosphere in the form of volcanic gases, thus completing the carbon cycle. Now, let's dive a little deeper and look at some of the implications of this. Remember, I said earlier that most of the carbon in the lithosphere is still there from when the earth originally formed. We can now see why volcanic activity is often seen as crucial for the formation of life, and why geologic activity is of particular interest when searching for life on other planets. Volcanic activity allows carbon to be released from rocks and into the rest of the carbon cycle. Thus, the presence of active volcanoes means that at least some carbon is being released into the atmosphere where it could potentially be used by living organisms. The presence of volcanoes does not in itself guarantee life, but we do know that any life out there would probably be carbon-based, so it would most likely occur on a planet that currently has or has previously had extensive volcanic activity. And that is precisely what we see when we look at the history of our own planet, which had extreme volcanic activity that first created our own atmosphere and established the beginnings of our carbon cycle. In addition to the origins of life, another major implication of the carbon cycle is climate change. By understanding how carbon moves throughout the earth, it becomes clear why human consumption of fossil fuels is such a big deal. Remember, about one fifth of the carbon that reenters the lithosphere does so through the formation of kerogen, which eventually turns into things like oil and natural gas. Normally, that carbon stays locked in the lithosphere for millions of years while geologic processes slowly eject it. But by burning them as fossil fuels, we drastically speed up the release of that carbon into the atmosphere. When we do that at the massive scale that has occurred since the Industrial Revolution, it is enough to throw a wrench in the whole carbon cycle, and we put carbon into the atmosphere faster than it can cycle out. The result is an excess of carbon dioxide that is having a measurable effect on earth's climate. The carbon cycle is an intricately balanced system that is required for life on earth, and likely required for life on other planets. By understanding how the carbon cycle works, we can better understand how life began, how it thrives, and how has the potential to cause great harm.
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