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The hydrologic and carbon cycles: Always recycle!

Hank explains how carbon and water are recycled through ecosystems and the biosphere. Created by EcoGeek.

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Video transcript

Brown hair: Adorable story alert! I was hanging out with my two year old second cousin in the ocean in Florida and we're jumping up and down with the waves and it's fun, but then the water got all calm and I said, "Oh no more waves," because that's the tier communication I'm going for with a two year old, but then he said, "Don't worry there will be more." I was like, "How do you know," and he was like, "It's a cycle." Yes, it is a cycle, the earth is filled with cycles. If my two year old second cousin knows it, you should know it, too. The universe is the great recycler, all the stuff that we've got on earth, every last particle of matter or calorie of energy has been around since the big bang. It just keeps getting re-purposed over and over again. When it comes to matter, at least, earth is essentially a closed system, all matter gets passed around in continuous biogeochemical cycles which are pathways for molecules like water or elements like carbon and nitrogen and phosphorous to move through all of the earth's various ecological and geological compartments. Of course we couldn't possibly talk about how all matter gets cycled around earth in one video because the earth is pretty big, but consider this an introduction to biogeochemical cycles, staring my two personal favorites, carbon and water. (upbeat music playing) I'm sure you already know about at least one kind of planet wide recycling because it's the most obvious to us, the hydrologic cycle which describes how water moves on, above and below the surface the earth driven by energy supplied by the sun and the wind. Talking about the hydrologic cycle it's most useful to think about all the water on earth being held in a series of reservoirs, the ocean for instance, or the atmosphere in the form of clouds, or in polar ice caps. Not only does water cycle through different places, it also takes different forms in different places in the cycle, liquid, solid or gas. Since it is a cycle, there is no beginning and there is no end, so where we start our discussion is arbitrary, but we're going to start it off with precipitation: rain, hail, snow, sleet, graupel, all that stuff is precipitation. It happens when water that's being held in the atmosphere condenses or turns from a gas into a liquid and then occasionally freezes into a solid right up in the air. The opposite of condensation of course is evaporation, the conversion of a liquid into a gas and when a substance converts straight from a solid to a gas that's sublimation and when it's from a gas to a solid that's deposition and now you know. Back to condensation, it's responsible for the formation of clouds, which happens when air containing water vapor rises and cools or is compressed to the point that it can no longer be a gas. At this point the vapor forms droplets. This is the same thing you see happening on your glass of ice tea on a humid day. The water in the air around the glass gets cold and turns from gas into liquid. So, a cloud is just a big pile of condensed water droplets, in a sense it's a gigantic flouting reservoir. Clouds are a handy feature of the hydrologic cycle because as they drift over the landscape they move water around the globe, so water that evaporates over the ocean can be deposited somewhere else. Otherwise if water always got deposited right where it evaporated the precipitation would be almost all right over the ocean because that's where most of the evaporation on earth takes place. So, wind moves clouds and as water keeps condensing clouds get heavier and heavier until our old friend gravity takes over and pulls the condensed droplets to the ground in the form of rain, or in the form of snow, or hail, or sleet, or graupel. Now the water's on the ground, but gravity continues to work on it, pulling it towards its resting place, whatever that might be, it either pulls the water across the surface of the land towards the lowest point, in a process called run-off or it pulls it underground. Water can be trapped or stored for a little while in places like, lakes and ponds and wetlands, but most of the water that falls as precipitation gets pulled lower and lower and lower as run-off through the creeks, streams and rivers until it reaches the ocean. In really cold places water of course freezes and hangs around as ice, in certain places for thousands of years at a time, like at the poles and glaciers and onmountain tops, but when it melts most of it, too, runs off into the oceans. So, you see where this is going, oceans are a big deal. They're pretty much the biggest deal, they're the reason that we have the hydrologic cycle in the first place. They're also the reason we have awesome stuff like weather and life on earth. The weird thing about oceans though is that they're salty and there is a reason for this! As water runs to the ocean it erodes minerals like salt from soil and carries it to the ocean. Now water heading to the ocean might not taste salty, but the salt's in there. But here's the thing, when the water evaporates again the salt doesn't evaporate with it, it gets left behind. You keep this up for a few billion years with the pure water evaporating from the ocean and then returning with tiny amounts of salt and that's your recipe for a billion cubic kilometers of brine. All this shows that the world's oceans are literally the last stop for all the liquid water on earth, the only way to get out of there is through evaporation and that leaves all your minerals behind. Now living things also have their role to play in the hydrologic cycle, in both plants and animals the breakdown of carbohydrates produce energy, produces water as a waste product so we lose water through evaporation from our skin, we also exhale water vapor and of course we pee it out. Indeed most organisms on earth are made mostly of water, although that water cycles in and out of us pretty quickly. In plants, water is sucked up through the roots and moves up to the leaves, the gas exchange organ where it evaporates quickly. This process is called evapotranspiration and since there are so many plants here on earth, it's responsible for a good amount of the water that enters the atmosphere. This process is essentially the opposite of condensation in that it turns liquid water into gas. The energy of the sun dries evaporation whether it's from the surface of the ocean or from tree tops and leaves and then once all that water evaporates into the atmosphere we're right back where we started. It's a cycle! So, now that you know a little bit about the hydrologic cycle it's a little bit easier to understand how the carbon cycle works. Carbon's one of the most abundant elements in the universe and here on earth it's always on the move, just like water jumping from one reservoir to the next. That's a good thing because a) all living things require carbon for their structure and to fuel their bodies and b) it's a big component in a bunch of non-living things as well. It's in rocks and the ocean, trapped in ice, plus it's in the atmosphere where it helps regulate the temperature. Without carbon dioxide earth would basically be a frozen wasteland, so lucky for us there's a whole [pam-flow] of carbon out there because we need it. Let's start out with the carbon in living things. If you were to take all of the water out of your body, carbon would constitute about half of what remained in the little pile of dust that used to be you. The first biological reservoir is plants. They absorb a bunch of carbon dioxide out of the atmosphere because they need it to photosynthesize. CO2 is also one of the bi-products of respiration, the process by which they use that energy. Plants take in carbon dioxide from the atmosphere during photosynthesize and then release CO2 back out into the atmosphere during their respiration process to make ATP for all their cellular functions. Right now, you're like wait, wait, wait, no, isn't the deal that plants get to take in the carbon dioxide and animals get to breath it out? Well, yes and no, it's just that plants take in more CO2 from the atmosphere than they give off through respiration, the rest is like their profit. It's what becomes the body of the plant. That's right, that big old massive tree, all of that mass came from gas, pretty cool. So, carbon absorbed by plants has three possible things, it can be respired back into the atmosphere, it can be eaten by an animal, or it can be present when the plant dies. And if a tree falls in the right kind of forest and it's not allowed to decompose normally because a bunch of other plants all fell right on top of it and they die and get buried and squished together and form rocks, like coal, we call these carbon rich geological deposits, fossil fuels. Lately, one of humanities very favorite pastimes is digging up all of this old carbon in the form of coal and oil and natural gas and burning it to fuel our ... pretty much everything's, but I'll get to that later. Another extremely important carbon reservoir is the ocean. Now carbon dioxide dissolves really easily in water and once it's in there a lot of it is used by phytoplankton, tiny plant like organisms that form the base of the marine food chain. They use carbon and photosynthesis and they also use it to make calcium carbonate shells and when these guys die their shells settle to the bottom of the ocean, pile up, become compressed and over time make rocks like limestone. Now, limestone obviously doesn't burn super well so it's not considered a fossil fuel but as limestone deposits are eroded by water the calcium carbonate is broken down to eventually form, among other things, carbon dioxide and carbonic acid. We make lime and cement by heating limestone which produces a pretty good amount of carbon dioxide. When we do burn fossil fuels, such as coal and petroleum products and natural gas, it also releases carbon in the form of carbon dioxide that's been stored for hundreds of millions of years in the geosphere, which is just a fancy word science-y word for earth rocks. This process is what started the atmosphere carbon dioxide levels rising like crazy in the past couple of hundred years and the excess of carbon dioxide in the atmosphere causes global climate change because CO2 in the atmosphere prevents some of the sun's energy from re-radiating back out into space. So, yeah, our planet is getting warmer because we've been burning through this massive reservoir of carbon that we have locked underground. This is causing all kinds of problems that we can see already and it's very likely going to keep causing bigger and bigger problems with time. The situation could be helped a lot if we would just stop unlocking all of that carbon and spitting it into the atmosphere, but in some respects we don't have control of the situation anymore because of ice. Remember how I said that carbon is often trapped in ice, well in places like Siberia, and Northern Canada, and Alaska and cold places that also have plants they contain huge carbon reserves that are trapped in permafrost, ground that's frozen year-round. These places are basically frozen wetlands that add another layer of dead plants matter each year. But as permafrost melts these dead plants decompose and huge amounts of carbon dioxide and methane are released into the atmosphere creating a positive feedback loop. Our carbon burning lifestyle is unleashing this other huge carbon reservoir which keep the whole greenhouse effect going with or without us, just saying. Sorry to end on such a frightening and depressing note, but the stability of global climate is not as stable as we would like it to be and the fact that we're throwing it out of whack is one of the most important reasons to study ecology in the first place.