Skip main navigation

New offer! Get 30% off your first 2 months of Unlimited Monthly. Start your subscription for just £35.99 £24.99. New subscribers only T&Cs apply

Find out more

Carbon, Phosphorus and Nitrogen Cycles

Geologist, Alasdair Skelton talks about the carbon, phosphorus and nitrogen cycles and how humans are perturbing them.
There are two parts to the carbon cycle. There’s a slow part and a fast part. Let’s begin by thinking about the slow part of the carbon cycle. This is how it works.
Carbon comes in the form of carbon dioxide to the atmosphere from volcanoes.
That’s why if you look at a piece of lava from a volcano, like this one. You can see it is full of holes, those are gas bubbles. Some of that gas was carbon dioxide.
It’s a tenth of a petagram of carbon that is added on average every year from volcanoes. But the amount of carbon dioxide in the atmosphere doesn’t increase. So, why not? That’s because it’s a cycle. There’s a second part to that slow cycle. That’s what makes it slow. The second part is the weathering of rocks. So what does weathering mean?
That is literally: rocks dissolve. What happens is that water and carbon dioxide in the atmosphere mix to make a weak acid. That acid, ever so slowly, dissolves the rocks. It’s particularly fond of attacking the pinkish crystals you can see in the rock. In front of you I’m holding some bigger ones. This is called feldspar. It reacts with that weak acid, and it dissolves. Some of it becomes clay.
Others become various ions: potassium, sodium. But it’s not just the rock that dissolves. The carbon dioxide does as well. When the carbon dioxide dissolves, it is soluble in water- -and it gets carried by rivers down to the oceans. And when it gets there, it reacts with another ion. That ion is called calcium. When they react together, they make a mineral. That mineral is called calcite. I’ve got some here.
These beautiful crystals are of the mineral calcite, calcium carbonate. Those are the building blocks of limestone. They form limestones at the bottom of the ocean.
So, let’s think what has happened: A tenth of a petagram of carbon added every year- -to the atmosphere from volcanoes, and the same amount is slowly taken away- -by the dissolving of rocks. That’s the slow carbon cycle. And then we’ve got the fast carbon cycle. This operates on time scales from seconds to hundreds, or perhaps thousands, of years. So what’s involved in that cycle? Like the trees, bushes, fields, grasslands… It also involves the oceans, seas, and lakes.
1.7 petagrams of carbon move between those reservoirs on average every year. That’s a lot more carbon moving around there, than in the slow cycle. But it’s moving much, much faster. Seconds to hundreds, or even thousands, of years. That, at least, is how it was. But it is not how it is.
In 2019, we released almost 8 additional petagrams of carbon to the atmosphere- -by burning fossil fuels.
Now remember, fossil fuels come from rocks.
Like this, coal, oil and gas. That means that they belong to the slow carbon cycle. The one that involves rocks. That means it is going to take hundreds of thousands of years for that carbon- -to get back where it came from.
We also added an additional petagram of carbon by changing how the land is used.
So, that makes 9 petagrams in total. That carbon doesn’t all stay in the atmosphere. Some of it ends up in the oceans, about a quarter. Some of it ends up taken up by the land, about another quarter. But it’s a full 4 petagrams of carbon that are added to atmosphere, every year. That was 2019. We will call that year “peak carbon”. So what do I mean by that? I mean the year that we released most carbon to the atmosphere. So how do I know it was 2019? I don’t. But it must be 2019 We must never, ever release so much carbon to the atmosphere ever again. Emissions simply must go down. Why?
For our own survival, and for the survival of all species on Earth.
One quarter of the carbon was released by us in the Anthropocene. That is since 1950. So why? During the same time period, since 1950- -global energy consumption has increased by a factor of 5. So what has caused that increase in energy consumption? Well, there are two main reasons. One of these has been an increase in population. That increase is only a factor of 3. So, there is a second reason. It is that we are consuming more energy per person on Earth today, than we were in the past.
The rise in population creates another problem. How do we feed so many people? One of the ways of doing so is by making the land more fertile. We can do that by phosphorus or nitrogen to the soil. But there is a problem. Phosphorous comes from rocks like, these.
These are the mineral apatite. It is phosphorus ore. It is not renewable. It takes geological processes to create phosphorous ore.
Known reserves will be exhausted within a few hundred years. So then we can turn to nitrogen. There is lots of nitrogen in the air we breathe. But there is another problem there. By taking nitrogen from the atmosphere and turning it into fertilizer, there is a by-product. That by-product is nitrous oxide. That is a very, very powerful greenhouse gas, meaning it makes the Earth warmer.

Geologist, Alasdair Skelton, explains that there are two parts to the carbon cycle.

The slow part of the carbon cycle operates on timescales of hundreds of thousands of years. On average, 0.1 petagram of carbon dioxide enters the atmosphere from volcanoes and is removed again from the atmosphere by the weathering of rocks, each year.

The fast part of the carbon cycle operates on timescales ranging from seconds to thousands of years. On average 1.7 petagrams of carbon is cycled between vegetation and soils, oceans, lakes and the atmosphere, each year.

Sitting on a rock outcrop alongside Lake Vättern in Sweden, Alasdair Skelton talks about 2019, a year which he calls peak carbon, meaning the year we emitted most carbon to the atmosphere. He explains that by burning fossil fuels and changing how the land is used, we emitted 9 petagrams of carbon to the atmosphere. Most of this carbon (8 petagrams) comes from burning fossil fuels and cement production. These belong to the slow carbon cycle, which means that it will take hundreds of thousands of years for that carbon to get back where it came from.

Surrounded by fields, Alasdair Skelton explains that one fourth of all carbon in the atmosphere was emitted by us since 1950. He links this to global energy consumption has increased by a factor of 5. He links this to population growth but because this has been by a factor of 3 (not 5) he explains that another reason is that we are consuming more energy per person.

With regards population growth, Alasdair Skelton explains that to feed so many people, we have turned to fertilizers which use phosphorus and nitrogen. Phosphorus comes from rocks and is non-renewable. Nitrogen is in plentiful supply but its conversion to fertilizers is coupled with emissions of nitrous oxide, a very potent greenhouse gas.

This article is from the free online

Taking on the Climate Crisis with Social Change

Created by
FutureLearn - Learning For Life

Reach your personal and professional goals

Unlock access to hundreds of expert online courses and degrees from top universities and educators to gain accredited qualifications and professional CV-building certificates.

Join over 18 million learners to launch, switch or build upon your career, all at your own pace, across a wide range of topic areas.

Start Learning now