Image in header: Weathering of rocks. Source: Climatescience.org
A slow carbon cycle
Earth’s climate has always varied throughout the planet’s history. Warmer and colder times alternate. Even though our planet has experienced climates that were quite spectacularly colder or warmer than today, the limits of a habitable planet have never been crossed. Such climate changes are largely determined by the concentration of greenhouse gases in the atmosphere, withCO2 in the leading role. The Earth has systems in whichCO2 is added to and removed from the atmosphere. So we are talking about the carbon cycle here.
Before humans existed using fossil fuels, the main addition of carbon to the atmosphere was the work of volcanoes. The magma comes from the Earth’s mantle. In that mantle, the pressure and temperature are very high, and calcium carbonate then spontaneously breaks down intoCO2 and Ca ions that form new rocks with silicon oxides. TheCO2 then starts escaping to the surface via volcanism.
But how does carbon get back into the mantle? This happens through a process we call silicate weathering. Some of theCO2 in the air goes to dissolve in rainwater, forming carbonic acid:
H2O+ CO2 -> H2CO3
This carbonic acid comes with the rain on rocks on land, and starts reacting with them, for example in the case of this calcium silicate rock:
CaSiO3 + H2CO3 +H2O-> Ca2+ + 2 HCO3-+ H2SiO4
The calcium ions and bicarbonate eventually reach the sea via groundwater and surface water. There, calcium carbonate (calcite) will form which will precipitate on the soil (spontaneous chemistry and also because living things create calcium skeletons).
Ca2+ + 2 HCO3--> CaCO3 +CO2
If you start noting the total chemical reaction from CO2 in the air to calcite on the seabed in a balanced way you get this:
In the chemical equations above, you can see that half of the original carbon from the atmosphere is sequestered in new calcite, and half forms backCO2 in the sea. The half that remains in the calcite on the sea floor will disappear back into the mantle in the very long term, thanks to plate tectonics. In this way, silicate weathering is Earth’s method of removing the greenhouse gasCO2 from the atmosphere over millions of years.
The rate at whichCO2 is added to the atmosphere (via volcanoes) and at whichCO2 goes back into the mantle (via silicate weathering) has varied throughout the planet’s history. The variable rate of silicate weathering actually acts like a thermostat, because weathering of rocks is faster when it is warm and/or when moreCO2 enters rainwater. In other words, during periods of warmer climate – for example 56 million years ago during the Paleocene-Eocene transition – more bicarbonate and calcium and magnesium ions entered the sea because the rocks weathered faster (bit more acidic rain and warmer). As a result, more calcite formed on the oceanic crust, which eventually disappeared back into the mantle. It’s a mechanism we call negative feedback, and it causes the more extreme climate fluctuations to flatten out over the long term. That way, Earth could guarantee a livable climate over billions of years.
Rocking the climate, but in moderation
While the principle of the thermostat is clearly very important for our planet, the reality is a bit more complicated. Indeed, apart from the amount ofCO2 in the air and a warmer or colder climate, there are many factors that influence this process. For example:
- The presence of complex life in the ocean: calcite formation today occurs 20 times faster via living creatures building calcium skeletons than by spontaneous chemical reactions without the intervention of life.
- Mountain formation: when large mountain ranges are formed by plate tectonics, rock weathering is considerably faster at that location.
- Low sea level on Earth: as sea water falls, more land is added, and so the area of rock weathering is greater.
- Plant growth: if the land is covered with vegetation, weathering of the underlying rock is delayed.
On top of that, the carbon cycle is by no means the only factor determining climate. So over the history of the Earth, we see many long-term climate fluctuations that we cannot easily understand. On even larger time scales (e.g. over the totality of the last 600 million years), we do see an additional trend: the average amount ofCO2 in the atmosphere is decreasing very slowly. This explains why we ended up in a colder world on average in more recent times (e.g. the last 3 million years). Humans are temporarily reversing this trend back to a warmer world by burning fossil carbon from the ground. But that is on a much shorter timescale. We can expect to have so littleCO2 left in the atmosphere for another 500 million years in the future that photosynthesis becomes impossible. Then, of course, all life for plants and animals will stop. The reason for that decliningCO2 trend is the very slow cooling of the Earth’s mantle, and thus diminishing volcanism.
Carbon: the whole picture
The whole carbon cycle that depends on the mechanisms described above is sometimes called the ‘slow carbon cycle’. In fact, there is also a much faster cycle related to life on Earth: photosynthesis and cellular respiration, and exchanges between water and air. There are very many sources out there that try to estimate the amounts of carbon circulating in the slow and fast cycles. I looked at many of them, and then tried to draw out some kind of consensus balance myself. You can see the result below.
Note that carbon reservoirs in soils and oceans are also much larger than the annual mobility of carbon.
If you express it in average amount per year, the share of volcanism and silicate weathering (the slow cycle) seems very small. But the fast cycle usually stays in balance over longer periods of time, so it is indeed the slow cycle that often managed to prevent the extremes in history. There are also exceptions to this. The huge rise of land vegetation in the Carboniferous was such an exception.
Another exceptional period was just over 600 million years ago, when the Earth entered an exceptionally cold climate more than once. This was from 720 million to 635 million years ago (just before the emergence of plant and animal kingdoms, in the so-called Cryogenian. It is much better known by the term snowball Earth (Snowball Earth) or slushball Earth. As the planet became almost completely covered in ice, one side of the slow carbon cycle was shut down: silicate weathering. Over millions of cold years, rocks could no longer weather in rainwater. After all, they were covered with a thick sheet of ice. However, the addition of CO2 by volcanoes continued. In this way, the greenhouse gas in the atmophere slowly increased, and the Earth was able to evolve back to a normal climate after millions of years.
Decision
There are many factors in the carbon cycle that occasionally go out of balance. This directly affects the climate. Despite this, the Earth manages to stay within livable limits, and the slow carbon cycle (volaknism and silicate weathering) keeps smoothing out extremes over the long term. We owe this to active plate tectonics and the right proportions of water and land on the surface. Life itself also takes on a major role.