Carbon – it’s the favourite element of many Earth system scientists, and the last two weeks of the course have both included bits about the global carbon cycle. I’ve been impressed by some of the interesting questions this has raised, so let me try and join some of the dots.
Week 2 introduced the long-term geological carbon cycle in which carbon dioxide is added to the atmosphere by volcanoes and metamorphic processes (the heating and pressurising of sedimentary rocks, which cooks out some of the carbon they contain). This input of carbon dioxide is balanced, over geologic timescales (here we’re talking about a million years or more) by an equivalent output of carbon dioxide. That output is due to the process of weathering, which liberates calcium and magnesium ions from silicate rocks, followed by the combination of those ions with carbon dioxide to form new carbonate rocks, deposited as sediments at the bottom of the ocean. (The chemically minded can think of this as, e.g.: CaSiO3 + CO2 => CaCO3 + SiO2.)
You might be wondering how the output of CO2 comes to balance the input? The answer is that the rate of silicate weathering is sensitive to the CO2 content of the atmosphere and the resulting temperature. If for example, volcanic activity increases and more CO2 enters the atmosphere, then CO2 and temperature levels will go up, but this leads to more silicate weathering and CO2 removal, until the system stabilises with the ‘sink’ for CO2 again matching the ‘source’.
Occasionally people ask if this negative feedback process will come to the rescue and balance our ‘anthropogenic’ CO2 emissions? The answer is: only very slowly! The removal of CO2 by silicate weathering is a tiny amount each year compared to the input of CO2 from fossil fuel burning. If left to natural processes (rather than geoengineers), it will take nearly a million years for accelerated silicate weathering to drag atmospheric carbon dioxide back down to its pre-industrial concentration.
In the meantime we need to think about much shorter-term processes in the global carbon cycle that are responding to our CO2 emissions, and that’s what Week 3 has tackled. To get a handle on this it first pays to know where carbon likes to live – a huge amount of it favours the deep ocean – 38,000 billion tonnes in fact. That is because carbon dioxide reacts with seawater and the resulting acid then dissociates into various dissolved forms (for the chemists: CO2 + H2O <=> H2CO3 <=> H + HCO3 <=> 2H + CO3). Roughly 2,000 billion tonnes of carbon are stored in all the vegetation and soils of the world. And before we started meddling with it, the atmosphere contained 600 billion tonnes of carbon.
Between each of these ‘reservoirs’ of carbon there are natural exchange fluxes. Roughly 100 billion tonnes of CO2 come out of the ocean each year and 100 billion tonnes go back in. It’s a similar story for the land surface, where CO2 goes into plants (through photosynthesis) and comes out again from plants, animals and fungi due to respiration (much of it from the breaking down of organic matter in soils). The crucial thing is that these exchange fluxes are naturally in balance.
When humans add CO2 to the atmosphere – most of it extracted as fossil fuels from the Earth’s crust – this creates an imbalance. The extra concentration of CO2 in the atmosphere makes both the land and the ocean take up more CO2. About a quarter of the CO2 we add to the atmosphere each year reacts with seawater and enters the ocean. Another quarter is taken up by plants, partly because photosynthesis works more efficiently in an atmosphere richer in CO2. These are referred to as the land and ocean ‘carbon sinks’, and we should be thankful to them every day, because without them the climate change challenge would be twice as large.
Several of you have been asking how these sinks might change in the future, as CO2 carries on going up and the world gets warmer? Of course we would like the sinks to grow, and whilst CO2 emissions keep increasing we expect the ocean sink at least to grow in absolute size. But the fraction of annual CO2 emissions that go into the ocean is expected to decline. That is because adding CO2 has the effect of acidifying the ocean and making it less able to take up extra carbon. Also warming makes CO2 less soluble in water (thinking of warming up a fizzy drink and watching it go flat). All this means that we cannot expect the ocean to mop up all of our CO2 pollution. When we have finished burning fossil fuels and adding CO2 to the atmosphere, and we have given the ocean time to find a new balance – which means in about the year 3000 AD – we expect somewhere between 20% and 40% of the CO2 we have emitted to still be in the atmosphere. The percentage gets larger the more CO2 we emit.
What about the land? Well, that is more uncertain. In the best case scenario the land will continue to take up CO2, but its storage capacity for carbon will saturate, and a new balance will be struck with more carbon in vegetation and soils than now. However, warming tends to reduce the storage of carbon on the land, because it accelerates respiration. Also frozen soils – the permafrost – may thaw and release some of its stores of carbon to the atmosphere. And fires may become more frequent and devastating in a warmer climate, returning more CO2 to the atmosphere via combustion. In the worst case scenario, the land could switch from being a sink to a net source of CO2 to the atmosphere. If that happens, we will have only the ocean to counteract the rise of CO2.
What we are talking about here are climate feedbacks involving the carbon cycle, and they are very topical. This week both Pierre Friedlingstein and I found ourselves having breakfast in the UK parliament talking about such feedbacks, because we helped out with a note on the topic of ‘Risks from Climate Feedbacks’ written for our politicians:
MOOC participants will hopefully be pleased to know that the MPs and peers present were very interested to hear about our course and the discussions it is triggering. You never know, perhaps one or two have even signed up…
Professor Tim
PS If you are wondering why it took me two weeks to write another blog entry, my excuse is I had to mark 240 exam scripts, which if nothing else revealed that my undergraduate class are wrestling with the carbon cycle as much as you are
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