Capturing carbon dioxide from thin air is the last thing we should talk about.
When I say this, I am deliberately expressing a double meaning. First, the energy requirements for carbon capture from thin air are so enormous, it seems almost absurd to talk about it (and there’s the worry that raising the possibility of fixing climate change by this sort of geoengineering might promote inaction today). But second, I do think we should talk about it, contemplate how best to do it, and fund research into how to do it better, because capturing carbon from thin air may turn out to be our last line of defense, if climate change is as bad as the climate scientists say, and if humanity fails to take the cheaper and more sensible options that may still be available today.
Before we discuss capturing carbon from thin air, we need to understand the global carbon picture better.
When I first planned this book, my intention was to ignore climate change altogether. In some circles, “Is climate change happening?” was a controversial question. As were “Is it caused by humans?” and “Does it matter?” And, dangling at the end of a chain of controversies, “What should we do about it?” I felt that sustainable energy was a compelling issue by itself, and it was best to avoid controversy. My argument was to be: “Never mind when fossil fuels are going to run out; never mind whether climate change is happening; burning fossil fuels is not sustainable anyway; let’s imagine living sustainably, and figure out how much sustainable energy is available.”
However, climate change has risen into public consciousness, and it raises all sorts of interesting back-of-envelope questions. So I decided to discuss it a little in the preface and in this closing chapter. Not a complete discussion, just a few interesting numbers.
Where is the carbon?
Figure 31.2: Estimated amounts of carbon, in gigatons, in accessible places on the earth. (There’s a load more carbon in rocks too; this carbon moves round on a timescale of millions of years, with a long-term balance between carbon in sediment being subducted at tectonic plate boundaries, and carbon popping out of volcanoes from time to time. For simplicity I ignore this geological carbon.)
Until recently, all these pools of carbon were roughly in balance: all flows of carbon out of a pool (say, soils, vegetation, or atmosphere) were balanced by equal flows into that pool. The flows into and out of the fossil fuel pool were both negligible. Then humans started burning fossil fuels. This added two extra unbalanced flows, as shown in figure 31.3.
The oceans circulate slowly: a chunk of deep-ocean water takes about 1000 years to roll up to the surface and down again. The circulation of the deep waters is driven by a combination of temperature gradients and salinity gradients, so it’s called the thermohaline circulation (in contrast to the circulations of the surface waters, which are wind-driven).
This slow turn-over of the oceans has a crucial consequence: we have enough fossil fuels to seriously influence the climate over the next 1000 years.
Where is the carbon going
Figure 31.3 is a gross simplification. For example, humans are causing additional flows not shown on this diagram: the burning of peat and forests in Borneo in 1997 alone released about 0.7 GtC. Accidentally-started fires in coal seams release about 0.25 GtC per year.
If fossil-fuel burning were reduced to zero in the 2050s, the 2 Gt flow from atmosphere to ocean would also reduce significantly. (I used to imagine that this flow into the ocean would persist for decades, but that would be true only if the surface waters were out of equilibrium with the atmosphere; but, as I mentioned earlier, the surface waters and the atmosphere reach equilibrium within just a few years.) Much of the 500 Gt we put into the atmosphere would only gradually drift into the oceans over the next few thousand years, as the surface waters roll down and are replaced by new water from the deep.
This isn’t the place to discuss the uncertainties of climate change in any more detail. I highly recommend the books Avoiding Dangerous Climate Change (Schellnhuber et al., 2006) and Global Climate Change (Dessler and Parson, 2006). Also the papers by Hansen et al. (2007) and Charney et al.(1979).
The purpose of this chapter is to discuss the idea of fixing climate change by sucking carbon dioxide from thin air; we discuss the energy cost of this sucking next.
The cost of sucking
Today, pumping carbon out of the ground is big bucks. In the future, perhaps pumping carbon into the ground is going to be big bucks. Assuming that inadequate action is taken now to halt global carbon pollution, perhaps a coalition of the willing will in a few decades pay to create a giant vacuum cleaner, and clean up everyone’s mess.
A. chemical pumps;
C. accelerated weathering of rocks;
D. ocean nourishment.
A. Chemical technologies for carbon capture
The chemical technologies typically deal with carbon dioxide in two steps.
Hurray for technical progress! But please don’t think that this is a small cost. We would require roughly a 20% increase in world energy production, just to run the vacuum cleaners.
B. What about trees?
C. Enhanced weathering of rocks
I like the small energy cost of this scheme but the difficult question is, who would like to volunteer to cover their country with pulverized rock?
D. Ocean nourishment
A final idea for carbon sucking might sidestep this difficulty. The idea is to persuade the ocean to capture carbon a little faster than normal as a by-product of fish farming.
While it’s an untested idea, and currently illegal, I do find ocean nourishment interesting because, in contrast to geological carbon storage, it’s a technology that might be implemented even if the international community doesn’t agree on a high value for cleaning up carbon pollution; fishermen might nourish the oceans purely in order to catch more fish.
J. Hansen et al (2007)
“Avoiding dangerous climate change” is impossible – dangerous climate change is already here. The question is, can we avoid catastrophic climate change?
David King, UK Chief Scientist, 2007
climate change ... was a controversial question. Indeed there still is a “yawning gap between mainstream opinion on climate change among the educated elites of Europe and America” [voxbz].
Where is the carbon? Sources: Schellnhuber et al. (2006), Davidson and Janssens (2006).
The rate of fossil fuel burning... Source: Marland et al. (2007).
Recent research indicates carbon-uptake by the oceans may be reducing. www.timesonline.co.uk/tol/news/uk/science /article1805870.ece,www.sciencemag.org/cgi/content/abstract/1136188, [yofchc], Le Quéré et al. (2007).
See also the Special Report by the IPCC: www.ipcc.ch/ipccreports/srccs.htm.
Wallace Broecker, climate scientist... www.af-info.or.jp/eng/honor/hot/enrbro.html. His book promoting artificial trees: Broecker and Kunzig (2008).
The best plants in Europe capture carbon at a rate of roughly 10 tons of dry wood per hectare per year. Source: Select Committee on Science and Technology.
Enhanced weathering of rocks. See Schuiling and Krijgsman (2006).
Ocean nourishment. See Judd et al. (2008). See also Chisholm et al. (2001). The risks of ocean nourishment are discussed in Jones (2008).