Climatologist Ken Caldeira was interviewed by Discovery Magazine in 2003.
Are there good ways to remove carbon dioxide from the air?
The most benign one is to grow more forests, but the amount of land available is too small. Our simulations suggest that by fertilizing the oceans you might be able to pull 1 billion tons of carbon per year out of the air. Later in the century, carbon emissions are predicted to range upwards of 20 billion tons a year, so ocean fertilization might be able to take care of 5 percent of that.
That comment on tree planting was made before Caldeira’s 2006 study that showed that only tropical tree planting is net beneficial.
You’ve also looked at injecting carbon dioxide into the deep ocean. How would that help?
Some 70 or 80 percent of the carbon dioxide that humans release into the atmosphere will eventually be absorbed by the oceans, but the transit time is several hundred years. One idea is to get the carbon into the deep ocean right away. The downside is that we don’t completely understand the impact of carbon dioxide on marine organisms such as coral, mollusks, and plankton, which make their shells or skeletons out of calcium carbonate. When water is enriched with CO2, it forms carbonic acid. Carbonic acid is corrosive to calcium carbonate. Studies suggest that coral reef growth might be reduced by 50 percent through the century because of the chemical effects of enhanced CO2. Plankton could also be seriously affected.If you put more CO2 into the atmosphere, the ocean is going to eventually become more acidic anyway, but the acid will be distributed fairly evenly over the surface of the ocean. When you inject into the deep ocean, however, CO2 is put in as a concentrated fluid, and the effects will be localized.
What about ambitious proposals, such as using giant mirrors in space to deflect sunlight?
My colleagues and I did computer simulations and found the engineering schemes worked well. They canceled most of the climate change and actually made plants grow more vigorously. But I think Earth is more complicated than our models and more complicated than our understanding of it. If humans attempt large-scale disruptions of the natural system, we’re likely to screw things up. What we really need are strong, long-term reductions in carbon emissions.Most of these plans would cost billions of dollars. Why should we be responsible for paying for something that won’t be a big environmental problem for 100 years or more? There’s no purely rational argument for why we should do something to help future generations. From an ethical point of view, however, we’re the stewards of our environment, and we have an obligation to hand over a planet that is in at least as good a shape as it was when we got it.
What is the most promising alternative energy source?
The key is having a portfolio of energy options. Wind is promising, and it would be helpful if it was coupled with a nationwide electrical distribution system. Fission power could be playing a much larger role, and I say that as somebody who used to demonstrate against it, in front of nuclear power plants.
You’ve criticized the Kyoto Protocol, the international accord to reduce greenhouse-gas emissions. Why?
It’s not enough to restrict emissions to several percentage points below the 1990 level, which Kyoto requires. Strong, long-term reductions in emissions are needed. Also, the Kyoto Protocol restricts emissions in industrialized countries but not in developing countries. Lastly, the amount that countries are allowed to pollute is based on historical patterns–how much those countries polluted in the past–so people who polluted a lot historically get to continue polluting a lot.I would like to see a per capita emission allotment. That would give an incentive for everybody around the world to reduce emissions. With a per capita system, people from Botswana could sell their emission allotment to people from the developed world, giving them capital. Unfortunately, what I would like would never fly politically.
…Life thrived when Earth was much warmer in the past. Why would it be bad if our planet warmed up again?
What is unprecedented is the rate of change. When the dinosaurs were around, the carbon dioxide level was as high as we are likely to make it in the next century, and global temperatures were also high. But that earlier increase occurred over millions of years, so ocean chemistry and ecosystems could adapt to the new climate conditions. We’re asking Earth to make the same kinds of transformations in a few hundred years.
Injecting CO2 directly into the deep ocean would engage the buffering process that ultimately must mediate the ocean’s uptake of most, if not all, of the accumulated and future anthropogenic CO2 emissions. Namely, the anthro. CO2 will be buffered by the dissolution of calcium carbonate sediments on the seabed. Because the saturation state of seawater for these carbonate minerals (aragonite and calcite, the constituents of many marine organisms’ shells) depends in part on pressure, almost all surface waters are “supersaturated” with respect to these minerals so they don’t tend to dissolve in surface and near-surface waters. And they won’t tend to dissolve for a long time because most of these waters won’t be undersaturated, even under quite steep emissions-growth scenarios. So the minerals which are most susceptible to dissolution by injected CO2 are those just above the depths in the ocean where the waters are close to saturation for the minerals – for calcite it’s, on average, some 3000-4000 meters deep, and for aragonite, it’s shallower, ~ 1000-2000 meters. There isn’t as much aragonite on the sea bed because most particles made of this mineral dissolve before they ever reach the seabed, or are not preserved long on the seabed.
The anthropogenic CO2 is entering the ocean at the surface where the main buffering mechanism is the conversion of dissolved carbonate ion ( CO3= ) to bicarbonate ion ( HCO3- ). It will penetrate to the deep ocean eventually, but only via ocean mixing processes that are relatively slow (time scale ~ centuries), and indeed very little anthro. CO2 is in the deep ocean so far.
So Ken is correct that an effective way to sequester CO2 is to inject it deep. Partly because the time scales of mixing will isolate it from contact with the atmosphere, thus it won’t have the chance to “outgas” CO2 for a while. And partly because at depth, the buffering mechanisms of carbonate particle dissolution may be engaged more effectively.
There are ecological risks as the deep ocean basins are not “dead” as was thought in the 18th and early 19th Centuries (one goals of the Challenger expeditions of the 1870s was to test the idea that the oceans were “dead” below a few hundred meters; the “azoic” theory). But most productivity in the ocean, the productivity that fisheries are based on, still occurs in the upper few hundred meters.
The other problem is the cost of injecting the CO2 into the deep ocean.
Concerns over the environmental risks of ocean acidification originated over just such proposals for deep injection of CO2 directly into the deep ocean. See papers such as
Brewer et al. for some discussion of such proposals. Many of these ideas originated in Japan, where there are few stable sedimentary basins suitable for geosequestration.