As even their critics admit, Levitt and Dubner have performed a useful service in drawing greater popular attention to geoengineering. Garden hoses to the sky,however, are not the only approach. Iron fertilization is simpler, cheaper and much more easily testable.
Most people are aware that CO2 and temperature are positively correlated in the long historical record but fewer people know that iron dust correlates negatively on the same scale – that is, temperature and CO2 levels are low when iron-dust is high. The graph illustrates.
The basic mechanism that appears to drive the association between low temperature, low CO2 and high iron-dust levels is that iron-rich dust sometimes sweeps off the continents into the oceans where it creates a plankton bloom. Phytoplankton take up CO2 in order to grow and as they die and produce fecal matter (I kid you not) carbon sinks to the lower depths or bottom of the ocean where it may remain for 100 to a 1000 or to even to millions of years (in the latter case eventually becoming oil).
A big advantage of iron fertilization as a way of reducing CO2 is that this process occurs naturally all the time and thus may be studied. It is also possible to run experiments. Indeed a dozen small-scale experiments over the past decade have already been run with all showing that iron fertilization does create phytoplankton blooms and some showing carbon sequestration. Interestingly, private firms looking for future carbon offset sources are driving much of the research into iron fertilization.