While I have no doubt that climate change is occurring and has been caused by human activity I am far less confident that we really understand the situation well enough to try to start deliberate climate engineering. Can I offer a radically different possibility: If measured by basic biomass, higher CO2 levels translate into significantly higher planetary biomass. Thus conditions for life on earth might be argued to be ideal at far higher CO2 levels than have prevailed for the last 500 thousand to a million or so years - essentially the global climate has been far too cold and dry for the optimal biomass. And the reason for that is largely because life itself sequesters atmospheric carbon into the ground as you note. How fortuitous and marvellous then that in the late stages of cyclic and intensifying glacial eras a species arises whose ecological purpose is to reverse that very problem. In a geological nanosecond of its arrival this species, Homo sapiens, digs to all the carbon life has sequestered over the last tens of millions of years and immediately begins putting it back into the atmosphere. This species, like all others is compelled by its biological drives - in its case it has a brain that cannot be sustained by local sunlight but requires burning of much older stored sources of sunlight such as found in wood, coal and oil, in order to cook food and provide it with a calorie supply unavailable in raw food. A very timely thing because the next glacial era that will set in within the next ten thousand years is forecast to be a planet killer, a global freeze over that would return Earth to the pre Cambrian conditions of the Cryogenic era in which the seas were frozen to the equator. Unfortunately this species also views itself as somehow transcendent over nature and its biology by virtue of its apparent rationality and freewill and it has interpreted its own actions as the outcome of moral failings rather than biological imperatives. In this context there is a great risk that large scale climate engineering attempts founded on current in vogue interpretations of climate change and our role in it may ultimately prove to be very misguided and harmful to the planet's ability to sustain life. Our best bet would be to try to ensure the health of as much of our terrestrial and marine eco systems as possible so they are best positioned to respond to and buffer the most severe impacts of climate change.